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MK5PFCCIRCUIT

ANALYSIS

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SETEC MK5PFCSlot Tech Magazine

D30

L4

C2

R23 R1

C16

C14

E1

R77

S1

D1

R85

E5

U14

U4

Q5

R18

6

R18

5

E4

X5b X6b E3 E2

L7

L5

C88

R90

L8

C105

U6

U1 U3

U5

U2

D26

D22

R18

4

C75

C76

C82

R176

C81

C56

C53

C86

R114

R18

9

R187

R17

7

R173

R179

R17

1

R17

8

R17

0

R118

R130

R11

9

R14

6

R113

R150

R12

6

R12

5

R11

1

R14

2 R124

R12

3

R15

6

R160

R16

2

R165

C106

Q9

R19

5

R144 R133 R134 R147

R11

7 R

136

R13

5

R137 R139 R140 R149

R11

6

R11

5 R

200

R197

R193

R191 R190

R18

2 R

183

R199

R14

3

R129 R128 R127 R198

D3

R121 R122 R154

C92

C98 C

93

R81

C64 C63

R20

2

C114

R20

5

R204 R203

C84

R92

R

93

R96

R10

0 R

97

R94

C90

R10

3

Q4

Q2

Q8

Q3

R87

R88

R158

R161 R157

R15

1

R168

R172

R18

8

R16

3

R194

R12

0

C78

C79

R13

2

R15

9

R141

R152

R181

R174

R192

R196

R13

8 R

148

R13

1

C72

D18

D17

R208

D19

R209

D16

C99

R21

1

R104

R102

D21

R216

R21

2

R214 R213

C95

C97

C91

C94

C102

C96

C59

R98

R99

R

95

R21

7

R91

R21

8

C116

R89

L6

U12

U7

C61

D5

D4

R82

D6 D7

D8

C117

D27

D28

K1

D33

C52

Q6

D12

U9

U10

U11

U8

R221

C118 D35

R220

R219

U13

D11

R84

C11

9

D36

R223

R22

2

C12

0

C74

C54

C10

8

C73

C

70

C87

C55

T1

R24

0 R

238

Q10

Q11

Q12

R23

5

R229

R233

R23

2

R23

4

R22

8

C12

1

C89

Q1

C101

C10

0

C67

C68

C57

C66

R231

R230

C58

D38

C12

2

R22

6

U15

R239

D37 R23

7

F3

R24

1

Q7

C5

H1

2

3

4 5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27 28 29

30 31

32

33

34

35

38

42

44

46

47 48

55

56

57 58

59

60 61

62

63 66

67

69

72

73

74 75

76

79

80

82

83

85

86 87

88

90

91

92

94

95

96

97

98 99

100

102 103

106

109 110

111

113

114

115

D40

D39

D41

R242

116 117 118

119 120 121 122 123 124

125 126 127

128 129 130 131

132

133

134

136

138

139

140

141 142

143

144

145

146

147 148

154

156

157

158

159 160

161

162

163

164

165 167

168

169 171

172

173 174

175

176

177 178 180

181

182

183

184

185

187

188

190 191 192

193 194

195

196

197

198

199

200

201

203

204

205

206

207

208

209

210 211

212

R86

R108

R243

D42

C125 R245 R244

213

D31 217

218

222

223

224

225

226

227

228

R236 C104

D43

X10

X9

X7 X8

Slot Tech Feature Article

florescent lamps or shutdown the monitor and youbegin to see its complexity.Top it all off with over-current protection andover-voltage protection,sprinkle a little temperaturecontrol on top and you havea complex system thatquickly reminds you thatthere are still some veryinteresting analog circuitsin this mostly digital world.

SETEC MK5PFCCircuit Analysis

The SETEC MK5PFCpower supply is one ofthe most complex

power supplies we see inthe gaming industry. Itincorporates the latestadvancements in modernpower supply design includ-ing Power Factor Correctionand DC to DC conversion.Add to that some very gam-ing-oriented features suchas the ability to control the

Slot Tech MagazineSETEC MK5PFC

AC (mains) InputFiltering and Monitor Control

Switched-mode power sup-plies are noisy. They gener-ate a lot of electromagneticinterference (EMI). Like allmodern power supplies,this one starts off with anEMI filter on the mainsinput (Although we don’tuse the term in NorthAmerica, in the rest of theworld, household AC cur-rent is referred to as the“mains.”). This filter systemlives on the appropriatelynamed “Filter Board.” It iscomprised of a network ofcapacitors and an inductor(C1-C6, L1) and preventsEMI from escaping thepower supply and taking afree ride on the mainswhere it might wreck havocon the rest of the system. Itdoesn’t actually change theAC current in any signifi-cant way.

R8 is a varistor, the self-sacrificing surge protectorthat absorbs the energyfrom things like lightningstrikes and freak powersurges. If you see that ithas blown up, you willnaturally have to replace it.

But don’t think badly of it.Don’t think it failed. It likelywas just doing its job. Youmight want to look at every-thing else plugged into thesame power source. It’susually a one-shot device. Ifit’s blown open, you mightnot even realize it becauseit did its job and the gamestill works perfectly. How-ever, you’ve lost a layer ofprotection and the next hitwill likely be fatal (andmaybe costly).

From here, the AC currentpasses through fuse F2 tothe female mains connec-tor. This is an unswitchedmains outlet. At the sametime, a double pole, single

throw mains switch com-pletely isolates both the hotand neutral lines when it’sin the off position. Whenthe switch is in the onposition, it applies themains to the Main Board ofthe power supply throughyet another line filter cir-cuit, this time comprised ofcoils L2 and L3, and capaci-tors C7-C11. It also appliespower to the monitor butnot directly.

Zero-Crossing

Before applying AC power tothe monitor, the AC currentpasses through a “zero-crossing” circuit made fromU1 and a TRIAC.

The “zero-crossing” is the instant in time that the voltage and current are bothzero. By switching only during this time, AC loads such as the monitor can besafely connected to the mains without excessive inrush current.


A zero-crossing circuitassures that the only timethat the monitor will actu-ally make the connection tothe mains is when theinstantaneous voltage of themains is precisely zero. Atthat exact moment in time,there is no current flowingthrough the mains and themonitor can be connectedsafely without causingexcessive inrush current orfreaking out the degaussingcircuit in a CRT monitor.Essentially, it’s like thedifference between hotplugging something and nothot plugging it. If themonitor’s power is con-nected only when the volt-age is zero, it can ramp upslowly as the sine wavevoltage increases. It’s amuch nicer and less de-structive way to turn thingson and prevents things likefuses that seem to blow forno reason or destruction ofinput rectifiers.

At the heart of the zero-crossing circuit is the zero-crossing detector itself, anMOC3083. The MOC3083consist of a normal, infra-red light emitting diodethat’s optically coupled to adetector. Sounds like anormal opto-isolator,doesn’t it? It would be ex-cept that on the same littlesilicon chip that containsthe photo-detector, there isa bit of circuitry that de-tects the zero crossing andgates the TRIAC (labeled D1on the schematic but that’san odd designation for aTRIAC) with an output frompin 6 only at the moment of

zero crossing.

The rest of the monitor’smains control circuit isstraightforward. The hotside of the AC power passesfrom pin 2 of the mainsswitch, through fuse F1and, when gated, throughthe TRIAC to the monitorreceptacle. The neutralconnection is between pin 1of the mains switch and themonitor receptacle.

But this so-called “FilterBoard” has another func-tion as well and it’s reallyimportant to realize thatthe monitor power is notonly controlled by the zero-crossing circuit but thatthe zero-crossing circuititself is controlled. It iscontrolled by the all-power-ful “Low Power” signal thatcomes from the game itself.

The Low Power signal is anactive low signal that


comes from an open collec-tor output on game itself.When the signal is low, itturns on the LED in theopto-detector-zero-crossing-IC-thingy, activating (gat-ing) the TRIAC and turningon the monitor. The bottomline is this, if this signal isnot active, your monitor willnot have AC power. Neitherwill the florescent lights butmore about that later.

Inrush Current Limiting

From the filter board, themains is connected to theAC input of the main boardof the power supply. Afterpassing through Fuse F3,the AC passes through yetanother line filter (L4 andassociated capacitors). Nowit’s time to apply the AC tothe input rectifiers—or is it?Not quite yet. One of themanufacturer’s specifica-tions for this power supply(driven by OEM require-ments, I assume) is that theinrush current be less than25 Amps peak when turnedon at either 120 Vac or 240Vac. In order to help ac-complish this, the MK5PFCuses a couple of seriesresistors and a relay toachieve a two step, “softstart” procedure.

When power is first appliedto the unit, relay contact K1is open. You can see thatthe contact is drawn on theschematic in its “normallyopen” position. The ACinput must pass throughtwo, high-wattage, ceramic,wire wound resistors (R77and R202, each 9.1 ohms, 7

watts) before it reaches thebridge rectifier, D1 (al-though I suppose it wouldbe more precisely correct tosay that the resistors areactually in the return path,between the bridge and theneutral side of the AC lineand that the AC currentpasses through Fuse F3,through the line filter to thetop side of the bridge andreturns through the resis-tors to neutral).

However you want to lookat it, there is a total of 18.2ohms of resistance in serieswith the AC input, limitingthe AC current to less than.5 Amp before the 7 wattdissipation rating of theresistors is exceeded.

The concept is this: Poweris applied to the unit. Atfirst, the current-limitedpower is applied, activatingthe power factor correctioncircuitry which, as you willread anon, controls thecharge rate of the primaryelectrolytic capacitor, themain culprit in the genera-tion of both high inrushcurrent and third harmon-ics, a pair of nasty phenom-ena we can do without,thank you very much.

After a few seconds, whenthe primary filter capacitor(C52) is fully charged andeverything has stabilized,relay K1 energizes “<click>”and the resistors are by-passed by the relay contact.The mains is now con-nected directly to the powersupply which then proceedsto connect power to the

loads such as switching onthe monitor AC and the +24Vdc outputs for the flores-cent lamps. Please keep inmind that the game cir-cuitry has ultimate powerthrough the Low Powersignal and that the loadswill not be energized untilthe signal is pulled low.

Also notice that there is a130 degree Celsius thermalswitch (S1) in series withthe resistors as well. Itopens at 130 degrees Cel-sius. That’s 266 degreesFahrenheit. The thermalswitch is visible in the lowerleft corner of the PCB. It isphysically mounted directlyon top of the two inrushcurrent limiting resistors,R70 and R202. It’s a safety.If the power supply doesn’tfire up immediately andenergize relay K1, theseresistors will get hot. If thetemperature exceeds 130 C,S1 opens and the currentflow stops. Keep this inmind when you’re trouble-shooting. If these resistorsare hot, don’t assume thatsomething is shorted on theAC input, drawing toomuch current through the

The two inrush current limitingresistors with the 130 degreethermal switch mounted on top.


resistors. ANYTHING thatprevents the power supplyfrom energizing relay K1(and that’s just about any-thing that fails in the powersupply) will result in theseresistors getting hot.

Power Failure Detection

Before we leave the AC areaand enter the DC world,look back at the top of theline filter L4 and find diodeD3, a common 1000 volt, 1amp 1N4007. This is thestart of our “power fail”detector. It is connected tothe hot side of the mainsand passes half the ACcycle through a string ofvoltage dividing resistors toa comparator circuit thatwe’ll look at later on. If thecomparator sees a singlemissing cycle or even a fewcycles, it will do nothing atall. However, if the powerreally has failed (perhaps ahalf-dozen missing cycles) itgenerates a PFAIL signal.This signals the game’scomputer which then doesall the housekeeping neces-sary to retain the integrityof the game prior to theimminent loss of power. Thepower supply itself willremain operational longenough to accomplish thesetasks (even to the point ofoperating the mechanicalmeters). It does this byvirtue of the energy storedin the primary filter capaci-tor, extended by instantlyshedding loads such as theflorescent lamps. This isone smart power supplyand it does it all with ana-log electronics as you’ll see

later.

To the Bridge and Beyond

We have finally made it tothe bridge rectifier, D1. Ofcourse the bridge rectifiesthe AC input, turning it intofull-wave, pulsating DC.Following D1, we find adeparture from traditionalpower supplies. The pri-mary filter capacitor is notconnected immediatelyfollowing the bridge rectifieras we have seen in allpower supply designs of thepast. Instead, a power fac-tor correction circuit isinserted between the outputof the bridge rectifier andthe primary filter capacitor,C52.

Let’s follow the positiveoutput of the bridge rectifierand see where it leads.There are two paths herefor the current to flow. Onepath passes through diodeD43 and then to C52. Butwhy do we need the diode?It’s already DC, isn’t it?Sure it is. It’s the output ofa bridge rectifier and bridgerectifiers turn AC into DC.Is the current being“double-rectified” or some-thing? Seems mysterious,doesn’t it?

The answer lies down theother path so let’s go backto the positive output of thebridge rectifier and follow itstraight across to coil L5.This is a large toroidal coil.From the right side of L5,we can follow the currentpath through diodes D12and D11 and then to the

positive lead of the C52, theprimary filter capacitor.What is going on here? Whyare there two paths andwhy do we have the “extra”diodes?

Harmonic Currents andActive Power Factor Correction

If you’re a regular reader ofSlot Tech Magazine, youknow all about harmonicsand switched-mode powersupplies. You know aboutthe power-sapping thirdharmonic and how it robsyour casino of power. If youneed a refresher, the topicwas covered extensively inthe August 2004 issue.

Harmonic currents are adirect result of the way inwhich a switched-modepower supply (SMPS) drawscurrent from the system.The input circuit of anSMPS is a bridge rectifierthat changes the 120 voltAC input to DC. A capacitorsmoothes this DC to elimi-nate voltage ripples and theresultant DC bus has avoltage of about 170 voltswhen the AC rms input is120 volts. Although the ACvoltage is a sine wave, therectifier draws its current inspikes. These spikes requirethat the AC supply systemprovide harmonic currents,primarily 3rd, 5th and 7th.These harmonic currents donot provide power to theSMPS, but they do take updistribution system capac-ity. The principal harmoniccurrent is the 3rd (180 Hz)and the amplitude of thiscurrent can be equal to or


even greater than that ofthe fundamental current.

We solve this problem withpower factor correction.Look at the circuit madefrom MOSFET Q1 and itsassociated driver, U7. Itkind of looks like it is itsown SMPS, doesn’t it?However, the drain of theMOSFET is connected tothe big toroid coil, L5.What’s this all about?

This, my friends, is a trickylittle circuit called a “boost”power supply. In this case,it’s more specifically calleda “follower boost.” We areusing the coil’s ability tostore energy, not as acharge (as we do with acapacitor) but in the form ofa magnetic field.

Our goal here is to changethe way the monitor’s filtercapacitor draws currentfrom the bridge rectifierand, subsequently, the AC(mains). We’re looking for away to boost the pulsatingDC output of the bridgerectifier so that instead ofcharging the filter capacitorwith narrow, harmonic-producing spikes of cur-rent, we have a steady flowof current flowing from thebridge rectifier into the filtercapacitor.

We accomplish this feat bypulsing MOSFET Q1. WhenQ1 is turned on, currentwill flow from the positiveoutput of the bridge recti-fier, through L5 andthrough Q1 to the highvoltage return path at the

negative side of the bridgerectifier (not technically“ground” in this case as theentire primary side is iso-lated, as usual, from thesecondary). The coil is ourload and it builds up a nicebig magnetic field. When Q1is turned off, the magneticfield collapses. This rapidlycollapsing magnetic fieldslices across the coils ofcopper wire and turns thecoil into an electric genera-tor in a process called “in-duction.” This newly gener-ated voltage (you can kindof think of the coil as abattery for this moment intime) is now in series withthe output of the bridgerectifier and, just like twoor more dry cell batteries inseries in a flashlight, thevoltages are added together.It’s called a “follower boost”circuit because this newlygenerated voltage is addedto the incoming voltage. Ifthe incoming AC rises, theboost follows along, risingas well. We don’t care aboutregulating the voltage atthis point because we’regoing to do that next withthe PWM part of the SMPS.

The result is that we aretaking a sine wave in andproducing a constant volt-age out and the upshot ofthis whole thing is thatinstead of charging the filtercapacitor only during thebrief peak period of the ACsine wave, we can keep aconstant charge on it andsubstantially reduce (oreliminate altogether) thethird-harmonic content ofthe system. This is known

as “active power factorcorrection” or PFC.

The diodes we were talkingabout at the beginning ofthis discussion (D11, D12and D43) are a sort of elec-tronic “anti-siphon” valve.They are used to ensurethat the current doesn’t“backflow” when, for ex-ample, the output voltage ofthe boost follower circuit ishigher than the outputvoltage of the bridge recti-fier.

The “brains” of the outfit isthe UCC38503 combinationPFC/PWM controller IC,U14. Because modernpower supplies often in-clude active PFC, this ICsimply includes both PFCand traditional PWM tech-nology in one package. Onthe PFC side, theUCC38503 samples boththe pulsating DC output ofthe bridge rectifier and thevoltage at the positive ter-minal of the filter through acouple of resistor voltagedivider networks. You’ll seea lot of these in this powersupply, where an extremelyhigh voltage (as high as+400 Vdc on the primary) ispassed through a dividernetwork of five or sevenresistors in order to cut thevoltage down before apply-ing it to the low voltageinput of a voltage compara-tor or other IC such as thisone. In this case, the volt-age sense input is pin 3.Pin 18 checks to see what’scoming out of the bridgerectifier. The UCC38503figures out what to do


based on these inputs andin turn, sends an outputfrom pin 12 to U7 which inturn controls Q1, turning itrapidly on and off, alter-nately storing and releasingenergy in L5 in order tomaintain a smooth flow ofcurrent into the primaryfilter capacitor, C52.

Are You High?

Yes. Very high. I am speak-ing of course, about thevoltage on the primary filtercapacitor. The schematicdiagram pegs it at +400Vdc. Of course, we’re even-tually going to cut thatdown to +24 Vdc. That’s theoutput voltage of the unit.But before we move on tothe secondary side ofthings, there is a very im-portant aspect of this de-sign that needs to bepointed out. U14, the

UCC38503 PFC/PWM Con-troller needs a power sourcebefore it can do anything atall. So does the MOSFETdriver, U7. Nothing’s goingto happen unless we getpower to these devices. Atthe moment, our only DCsupply is the +400 Vdc at

the positive terminal of theprimary filter capacitor.

To accomplish this task, wehave a power supply withina power supply here. It’s aremarkable little high volt-age, three terminal linearregulator (U13, a type

UCC38503 PFC/PWM Controller


VB408) that takes the +400volt, unregulated voltage atthe primary and regulates itto an output voltage thatcan be anything between+1.25 Vdc to just 30 voltsbelow the input voltage. Inthis case, we can create anice regulated power supplyof +13.25 volts DC by peg-ging the reference voltage atpin 1 with a 12 volt Zenerdiode, D38. The output ofthe device is 1.25 volthigher than the referenceinput at pin 1. The outputcurrent is limited to just 40milliamps but it’s enough topower the few low voltagethings we need to operatebefore the main powersupply output comes online, specifically U7 andU14 as well as an LM339Quad Comparator (U2) andthe relay, K1 (remember theinrush current limitingsystem and the relay con-tact that bypasses the re-sistors? All of that has tooperate BEFORE the powersupply kicks in!).

PWM Controller

We have looked at the PFChalf of the UCC38503combination PFC/PWMcontroller IC. Let’s continueby looking at the other halfof U14. I guess I’d have tosay that this is the only“boring” part of this powersupply. The PWM controllerpart of U14 is totallynormal in every respect. Pin10 is the “Gate 2” outputthat controls MOSFET Q5,the primary switchingtransistor that switches theprimary current on and off

through the primarywinding of powertransformer T1. Totallynormal.

You can see the typicalregulation feedbackprovided by the transistorhalf of an opto-isolator (U4)and you can see that theLED half of U4 is poweredby the +24 VDC output ofthe supply with afrequency-compensatedTL431 (U9) providing a nicereference voltage on thecathode of the device.

We light up the LED in theopto-isolator with voltagefrom the secondary outputof the power supply. Thehigher the voltage, thebrighter the LED shines.We read the brightness ofthe LED with thephototransistor in the opto-isolator, which is connectedto the PWM control circuitryin U14 on the primary sideof the transformer. In thisway, the secondary can“talk” to the primarywithout actually touchingit.

But in order to maintaintight voltage regulation, weneed to go just one stepfurther. We need to controlthe brightness of the LEDunder a variety of changingload conditions of both highand low frequencies. Thereis another element in thechain of regulation and inthis case it’s U9, a typeTL431. Get to know theTL431 because there arefour of them in this powersupply.

You can think of the TL431as a sort of programmableZener diode. It is a “shuntregulator” that can beprogrammed to be anyvoltage from a minimum of2.5 volts to a maximum of37 volts. Inside the device,an internal 2.5 voltreference is compared tothe voltage that is appliedat the reference pin input.This reference voltage isderived by a resistor voltagedivider (R96, R97, andR100). The TL431 providesthe gain that is needed atlow frequencies so that theLED in the opto-isolator willproduce enough of a changein brightness in order tosignal the primary side andcompensate for the lowfrequency changes in theload.

But this gain is not neededat high frequencies. Thegain of the opto-isolatoritself (the CTR or currenttransfer ratio) works justfine, without any assistancefrom the TL431’s gain,thank you very much. Thisleads to a sort ofengineering dilemma wherehigh frequency changes inload can produce largervoltage swings than lowfrequency loads, makingtight regulation impossible.

Compensation

In this case “compensation”is “frequencycompensation” which is theway we can control thefrequency response ofvarious circuitry. By usinga combination of resistors


and capacitors primarily,we can integrate the variousload frequencies and “tell”the TL431 how to behave atcertain frequencies. In thispower supply, thecompensation network ismade from C119, R103 andC89, a 1 uf, bipolarcapacitor. Thecompensation networkallows the TL431 tomaximize its contribution atvery low frequencies and toremove its influence athigher frequencies. Theconnection of C89 andR103 between the cathodeand the reference terminalof the TL431 allowsmaximum loop gain at DCfor the best voltageregulation.

What else can you sayabout this totally normalSMPS design? The output ofpower transformer T1 isrectified by D30 and filteredby C88 (2200 mF 35 V).After passing through achoke (L8), C106 (also 2200mf 35V) provides additionalfiltering. At the same time,of course, C88 and C106are “reservoir capacitors”that, along with the energystored in C52 (the primaryfilter capacitor) will be allthe energy that the powersupply has in case of apower failure.

Cut the Juice, Bruce!

Do you remember the LowPower signal from lastmonth? Remember that theLow Power signal must bedragged to ground in orderto energize the monitor AC

power. Well, the Low Powersignal is doing somethingelse at the same time,something sort of unrelated(electronically speaking) tomonitor control butsomething that a slotmachine needs to take careof and that is surviving ablackout. In the case of animmediate and unpredictedloss of AC power, a slotmachine has some seriousbusiness to attend to beforethe energy stored in thepower supply’s electrolyticfilter capacitors is fullydissipated. Mostly, the CPUsimply has to store a smallamount of data (things likecustomer credits andcurrent game condition)and perform an orderlyshutdown but in somecases, the slot machinemight even want to run longenough to continue toincrement the hard coinmeters (theelectromechanical unitsthemselves) until thecorrect count is obtained.

The MK5PFC makes thispossible by quicklyshedding some of the +24VDC load and it does it witha remarkable little devicecalled a UCC3913 NegativeVoltage Hot Swap Manager,also known as a circuitbreaker! Both of theseterms are familiar to us, ofcourse. We know all abouthot swapping (and thedamage it can cause in aslot machine) and a circuitbreaker is, well, a circuitbreaker.

When the Low Power signal(it comes from the slotmachine, remember?) goeshigh, the LED in U1 on thefilter board turns off. Wecovered that in part one.But what we didn’t cover iswhere the LED in U1 getsits power source at theanode. This is very clever.Let’s look at the entirecontrol circuit.

U13 is the UCC3913. Let’sfollow the green path. Thisis the +24 VDC power bus.It’s the output from thesecondary winding of T1,rectified by D30 and filteredby C88. It’s the actualoutput of the power supply,its Raison d’être. The +24VDC bus is connected tothe florescent lamps atconnectors X7 and X8, pin4. However (and here comesthe interesting part) weneed a return path tocomplete the circuit for theflorescent lamps. Thereturn path is through pin2. Follow the checkeredgreen path, remembering asyou do that this is thereturn path and we’reheaded for groundsomewhere, the shorter thepath (least resistance) thebetter. In this case, theshortest path (the onlypath) is through MOSFETQ7 and its source resistorR236 (just 15 milliohmsused, naturally, for over-current sensing) to ground.

Naturally, Q7 has to beturned on for this tohappen and you can seewhere all this is headed.U13 is the thingy that


controls the gate voltage ofQ7. Obviously, U13 has tohave power in order for thisto happen. That powersource (Vdd-pin3) comesfrom the +24 VDC busthrough transistor, Q6. It’sa PNP transistor with itsemitter connected to the+24 VDC bus and its base(which must be draggeddown in voltage in order toenergize Q6 because it’sPNP) connected to—TADA!—the Low Power signal!That’s an interestingconnection between thecircuit that controls themonitor AC and the circuitthat controls the florescentlamps. At the same timethat the Low Power signal islighting the LED in U1(thus controlling themonitor) it is pulling downthe voltage at the base ofQ6, shooting the powersource to U13 whichenergizes Q7, completingthe return path to groundand lighting the florescentlamps.

In the case of a powerfailure, the Low Powersignal goes high. Thisinstantly shuts off theTRIAC providing AC powerto the monitor (which atfirst glance seems sort ofsilly since there actually isno AC at the moment butit’s part of the whole loadcontrol circuit) and, at thesame time, turns off theflorescent lamps. Byshedding some of the loadfrom the +24 VDC powerbus, the energy stored inthe power supply will besufficient to take care of

business before everythingdecays to zero.

Correction

Actually, it’s more of anaddendum but I admit toan error in our discussionof U13, the type VB408. Ihad mentioned that theoutput current is limited tojust 40 milliamps butmentioned that it wasenough to power the fewlow voltage things we needto operate before the mainpower supply output comeson line. It is not. Notwithout some assistance.What I had failed tomention is a pretty darnedclever little part of this“power supply within apower supply.”

If you recall from ourdiscussion, coil L5 is anenergy storage device usedin the PFC circuit. Welooked at L5 as part of thePFC circuit but there is yetanother winding on L5,wound around the sametoroidal ferrite core. It’sconnected to pins 5 and 6and can be found on theschematic diagram just tothe right and below thevoltage regulator, U13.Once the PFC circuit kicksin (and Q1 is operating)there is a ton of energy inL5. The winding betweenpins 5 and 6 simply picksup some of this energy.Sounds like a transformer,doesn’t it? For all intentsand purposes, it is. It’s justa really cool and efficient,high-frequency, toroidaltransformer that keeps all

of its precious energytightly held within itsdonut-shaped core until wetap into it when we need to.

And we need it now. Oncethe PFC circuit has kickedin, we take this low voltageoutput from L5 and rectifyit with D17. From there, thecurrent passes through R82and onto the power bus.Diode D19 prevents thecurrent from flowingbackward when the outputvoltage of this little “supplywithin a supply” is greaterthan the output voltage ofthe regulator. Also, we can’tallow the output voltage ofU13 to exceed the inputvoltage as can occur whenthe main AC power isremoved. The primary filtercapacitor can (will)discharge faster than thesecondary filter capacitorsbecause it uses the last bitof its energy charging them!In general, voltageregulators don’t do wellwhen the output voltageexceeds the input voltage.

By their Grounds Ye ShallKnow Them

As with all switched-modepower supplies, it is veryimportant to realize theisolation between theprimary circuits and thesecondary circuits. In thispower supply, there is quitea bit of low-voltage circuitryconnected to the primaryreturn (0v) which is totallyand completely isolatedfrom the secondary ground!This “hot return” (meaningthat although it represents


a common return path (0v)for all of our primarycircuitry, it is hot in respectto Earth ground) is markedon the schematic diagramwith a black and redcheckerboard pattern.When you are makingvoltage measurements inthe primary, you must haveyour meter groundconnected to the properpoint. If you are using anoscilloscope, you must havethe power supply pluggedinto an isolationtransformer beforeconnecting your ‘scopeground to this point or youwill vaporize portions ofboth the power supply andthe oscilloscope ground.

The “cold return” is thesecondary ground. It’smarked on the schematicwith a blue and blackcheckerboard. This istotally normal andconnected to all of thegrounds throughout the slotmachine, including theEarth ground and all DCgrounds everywhere.

As you look at the tworeturn paths (the primary,with its red checkerboardand the secondary in blue)the schematic diagram sortof resolves itself and youcan more readily visualizethat the circuits in thelower left hand corner of theschematic (U1, U3 andassociated components—they’re fault detectioncircuits that we’ll get toshortly) actually belong onthe right side of theschematic (to the right of

the secondary winding ofpower transformer T1) ifyou wanted to follow hardand strict rules of drawinginputs (the primary circuits)on the left and outputs (thesecondary(s)) on the right.Once you realize that, it’smuch easier to visualizehow the circuits actuallyoperate and the schematicdiagram is way lessintimidating. Of course,there is no way to draw theschematic that way in anysort of acceptable aspectratio. It would be way toowide. Honestly, it’s amiracle the engineers wereable to fit it all on one pageand it is, in fact, a verywell-drawn schematicdiagram. I’ve just made iteven better by adding thecolored busses so we canidentify, at a glance, theoverall structure of theunit.

Fault Detection andProtection

We want to keep an eye ona few different functions.We certainly want to look atthe output voltage to makesure it doesn’t go too high.If something fails and the24 VDC output climbs ashigh as 28 volts, we want todo something about it(pronto) before anythingbecomes damaged.Likewise, if the outputvoltage drops below 22.5VDC, we’d like to know thatas well. It’s not likely tocause any damage but ifthe voltage is dropping, wewill want to start sheddingloads and at the same time,

inform the CPU so it canstart an orderly shutdown.We also measure theinternal temperature as well(you’ll see why in a minute).

All of these are analogmeasurements but wereally don’t need tomeasure these actualvalues so much as wesimply need to know whenwe have crossed a presetthreshold. As long as theoutput voltage is between22.5 and 28, we really don’tcare what the actual voltageis.

This comparison betweentwo values (our sampledvalue and the preset)perfectly describes thefunction of a comparatorand that’s what U3 is. It’san LM339 quadcomparator. The “quad”part means that there areactually four identicalcomparators in a singlepackage so what appears atfirst to be four individualdevices is really just onecomponent.

In a single-endedconfiguration like this one,its operation couldn’t besimpler. There are twoinputs and one output. Thetwo inputs are labeled +and -. The + input is alsoknown as the “non-inverting input” while the –input is also called the“inverting input.” TheLM339 compares the twovoltages at the inputs. If the– input is a higher voltagethan the + input, the


output pin goes to zerovolts (the voltage at pin 1).

However, rather thanthinking of it as “puttingout zero volts” it’s muchbetter to think of the outputas what it really is, aconnection to ground. It’san open collector output,Q8 on the schematic of thecomparator. When the –input is higher in voltagethan the + input, thecomparator is “activated”and the output is connectedto ground through thetransistor. Anythingconnected to the output pinwill become grounded. It isa “current sink.” It is NOT acurrent source.

On the other hand, if the +input is a higher voltagethan the – input, the outputpin essentially becomesdisconnected fromeverything (Q8 is turned off)and will be just swinging inthe breeze with its opencollector. Of course, thevoltage on the pin willswing up to the voltagedetermined by what everpull up resistor or resistorvoltage divider network wehave on the output. Wedon’t really even care whatthis voltage is all the time.We can often just think of itin digital terms as being“high” or even just “notgrounded” and leave it atthat.

With this concept firmly inplace, let’s start on the leftside with U15, the LM35CTemperature Sensor. Thispower supply has an

Inside the LM339 Comparator


internal temperaturesensor that is part of theover-current protectionsystem. It’s is a reallygreat place to see thecomparator in action. In anutshell, if too muchcurrent is drawn from thepower supply for anextended period of time(causing the power supplyto overheat) or if themachine is operating in anenvironment that exceedsthe maximum temperaturerating (causing the powersupply to overheat) wewant to turn off the powersupply. U15 has just threeleads: a power supplyinput, a ground and anoutput. Want to guess howit works? If you said “theoutput voltage changeswith temperature” you’reright. It operates in a rangeof -55 to +150 C. The higherthe temperature, the higherthe output voltage (it risesat 10 millivolts per degreeCentigrade). You can seethe output is connected topin 10, the – input of theLM339, U3A.

For the other input to thecomparator, we generate a+2.5 VDC reference voltageusing a TL431. Thisreference gives us a preciseand unchanging voltage towhich we can compareother voltages. In this case,we set the “trip” level of thecircuit with a voltagedivider made from R124and R231.

As long as things are cool,the – input voltage is lowerthan the + input and the

comparator output sits atabout 3 volts (R142 andR111 form a voltage dividerthat does this). However, ifit becomes too hot insidethe power supply, thevoltage at the – input willexceed the reference voltageat the + input and theoutput at pin 13 will go toground.

This low signal is felt at the– input of U3B at pin 6. The+ input of U3B is tieddirectly to the 2.5 VDCreference voltage so as the –input drops from 3 volts tozero, the output voltage ofU3B (pin 1) swings high.This signal is connected tothe + input of U3C at pin 9.As the voltage rises from 0(which is what it will be ifthe temperature is OK) andpasses the +2.5 VDC

reference (connected to the– input of U3C at pin 8) itwill trigger the output ofU3C at pin 14 to go high aswell. As you can see, theoutput of U3C is connectedto the gate of MOSFET Q10so this high signal will turnon the device.

Please remember that whatis REALLY happening hereis that the gate of Q10 isnormally HELD DOWN TOZERO VOLTS by theACTIVE output of U3C. It isonly when the temperatureRISES that the output ofU3C goes high (meaning itsinternal open collectortransistor is now turned off)and the gate is allowed tobe pulled high by resistorR229. This “negative logic”is carried out throughoutthe design of this powersupply. The active devices


are almost never sourcingcurrent (Q6 being thenotable exception as itsources the Vdd for U12).The current source insteadcomes from some sort ofpower bus, through aresistor or some resistors asa voltage divider. We shuntthis current to ground (or 0volts or whatever you wantto call it here) when thecomparator’s – inputvoltage exceeds the voltageat the + input.

So, after all of this, here iswhere we stand: If thetemperature is normal, Q10will be off. If it gets too hot,Q10 will be turned on.

What is Q10 doing and whyis it so important in thischain of events? The drainof Q10 is connected to thecathode of the LED in opto-isolator U5. When Q10turns on, the LED lights up.Directly above the LED halfof U5 is the phototransistorhalf of U5, connected (asyou can see by the redcheckered return path) tothe primary side of thepower supply. Now we’regetting somewhere becausewe have not only detectedthe high temperaturecondition but we have away to tell the primary sideof things (where all theaction is!) that we have aproblem and that it mightbe a really good idea to shutdown before things get anyhotter.

So turn your attention nowto the phototransistor halfof U5 and the + input of U2

to which it is connected. Asyou can see, if thephototransistor in U5 isturned on by the light fromthe “high temp” LED half ofU5, it’s going to drag the +input down to zero voltsand, since the – input willnow be higher in voltagethan the + input, theoutput of U2D at pin 13 willgo low. This will drag downthe gate of MOSFET Q4,turning it off and when thathappens, finally, at longlast, we arrive at the finalgoal of this circuit.

When Q4 is turned off, therelay, K1, drops out. Do youremember K1 from part oneof this discussion? Relaycontact K1 shorts out the18.2 Ohms of seriesresistance on the AC input.If the temperature rises toohigh, the relay drops outand the power supplyenters an operating modethat places resistors inseries with the AC inputonce again.

It’s also time to shut downthe power supply. This iseasily accomplished be-cause U14, the UCC38503PWM Controller has an“Enable” input. Pin 4 has tobe high (typically, it’saround 8 volts) for thedevice to operate. All wehave to do is to drag pin 4low and the GT2 output atpin 10 turns off, turning offthe entire primary circuit.Of course, the +24 VDCoutput goes down as well.

As covered previously, whenan over-temperature condi-

tion occurs, U2, pin 13 goeslow. The cathode of diodeD28 is connected to thispin. The anode of D28 isconnected to the Enable pinof U14. When U2, pin 13goes low, it drags down theEnable pin and shuts downthe power supply.

OVP

In this same area, we findthe over-voltage protectioncircuit as well. Like theOver-Temperature control,it’s U3. It’s the one remain-ing section of U3, inputpins 4 and 5 with the out-put at pin 2. Like an over-temperature condition, anover-voltage condition in apower supply needs to bedealt with swiftly and force-fully. Over-temp mightdestroy a power supply buta severe over-voltage condi-tion has the potential ofcreating a lot of damage allover the system so we needto take care of it right away.

Pin 4, the inverting input ofthe comparator, is alwayslooking at the +24 VDCoutput of the power supply.It’s the green colored busson the schematic diagram(if you don’t have lastmonth’s schematic handy,you can download a copyfrom tinyurl.com/mk5pfc).It does it through a voltagedivider made from resistorsR113, R150 and R216. Ireally like the way theseengineers think at SITECbecause the total resistanceof all three of these resis-tors is very close (less than5% away) to 24K ohms.


That makes it easy to seethat you have one volt perohm, making the voltage(when everything is normal)at pin 4, 2.2 volts. Comparethat to the 2.5 volt refer-ence and you can see thatwhen all is well, the outputof U3, pin 2 will be highand we will not be turningon the LED in U5. As yourecall from the Over-Tem-perature Sensor discussion,as soon as we energize theLED in U5, the power sup-ply is going to shut downand you can easily see nowhow that’s going to happen.When the power supplyoutput rises above +28VDC, the voltage at pin 4rises above 2.5 volts, mak-ing the “-“ input higher involtage than the referenceat the “+” input and forcingthe output at pin 2 to go toground. Of course, thiscompletes the ground con-nection for the LED in U5,turning it on and subse-quently (and immediately)turning off the power sup-ply as explained earlier inthe Over-Temperature Sen-sor discussion.

Recap: Bad things such asover-temperature or overvoltage will turn on U5.

Let’s go back to relay K1and recall that when badthings happen to the powersupply and energize U5, K1drops out and no longershorts out the 18 ohmsworth of resistance that isin series with the mains atthe AC input. Let’s see howthe power supply is going toreact to this series resis-

tance.

Naturally, putting 18 ohmsin series is going to have anoticeable effect on things.Specifically, the unregu-lated primary voltage isgoing to drop. This is wherehaving the colored bussesreally comes in handy be-cause if you follow the sortof pink colored primary DCbuss all the way to the left,you’ll see a voltage dividermade of seven resistors,R199, R214, R213, R121,R122, R154 and R156. Ourdivided voltage is applied topin 6, the inverting input ofone of the four devicesinside integrated circuit U2,an LM339.

You can see from the factthat these guys are all .5%resistors that this is some-thing important. This iswhere we keep an eye onthe primary DC voltage.This is another comparatorcircuit. Our reference volt-age is at pin 7, connected tothe 2.5 volt reference bussthrough precisely 1000ohms (again, .5% precisionresistors are used).

It is worthy to note that the12 volt power supply for theLM339 U2 and the refer-ence voltage generated byU8, the TL431 comes fromU13, the three-terminalregulator we discussed inpart 1. Because the 12 voltoutput of this regulator isso low in comparison to the+360 VDC or so that is itsinput (when the mainsinput is 240 VAC) the out-put of the regulator will

remain perfectly stable,regardless of the fact thatwe may have dropped theprimary DC voltage some-what with the inclusion ofthe 18 ohms of series resis-tance or even in the case ofa serious problem with themains input. This +12 voltsupply will remain up andperfect for quite some time,even as the world collapsesaround it as the input onlyneeds to be a few voltshigher than the output inorder to function perfectly.

Normally (when everythingis operating properly) thevoltage at pin 6 is higherthan pin 7, activating theoutput and dragging pin 1down to ground. Notice thatit is the “hot ground” on theprimary side since that’swhat we’re looking at here.This turns on the LED halfof opto-isolator U6.

When we throw the 18ohms of resistance in serieswith AC input, the unregu-lated primary voltage drops.The voltage at U2, pin 6drops below the referencevoltage and the output atpin 1 will swing high, leav-ing the LED without a re-turn path, its cathodeswinging in the electronicbreeze without a path toground. Of course, thisturns off the LED.

Resistor R160 straps theoutput voltage back into thenon-inverting input, assur-ing that the output at pin 1remains latched “off” andthat the circuit doesn’treassert itself. The specific


circumstances of thisreassertion do not warrantdiscussion in this article.

So to recap, if the primaryvoltage is good, U6 LED ison. If the primary voltagebegins to drop (whethercaused by the 18 ohms ofseries resistance OR by anactual drop in primaryvoltage caused by a signifi-cant drop in the mainsvoltage) U6 LED is off.

If we’re driving the LED halfof an opto-isolator we mustbe getting ready to talk tothe outside world. We are,but in a surprise move, allof this primary voltagedetection that we just per-formed has absolutely noth-ing to do with the actualpower supply. That is tosay, it has nothing at all todo with the generation ofthe +24 VDC output of thepower supply. It is a detec-tion circuit that talks to theslot machine itself. It exam-ines the condition of theprimary DC voltage (fromwhich it infers the conditionof the mains-when we addthe series resistance, we areactually “tricking” thepower supply into thinkingthat the mains voltage isdropping) and signals theCPU in the slot machinethrough the status of the“Power Fail” signal. You cansee it on the schematicdiagram at pin 9 of connec-tor X9. Follow it back andyou’ll see it’s connected tothe drain of Q2 so obvi-ously, when Q2 turns on,the PFAIL signal will go low.

We’ll back up and look atthe entire circuit in just amoment but I want topause and reinforce thisconcept to those of you whohave been wondering justhow it is possible that youcan have a power supplythat works perfectly on theworkbench, producing aperfect +24 volt outputunder a massive test loadbut fails to operate in a slotmachine. This circuit istotally separate from the+24 VDC power supply. Thecircuit lives in this box andon this PCB because this isthe only place where theslot machine actually physi-cally touches the mainsand, as you have seen andas you will continue to seein this article, we need towatch the mains so that incase of power failure, wecan tell the slot machine tostore data and perform anorderly shutdown. This isone of the ways we do it.This circuit really has noth-ing at all to do with gener-ating the +24 VDC output.It just lives in the same box,looking at the primary DCvoltage and telling the CPUwhat is happening throughthe “Power Fail” outputsignal.

Let’s go back to opto-isola-tor U6. This time, let’s lookat the transistor half. Weare now on the isolated,secondary side of course,where everything in the slotmachine is all groundedtogether.

This is an interesting designin that you see the transis-

tor in the source pathrather than simply actingas a ground switch. If thetransistor in U6 is on, the+24 VDC output buss isconnected through thetransistor to pin 6, theinverting input of U1. A 6.8volt Zener diode pegs thevoltage at no higher than6.8 volts. I have an engi-neering issue with thiscircuit as there is no resis-tive current limiting hereother than semiconductorjunctions. I am going to runthis by the engineers atSETEC for comment.

This circuit compares thevoltage at pin 6 to the refer-ence voltage at pin 7, thenon-inverting input. As longas the power is good, pin 6will be higher than pin 7and the comparator will beactive, its output voltagedragged to ground at pin 1.When the power fails, U6turns off and the voltage atthe non-inverting input (thereference voltage) will ex-ceed the voltage at pin 6.The comparator “turns off”and pin 1 is pulled up tothe +24 volt buss by R187.

Of course, you can see thatwhat we are really doinghere is controlling the gateof MOSFET Q9, which isconnected directly to thecomparator output at pin 1.When everything is normal,pin 1 is low and Q9 is off.Upon power failure, Q9turns on.

Now, it’s on to the nextstage. It’s also U1, this timepins 4 and 5 are the inputs


and pin 2 is the output.Our reference voltage isconnected to the “+” inputwhile the voltage at the “-“input is determined by Q9.If Q9 is off (normal opera-tion) the voltage at theinverting input will behigher than the referenceand the comparator outputat pin 2 will be low. Uponpower failure (when Q9 isturned on) the voltage atthe inverting input dropsand the comparator turnsoff, allowing the voltage atpin 2 to rise, pulled up byR179. This, of course, gatesMOSFET Q2 and when Q2turns on, it generates the“Power Fail” signal, morecorrectly referred to as“Power Fail Not” as it is an“active low” output. You’llnotice that the signal islabeled PF with a bar overthe top. The bar on topindicates an active lowsignal. Oddly enough, thesame signal is labeled“PFAIL” at the connector,without the bar over thetop. I think that’s just anoversight.

That’s it. That is the wholepoint of that circuit. Youcan see that it has nothingto do with the creation ofthe +24 VDC output nordoes it have to ability toperform any sort of shut-down of the power supplyon its own. It can only talkto the CPU. It’s a sort oftattletale circuit, a RatFink, a Stool Pigeon, asquealer (I gotta stopwatching old prison mov-ies).

Output Monitor

While we’re in this area,let’s take a look at anothercircuit that is used to moni-tor something, this time the+24 VDC output itself. Wehave already seen how thepower supply will respond(immediately) to an over-voltage condition by shut-ting down the power supplybefore damage can occur.On the other hand, in thecase of a loss of the +24VDC output, we don’t haveto worry about damage andwe don’t have to shut downthe power supply. It’s al-ready going down! However,if the +24 VDC output isn’tup and running (or, morespecifically, if it is in theprocess of failing duringotherwise normal operation)it would be nice to let theCPU know about it so it cando some housekeeping.

The output monitor usesthe remaining two sectionsof U1, the LM339 QuadComparator. At first glance,it may look like this is somesort of circuit without aninput. You can see that thenon-inverting input at pin 9is connected to the refer-ence buss but what’s upwith the “-“ input at pin 8?It looks like it’s just hookedup to the +24 VDC bussthrough some more of those.5% precision resistors,R173 and R208 (both 1.8 kin series for a total of 3600ohms) and R181 and R209,both 2k ohms in parallel fora total of 1000 ohms. Theuse of precision resistorstells us we’re monitoring

something important hereand in this case, we’relooking at the +24 VDCbuss itself, the same bussthat is powering U1. That’sour “input!”

Of course, first semesterelectronics students learnall about the voltage dividerand we have sure seen a lotof them in this power sup-ply. This is another classicexample of why we learnabout this stuff in school.The problem is that inschool, we aren’t shownWHY or WHERE we usevoltage dividers. This is abeautiful (and easy to un-derstand) example of volt-age division using resistors.When the output voltage is+24 VDC, the voltage at pin8 will be 5.2 volts. This ishigher than the referencevoltage at pin 9 and so thecomparator is active; theoutput at pin 14 is zerovolts. You can see that thisoutput is connected to thegate of MOSFET Q8. Aslong as the output voltageof the power supply isgreater than 22.5 VDC, Q8remains off. However, if theoutput voltage drops below22.5 VDC, the referencevoltage at the “+” input willbe the higher than thevoltage at the “-“ input andthe comparator turns off.Resistor R189 pulls up thegate and Q8 turns on.

Recap: +24 VDC outputnormal, Q8 off. Output lessthan +22.5 VDC, Q8 on.

When Q8 turns on, it com-pletes the ground connec-


tion for resistor R195. Whatthis is really doing is drag-ging down pin 10 (as part ofa voltage divider-again!), theinverting input of the fol-lowing comparator stage.It’s the normal deal as we’veseen again and again in thisdesign: pin 11 (the + input)is tied to a reference voltageand the condition of the –input determines the out-put. When the voltage atpin 10 drops below thereference voltage at the +input, the output at pin 13swings high and gates thefollowing MOSFET, in thiscase Q3.

This creates our “OutputFail” signal. Like the PowerFail signal, it’s an “activelow” signal that speaksdirectly to the CPU througha dedicated connection atX9, pin 2. If the outputvoltage drops below 22.5VDC, this output goes low.The CPU then decides whatto do next. Again, this sig-nal actually has nothing todo with the generation ofthe +24 VDC directly nordoes it have any sort ofprotection or shut-downfacilities of its own. If thereis any shutting-down to do,these two circuits (PowerFail and Output Fail) don’tdo it directly. They can onlytell the CPU what is hap-pening.

Remember the Mains!

Of all the things that can gobad while a machine is inoperation, loss of AC poweris by far the most commonoccurrence. From the

machine’s point of view,this happens a lot. It hap-pened a bunch of timesbefore the machine evermade it to the slot floor!Every time the power switchis turned off or the machineis unplugged or discon-nected from the mains(accidentally or otherwise)the machine (obviously)loses power but as we haveseen, the entire systemdoesn’t actually lose powerimmediately. Because of theenergy stored in the electro-lytic capacitors, we have alittle bit of time to putthings in order before welose power completely.

Our goal here is to detectwhen we have lost the ACinput power right away,before the +24 VDC outputof the power supply fails.However, we don’t want tojump the gun and startshutting things down toosoon. We don’t want theloss of a single cycle of AC(or even two or three cycles)to trigger a shutdown. Amomentary loss of ACshouldn’t affect the ma-chine if the +24 VDC outputremains perfect. On theother hand, you don’t wantto delay in starting theshutdown procedure if theproblem is a genuine loss ofAC power. By the time theoutput of the power supplyreally is affected, it may betoo late to do anythingabout all the data in theCPU if we haven’t com-pleted housekeeping beforethe power supply craps out.

What we need then is a

compromise and that’swhat the MK5PFC does. Itlooks at the AC input andafter around a half-dozenmissing cycles decides thatthat there is a real andpersistent loss of AC powerand it does somethingabout it.

And what it does is prettydarned interesting and now,since you already know allabout how all the otherstuff in the power supplyoperates, we can see howthe AC input detectionworks and what it does tothe power supply.

Let’s start with the detec-tion circuit. We want toknow when we have lost ACpower so we use a singlediode (D3, a common1N4007) to look at thepositive half cycle of thewaveform. We’ll get 50 or 60pulses a second here. Thevoltage of this pulse isdivided down to a morereasonable level and ap-plied to the inverting inputof U2 at pin 8. The trickhere is that we want to beable to miss one or two oreven a few pulses withouttriggering the CPU to begina machine shutdown. Thisis accomplished by hangingan electrolytic capacitor(C67-22uf) from pin 8 toground. The larger thevalue of the capacitor, thelonger the time constantand so this value was cho-sen to allow the system toremain operational with theloss of a few cycles buttrigger the “Power Fail”signal when there is a real


loss of AC power as thevoltage on C67 (and pin 8)will quickly bleed offthrough R212 and theinput structure of U2 whenthe AC goes missing. As thevoltage on pin 8 dropsbelow the reference on pin9, the output at pin 14 goeshigh, gating MOSFET Q11.

At this point, a couple ofthings are going to happenmore or less simulta-neously. One is that theunit will generate the“Power Fail” signal that tellsthe CPU to store data andshut down properly. Theother is that the powersupply itself will shut down,killing the +24 VDC output.

Let’s look at generating the“Power Fail” signal first.This is easy because we’vealready seen how a drop inthe primary DC voltage willtrigger the “Power Fail”signal. We just have to seehow the two circuits areconnected and the waythey’re connected is bydiode D26. The anode isconnected to the “-“ inputat U2, pin 6. This is the pinthat is also watching theprimary DC voltage. Thecathode of D26 is connectedto the drain of MOSFETQ11. When Q11 is ener-gized (due to a loss of ahalf-dozen cycles of ACpower) the cathode of D26is essentially grounded,dragging the inverting inputat U2 pin 6 down in voltage.The result is precisely thesame as it is when theprimary DC voltage drops,eventually resulting in the

active low “Power Fail”signal being generated.

At the same time, when ACpower is lost and Q11 hasenergized, it will drag downthe voltage at U2 pin 4, theinverting input. This willturn off the comparator,allowing Q12 to turn on (itgets its gate voltage fromR232 and R233).

Once Q12 is turned on,we’re really going to killsome things. Firstly, take alook at diode D27. Thecathode is connected to thedrain of Q12 and anode isconnected to our old friend,the Enable input of U14,the UCC38503 PWM con-troller IC. When Q12 isturned on, we’re going tokill the +24 VDC powersupply immediately. We aregoing to turn off all primaryactivity and that’s a goodthing because we don’twant to waste any of ourprecious stored energymessing around. Bygrounding the Enable inputthrough Q12, we turn off allPWM activity and now relyon the charge stored in thesecondary electrolytic filtercapacitors.

Secondly, you can seethrough the connection ofdiode D39 between theexact same drain of Q12and the gate of Q4 thatwhen Q12 turns on due to aloss of AC power, it will alsodrop out relay K1. You cango right back to the verybeginning of this discussionif you need to refresh your-self on all that that entails.

The Stuff That Fails

So, that’s how the unitworks. It’s hard not to callit a complex system full ofcircuits that create, control,regulate, signal and shutdown but broken up into itsindividual functions (andwith familiarity and anannotated schematic dia-gram) it becomes manage-able. This is not a cheap,disposable power supplybut they can fail from yearsof 24/7 operation inside ahot slot machine. Some-body has to repair thesethings and now that youknow all about the unit,that someone can be you.

On the other hand, manytechnicians (me included)are just as happy to have alist of the stuff that fails aswe are to have a detailedand exact knowledge of howthe system operates. As atechnician, my job is to fixstuff as quickly and accu-rately as possible. My job isnot to prove to my co-work-ers how smart I am. Ifsomeone can tell me whatto replace, I am happy tohave the advice and if Ihave it, I am happy to shareit.

To that end, I asked thefolks at Aristocrat andSETEC to name the topdozen faults for MK5PFC.You can now totally forgeteverything you just learnedabout this remarkablepower supply.

- STM


MK5PFC High Failure Items

Low output voltage -> C89 leakyUnit unstable -> C52 failedNo power up -> C66 leakyT1 noisyNo power up -> Q1 failedNo power up -> U7 failedNo power up -> R127 open circuitR77, R202 wire woundU13 Voltage RegulatorU14 Surface Mount ICD1 Bridge RectifierD11, D12, D43, D50 DiodesPin pushed back in X3S1Monitor fuse holder damaged in transit, no monitor O/P

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