Unitrode - UC3854A/B and UC3855A/B Provide Power Limiting With Sinusoidal Input Current for PFC Front Ends

by Laszlo Balogh

This design note focuses on one of the major im-provements introduced to the industry standardUC3854 high power factor boost controller. Thenew UC3854A/B versions eliminated the need forexternal components to clamp the voltage andcurrent error amplifier outputs and optimized thevoltage levels of some of the sense circuitry. Allof these issues are already covered by DN-39design note (present release is version E). Thefollowing aspects were expressed implicitly, inprevious literature, and are now described in fur-ther details.What makes intelligent power limiting possiblewith the UC3854A/B and with the newerUC3855A/B ZVS high power factor controllers, isthat the maximum value of the multiplier outputcurrent is not directly related to the current of theRSET resistor any more.Instead, it is limited to be equal or smaller thantwice the instantaneous value of the IAC current.This new feature provides a very delicate and ef-fective way to limit input power to the power fac-tor corrector front-end while the converter stillmaintains a sinusoidal input current waveform. Ithas to be emphasized here that the power limit-ing scheme of the UC3854A/B does producesinusoidal input current waveform even if theload is a negative impedance, like DC-DC con-verters. In these cases, special care has to betaken to guarantee that the output voltage of theboost power factor corrector is greater than thepeak value of the input line voltage at all operat-ing equilibrium. This can be insured by settingthe power limiting of the DC-DC converter belowthe maximum power handling capability of thePFC stage.In order to establish a straightforward design pro-cedure for the multiplier setup, first a basic rela-tionship should be shown. The ratio of themultiplier output current, (IMO) and the IAC cur-rent is constant within one cycle of the AC inputbecause:

IAC(t) = VRMS 2 sin ( t)RAC (1) and

IMO(t) = IAC(t) (VEA 1.5)K (A VRMS)2 (2)

where,

VRMS is the RMS value of the AC input voltage; VEA is the voltage error amplifier saturation volt-

age (VOH);K is the multiplier constant (K = 1);A is the divider ratio to the VRMS pin of the IC.The ratio of IMO(t) to IAC(t) is given as:

R = IMO(t)IAC(t) = VEA 1.5

K (A VRMS)2(3)

which is determined only by the RMS value ofthe input voltage and stays constant within oneline cycle.In the case of a well executed design, the ratiowill be equal to two - right at the minimum inputvoltage where the rated output power is still ex-pected to be delivered. Figure 1 shows the opti-mal ratio of IMO and IAC as a function of thenormalized input voltage. The horizontal axis of

Design Note

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0

0.5

1

1.5

2

0.5 1 1.5VRMS, NORM

IIMOAC

Figure 1. Ideal IMOIAC Ratio

SLUA196A - JUNE 1995 - REVISED NOVEMBER 2001

the graph is normalized for the minimum linevoltage where full power can be obtained. Forvalues below unity the converter will work inpower limited mode.To achieve precise power limiting the highlightedcomponents of Figure 2 have to be calculated.1. From the maximum peak input voltage and the

highest allowable IAC value calculate RAC.

RAC = VRMSmax 2IACmax (4)where IACmax 600A is given in the datasheet.

2. The factor "A" of equation (2) can be determinedfrom the specified minimum input voltage valuewhere the circuit has to supply full rated power.At that point, the voltage error amplifier output(VEA ) is about to saturate, and IMO shall be at itstheoretical maximum which is 2 IACmax. The re-quired value of "A" can be expressed from (3)using the conditions previously stated:

A = 2.25VRMSmin (5)Now every parameter is given for equation (2),and IMO is known for all operating conditions.

3. The next step is to calculate the peak value ofthe multiplier current (IMOmax) at the minimum in-put voltage (VRMSmin). From combining (2), (4)and (5),

IMOmax = VRMSmin 2 (VEA 1.5)K RAC 2.25 (6)4. IMOmax will define the maximum value of the in-

put current (IINpeak) which occurs at the peak ofthe minimum line voltage, (VRMSmin) and at maxi-mum load. The required peak input current isgiven as:

IINpeak = PLIMITVRMSmin 2 (7)

The relation between IMOmax and IINpeak is de-fined by the two resistors RSENSE and RMOaccording to:

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Figure 2. Multiplier Set Up Components in a Typical PFC ApplicationUDG-95099

Design Note

2

IINpeak = IMOmax RMORSENSE (8)5. The last parameter to be calculated is the value

of RMO from equations (6), (7) and (8), assumingthat RSENSE is already selected based on the al-lowed power dissipation of that resistor.

RMO = 2.25 K PLIMIT RAC RSENSEV2RMSmin (VEA 1.5)

(9)

After all design parameters are defined, the nor-malized input RMS current values and the nor-malized input power can be calculated as thefunctions of the normalized input voltage.

As Figure 3 demonstrates, exact, and constantpower limiting can be realized by the UC3854A/Bhigh power factor controller ICs for the entire in-put voltage range. That is caused by the inputvoltage feedforward term in the multiplier equa-tion, (2).Design ExampleThe following example is to illustrate how to exe-cute the procedure described above. The designexample has the following start up parameters:VIN = 70 . . . 132 VACRMSVINmin = 90 VACRMS (where full output power still

obtainable)POUT = 250W (power limit of the load con-

verter)PLIMIT = 275W (set PFC power limit 10%

above load converter) = 0.95 (expected efficiency)

For optimum results follow the step-by-step de-sign guide given below:

Step 1.

RAC = 132 2600 106

= 311.1 103

RAC = 330k

Step 2.

A = 2.2590 = 0.01667A = 16.67mV/VRMS

Step 3.

IMOmax = 90 2 (6 1.5)1 330 103 2.25

= 771.4 106

IMOmax = 771.4A

Step 4.IINpeak = 27590 0.95 2 = 4.55IINpeak = 4.55A

Step 5.Assume the maximum power dissipation of thecurrent sense resistor PRS = 0.5W, then:

RSENSE = PRS VINmin2 2

POUT2(10)

RSENSE = 0.5 902 0.952

2752 = 0.04833

RSENSE = 47m

Step 6.

RMO = 2.25 1 275 330 103 47 103

902 0.95 (6 1.5) = 277.1RMO = 270

The design just completed will exhibit the re-quired power limiting characteristics for the entireoperating input voltage range. The described cal-culations can be easily programmed as it isshown in the Mathcad worksheet in the Appen-dix.

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0

0.5

1

0.5 1 1.5

PNORM

IIN, NORM

VRMS, NORM

Figure 3. Power Limit and Maximum InputCurrent Values as a Function of Input

Voltage (All Normalized)

Design Note

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APPENDIXThis MathCAD file calculates the power limiting characteristic

of the UC3854A/B and UC3855A/B high power factor controllersfor wide input voltage range application.

INPUT PARAMETERS:VINmin := 70 Input voltage (RMS) value,

where the controller startsoperating.

VINlim := 90 Minimum input voltage(RMS) where the circuitmust deliver its rated outputpower.

VINmax := 270 Maximum input voltage(RMS).

PLIM := 275 110% of the load power re-quirements.

:= 0.93 Expected efficiency of thePFC stage.

IACmax := 600 10-6 Maximum value of the IACcurrent as defined in thedatasheet.

VEAsat := 6 Output voltage of the volt-age amplifier when it issaturated high.

K := 1 The multiplier constant as itis given in the datasheet.

RS := 0.047 The current sense resistorvalue, defined previouslybased on the acceptablemaximum power dissipation.

DESIGN PROCEDURE:Step 1. Estimate RAC resistance:

RACest = VINmax 2IACmax RACest = 6.364 105

Pick the closest higher standard value:RAC := 6.8 105

Step 2. Divider ratio of the input RMS voltage tothe VRMS pin (8) of the IC.

A := 2.25VINlim A = 0.017 [mV/VRMS]

Step 3. Determine the maximum multiplier outputcurrent. It occurs at the peak of the mainscycle at the minimum input voltage ampli-tude where full rated power is stillobtainable.

IMOmaxpk := VINlim 2 (VEAsat 1.5)K RAC 2.25

IMOmaxpk = 3.744104

Step 4. Calculate the highest peak input currentvalue as defined by the rated output powerand the minimum input voltage amplitudewhere full rated power is still obtainable.

IINpk := Plim 2VINlim

IINpk = 4.646

Step 5. Estimate RMO resistance.

RMOest := IINpkIMOmaxpk RS RMOest = 583.367

Pick the closest lower standard value: RMO := 560DESIGN VERIFICATION:Step := 20 Number of points to calculate. i := 0..step Step variable.Input voltage range definition:

VINRMS(i) := VINmin + VINmax VINminstep i

IAC current as a function of the input voltage:

IACRMS(i) := VINRMS(i)RACIMO current as defined by the input voltage andapplying the IMO < 2 IAC limit.

IMORMSest(i) := IACRMS(i) (VEAsat 1.5)K (A VINRMS(i))2

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IMORMS(i) := if (IMORMSest(i) > 2 IACRMS(i), 2 IACRMS(i), IMORMSest(i))

Calculating the maximum input RMS current levellimited by the multiplier output current.

IINRMS(i) := IMORMS(i) RMORSThe power limit of the power factor corrector.

PINmax(i) := VINRMS(i) IINRMS(i)

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50 100 150 200 250 3000

100

200

300

400

V (i)INRMS

IMORMS(i)

IACRMS(i)

Figure 4. The IAC and IMO currents (in Amps)as a function of the RMS input voltage.

V (i)INRMS

IINRMS(i) 100

PINmax(i)

50 100 150 200 250 3000

100

200

300

400

Figure 5. Input power [W] and input RMScurrent [A] (multiplied by 100 to fit the samescale) as a function of input RMS voltage.

Graphical representation of the obtained data:

UNITRODE CORPORATION7 CONTINENTAL BLVD. MERRIMACK, NH 03054TEL. (603) 424-2410 FAX (603) 424-3460

Design Note

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