Lecture II

Types of Diode– Signal Diode

-- Power Diode – Fast recovery diode

– Schottky Diodes

– LED Diode--Zener Diode

--LASER Diode

Max properties: similar to regular power diodes but recover time as low as 50ns• The following is a graph of a diode’s recovery time. trr is shorter for fast recovery diodes

Fast Recovery Diodes

Max properties: 1500V, 400A, 1kHz• Forward voltage drop of 0.7 V when on

Power Diodes

What do we use diodes for?

Used to Block DC Voltage

Non-linear mixing of two voltages (e.g. AM)

Turn AC into DC (voltage rectifier)

Voltage multiplier (ex. double input voltage)

Protect circuits by limiting the voltage (clipping and clamping)

• Max properties: 400V, 400A

Schottky Diodes• Lower voltage drop when conducting than regular diodes

• Very fast recovery time

• Ideal for high current low voltage applications

BJT–Bipolar Junction Transistors is a current controlled device

• Types of Transistors

IGBT–Insulated Gate Bipolar Transistor is a voltage controlled device

FET–Field Effect Transistor is a voltage controlled device

MOSFET–Metal Oxide Semiconductor is a voltage controlled device

IGBT-NPN IGBT + Diode

R4

10K

+10V

100K

R3

10V

R21K

hfe = 100Q1 2N2222

Inverting Amplifier

Inverting Amplifier Simulation

IC = hfeIB then transistor is in active mode

Ib> Ic/hfe then transistor is in saturation

b

cfe I

Ih

•.

0.000us 100.0us 200.0us 300.0us 400.0us 500.0us

12.50 V

10.00 V

7.500 V

5.000 V

2.500 V

0.000 V

-2.500 V

-5.000 V

-7.500 V

A: v1_1

B: q1_3C: c2_2

When transistor is driven hard in saturation then it acts as a unidirectional switch.

Transistor is driven in Active Mode.

• Linear Voltage Regulator

C

B

+

C1100uF

A

+10V

10kHz

V1-200m/200mV

C21uF

Q1BC547A

R520K

R41k

R32K

R22K

R11k

C

B

A

MOSFET

Measured input characteristics ( ID vs. VGS ) for (a) nMOS transistor

As Can be seen from it’s input characteristicsMOSFET is a voltage controlled device

Extremely fast switching speed with lower switching losses compare to BJT & IGBTbut act as small resistance RDS(on) in on state

In High Voltage Range, due to high RDS(on) conduction losses becomes very high compare to BJT and IGBT and hence MOSFET are used in low voltage Switching circuits.

IGBT

IGBT-NPN IGBT + Diode

Equivalent circuit model of an IGBT

IGBT has Input characteristics of MOSFET and Output Characteristics of BJT

It is rugged like BJT with low conduction losses but switching speed similar to MOSFET with lower switching losses than BJT

High Efficiency Power SwitchesIt all started with the mighty Bipolar Junction Transistor (BJT) in 1970, the first fully controllable and commercially viable power switch. BJTs gave rise to switchmode power conversion by late 1970s and made this class of converters a reality. Then came the Metal Oxide Field Effect Transistor (MOSFET) by the late 1970s, a new class of power devices that offered significant performance improvements

compared to BJTs and extended the limits of power conversion technology as we know it today. In the 1980s, a new addition to the family of power devices was made with the invention of the Insulated Gate Bipolar Transistor (IGBTs). The IGBT was built on the core MOSFET technology and extended MOSFET operation into high power applications.

A BJT is a current controlled device where the device is turned on and off by controlling the current into its base junction (IB). A base drive circuit is normally employed to achieve that (Fig. 1). When a BJT is turned on, a forward voltage drop is developed across the device terminals (collector-to-emitter), which gives rise to power loss (conduction loss) and thus heat loss while the device is conducting current.

However, this voltage drop is relatively low and is easily tolerated by the external circuitry.

The BJT with drive and control circuitry

Although BJTs were the only real power transistors throughout the 1970’s, they had considerable performance and control limitations, namely:

2. BJTs have relatively slow switching characteristics limiting their maximum operating switching frequencies to the low kHz range.

3. The BJT is a negative temperature coefficient device, meaning that the device is liable for thermal runaway as the device temperature increases

1. The drive circuitry was complex and quite sizeable since the base current require to turn-on and off the device is quite high.

The above limitations accelerated the pace of searching for a new power device and yielded to the development of the power MOSFET.

Metal Oxide Field Effect Transistors (MOSFETs)

A MOSFET is a voltage controlled device where the device is turned on and off by controlling the voltage across the gate-source junction (VGS) (Fig. 2). Unlike BJTs, MOSFETs have high input impedances and thus require minimal gate currents to turn-on and off. This greatly simplifies the drive circuitry and reduces its cost. In addition, MOSFETs have very fast switching characteristics allowing them to operate into the MHz range.

MOSFET with drive and control circuitry

In general, MOSFETs extended the performance advantages of BJTs in terms of high-current handling capability while eliminating their drawbacks in terms of:

1. Ease of control: Simpler and lower cost drive circuitry.

2. Faster switching behavior: allowing operation into the MHz range. This resulted in the miniaturization of power converters as the size of the passive and filtering components was greatly reduced.

2. High on-resistance especially at high breakdown voltages, which limits their current handling capabilities. The resistive like behavior of MOSFETs entails that the voltage drop increases with increasing currents (IR drop) as well as increasing temperatures (increased resistance). One way to alleviate the forward drop limitation is to parallel MOSFETs to reduce the equivalent resistance, similar to connecting resistors in parallel (Fig. 3). Just like resistors, the equivalent resistance of the parallel configuration is divided down by the number of parallel devices. This greatly reduces the conduction losses and increases the current carrying capability of power MOSFETs.

3. High current carrying capabilities: The positive temperature coefficient characteristics allow MOSFETs to be paralleled which further increases their current handling capability.

The above has lead to MOSFETs becoming the devices of choice for power designers and to the elimination of BJTs in low to medium power applications.

Well it is all good to be true for MOSFETs. MOSFETs do have limitations, namely:

1. Relatively low breakdown voltages: The breakdown voltage limit of the metal oxide junction is limited to 1000-1200V. This has limited the use of power MOSFETs in high voltage applications.

Parallel connected MOSFETs for increased current handling capability and reduce conduction loss

An IGBT is a “combo” device derived from the power MOSFET and BJT technologies. In fact, the IGBT is a “spinoff” from power MOSFETs invented to extend the performance limitations of power MOSFETs into high voltage and high power applications. It combines the low conduction loss of a BJT with the relatively high switching speed of a MOSFET.

The IGBT physical structure is a true representation of the “combo” technology the device incorporates having a MOSFET input stage and a BJT output stage (see Fig. 4).

Insulated Gate Biploar Transistors (IGBTs)

IGBT symbol and equivalent internal structure showing the input MOSFET and the output BJT

Similar to power MOSFETs, the IGBTs are voltage controlled devices and have high input impedances thus requiring minimal gate drive circuitry. In addition, IGBTs and can turn-on and off at speeds comparable to MOSFETs. Unlike MOSFETs, however, IGBTs exhibit forward voltage drops and current densities similar to BJTs while switching much faster. In addition, their breakdown voltages are much higher than MOSFETs with readily available devices having breakdown voltages in excess of 5000V.

In summary, the performance improvements of IGBTs over MOSFETs and BJTs are summarized below:

5. High breakdown voltages approaching few thousand volts. The above performance improvements make IGBTs the devices of choice in high voltage and high power applications. Specifically, IGBTs are better suited for voltages in excess of 600V and for power levels in excess of 5-10kW.

1. MOSFET input stage which entails high input impedance, minimal drive requirements, and simple and low cost drive circuitry

2. Fast switching behavior comparable MOSFETs

3. Low forward voltage drop resulting in lower conduction losses

4. High current carrying capabilities. Presently, IGBTs are available in current ratings in excess of 1000A.

AC to DC and AC to DC to again AC

Why do we require to rectify ac voltage ? then again invert back to AC?

Most of the electronic circuits work on DC power supply and incoming power to factory or house being AC such as computer TV etc., we need to convert it to DC

Also circuits like battery chargers, mobile chargers do need to convert AC in to DC

In AC drives, motor speed is controlled by changing motor supply frequency to adjust it’s speed

In UPS or Uninterruptible Power Supply, AC input is rectified and then inverted to give out a clean output free from mains disturbances and battery is used in parallel with rectified voltage to give AC output to sensitive or critical load in absence of Mains supply such as computer supply, medical equipment etc.

When AC input is rectified and filtered by capacitors or L-C filter it produces harmonic currents as rectifier load acts as a non-linear load.

This requires reducing the harmonics below acceptable limit which is achievedBy Unity Power Factor Circuit or commonly called as UPF circuit.

Fundamentals of Switch Mode Power Supply

VOUT = D ⋅ VIN where

BUCK BOOST (Inverting)

Comparison of Linear Versus Switch Mode Power SupplyThe advantages and drawbacks of both technologies

Size: - A 50W linear power supply is typically 3 x 5 x 5.5”, whereas a 50W switch-mode can be as small as 3 x 5 x 1”. That’s a size reduction of 80%.

Weight: - A 50W linear weighs 4lbs, a corresponding switcher is 0.75lb. As the power level increases, so does the weight. I personally remember a two-man lift needed for a 1000W linear. Today one can carry a 2000W in his carry on luggage when he flies!

Input Voltage Range: - A linear has a very limited input range requiring that the transformer taps be changed between different countries. Normally on the specification you will see 100 to 240VAC.

This is because when to input voltage drops more than 10%, the DC voltage to the shunt regulator drops too low & the power supply cannot deliver the required output voltage. At input voltages greater than 10%, too much voltage is delivered to the regulator resulting in over heating. If a piece of equipment is tested in the US and shipped to Europe, or even to Mexico in some cases, the transformer “taps” have to be manually changed. Forget to set the taps? The power supply will most certainly blow the fuse, or may well be damaged.

Most switch-mode supplies will operate anywhere in the world (85 to 264VAC), from industrial areas in Japan to the outback of Australia without any adjustment. The switch-mode supply will also be able to withstand small losses of AC power in the range of 10-20ms without affecting the outputs. A linear will not. No one will care if the AC goes missing for 1/100th of a second when charging your phone, it will take 100 of these interruptions to delay the charge by one second! Having a piece of equipment reboot 100 times a day will cause some heartbreak!

Efficiency: - A linear power supply because of its design will normally operate at around 60% efficiency for 24V outputs, where as a switch-mode is normally 80% or more. Efficiency is a measure of how much energy the power supply wastes. This has to be removed with fans or heat sinks from the system.

For a 100W output linear, that waste would be 67W. A 100W switch-mode would be just 25W. 67W – 25W = 42W is the extra power lostDoesn’t sound much, but don’t try touching a 40W light bulb! If the equipment were running 24 hours a day, then the extra losses would be 367kW hours, even at $0.1 per kW hour, that’s an extra $37 a year for a power supply that costs around $80.

As a quick note, in Europe, they are trying to limit those losses of all power supplies used by consumers particularly when operating off load (as many products are left plugged in 24 hours a day). Imagine 250 million power supplies eating up a couple Watts. That equates to the output of a whole power station!

Reliability: - If reliability is calculated using a part count method, then the linear power supply will win. With the design & quality improvements made in the last few years with switch-mode parts & technology, in reality this advantage has been negated. I have demonstrated life testing data showing no failures after over 1,000,000 hours on some Lambda (reputed company)products.

Electrical Ripple and Noise: - This is where the linear really scores! The linear obviously is a lot “quieter”, by up to a 10,000 times. The topology of the switch-mode supply with its high frequency switching technology had to have a downside right? So if the noise is 10,000 times worse, how can anyone use it? Sounds so bad. In truth, there are some applications (studio mixers and very sensitive test equipment) where low electrical noise is critical. The others?

One of my first sales calls in the USA was to a manufacturer who built semiconductor fabrication equipment. They used 8 really big linear units in a large box measuring 2x3x4 feet, it was heavy & actually was dictating the size of their end equipment. I told the engineer that I could replace all eight units with two modular products measuring 5x5x10”. He laughed and said the noise would be too great. I sent him samples and went to visit three weeks later. He was delighted with theperformance and has been a long term Lambda customer ever since.

5V linear 5V switch-mode Transient Response:Transient response is how a power supply reacts to a (fast) change in load. If the output load quickly changes from say full load to half load, the output voltage of the power supply will rise (overshoot) before the internal control circuit has time to compensate, and then undershoot a little less as the circuit over compensates. The length of time is takes from the instant of the load change to the time the output voltage settles back into the load regulation limits can be critical to some loads. Here the linear again outperforms the switch-mode.

For a 50% change in load the switch-mode will often take 3000us to recover. A linear supply will recover in 50us. Is this critical for all applications? There are a few specialized technologies where this is important and most engineers will advise you if this is critical. For the other instances on board capacitors at the end load & the inductance of cables is enough to reduce overshoot down ten-fold to where it no longer is a concern.

As linear power supplies are “quieter” and do not need these capacitors, they simplify the system filtering, and allow more of the system leakage current “budget” to be used for other parts like monitors. The overall size of the system filter can also be reduced. How much that impacts cost & performance varies from customer to customer.

Low leakage currents and Conducted EMI: -A widely used technique in the design of switch-mode power supplies is to connect special capacitors from the AC input terminals to Earth. This is an cost effective method to reduce noise from being fed back through the input wires and potentially affecting other equipment. These capacitors have a side effect of allowing a “leakage current” to be passed through the Earth or ground cable. Many safety specifications have limits on the amount of this current that is allowed. UL1950 allows 3mA, medical industrials less than a tenth of that. The gaming industry is even tighter.

Some switch-mode power supplies (like Lambda’s Vega series) are now available with increased internal filtering that allows for low leakage versions to be offered to meet medical specifications.

Linear Switching

FunctionOnly steps down (buck) so input voltage must be greater than output voltage

Step up (boost), step down (buck), inverts

Efficiency

Low to medium, but actual battery life depends on load current and battery voltage over time. Efficiency is high if difference between input and output voltages is small

High, except at very low load currents (μA), where switch-mode quiescent current (IQ) is usually higher

Waste heatHigh, if average load and/or input to output voltage difference are high

Low, as components usually run cool for power levels below 10 W

ComplexityLow, usually requiring only the regulator and low-value bypass capacitors

Medium to high, usually requiring inductor, diode, and filter caps in addition to the IC; for high-power circuits, external FETs are needed

SizeSmall to medium in portable designs, but may be larger if heatsinking is needed

Larger than linear at low power, but smaller at power levels for which linear requires a heat sink

Total cost LowMedium to high, largely due to external components

Ripple/NoiseLow; no ripple, low noise, better noise rejection

Medium to high, due to ripple at switching rate

Table 1: Comparison of the characteristics of switching and linear regulators.

As you can see, depending upon what is critical to the Customer will influence the decision to go with either a switch-mode or a linear power supply. It is often worth challenging the use of a linear. Sales of linear supplies (>10W) fall every year as technology adapts and improves.

Sinusoidal voltage and non-sinusoidal current give a distortion power factor of 0.75 for this computer power supply load.

AC power flow has the three components: real power (also known as active power) (P), measured in watts (W); apparent power (S), measured in volt-amperes (VA); and reactive power (Q), measured in reactive volt-amperes (var).The power factor is defined as:

  In the case of a perfectly sinusoidal waveform, P, Q and S can be expressed as phasors that form a phasor triangle such that:

                 If      is the phase angle between the current and voltage, then the power factor is equal to the cosine of the angle,             

displacement power factor DPF =

distortion factor DF And PF = DF*DPF

Unity Power Factor