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DRIVE AND SNUBBERDRIVE AND SNUBBERCIRCUITSCIRCUITS

Mohd Shawal Bin JadinMohd Shawal Bin Jadin

Ext : 2321Ext : 2321

BEE4223BEE4223

Power Electronics & Drives SystemsPower Electronics & Drives Systems

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LEARNING OUTCOMESLEARNING OUTCOMES

At the end of the lecture, studentshould be able to:

Identify the driver circuit for

electronics switches

Outline the importance of snubber

circuit in such that to smoothened thesmoothened theswitching transitionswitching transition

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DRIVER CIRCUIT (BASE / GATE)DRIVER CIRCUIT (BASE / GATE)

InterfaceInterface between control (low power electronics) and (highbetween control (low power electronics) and (high

power) switchpower) switch.

Functions:

amplifies control signal to a level required to drive poweramplifies control signal to a level required to drive powerswitchswitch

provides electrical isolation between power switch andprovides electrical isolation between power switch andlogic levellogic level

Complexity of driver varies markedly among switches.MOSFET/IGBT driversMOSFET/IGBT drivers are simplesimple but GTO driversGTO drivers are very

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ELECTRICAL ISOLATION FOR DRIVERSELECTRICAL ISOLATION FOR DRIVERS

Power semiconductor devices can becategorized into 3 types based on theircontrol input requirements:

a) Current-driven devices BJTs, MDs, GTOs

b) Voltage-driven devices MOSFETs, IGBTs,MCTs

c) Pulse-driven devices SCRs, TRIACs

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CURRENT DRIVEN DEVICES (BJT)CURRENT DRIVEN DEVICES (BJT)

Power BJT devices have low current gain due toconstructional consideration, leading current than

would normally be expected for a given load orcollector current.The main problem with this circuit is the slow turn-

off time.

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CIRCUITSCIRCUITS

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::THYRISTORSTHYRISTORS

Pulse transformer is used for isolationused for isolation. R1is to limitlimitthe gate currentthe gate current

Normally a pulse with length 10us with amplitude of50mA is sufficient to turn-on the thyristors. It is quiteIt is quitecommon to fire the thyristors with successive pulsescommon to fire the thyristors with successive pulsesto ensure proper turn-onto ensure proper turn-on.

It is not possiblenot possible to tturn-offurn-offa thyristor with the abovecircuit

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EXAMPLE: SIMPLE MOSFET GATE DRIVEREXAMPLE: SIMPLE MOSFET GATE DRIVER

Note: MOSFET requires VMOSFET requires VGSGS =+15V for turn on=+15V for turn onand 0V to turn off.and 0V to turn off. LM311 is a simple amp withopen collector output Q1.

When BB11 is highis high, Q1 conducts. VGS is pulled to

ground. MOSFET is offMOSFET is off.

When BB11 is lowis low, Q1 will be off. VGS is pulled toVGG. If VIf VGGGG is set to +15V, the MOSFET turns on.is set to +15V, the MOSFET turns on.

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EXAMPLE: SIMPLE JFETEXAMPLE: SIMPLE JFET

GATE DRIVERGATE DRIVER

Example of having a chopper circuit that requiresswitching to control output voltage

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EXAMPLE: SIMPLE JFET GATE DRIVEREXAMPLE: SIMPLE JFET GATE DRIVER

Left side of the driver circuit is part where to control switching output. R2, C1 to control on time

R3 to control off-time

R5 to control pulse length (switching frequency)

D1 to make sure ON/OFF independently

RR22, C, C11 + R+ R55RR33, C, C11 + R+ R55

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SNUBBERSSNUBBERS

PCB construction, wire loops creates stray inductance,wire loops creates stray inductance, LLss..

Using KVL,Using KVL,

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SNUBBERSSNUBBERS

From previous equation, the voltage across the switch isthe voltage across the switch isbigger than the supply (for a short moment).bigger than the supply (for a short moment).

The spike may exceed the switch rated blocking voltagespike may exceed the switch rated blocking voltageand causes damageand causes damage due to overvoltage.

To preventTo prevent such occurrence, a snubber is put across thea snubber is put across the

switchswitch. An example of a snubber is an RCD circuit shownbelow.

Snubber circuit smoothened the transition and makeSnubber circuit smoothened the transition and makethe switch voltage rise more slowly.the switch voltage rise more slowly. In effect itdampens the high voltage spike to a safe value.

Switches and diodes requires snubbersSwitches and diodes requires snubbers. However, newgeneration of IGBT, MOSFET and GCT do not require it.

RCD SNUBBERSRCD SNUBBERS

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RCD SNUBBERSRCD SNUBBERS

In general, snubbers are used for:

turn-on:turn-on: to minimize large overcurrents through the device atto minimize large overcurrents through the device atturn-onturn-on

turn-off:turn-off:to minimize large overvoltages across the device duringto minimize large overvoltages across the device duringturn-turn-

off.off.

Stress reductionStress reduction: to shape the device switching waveformto shape the device switching waveformsuch that the voltage and current associated with the device aresuch that the voltage and current associated with the device are

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