1

Power Switches and Their Drive Circuits

  Bipolar Transistors   Base Drive Circuits   Power MOSFETs   Gate Drive Circuits

2

Bipolar Transistors

Bipolar transistor as a power switch   During switching off, Vce overlaps with Ic

resulting high switching losses (Could be reduced by snubber circuits)

Vce Ic

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Bipolar Transistors

Why are bipolar transistors so slow in turning off?

  Stored charges in forward biased CB junction (saturation, similar to the reverse recovery in diodes (PN junctions))

  Inductive loading (leakage inductance of transformer)

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Ideal Base Current Waveform

ON OFF OFF

Base current spike for fast turn-on

Reverse base current spike to minimize storage time and

speed up turn-off

Base current to turn on the transistor with lowest β at

maximum collector current

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Base Drive Circuits

C1 & D1 provide high current for fast turn on, R2 discharges C1

R1 determines steady on base current

C2, L, and 4.7V zener provide negative base current for fast turn off

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Baker Clamps

These diodes prevent BC junction being forward biased

For speed up turn off, a path allows reverse biasing of BE junction

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Proportional Base Drive Q1 turns off, stored magnetization current provides a short impulse to turn Q2 on by flyback action

Q2 remains on due to positive feeback (base current is proportional to collector current)

When Q1 is on, Q2 is turned off by reverse base current

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Wood Base Drive

-ve voltage clamp

Reverse base current

Forward base current

Charged up during on time

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Power MOSFETs

  Voltage controlled devices   No reverse recovery because using majority

carriers   Drain current turnoff time is so fast that the

dissipation due to the overlap of falling current and rising voltage is generally negligible

  Vds ON could be quite high, 2V to 3V, compared with Vce ON , 1 V, of bipolar

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Required Gate Current

Voltage controlled device. Zero current? No, not a high speed switching!

178 V

150 pF

1800 pF 10 V gate pulse

I2

I1

For a rise time of 50 ns, I1 = 360 mA and I2 = 564 mA, i.e., required gate current is 0.924 A!!

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Gate Rise and Fall Time for Desired Drain Current Rise and Fall Time

  Say Vth = 2.5 V, Vg1 = 5 V for fully turn on of drain current

  10 V gate pulse of 50 ns rise time, 10 V

Vgs

Ids Drain current rise time is only 12.5 ns!!

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Gate Drive Circuits

  Required to absorb (sink) and deliver (source) current from gate

  For PWM chips can only sink OR source current Cin

PWM Chip

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Gate Drive Circuits

  Some PWM chips generate non-overlap gate pulse with totem pole outputs

B

A

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Snubber Networks

  Turn-Off Switching Losses   Turn-Off Snubbers   Non-Dissipative Snubbers

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Turn-OFF Switching Losses

  Bipolar power switches usually have slow turn-off time

  Overlap of falling current and rising voltage accounts for most of the switching losses

Ic

Vce

Pd

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Turn-OFF Switching Losses

  Power MOSFET has shorter current fall time

  But high -di/dt means higher voltage spike generation due to transformer leakage inductance

Id

Vds

Pd

Typical waveforms of a Forward converter

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Turn-Off Snubbers

Snubber

C slows down the rise time Typical waveforms of a Forward

converter with snubber

Id

Vds

Pd

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2Vdc

Ip

tf

Turn-Off Snubbers

Vdc = 200 V, Ip = 4 A, tf = 0.3 µs, T = 10 µs Transistor dissipation without snubber PDQ = (2 × 200 × 4)/2 × 0.3/10 = 24 W

Drain voltage rises instantly with snubber

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Turn-Off Snubbers

  Assume that half of the current flows into the snubber and drain rises to 2 Vdc at tf

  C = ((Ip/2) tf)/2 Vdc = 1.5 nF   C should be discharged completely during TON

  R = TON min/3C   Let the nominal duty cycle be 0.4, and

minimum duty cycle be 0.3 for 30% high line, therefore TON min = 3 µs

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Turn-Off Snubbers

  R = 666 Ω   The dissipation of snubber resistor is   PDR = ½C(2 Vdc )2/T = 12 W

2Vdc

Ip/2

tf Drain voltage rises to

2Vdc in tf

with snubber

Half of the current flows to snubber capacitor

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Turn-Off Snubbers

  The dissipation of transistor with snubber can be estimated as

  PDQ = ((Ip/2)(2 Vdc tf)/(6T) = 4 W

In practice, if the transistor is still too warm, increase C and have more dissipation on R.

Dissipation on R CANNOT be reduced by decreasing its value!!

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Snubber Returned to Positive

  Voltage stress on C becomes Vdc instead of 2 Vdc

  For the same effect, double in value

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Non-Dissipative Snubbers

Vdc

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Protections

  Inrush Limiting   Start-Up Methods   Soft Start   Output Overvoltage Protection   Current Limit Circuits   Output Current Limiting

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Inrush Limiting

Resistors for inrush current limiting

R = Vp/I

I is the half-cycle surge current ratings of

diodes

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Inrush Limiting

  For reducing dissipation, negative temperature coefficient thermistors (NTC) can be employed as inrush current limiting resistors

  Protection may NOT work properly if SMPS is switch off and back on rapidly!

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Active Inrush Limiting

  For high-power applications, dissipation of inrush resistor can be eliminated

Assume that converter is soft start, i.e., a delay for turning on the triac

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Dissipative (Passive) Start Circuit

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Active Start Circuit

Transistor is off after start up

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Impulse Start Circuit

Vz - VA < VD < Vz - Vstart

Vz

VD

VA

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Soft Start

  Resulting lower stress on converter components   Progressively increase the duty cycle of the

PWM controller during start up   Coupled with under voltage lock out in modern

controllers   Start up should be slow enough to ensure that

there is no overshoot in the output during turn on

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Soft Start Unitrode Application Note U-134

Soft start capacitor

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Output Overvoltage Protection

  SCR Crowbar overvoltage protection

SCR is triggered on when Vo > VZ

VZ

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Output Overvoltage Protection

  SCR Crowbar with high precision

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Current Limit Circuits

  Pulse-by-pulse over-current limiting for primary-reference direct drive designs

PWM IC

Resetting the output

driver

Current sense resistor

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Output Current Limiting

  Non-dissipative current limit circuit for control on the output side

Vout Vin