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Practical Considerations in

Implementing Switching Power-Poles

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• Gate Driver ICs

• Design Considerations

– Thermal Considerations

– Magnetic Components

– Capacitors

– Selection of Switching Frequency

• Diode Reverse Recovery Characteristic

– Conduction Losses

– Increase in Switching Losses

Gate Drivers

• In order for PE converters to operate

efficiently, switching losses must be

minimised.

• Resonant converters can help but might

not be possible at high switching

frequencies or if feedback control is

needed

• Need to drive the switches on and off as

quickly as possible

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Initial conditions for drive ccts

• The drive circuit must provide adequate

drive power (e.g., IB for BJTs or Vgs for

MOSFETs) to keep the switch fully in the ON

state where conduction losses are low

• The drive circuit should also provide the

ability to quickly turn the power switch OFF

and keep it OFF even in the presence of

substantial switching noise

• The drive circuit is solely the interface

between the control circuit and the power

switch (we are not considering how to create

the control signal). 3

• The drive circuit may need to output

significant power (compared to control

circuits). e.g., power BJTs have low values of β

(typically around 10), so the IB supplied by the

drive circuit can be substantial if Ic is large.

• The basic form of a drive circuit depends on:

– Does the drive circuit output need to be unipolar or

bipolar (unipolar circuits are simpler than bipolar

circuits)?

– Is the drive output a current source or a voltage

source?

– Is electrical isolation required between the control

signal and the switch drive signal?

– Maximum switching frequency?

– Can it provide the necessary (peak) gate current?4

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• Additional functionality may be required

from the drive circuit:

– Switch over-current protection

– Creation of blanking times

– Wave-shaping of control signals to

enhance switch on and/or switch off

performance

– Number of independent output channels

– Maximum charge per pulse (Qout/pulse);

output caps of the driver must be able to

deliver the gate charge for charge and

discharge5

Choosing the wrong one – Resistor that

is!

• Overheated/burnt – low peak power, no-

surge proof resistor

• EMI – small gate resistors used

• Gate oscillations – high resistors

• Increased Switching losses – High

resistors

• So what values?

– Use switch data sheets – Rt

– Select Rt < Rg <2Rt

– Also Rg(off) = 2Rg(on)6

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MOSFET (& IGBT) drivers

• MOSFETs (& IGBTs) are voltage controlled devices

where an input voltage determines the output current

• Commonly, a VGS that is 5 – 15 V higher than VT is

sufficient to ensure the MOSFET is fully on.

• Switching a MOSFET off requires reducing VGS below

VT

• The charge associated with the gate capacitance can

require high pulse currents for high frequency

applications and drive circuit must be able to source

and sink

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Totem pole

• A very common circuit

configuration used to

provide the high

source and sink

current needed is the

totem pole

arrangement (double

emitter follower; push-

pull buffer)

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Electrically Isolated drivers

• Sometimes need to electrically isolate the

control signal from the output signal to the

switch

• This is usually due to the position of the

switch in a power circuit where the source (or

emitter) terminal voltage shifts high or low

depending on whether it has been switched

on or off.

• To ensure correct operation, the drive signal

must be able to float with the rising and falling

source terminal voltage.9

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Gate Driver Integrated Circuits (ICs) with Built-

in Fault Protection

12extV VCCV

cv

S

12extV VCCV

cv

S

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Optically isolated drivers

• Optocoupler IC with high speed

phototransistor

• The drive cct is then constructed on the

floating side with its own DC Power

supply

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Transformer Isolated

• Transformer isolation – no need for a dc power

source

• Works well for HF

• Core saturation is a problem – secondary V is bipolar

– so need to worry about negative VGS

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Current controlled devices - BJTs

• Power BJTs are current controlled devices. To

ensure a BJT is fully saturated (switched on), the

current through the base must be at least equal to

IC/β.

• Switching speed determined by how quickly the drive

signal can source and sink the charge associated

with the capacitance effect in forward biasing the

base-emitter p-n junction

• For fast switching in power BJTs need a base drive

signal that provides an initial very high current to

quickly charge (or discharge) the base-emitter

capacitance, which falls to a steady-state value

capable of maintaining saturation.13

A simple but effective cct

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• When the drive voltage falls to zero for turn-

off, the charged capacitor creates a high

negative current pulse that quickly discharges

the base-emitter capacitance.

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• Gate driver ICs:

– Save 30% on parts count

– 50% on PCB space

– The input signal Vc from the controller is

referenced to the logic level ground

– Vcc is the logic supply voltage

– Vext to drive the gate is supplied by the isolated

power supply referenced to the MOSFET source

S.

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Quiz #1

In the switching power-pole shown above, the

negative terminal of the input voltage is

grounded and the control voltage to control the

transistor is a logic-level signal generated with

respect to Ground.

In the output stage of the gate-driver IC shown in

the earlier slide, the power supply Vext must be

isolated from ground.

A. True

B. False

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Figure 2-10 Capacitor ESR and ESL.

C ESL ESRC ESL ESR

max max

ˆrms

p

w

LIIA

k J B

,

max max

conv y y rms

p

w s

k V IA

k B J f

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• Switching Frequency

• Selection of Transistors and Diodes

• Magnetic components

• Capacitor Selection

DESIGN CONSIDERATIONS

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PSpice Modeling:

Frequency

1.0KHz 3.0KHz 10KHz 30KHz 100KHz 300KHz 1.0MHz

I(L2) I(L1) -I(V3)

0A

10A

20A

30A

40A

50A

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Simulation Results: Individual and Total Admittances

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Thermal Design

( )j a jc cs sa dissT T R R R P

jcR

dissP

(b)

csR sa

R

aT

aTsTcTjT

(a)

chip

heat sinkisolation pad

jTcT

sTaT

ambient

case

jcR

dissP

(b)

csR sa

R

aT

aTsTcTjT

(a)

chip

heat sinkisolation pad

jTcT

sTaT

ambient

case

Magnetics and

capacitors

size

Heatsink

Sf

Magnetics and

capacitors

size

Heatsink

Sf

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Design Tradeoffs

Size of magnetic components and heat sink

as a function of switching frequency

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Diode Reverse Recovery and Power Losses

Diode Forward Loss: , (1 )diode F FM oP d V I

Diode Reverse Recovery Characteristic:

Diode Switching Losses:, ,

1( )2

diode sw RRM b d neg sP I t V f

0

rrt

at bt

RRMI

t

t0

FMV

,d negV

rrQ

0

rrt

at bt

RRMI

t

t0

FMV

,d negV

rrQ

inV

0

GGV

DiD

0I

Di

S

G DSv

Concept Quiz #2

In the switching power-pole shown in the

earlier slide, what is vd,neg equal to in

magnitude:

A. Essentially Vin

B. 0

C. Depends on the diode characteristic

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DSv

0

0

0 t

t

t

rit

inV

fvt

a rrt t

oI

Di

RRMI

swp

fvt

DSv

0

0

0 t

t

t

rit

inV

fvt

a rrt t

oI

Di

RRMI

swp

fvt

Effect of Diode Reverse Recovery Current:

inV

0

GGV

DiD

0I

Di

S

G DSv

Summary

• Practical Implementation Considerations

– Gate Driver ICs

– Design Considerations

• Thermal Considerations

• Magnetic Components

• Capacitors

• Selection of Switching Frequency

– Diode Reverse Recovery Characteristics

• Conduction Losses

• Increase in Switching Losses

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Quiz #1

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A. Doubles

B. Remains the same

C. Becomes one-half

In a switching converter, the switching power loss in a MOSFET is 100 W.

It is decided to redesign this converter and replace the original MOSFET with a new one

that has twice the switching speed, that is, it has one-half the voltage and current rise-

times and fall-times compared to the original MOSFET.

This new converter is to be operated at twice the switching-frequency compared to the

original converter.

Question: What happens to the switching power loss in the MOSFET of the new

converter, compared to that in the original converter?

Quiz #2

In the MOSFET of a switching power-pole, the average power dissipation is 30 Watts. The junction temperature of this MOSFET must not exceed 1000C, and the ambient temperature is given as 400C. Calculate the maximum thermal resistance that can be present between the ambient and the MOSFET junction.

A. 20 C/W

B. 3.330 C/W

C. 4.660 C/W

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