ECE1750, Spring 2018

Week 5 – MOSFET Gate Drivers

1

Power MOSFETs(a high-speed voltage-controlled switch)(a high-speed, voltage-controlled switch)

D: DrainD: DrainIf desired, a series blocking diode can be inserted here to prevent reverse current

D

G: Gate

Switch closes when VGS ≈ 4Vdc

G

S: Source

N channel MOSFET equivalent circuit

e GS dcS

• Controllability? - Controlled turn on, controlled turn off (but there is an internal antiparallel diode). Thanks to the diode it can conduct in both directions but it cannot block D-S voltages in which VD<VS.

2

g

• Controlled through the gate by voltage. If VGS>VGS,th it conducts. Otherwise, it does not conduct (in the forward direction).

We Avoid the Linear (Lossy) Region, Using We Avoid the Linear (Lossy) Region, Using Only the On and Off States

Ideal MOSFET “off”

Ideal MOSFET “on”

DD

SS

when VGS = 20 V when VGS = 0 V

3

Power MOSFETSMain characteristics• Static Characteristic

Ohmic

i

RegionON State

Active Region

iD iD

Higher

ON State

gVGS

v

Lower VDS

4

vDS vGSCutoff (OFF State)VGS<VGS,th

VGS,th

Main characteristics – Behavioral modelsPower MOSFETS

• Static model

ON h VGS>VGS h(sw) sw =

ON when VGS>VGS,thOFF when VGS<VGS,th

• Dynamic ModelD

Capacitances (particularly Cgs) need to be charged or

rd(T)

C d

iD

D g ) gdischarged to turn the MOSFET ON or OFF,

respectively

RDS,ONCgs

CgdCds

G

5S

We Want to Switch Quickly to Minimize Switching LossesTurn Off Turn On

VDS(t)

Turn Off

VDS(t)

Turn On

I(t) I(t)∆toff ∆ton

0 0

I(t) I(t)∆toff ∆ton

pLOSS(t) pLOSS(t)

0 0

Energy lost per turn off

Energy lost per turn on00

6

Turn off and turn on times limit the frequency of operation because their sum must be considerably less than period T (i.e., 1/f)

Consider, for example, the turn off

VDS(t)

Turn Off

VEnergy lost per turn off is proportional to

0

V • I • ∆toff ,

so we want to keep turn off (and turn on) times as small

I(t)

I

(and turn on) times as small as possible.

The more often we switch, the more

p OSS(t) ∆toff

0“energy loss areas” we experience per second.

Thus, switching losses (average W) pLOSS(t) ∆toff

0Energy lost per turn off

, g ( g )are proportional to switching frequency f, V, I, ∆toff, and ∆ton.

7And, of course, there are conduction losses that are equal to the product of RDSon and I squared

Switching losses• With a resistive load, voltage and current waveforms are approximately

like (linear commutation)

turn-on losses: turn-off losses:

offVI ffV I

Total switching losses:

( ) offonon off

on on

VIp t t V tt t

( ) off onoff on

off off

V Ip t t I tt t

0 0

1 ( ) ( ( ) ( )) ( )6 6

T T on off on offsw on off on off sw

I V I VP p t dt f p t p t dt f t t f t

T

Switching losses• With in inductive load and an ideal switch, voltage and current

waveforms are approximately likeMore realistic behavior

Approximate behavior

turn-on losses: turn-off losses:

offV I

Total switching losses:

( ) 0onp t , ,

( )( / 2) ( / 2)

off onoff on off

L off L off

Ip t I t V tt t

,0

1 ( ) ( )2 2

T on off on offsw L off sw

I V I VP p t dt f t f t

T

Advantages of Operating Above 20kHzYes switching losses in power electronic switches do increase withYes, switching losses in power electronic switches do increase with operating frequency, but going beyond 20kHz has important advantages. Among these are

• Humans cannot hear the circuits

• For the same desired smoothing effect, L’s and C’s can be smaller because, as frequency increases and period T decreases, L’s and C’ h d di h l l f ti

• Humans cannot hear the circuits

C’s charge and discharge less energy per cycle of operation. Smaller L’s and C’s permit smaller, lighter circuits.

• Correspondingly, L and C rms ripple currents decrease, so current p g y, pp ,ratings can be lower. Thus, smaller, lighter circuits.

• AC transformers are smaller because, for a given voltage rating, the peak flux density in the core is reduced (which means transformerpeak flux density in the core is reduced (which means transformer cores can have smaller cross sectional areas A).

)cos()sin()( maxmax tNABtBdNAdBNAdNtv

10

)cos()( max tNABdt

NAdt

NAdt

Ntv

Thus, smaller, lighter circuits. N = number of turns, ϕ = magnetic flux, B = magnetic field, A = x-sectional area

Fast Switching Frequencies• Previous slide was just to illustrate how, with increased

switching frequency, one can reduce the size of AC transformer cores needed:transformer cores needed:

)cos()sin()( maxmax tNAB

dttBdNA

dtdBNA

dtdNtv

dtdtdt

Thus, smaller, lighter circuits. N = number of turns, ϕ = magnetic flux, B = magnetic field, A = x-sectional area

• The drawback, of course, to high frequency switching is increased power loss, since: P (loss) = P x number of switching eventsPTotal (loss) = Pswitching loss x number of switching events

(or, the switching frequency)

• This is the downside of high-frequency switching. Thus, one must work to ensure overall losses are reduced by working to reduce the individual switching transition time. 11

• MOSFETS are usually controlled with a pulse-width modulation (PWM) strategy.

• In essence, the PWM process involves comparing a sawtooth (or any other , p p g ( yperiodic function with linear transitions) with a voltage level. If the voltage level is constant, it will tend to produce a constant (dc) output.

Sawtooth or triangleadjustable analog input (duty cycle control)

Linear portions of sawtooth allows to directly translate voltage levels into time intervals (on-times)

12V

Gate Driver

( TC1427)(e.g. TC1427)

12Comparator

(e.g., LM393)

• PWM

D = 8/12 = 0 75D 8/12 0.75

Gate Driver

( TC1427)(e.g. TC1427)

13Comparator

(e.g., LM393)

• PWM

D = 6/12 = 0 5D 6/12 0. 5

Gate Driver

( TC1427)(e.g. TC1427)

14Comparator

(e.g., LM393)

• PWM

D = 4/12 = 1/3D 4/12 1/3

Gate Driver

( TC1427)(e.g. TC1427)

15Comparator

(e.g., LM393)

MOSFETS are Very Sensitive to Static Electricity

• Touching the gate lead before the MOSFET is properly mounted will likely ruin the MOSFET

• But it may not fail right away. Instead, the failure may be gradual. Your circuit will work, but not correctly. Performance gradually deteriorates.

• When that happens, you can spend unnecessary hours debugging

• Key indicators of a failed MOSFET are

• Failed or burning hot driver chip• Burning hot gate driver resistor (discolored or bubbled up)Burning hot gate driver resistor (discolored, or bubbled up)

• Board scorches or melts underneath the driver chip or gate driver resistor

16

Avoid these problems by mounting the MOSFET last, by using an antistatic wristband, and by not touching the gate lead

MOSFET Switch Turn-Off Overshoot..200kHz, 0.01µF snubber

VDSSimple snubbercircuit

200kHz no snubber 100kHz 0 01µF snubber

VGS

200kHz, no snubber 100kHz, 0.01µF snubber

VDS VDS

VGS VGS

200kHz, 0.0022µF snubber 50kHz, 0.01µF snubber

VDS VDS

17VGS VGS

MOSFET Safe Operating Area (SOA)

Pulsed drain current must never be exceed

Maximum continuous drain

current can be exceeded but onlymust never be exceed exceeded but only

for a brief time

Thermal limit (power limit) can be exceeded but only for a brief timeOperation naturally

limited by RDS on

Breakdown voltage must never be

limited by RDS,on (Ohm’s law)

18

must never be exceeded

MOSFET Datasheet

19

Small 10Ω , 100W Resistor. 22Vdc. Vpeak = 240V.

MOSFET opens

VDS

VGS

ON

OFF

DT

20Note: Ringing is caused by the interaction of parasitic capacitors and inductors.

DT

Small 10Ω , 100W Resistor. 22Vdc. Vpeak = 40V.

0 022 S C0.022µF Snubber Cap

MOSFET opens

21

Small 10Ω , 100W Resistor. 22Vdc. Vpeak = 60V

0 0068 S C0.0068µF Snubber Cap

MOSFET opens

22

Gate Drivers

23

Integrated PWM

TT CRf

2.1

MC34060A, Fixed Frequency, PWM, Voltage Mode Single Ended Controller

241

T Tf

C R