Drivers What is a driver MOSFET capacitances Diver operation - - PDF document

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Mor M. Peretz, Switch-Mode Power Supplies

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Drivers

• What is a driver
• MOSFET capacitances
• Diver operation
• Gate capacitance
• Gate drive calculation
• Driver types
• High-side driver
• Isolation
• Optocoupler
• Boot-strap supply

Mor M. Peretz, Switch-Mode Power Supplies

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Driver

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Mor M. Peretz, Switch-Mode Power Supplies

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MOSFET capacitances

Q Vgs Vt Qgs+Qgd Qgs+Qgd Miller capacitance 15V Q1 Q2 Q3

Mor M. Peretz, Switch-Mode Power Supplies

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Miller effect

VCgs(0) = 0 VCgs(∞) =VGSmax VCgd(0) = -VO VCdg(∞) = VGSmax Cgs Cgd Ig Vs Rg G Vo ( )

total GS gs GS O gd

Q V C V V C   

max max

(1 )

total

• eq

gs gd GS GS

Q V C C C V V        

1 1

m

C C k C k         

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Mor M. Peretz, Switch-Mode Power Supplies

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Miller effect

t

Cgs Cgd Ig Vs Rg G Vo

Mor M. Peretz, Switch-Mode Power Supplies

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Required drive calculation

Equivalent capacitance

t t ? ? VS Ig

Assuming constant drive current

max total g

• n

eq gs

Q I t C V    

max eq gs g

• n

C V I t 

max

(1 )

• eq

gs gd GS

V C C C V   

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Mor M. Peretz, Switch-Mode Power Supplies

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Required drive calculation

Gate charge

3 g r

Q I t  For t1 For t2

2 g

Q I t V I R       

Mor M. Peretz, Switch-Mode Power Supplies

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Driver types

pnp npn Vc Rg s d s d p channel n channel n channel n channel Vc s d s d

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Mor M. Peretz, Switch-Mode Power Supplies

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High-side driver

Vb VGS Gate driver

Mor M. Peretz, Switch-Mode Power Supplies

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High-side driver

Transformer

Rs Problem of large Don Possible solution: DC restorer

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Mor M. Peretz, Switch-Mode Power Supplies

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High-side driver

Optocoupler

VC - floating supply Driver VC

Mor M. Peretz, Switch-Mode Power Supplies

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High-side driver

Potential offset

Driver +VC Re floating supply Logic signal is referred to GND FET is used to sustain the high voltage

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Mor M. Peretz, Switch-Mode Power Supplies

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High-side driver

Boot-strap supply – floating capacitor

Driver Internal circuit in C +15V Rg Re

Mor M. Peretz, Switch-Mode Power Supplies

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High-side driver

Boot-strap supply – commercial product

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Mor M. Peretz, Switch-Mode Power Supplies

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High-side driver

Boot-strap supply – commercial product

Mor M. Peretz, Switch-Mode Power Supplies

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Current sensing

• Resistive sensing
• Current transformer
• Measuring DC
• Reset
• Design
• Hall effect

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Mor M. Peretz, Switch-Mode Power Supplies

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Resistive sensing

R

I

R

V

R

I

R

V L R t t

R

I

R

V Parasitic inductance

Mor M. Peretz, Switch-Mode Power Supplies

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Current transformer

DC component via primary lost DC component may saturate transformer

L

I

• ut

I t t

Iout T IL L

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Mor M. Peretz, Switch-Mode Power Supplies

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Current transformer

Pulse current - Problem due to reset

S

I

m

I

m

L n : 1

R

I

R

V   IS 

R

V R  

m L

I IS  n : 1 t

R

V

Mor M. Peretz, Switch-Mode Power Supplies

[7-20]

Pulse current transformer

1 : n 1 : n R

R

V I t t ? VR

R

I V R n  

m

I

2

V

R

V

S

R

r

R

Rr > Rs Reset with resistor

1

S

2

S

2

V t

R

V sonant Re effect

2 1

S S 

pk

V

Vpk may damage diode

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Mor M. Peretz, Switch-Mode Power Supplies

[7-21]

Pulse current transformer

Reset with clamp

m

I R

R

V

reset

t sonant Re effect ) V V (

D Z  Mor M. Peretz, Switch-Mode Power Supplies

[7-22]

Measuring inductor current Combining two sensors

RS VO

L

I t

FET

I

D

I

FET

I

D

I

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Mor M. Peretz, Switch-Mode Power Supplies

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Design of current transformer

Lm2 VR I n VD V2

Transformer saturation is a function of the voltage

max R e

dt I B V R nA n

v

 



av

• n

av on s

V D Vdt V t f  



2

V

R D

V V 

av

V

• n

t

S

T

max max av e s

I RD B nA f 

Mor M. Peretz, Switch-Mode Power Supplies

[7-24]

Hall Effect sensor

I n

• V

S

R

f

I Active Device Hall effect sensor

Zeroing the flux by feedback

f

• S

I I n I V R n   

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Mor M. Peretz, Switch-Mode Power Supplies

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Mor M. Peretz, Switch-Mode Power Supplies

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Mor M. Peretz, Switch-Mode Power Supplies

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