1 Mor M. Peretz, Switch-Mode Power Supplies [5-4] Transistor turn - - PDF document

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1 Mor M. Peretz, Switch-Mode Power Supplies [5-4] Transistor turn - - PDF document

Nov 03, 2022 159 likes •263 views

Mor M. Peretz, Switch-Mode Power Supplies [5-1] Parasitic effects Snubbers and clamps Turn on and turn off effects Turn on - diode reverse recovery Turn off dI/dT effect on transistor Turn off diode forward recovery

slide-1
SLIDE 1

1

Mor M. Peretz, Switch-Mode Power Supplies

[5-1]

Parasitic effects

Snubbers and clamps

  • Turn on and turn off effects
  • Turn on - diode reverse recovery
  • Turn off – dI/dT effect on transistor
  • Turn off – diode forward recovery
  • Mosfet gating
  • Transistor protection - Clamp
  • Flyback leakage problem
  • Clamp configuration
  • Component selection
  • Diode protection – Snubber
  • Switching snubbers and lossless (ZVS) snubber

Mor M. Peretz, Switch-Mode Power Supplies

[5-2]

Diode on – bidirectional current flow

Diodes reverse recovery

Vin L C R Vx Vo

Mor M. Peretz, Switch-Mode Power Supplies

[5-3]

Diodes recovery – spike and oscillations

Lstray Lstray Lstray ESR Lstray Lstray ESR

Diode on Diode off

slide-2
SLIDE 2

2

Mor M. Peretz, Switch-Mode Power Supplies

[5-4]

Transistor turn off - parasitics

Lmain Lstray Lstray Lstray Lstray Lstray Co Vo Lmain ESR

Transistor

  • n

Transistor

  • ff

LS LD RG G S D

Mor M. Peretz, Switch-Mode Power Supplies

[5-5]

Transistor turn off – diode forward recovery

I

I t t VD VPK VF

Mor M. Peretz, Switch-Mode Power Supplies

[5-6]

Mosfef gating

RL

RG RL CGS LS Vgs

gs

V (real) V'gs Vgs Depend on Q

G L gs S

R R C L Q  

LS LD RG G S D

slide-3
SLIDE 3

3

Mor M. Peretz, Switch-Mode Power Supplies

[5-7]

Flyback leakage problem

Vin Llkg VO Cdss

Voltage spike can develop HIGH voltage Vc - ? V - ?

Mor M. Peretz, Switch-Mode Power Supplies

[5-8]

Clamp

Vin Llkg VO Cdss

Zener diode D – C - R

Mor M. Peretz, Switch-Mode Power Supplies

[5-9]

Clamp configurations

Vin Vin

1 2 c in

  • n

V V V n  

1 2 c

  • n

V V n 

slide-4
SLIDE 4

4

Mor M. Peretz, Switch-Mode Power Supplies

[5-10]

Parasitic inductance and physical placement

Vin L2 L3 L4 L1 C

Vin Cc Rc Llkg

Connection of the clamp/snubber should be directly

  • n the element

Mor M. Peretz, Switch-Mode Power Supplies

[5-11]

Clamp design

Equivalent circuit Assuming small ripple on the clamp capacitor (will be designed that way)

av

V

Cc

V V 

s

T Current waveform

Mor M. Peretz, Switch-Mode Power Supplies

[5-12]

Calculation of clamp components

_ av c cc av

I R V 

av

I

1 2

  • cc

Lkg

n V V n L 

c c s

C R T 

1 2

  • cc

p p Lkg

n V V dI n dt L  

2

pk p av s

I t I f   

pk

I

slide-5
SLIDE 5

5

Mor M. Peretz, Switch-Mode Power Supplies

[5-13]

Design procedure

  • 1. Select VCc av > Vo’
  • 2. Calculate Ip av
  • 3. Select
  • 4. Select Cc T > T

s

  • 5. Adjust values

_ cc av c av

V R I 

2

pk p av s

I t I f   

Mor M. Peretz, Switch-Mode Power Supplies

[5-14]

Diode turn off snubber

Vo Lstray Lstray CD

Diode on Diode off snubber

Mor M. Peretz, Switch-Mode Power Supplies

[5-15]

Diode turn off snubber

CD Lstray CS RS VO

D

V

  • V

snubber no snubber good snubber bad

D

V

O

V e arg l very is CS

Cs > CD

slide-6
SLIDE 6

6

Mor M. Peretz, Switch-Mode Power Supplies

[5-16]

Snubber design

CD Lstray CS RS VO

Required values Ipk (reverse current) Stary inductance

 

2 2

2 2

stray pk s

  • L

I C V V   

s s s

C R T  Resistor for damping Cs needs to discharge Use simulation to optimize the snubber

Mor M. Peretz, Switch-Mode Power Supplies

[5-17]

Switching snubber

control

GS

V

S

V

S

V

S

I

d

J

switching

P t t t

control

GS

V

S

V dt dV

S

I

p

J t t t dt dI

The objective: avoid overlap of current and voltage Slow the rise of either current or voltage

Mor M. Peretz, Switch-Mode Power Supplies

[5-18]

Switch snubber

  • V

C

  • V

C

dss

C

Turn off dV/dT (at turn off) can be slow down by adding external snubber capacitor C

dss

dV I dt C C  

slide-7
SLIDE 7

7

Mor M. Peretz, Switch-Mode Power Supplies

[5-19]

Switch snubber

  • V

C

Problem at turn on

2 2

2 2

O O C d s

CV CV E P f   

Dissipated through the transistor Solution

CS RS VO Mor M. Peretz, Switch-Mode Power Supplies

[5-20]

Switch snubber design

CS RS VO RDSon

If Rds on < Rs most energy will be lost to Rs  Heat Selection of Cs  Selection of Rs  to ensure reset

1 4

  • n
  • n

s s s s

T t t R C R C   

Question: how VDS will look if Cs is not fully discharged?

Mor M. Peretz, Switch-Mode Power Supplies

[5-21]

Lossless (ZVS) snubber

2 VC 2 VC

C1 C2 L C1 , C2 of transistor plus external

1

Q

2

Q

L

I

1 DS

V

2 DS

V delay t t t t t

slide-8
SLIDE 8

8

Mor M. Peretz, Switch-Mode Power Supplies

[5-22]

Mor M. Peretz, Switch-Mode Power Supplies

[5-23]

Mor M. Peretz, Switch-Mode Power Supplies

[5-24]