Download Parasitic effects Diodes reverse recovery Diodes recovery – spike

[5-1]
Mor M. Peretz, Switch-Mode Power Supplies
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
[5-2]
Mor M. Peretz, Switch-Mode Power Supplies
Diodes reverse recovery
L
Vin
Vx
Vo
R
C
Diode on – bidirectional
current flow
Mor M. Peretz, Switch-Mode Power Supplies
Diodes recovery – spike and oscillations
Diode on
Lstray
Lstray
ESR
Lstray
Diode off
Lstray
Lstray
1
ESR
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[5-4]
Mor M. Peretz, Switch-Mode Power Supplies
Transistor turn off - parasitics
Lmain
Transistor
on
Lstray
Lstray
D
Lmain
Lstray
Transistor
off
LD
Co
Vo
G
Lstray
ESR
Lstray
RG
LS
S
[5-5]
Mor M. Peretz, Switch-Mode Power Supplies
Transistor turn off – diode forward recovery
I
I
t
VD
VPK
VF
t
[5-6]
Mor M. Peretz, Switch-Mode Power Supplies
Mosfef gating
RL
D
RL
RG
CGS
LD
G
RG
Vgs
LS
LS
S
Vgs
V'gs
2
Depend on Q
Q
LS
C gs
RL  R G
Vgs (real)
[5-7]
Mor M. Peretz, Switch-Mode Power Supplies
Flyback leakage problem
VO
Vin
Voltage spike can
develop HIGH voltage
Llkg
Vc - ?
Cdss
V-?
[5-8]
Mor M. Peretz, Switch-Mode Power Supplies
Clamp
Zener diode
VO
Vin
D–C-R
Llkg
Cdss
[5-9]
Mor M. Peretz, Switch-Mode Power Supplies
Clamp configurations
Vin
Vc  Vin 
n1
Vo
n2
Vin
Vc 
3
n1
Vo
n2
[5-10]
Mor M. Peretz, Switch-Mode Power Supplies
Parasitic inductance and physical placement
Vin
L1
Vin
Rc
Cc
Llkg
C
L4
L2
L3
Connection of the clamp/snubber should be directly
on the element
[5-11]
Mor M. Peretz, Switch-Mode Power Supplies
Clamp design
Equivalent circuit
Current waveform
Assuming small ripple on the clamp capacitor (will be
designed that way)
V
VCc
Vav
Ts
[5-12]
Mor M. Peretz, Switch-Mode Power Supplies
Calculation of clamp components
Vo
n1
 Vcc
n2
LLkg
I av Rc  Vcc _ av
I pk
Cc Rc  Ts
I av
dI p
I av 
4
I pk  t p
2
 fs
dt p
Vo

n1
 Vcc
n2
LLkg
[5-13]
Mor M. Peretz, Switch-Mode Power Supplies
Design procedure
1. Select VCc av > Vo’
I av 
2. Calculate Ip av
Rc 
3. Select
Vcc _ av
I pk  t p
2
 fs
I av
4. Select Cc T > Ts
5. Adjust values
[5-14]
Mor M. Peretz, Switch-Mode Power Supplies
Diode turn off snubber
Vo
Diode on
Lstray
snubber
Diode off
CD
Lstray
[5-15]
Mor M. Peretz, Switch-Mode Power Supplies
Diode turn off snubber
VD
CS RS
CD
no snubber
bad snubber
Vo
VO
Lstray
VD
good snubber
Cs > CD
5
CS is very l arg e
VO
[5-16]
Mor M. Peretz, Switch-Mode Power Supplies
Snubber design
CS RS
Required values
Ipk (reverse current)
Stary inductance
VO
CD
Lstray
Lstray I pk 2
2

Cs Vo  V 
2
2
Resistor for damping Cs Rs  Ts
Cs needs to discharge
Use simulation to optimize the snubber
[5-17]
Mor M. Peretz, Switch-Mode Power Supplies
Switching snubber
control
control
VGS
VGS
t
t
VS
IS
VS
VS
dI
dt
IS
dV
dt
t
t
Jd
Jp
Pswitching
t
t
The objective: avoid overlap of current and voltage
Slow the rise of either current or voltage
[5-18]
Mor M. Peretz, Switch-Mode Power Supplies
Switch snubber
Vo
C
dV/dT (at turn off) can be slow down by adding external
snubber capacitor C
Vo
Turn off
6
Cdss
C
dV
I

dt C  Cdss
[5-19]
Mor M. Peretz, Switch-Mode Power Supplies
Switch snubber
Problem at turn on
Vo
EC 
C
CVO 2
2
Pd 
CVO 2
 fs
2
Dissipated through the transistor
Solution
VO
CS
RS
[5-20]
Mor M. Peretz, Switch-Mode Power Supplies
Switch snubber design
VO
CS
RDSon
RS
T
1
 ton
Rs Cs
If Rds on < Rs most energy will
be lost to Rs  Heat
Selection of Cs 
Selection of Rs  to ensure reset
ton  4 Rs Cs
Question: how VDS will look if
Cs is not fully discharged?
[5-21]
Mor M. Peretz, Switch-Mode Power Supplies
Lossless (ZVS) snubber
Q1
C1
L
VC
2
t
Q2
delay
t
C2
C1 , C2 of transistor
plus external
VC
2
IL
t
VDS1
t
VDS2
t
7
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
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