To:
PI-Taiw
wan FAE Sta
aff
From::
PI-San Jose Applic
cations Engineering
Date:
c-15
30-Dec
Revis
sion:
A
Re:
RCD snubber
s
- Re
educing snu
ubber resisto
or dissipatio
on and output rectifier d
diode
peak in
nverse volta
age stress in
n a fly-back power supp
ply
Introduction:
Volta
age waveforrm across output
o
rectiffier diode in
n a flyback converter sshows a vo
oltage
ring at the insta
ant of turn-O
ON of the primary
p
side
e MOSFET. This ringin
ng is cause
ed by
reson
nant interac
ction betwe
een the leak
kage reacta
ance of the
e secondaryy winding, trace
induc
ctance of th
he secondarry side circu
uit interconn
nections, ou
utput rectifie
er diode revverse
recov
very and dio
ode junction
n capacitance. The ressulting reverrse voltage can exceed
d the
PIV (Peak
(
Inverrse voltage)) rating of th
he output re
ectifier diode and the rringing resu
ults in
incre
eased EMI.
A red
duction in th
he peak voltage and damping
d
of the resonant ring can be achieve
ed by
conn
necting an RC circuit across the output recctifier diode
e. The addition of thiss RC
snub
bber circuit results in a slight dro
op in efficie
ency due to
o power diissipation in
n the
resistor. This RC
C circuit can
n also lead to
t increase in the initial peak curre
ent at the in
nstant
of turrn-ON of the
e primary sid
de MOSFET
T.
Choic
ce of components us
sed for the RC circuit is a trad
deoff betwe
een losses, EMI
perfo
ormance imp
provement and
a voltage
e stress on th
he output re
ectifier diode
e.
Redu
uction in diode
d
peak inverse voltage
v
is ttypically acchieved by increasing
g the
capa
acitance in the
t RC snu
ubber howe
ever increasse in capaccitance resu
ults in incre
eased
loss in the snubb
ber resistor.
Addin
ng a diode in parallel with the sn
nubber resisstor reduce
es loss in th
he resistor. This
allow
ws for an inc
crease in sn
nubber capa
acitance wh
hile simultan
neously kee
eping the lo
oss in
the snubber
s
resistor low.
Use of a diode in
i parallel with
w the resis
stor is there
efore recom
mmended fo
or designs w
where
p
voltage across th
he output re
ectifier diod
de is to be reduced byy increasing
g the
the peak
snub
bber capacita
ance and th
he loss in the
e snubber rresistor is to
o be reduced
d.
CONF
FIDENTIAL: The
T informatio
on in this reporrt contains confidential and proprietary in
nformation of P
Power
Integrrations. By receiving this re
eport, the custtomer agrees to use this infformation for the sole purpo
ose of
addressing the issues reviewed in
n the report an
nd to keep the report’s conte
ents confidentiial.
 Pag
ge 1 of 6
COMPA
ANY CONFIDE
ENTIAL
23-Decc-2015
RCD snubber - Reducing output diode voltage stress in a fly-back power supply
Explanation:
Figure.1 shows the equivalent circuit of the flyback converter power stage. Lm is the
magnetizing inductance of the primary and NP and NS represent the ideal transformer.
Whenever the primary side MOSFET turns ON while the secondary side diode is still
conducting, the voltage across the secondary side diode is rapidly reversed. The diode
goes from a conducting state to a reverse biased state and the magnitude of the reverse
current depends on the diode reverse recovery characteristics. Energy is stored in the
leakage reactance Llk2. Following the reversal of voltage across the secondary winding,
a resonant ring is seen at the anode of the reverse biased output diode. Llk2 and
junction capacitance Cj determine the frequency of the resonant ring seen across the
diode. This ringing can generate conducted and radiated EMI. The ring can exist for a
prolonged period of time due to low inherent damping in the circuit and the overshoot in
voltage across the output diode that is caused by this ring can exceed diode rating in
some designs and lead to diode damage.
Figure 1 Equivalent circuit of the flyback converter
Figure.2 shows one example of the voltage at the anode of the rectifier diode without any
RC snubber connected across the diode for a 19V output power supply.
CONFIDENTIAL: The information in this report contains confidential and proprietary information of Power Integrations.
By receiving this report, the customer agrees to use this information for the sole purpose of addressing the issues
reviewed in the report and to keep the report’s contents confidential
 Page 2 of 6
COMPANY CONFIDENTIAL
30-Dec-2015
RCD
D snubber - Reducing output
o
diode
e voltage strress in a fly--back powerr supply
Figure
e 2 Undamped
d ringing at the
e anode of the
e output rectifie
er diode
Damping of the resonant rin
ng is achiev
ved using a RC circuit cconnected a
across the d
diode
as sh
hown in Figu
ure.3. The capacitance
c
e used in the
e RC circuitt is considerrably higherr than
the ju
unction capa
acitance of the
t output rectifier
r
diod
de.
R1
10
0.5 W
C1
D1
1
1 nF
200 V
Sch
hottky Diode
Figure
F
4 Dam
mping of resonan
nt ring with the a
addition of the R
RC snubber
Figure
e 3 RC snubbe
er connected
As shown
s
in Figure.4, the
e peak amplitude of tthe first ressonant ring
g is dramattically
reduc
ced with the
e addition of
o the RC sn
nubber acro
oss the diod
de. For the e
example sh
hown,
the fiirst peak re
educed from
m 160V to 120V with a 10Ω resisto
or and 1nF capacitor in the
RC snubber
s
circuit. Furth
her reduction in the magnitude off the resonant ring ca
an be
achie
eved with in
ncrease in the
t capacita
ance used in the snub
bber as sho
own in Fig.5
5 and
Fig.6
6
Figure 5 10Ω + 1nF RC
C snubber
Figure 6 10Ω + 1.5nF RC Snubber
CONFIDENTIAL: The
e information in this report conttains confidentia
al and proprieta
ary information o
of Power Integrrations.
By rec
ceiving this repo
ort, the custom
mer agrees to use this informa
ation for the solle purpose of a
addressing the issues
review
wed in the report and to keep the
e report’s conten
nts confidential
 Page 3 of 6
COMPANY
Y CONFIDENTIIAL
30-Decc-2015
RCD
D snubber - Reducing output
o
diode
e voltage strress in a fly--back powerr supply
The values
v
of th
he resistor and
a capacita
ance used in the RC ssnubber circcuit are adju
usted
until the peak re
everse volta
age across the output rectifier dio
ode is sufficciently below
w the
PIV rating
r
of the
e diode and
d conducted and radiate
ed emission
ns are below
w the appliccable
limits
s.
In ea
ach switchin
ng cycle, the snubber capacitor iss charged a
and discharrged through
h the
snub
bber resistor which res
sults in los
ss in the re
esistor. Inccrease in th
he value o
of the
capa
acitance res
sults in incrrease in RM
MS current through the resistor tthus resultin
ng in
incre
eased loss.
If a diode
d
is con
nnected acro
oss the resiistor in the R
RC snubbe
er, part of the current flo
ow in
the snubber
s
circuit is throug
gh the diode
e which resu
ults in a net reduction in
n current thrrough
the snubber
s
res
sistor and hence
h
a re
eduction is tthe loss in the snubb
ber resistor. The
addittion of the diode changes the resis
stance in the
e RC circuitt depending
g on the dire
ection
of cu
urrent flow in
n the circuit and hence has the effe
ect of chang
ging the dam
mping offere
ed by
the circuit
c
however without significantly
y altering th
he magnitud
de of the peak ring vo
oltage
for th
he first ring which
w
happe
ens to have the highestt amplitude..
Figurre.7 shows the locatio
on and conn
nections of the additio
onal diode in the prop
posed
RCD
D snubber circuit.
ov ery Diode
Fast Reco
D2
R1
10
0.5 W
C1
1 nF
200 V
D1
ottky Diode
Scho
e 7 RCD Snub
bber for output rectifier
Figure
Figurre.8 and Figure.9 sho
ow one exa
ample of ch
hange in ccurrent flow
wing through
h the
resistor with and
d without the
e diode adde
ed across th
he resistor.
Figure
e 8 Current through snubbe
er resistor witho
out
paralle
el diode
Figurre 9 Currentt through snu
ubber resistor with
diode
e in parallel witth resistor
CONFIDENTIAL: The
e information in this report conttains confidentia
al and proprieta
ary information o
of Power Integrrations.
By rec
ceiving this repo
ort, the custom
mer agrees to use this informa
ation for the solle purpose of a
addressing the issues
review
wed in the report and to keep the
e report’s conten
nts confidential
 Page 4 of 6
COMPANY
Y CONFIDENTIIAL
30-Decc-2015
RCD
D snubber - Reducing output
o
diode
e voltage strress in a fly--back powerr supply
Table
e.1 compare
es current th
hrough the snubber
s
ressistor for thre
ree different combinatio
ons of
the snubber
s
circ
cuit under th
he same operating condition. Addittion of the d
diode acrosss the
resistor has the effect of red
ducing the RMS
R
curren
nt through the resistor w
without affe
ecting
the im
mprovemen
nt in peak inv
verse voltag
ge achieved
d through the
e increase iin capacitan
nce
Resis
stor
Cap
pacitor
Dio
ode in parallel
p
snu
ubber Resis
stor
w
with Negativve peak RMS
cu
urrent
Anode voltage
through ressistor
10Ω
1nF
F
No
-114V
224.7mA
10Ω
1.5n
nF
No
-106V
268.4mA
10Ω
1.5n
nF
Ye
es
-109V
214.2mA
Figurre.10 shows the diode
e anode vo
oltage, curre
ent through
h the snubb
ber resistorr and
curre
ent through the snubbe
er diode und
der one ope
erating cond
dition for the example used
for th
his note.
CONFIDENTIAL: The
e information in this report conttains confidentia
al and proprieta
ary information o
of Power Integrrations.
By rec
ceiving this repo
ort, the custom
mer agrees to use this informa
ation for the solle purpose of a
addressing the issues
review
wed in the report and to keep the
e report’s conten
nts confidential
 Page 5 of 6
COMPANY
Y CONFIDENTIIAL
30-Decc-2015
RCD snubber - Reducing output diode voltage stress in a fly-back power supply
Conclusion
For some of the high power switching power supplies, loss in the snubber resistor may
be considerable and may require high power resistors to handle the power dissipation. In
such designs use of the RCD snubber can effectively keep the loss in the snubber
resistor low.
Document Distribution
This document is for internal use only. Only provide to customers once D. Chen and M.
Khan have given approval. Do not provide to distributors or anyone outside Power
Integrations without express permission.
Feedback
Please send feedback or comments to E. Hwang, D. Chen, or R. Joshi
Revision History:
(Rev A. 12302015) First release
CONFIDENTIAL: The information in this report contains confidential and proprietary information of Power Integrations.
By receiving this report, the customer agrees to use this information for the sole purpose of addressing the issues
reviewed in the report and to keep the report’s contents confidential
 Page 6 of 6
COMPANY CONFIDENTIAL
30-Dec-2015