4. Snubber design (RC snubber)

RC snubber Secondary side diode

3

4

C102220nF

275VAC

LF10123mH

C101 220nF

275VAC

RT15D-9

F1 FUSE

250V

2A

C103100uF

400V

R102

30kΩΩΩΩ

R105

40kΩΩΩΩ

R10356kΩΩΩΩ2W

C10410nF

1kVD101

UF 4007

C106

47nF50V

C10522uF

50V

D102

TVR10GR104

5ΩΩΩΩ

1

2

3

4

5

T1

EER3016

BD101

2KBP06M3N257

1

2

R101560kΩΩΩΩ

1W

IC1

FSDM07652R

Vstr

NC

Vfb

Vcc

Drain

GND

1

2

34

5

6

ZD10122V

8

10

D202MBRF10100

C201

1000uF

25V

C202

1000uF25V

L201

12V, 2.5A

6

7

D201MBRF1045

C2031000uF

10V

C204

1000uF

10V

L202

5V, 2A

R201

1kΩΩΩΩ

R202

1.2kΩΩΩΩ

R204

5.6kΩΩΩΩ

R2031.2kΩΩΩΩ

C205

47nF

R2055.6kΩΩΩΩ

C301

4.7nF

IC301

H11A817AIC201

KA431

+

-

4. Snubber design (RC snubber)

Why Diode voltage oscillates when turned off?

Oscillation between the leakage inductance and diode output capacitance

Need to be damped with additional resonant network

Cout

Llk

Cout

Llk

Cout

Llk

3Cout

outrout

outlk

out

lk

CfC

CL

C

LR

π2

1===

Z

R

4. Snubber design (RC snubber)

(1) Measure the original resonance period (Tr) of the diode voltage waveforms.

(2) Find a capacitor value that doubles the resonance period when connected in parallel with the diode.

Tr=46ns

8

10

D202MBRF10100

C2011000uF

25V

Csn

With 680pF capacitor, the resonance period is approximately doubled (46ns 96ns)

4. Snubber design (RC snubber)

(3) Calculate the snubber resistor with the following equation.

8

10

D202

MBRF10100

C201

1000uF

25V

680pF 30

4. Snubber design (RCD snubber)

RCD snubber Primary side

4. Snubber design (RCD snubber)

Experimental results (Csn=2.2nF, Rsn=56kΩΩΩΩ, fs=66kHz)

WkuVV

VfIL

WkR

V

ROsn

snspklk

sn

sn

256.065122

122661.1)3(

2

1

2

1

266.056

122

22

22

=−

×××=−

==

WkuVV

VfIL

WkR

V

ROsn

snspklk

sn

sn

360.065143

1436641.1)3(

2

1

2

1

365.056

143

22

22

=−

×××=−

==

4. Snubber design (RCD snubber)

Design Procedure

Measure the leakage inductance : measure the primary side inductance with all other windings shorted

– LCR meter is not always correct (Normally 50% error) especially the leakage inductance is small

Determine the snubber capacitor voltage (Vsn) considering voltage margin of BVdss

Calculate the snubber resistor using

Determine the snubber capacitor considering the snubber capacitor voltage ripple (1~10nF)

If the measured drain voltage is different from the designed value, leakage inductance should be calibrated using

2

)(2

pkslk

ROsnsnsn

IfL

VVVR

−=

2

)(2

pkssn

ROsnsnlk

IfR

VVVL

−=

4. Snubber design (RCD snubber)

Design Example (VRO=65V, Vin=265Vac(VDC=370V), Ipk=1.5A)

Measure the leakage inductance with LCR meter: 5uH @ 70kHz

Determine the snubber capacitor voltage considering voltage margin of BVdss : Vsn=182V (Vsn+VDC=182+370=552V :85% of 650V)

Calculate the snubber resistor

Ω=

××

−××=

−= k

kuIfL

VVVR

pkslk

ROsnsnsn 57

5.1665

)65182(1822)(2

22

4. Snubber design (RCD snubber)

Determine the snubber capacitor considering the snubber capacitor voltage ripple : Csn=2.2nF

Measured the peak drain voltage : 520V (Vsn=146V)

uHkkIfR

VVVL

pkssn

ROsnsnlk 3

5.16656

)65150(1502)(2

22=

××

−××=

−=

Rsn=56kΩΩΩΩ, Csn=2.2nF

Vsn=520-370=150V

# Vsn is different from the

designed value

Recalculate the leakage inductance with measured Vsn

4. Snubber design (RCD snubber)

Recalculate snubber resistor

Ω=

××

−××=

−= k

kuIfL

VVVR

pkslk

ROsnsnsn 95

5.1663

)65182(1822)(2

22

Rsn=96kΩΩΩΩ, Csn=2.2nF

Vsn=543-370=173V

The drain voltage is lower than the designed value by 9V due to the

stray resistance (Measured:543V, Designed:552)