Application Note
SiC MOSFET

Snubber circuit design methods
SiC MOSFET is getting more popular in applications where fast and efficient switching is required, such as power supply applications. On theother hand, the fast switching capability causes high dv/dt and di/dt, which couple with stray inductance of package and surrounding circuit,resulting in large surge voltage and/or current between drain and source terminals of the MOSFET. The surge voltage and current have to becontrolled to not exceed the maximum rated voltage/current of the device. This application note illustrates a way to design snubber circuit, which isone of the methods to suppress surges voltages and currents.

Surge voltage occurring in Drain-Source
When a MOSFET turns on, current stores energy in the stray

When LS turns off, IMAIN flows through the loop form by LMAIN ,inductance of the wire on the PCB layout. The stored energy

CDCLINK and parasitic capacitance of HS and LS, as shown by

resonates with the parasitic capacitance of the MOSFET, anddotted line. Where CDCLINK is bulk capacitor placed in parallelthat produces surge current. Figure 1 illustrates the path ofwith input HVdc-PGND. During the turn off of LS, surge voltage

the ‘ringing current’ in a half bridge topology, which has highoccurs in drain-source of LS by resonant phenomenon betweenside switch (HS) and low side switch (LS). When LS turns on,

LMAIN and parasitic capacitance of the MOSFET COSS

current IMAIN flows from VSW through the stray inductance LMAIN.

（CDS+CDG）. The maximum voltage VDS_SURGE is as shown in(1). Where VHVDC is the applied voltage on HVdc terminal andROFF is resistance when the MOSFET turns off. (*1).
−1𝑉 ∗𝑒 −(𝑎/𝜔)[tan (𝑎/𝜔)+𝛷]＋𝑉HVDC1+(𝑎⁄𝜔 )2HVdc

𝑉DS_SURGE = 𝐴(1)

where:HS(High Side)

𝑉A = √𝑉HVDC ２ + (𝑎/𝜔)2 ∗ (2 ∗ 𝑅OFF ∗ 𝐼MAIN ｰ𝑉HVDC )Φ=𝑡𝑎𝑛−1

CDCLINK

Vsw

IMAIN

𝑎=LS(Low Side)

11√𝐿MAIN∗𝐶OSS√𝐿MAIN/𝐶OSS 2∗ √1 − (2𝑅OFF)

Figure 2 shows surge waveforms when ROHM’s SiC MOSFETPGND(SCT2080KE) turns off with 800V applied on HVdc. According

LMAIN

to the waveform, VDS_SURGE reaches 961V and ringing

PGND

frequency is about 33MHz, which brings LMAIN of 110nH.

Figure 1 Current path when turn-off surge occurs© 2020 ROHM Co., Ltd.

𝑉HVDC(𝑎/𝜔)∗(2∗𝑅OFF ∗𝐼MAIN −𝑉HVDC )2∗𝑅OFF∗𝐶OSS𝜔𝑆𝑈𝑅𝐺𝐸 =IMAIN

2

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Snubber circuit design methods

It would be the best if stray inductance is minimized as much as
VDS=800V, RG_EXT=3.3ohm

60

possible. However it is not always realistic because it might

961V

make the heat dissipation condition worse. Instead, placing the50IDsnubber capacitor as close as possible to the MOSFETVDS

400

20200

100033MHz

400.65400.75Time [us]400.85connection inductance and clamps surge voltage while theMOSFET turns off.

1200

SCT2080KE

VDS=800V, RG_EXT=3.3ohm

without C Snubber

1000-10400.95

ID901V

601uF C Snubber

961V

50VDS

800Figure 2. Turn-off waveform with surge

40600

30[A]

-200
400.55

capacitor also absorbs the energy stored in the minimized[A]

600minimize the stray inductance of the circuit. The snubber

ID30[V]

40VDS80040020ID

1000

[V]

SCT2080KE

VDS

1200

Application Note200

100044.6MHz

Next step, a snubber capacitor CSNB is introduced, as shownin Figure 3. This capacitor makes LMAIN neglectable. The
waveform of the surge voltage when LS turns off is shown inFigure 4.
-200
400.55

HVdc

400.65400.75Time [us]400.85-10400.95Figure 4. Reducing turn-off surge by C snubber

CSNB

CDCLINK

Variety and selection of snubber
There are two methods of snubber circuits: passive snubber,LSNB

which consists of passive components such as resistor, inductor,
capacitor and diodes; and active snubber, which utilize
semiconductor switch(*1 ） . In this application note, passivePGND

snubber is chosen, due to its simplicity and cost effectiveness.
Figure 5 shows different snubber examples: (a) C snubber,

Figure 3 C Snubber

where the capacitor CSNB is connected in parallel

MOSFET bridge; (b) RC snubber where the resistor RSNB andSurge voltage is reduced by more than 50V (reaching 901V)

capacitor CSNB are connected in parallel to each MOSFET; (c)and ringing frequency increases to 44.6MHz. It is because CSNB

Discharge RCD snubber, where a diode is added to RC

is placed close to the switches and as a result, the straysnubber; and (d) non-discharge RCD snubber, where theinductances (LSNB) involved in the switching path is reduced. Indischarging path is changed from the discharge RCD snubber

this case, LSNB is about 71nH according to the equation (1).
© 2020 ROHM Co., Ltd.

to thepresented in (c).

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Snubber circuit design methods

Application Note

In principle, the snubber has to be placed as close as possible

parallel.

to the MOSFET in order to maximize its effectiveness.

(d) Non-discharge RC snubber: RSNB dissipates only the energy
absorbed by CSNB produced during the overvoltage, it means(a) C snubber: it has fewer components but has relatively longer

the snubber doesn’t discharge all energy stored in CSNB at every

wires. It is more suitable for 2in1 module rather than circuit withswitching transient. Thus the energy consumption at RSNB does

discrete components.

not much increase at high switching frequencies. Therefore a(b) RC snubber: it can be placed close to the MOSFET,large CSNB can be implemented, which realizes a highly

however the energy stored in CSNB has to be dissipated by RSNB

effective snubber circuit. But it is also to be noted that thisduring every switching transient

method requires very complicated wire layout which can beof the MOSFET. If theswitching frequency is high enough, RSNB would dissipate large

realized by more than 4 layers PCB.

amount of energy (several watts), which limits the size of CSNB.

Everysnubbercircuit

has

bothadvantages

andAnd, as results, the suppressing surge capability of the snubber

disadvantages, and should be chosen according to circuit

is reduced.

topology and power.
HVdc

HVdc

Designing C snubber

CSNB
RSNB

C snubber circuit (Figure 6) absorbs energy stored at LMAIN.

CDCLINK

CDCLINK
CSNB

CSNB

The stray inductance of the snubber path LSNB has to be less

RSNB

than LMAIN. Larger CSNB makes snubber more effective because(a)energy stored at CSNB is not discharged. Series inductance ofPGND

PGND

the capacitor (ESL), which adds on LSNB, has to be minded(b)because ESL normally increases with the capacitor size.
Capacitor should be selected based on electrostatic capacity
HVdc

HVdc

calculated using eq. (2). VDC_SURGE is defined as the maximum

CSNB

CSNB
RSNB

surge of HVdc. With an assumption that all energy stored at

RSNB

CDCLINK

LMAIN is transferred to CSNB.

CDCLINK

RSNB

RSNB

CSNB

𝐶SNB >CSNB

PGND

𝐿MAIN ∗𝐼MAIN 2(2)𝑉DS_SURGE 2 −𝑉HVDC 2

PGND

HVdc(c)(d)Figure 5 Snubber circuits

CSNB(a) C snubber, (b) RC snubber,(c) Discharge RCD snubber, (d) Non-discharge RCD snubber

CDCLINKVsw

IMAIN(c) Discharge RCD snubber: RSNB dissipates energy as
much as in (b) during turn ON, but CSNB surge absorption

LSNB

capability is more effective than (b) because surge current flows
through the diode. The recovery characteristic of the diode must
be considered, high di/dt in the snubber circuit can be occurred

IMAIN

during the switching transient. Therefore, stray inductancesPGND

should be minimized as much as possible to limit over voltages.

Figure 6 C snubber

It is also the same effect if RSNB is connected with CSNB in© 2020 ROHM Co., Ltd.

PGND

LMAIN

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Snubber circuit design methods

Application Note

Designing RC snubbervoltage.

Figure 7 shows the current loops when RC snubber works. CSNB𝜔SNB =is determined by equation (2) and RSNB is obtained from1
𝑅𝑆𝑁𝐵 ∗𝐶𝑆𝑁𝐵≪ ω(5)

SURGE

equation (3).

Designing discharge RCD snubber
𝑅SNB <−1Design procedure of discharge RCD snubber is basically the(3)𝑓𝑆𝑊 ∗𝐶𝑆𝑁𝐵 ∗ln[(𝑉𝐷𝑆_𝑆𝑈𝑅𝐺𝐸 −𝑉𝑆𝑁𝐵 )⁄𝑉𝐷𝑆_𝑆𝑈𝑅𝐺𝐸]

same as RC snubber. Besides resonant frequency is not needto be minded because surge is absorbed by Diode. For diode, afast recovery diode type is suitable.

HVdc

LSNB

Designing non-discharge RCD snubber

CSNB
RSNB

Non-discharge RCD snubber only consumes energy from thesurge voltage, as a result the power dissipation of RSNB is

CDCLINKVsw

reduced and the selection options for RSNB is wider. Thus, CSNBIMAINcan be increased, and therefore, the clamping effectiveness.
CSNB and RSNB are determined by equation (2) and (3)RSNB

respectively. Power consumption of RSNB is determined byCSNB

following equation (6) which does not have the second term ofLSNB

equation (4) including CSNB and fsw. This leads efficient
clamping capability and makes higher frequency of fsw possible.

IMAIN
PGND

LMAIN

PGND

𝐿

𝑃SNB = 𝑇𝑅𝐴𝐶𝐸

×𝐼𝑀𝐴𝐼𝑁 2 ×𝑓𝑠𝑤2(6)Figure 7 RC snubber
HVdc

Where:
fSW: Switching frequency, and

VSNB: Discharge voltage of snubber (0.9xLSNB
CSNB

VDS_SURGE）
RSNB

After determining the value of RSNB, the resistor size has to be
selected based on the power dissipation calculated by equation

Vsw

(4).

RSNB

𝐿

𝑃SNB = 𝑇𝑅𝐴𝐶𝐸

×𝐼𝑀𝐴𝐼𝑁 2 ×𝑓𝑠𝑤
2𝐶+ 𝑆𝑁𝐵

2×𝑉𝐻𝑉𝐷𝐶 ×𝑓𝑠𝑤
2

CDCLINK

(4)
CSNB
LSNB

This equation says that, the higher the fSW or VHVDC the higher
the power dissipation of RSNB must be. In case of PSNB is too

PGND

LMAIN

high for the resistor, CSNB needs to be decreased.
PGND

In addition, resonant frequency ωSNB of RSNB and CSNB has tobe lower enough than resonant frequency of surge ωSURGE, asFigure 8. Discharging route of non-discharge RCD snubberpresented by (5). Thus the RC snubber can absorbs surge© 2020 ROHM Co., Ltd.

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Snubber circuit design methods

Application NoteFigure 8 shows discharging route right after snubber action ofThe peak voltage using snubber reaches 1069V which is 12%

non-discharge RCD type. The discharging current flows in thelower when no snubber is used. Without snubber the peakupper arm to PGND and in lower arm to HVdc through RSNB,

voltage reaches 1210V.

which is not highly influenced by the wire inductance. On theFigure 10 shows the influence of the snubber on the efficiency

other hand, wire inductance LSNB has to be as small as possibleusing a buck converter with the condition of input voltage 400V,

to avoid over voltages due to the high di/dt.

output voltage 200V, RG_EXT 6.8Ω and frequency 100 kHz.

Figure 9 shows turn-off surge comparison both VDS and ID.

Load power was varied from 1kW to 4.8kW. Though snubber(a) is a test circuit and (b) is the VDS and ID during the turn-off

made slightly worse efficiency by 0.4% below 4kW range, ittransient of the MOSFE.T. The experimental results have been

improved the efficiency by 0.15% above 4kW range. This isobtained using ROHM’s evaluation board (part number

because power loss due to surge voltage increases as load

P02SCT3040KR-EVK-001) with SiC MOSFET (SCT3080KR) topower increases. With the snubber, the surge voltage isverify effectiveness of the non-discharge RCD snubber. The

suppressed and therefore the switching loss is reduced.

test have been performed with a RG_EXT =3.3Ω, E =800V anddrain current ID =ca.70A.

98.5
98.0

Efficiency [%](HS)

RG_EXTHighSideLE

(LS)

VG

RG_EXT

Buck Efficiency
Input:400V, Output:200V
L=250uH, Rg=6.8ohm, fsw=100kHz

Low
Side

SCT3080KR

with Snubber
without Snubber

1000

4000

97.5
97.0

96.5
96.0
95.50LMAIN(a) Circuit of double pulse switching test1200

Package influence on surge

80

80060[V]

ID

600

40VDS

1000

VDS_With Snubber
ID_Without Snubber
ID_With Snubber

1069V

5000

Figure 10 Efficiency comparison of buck converter

VDS=800V, RG_EXT=3.3ohm
100
VDS_Without Snubber

1210V

2000 3000
Pout [W]400

20Package influences on turn-off surge voltage. Figure 11
shows two packages for SiC MOSFET in ROHM. (a) is200[A]

conventional TO-247N(3L), and (b) is TO-247-4L having driverID

source pin which is being popular in the market as today.

0VDS
0-20-200
26.55

26.65

26.75
Time [us]26.85-40
26.95(b) Waveform of VDS, ID during turn off

(a)

(b)

Figure 9 turn-off surge comparison（with/without snubber）
Figure 11 Package example for SiC MOSFET(a) TO-247N, (b) TO-247-4L© 2020 ROHM Co., Ltd.

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Snubber circuit design methods

Application Note

4L type changes route of driving circuit and realizes faster

References：switching than 3L type. Due to the fast switching speed, 4L type

*1 「The Fundamentals of Switching-Mode Converters」

tends to have larger surge voltage during turn-on and turn-off

P95~P107

compared with conventional 3L. For more details please refer toK. Harada, T. Ninomiya, W. Gu

the application notes “Improvement of switching loss by driver

CORONA PUBLISHING CO., LTD., Feb. 1992
*2 「Gate-Source Voltage Surge Suppression Methods」

source terminal”.
Figure 12 shows wave form comparison of the surge voltage

Application Note（No. 62AN010ERev.002）

between 3L type and 4L type during the turn-off transient. The

ROHM Co., Ltd. April 2020

test conditions are: VDS＝800V, RG_EXT ＝3.3Ω, ID＝65A. The
result shows that the 4L type has a higher surge voltage(1210V) than the 3L type (957V).

VDS=800V, RG_EXT=3.3ohm
100
VDS_4LVDS_3L
ID_4L
ID_3L

957V

1000
80060[V]80600

40VDSID

400

20200[A]

1210V

ID

12000VDS0-200
26.55-20-4026.65

26.75
Time [us]26.85

Figure 12 Turn-off surge comparison without snubber（TO-247N VS. TO-247-4L）
The VDS ringing could cause unexpected surge voltage on thegate-source voltage VGS and exceeds max limitation of VGS.
Because the ringing passes not only through CDS, but alsothrough CDG and CGS, as is shown in Figure 7 and Figure 8.
ROHM offers countermeasures against the surge in anotherapplication note(*2) but if the measure is insufficient, snubber
circuit should be effective solution.
As described above, gate signal of MOSFETs in bridge
topology may influence to each other and may cause
unexpected surge voltage. PCB layout should be taken inaccount to prevent the surge.

© 2020 ROHM Co., Ltd.

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NoticeNotes
1) The information contained herein is subject to change without notice.
2) Before you use our Products, please contact our sales representative and verify the latest specifications :3) Although ROHM is continuously working to improve product reliability and quality, semiconductors can break down and malfunction due to various factors.
Therefore, in order to prevent personal injury or fire arising from failure, please take safetymeasures such as complying with the derating characteristics, implementing redundant andfire prevention designs, and utilizing backups and fail-safe procedures. ROHM shall have noresponsibility for any damages arising out of the use of our Poducts beyond the rating specified by
ROHM.
4) Examples of application circuits, circuit constants and any other information contained herein areprovided only to illustrate the standard usage and operations of the Products. The peripheral
conditions must be taken into account when designing circuits for mass production.
5) The technical information specified herein is intended only to show the typical functions of andexamples of application circuits for the Products. ROHM does not grant you, explicitly or implicitly,
any license to use or exercise intellectual property or other rights held by ROHM or any otherparties. ROHM shall have no responsibility whatsoever for any dispute arising out of the use ofsuch technical information.
6) The Products specified in this document are not designed to be radiation tolerant.
7) For use of our Products in applications requiring a high degree of reliability (as exemplifiedbelow), please contact and consult with a ROHM representative : transportation equipment (i.e.
cars, ships, trains), primary communication equipment, traffic lights, fire/crime prevention, safety
equipment, medical systems, servers, solar cells, and power transmission systems.
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9) ROHM shall have no responsibility for any damages or injury arising from non-compliance withthe recommended usage conditions and specifications contained herein.
10) ROHM has used reasonable care to ensurH the accuracy of the information contained in thisdocument. However, ROHM does not warrants that such information is error-free, and ROHM
shall have no responsibility for any damages arising from any inaccuracy or misprint of suchinformation.
11) Please use the Products in accordance with any applicable environmental laws and regulations,such as the RoHS Directive. For more details, including RoHS compatibility, please contact aROHM sales office. ROHM shall have no responsibility for any damages or losses resulting
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