Snubber Circuit Design For Igbt

The effect on turn-off surge voltage is moderate. As opposed to the RC snubber circuit, a snubber diode has been added. This allows the snubber's resistance to increase and consequently avoids the IGBT load conditions at turn-on problem.

Since the power dissipation loss of this circuit (primarily caused by the snubber's resistance) is much greater than that of a discharge suppressing snubber circuit, it is not considered suitable for high frequency switching applications.

The power dissipation loss caused by the resistance of this circuit can be calculated as follows:

L: Wiring inductance of main circuit, lo: Collector current at IGBT turn-off, Cs: Capacitance of snubber capacitor, Ed: DC supply voltage, f :Switching frequency

Discharge suppressing RCD snubber circuit

v jflW

The effect on turn-off surge voltage is small Suitable for high-frequency switching Power dissipation loss caused by snubber circuit is small.

The power dissipation loss caused by the resistance of this circuit can be calculated as follows:

Inverter

L: Wiring inductance of main circuit lo: Collector current at IGBT turn-off f :Switching frequency

Table 5-4 Lump snubber circuits

Snubber circuit schematic

Circuit features (comments)

Main application

C snubber circuit

This is the simplest circuit The LC resonance circuit, which consists of a main circuit inductance coil and snubber capacitor, may cause the C-E voltage to oscillate.

Inverter

C snubber circuit

This is the simplest circuit The LC resonance circuit, which consists of a main circuit inductance coil and snubber capacitor, may cause the C-E voltage to oscillate.

Inverter

Snubber For Igbt
Table 5-5 Guidelines for determining lump C snubber circuit capacity

'——Item Module rating ' —■—

Drive conditions*1

Main circuit wiring inductance (^H)

Capacitance of snubber capacitance Cs (^F)

-Vge(V)

Rg(q)

600V

50A

15 max.

68 min.

0.47

75A

47 min.

100A

33 min.

150A

24 min.

0.2 max.

1.5

200A

16 min.

0.16 max.

2.2

300A

9.1 min.

0.1 max.

3.3

400A

6.8 min.

0.08 max.

4.7

1200V

50A

15 max.

22 min.

0.47

75A

9.1 min.

100A

5.6 min.

150A

4.7 min.

0.2 max.

1.5

200A

3.0 min.

0.16 max.

2.2

300A

2.0 min.

0.1 max.

3.3

*1: Typical standard gate resistance of U series IGBT is shown.

*1: Typical standard gate resistance of U series IGBT is shown.

6MBI300U-120

Ed=600V, Vge=+-15V, Ic=300A, RG=2.2Q, Tj=125C, Ls=65nH VCE: 200V/div, IC:100A/div, VGE: 20V/div, t:200ns/div

Fig. 5-7 Current and voltage waveforms of IGBT in lump snubber circuit at turn-off 2.3 Discharge-suppressing RCD snubber circuit design

The discharge suppressing RCD can be considered the most suitable snubber circuit for IGBTs. Basic design methods for this type of circuit are explained in the following.

1) Study of applicability

Figure 5-8 is the turn-off locus waveform of an IGBT in a discharge-suppressing RCD snubber circuit. Fig. 5-9 shows the IGBT current and voltage waveforms at turn-off.

Fig. 5-8 Turn-off locus waveform of IGBT

Igbt Switching Waveforms

(pulse)

rbsoa

The discharge-suppressing RCD snubber circuit is activated when the IGBT C-E voltage starts to exceed the DC supply voltage. The dotted line in diagram Fig. 5-8 shows the ideal operating locus of an IGBT. In an actual application, the wiring inductance of the snubber circuit or a transient forward voltage drop in the snubber diode can cause a spike voltage at IGBT turn-off. This spike voltage causes the sharp-cornered locus indicated by the solid line in Fig. 5-8.

The discharge-suppressing RCD snubber circuits applicability is decided by whether or not the IGBTs operating locus is within the RBSOA at turn-off.

Igbt Testing Waveform
Fig. 5-9 Voltage and current waveforms at turn-off

The spike voltage at IGBT turn-off is calculated as follows:

Vfm:

dIc/dt:

Dc supply voltage

Transient forward voltage drop in snubber diode

The reference values for the transient forward voltage drop in snubber diodes is as follows:

600V class: 20 to 30V

1200V class: 40 to 60V

Snubber circuit wiring inductance

Maximum collector current change rate a IGBT turn-off

2) Calculating the capacitance of the snubber capacitor (Cs)

The necessary capacitance of a snubber capacitor is calculated as follows:

L: Main circuit wiring inductance Io: Collector current at IGBT turn-off VCEP: Snubber capacitor peak voltage Ed: DC supply voltage

VCEP must be limited to less than or equal to the IGBT C-E withstand voltage.

3) Calculating Snubber resistance (Rd)

The function required of snubber resistance is to discharge the electric charge accumulated in the snubber capacitor before the next IGBT turn-off.

To discharge 90% of the accumulated energy by the next IGBT turn-off, the snubber resistance must be as follows:

f: Switching frequency

If the snubber resistance is set too low, the snubber circuit current will oscillate and the peak collector current at the IGBT turn-off will increase. Therefore, set the snubber resistance in a range below the value calculated in the equation.

Irrespective of the resistance, the power dissipation loss P (Rs) is calculated as follows:

4) Snubber diode selection

A transient forward voltage drop in the snubber diode is one factor that can cause a spike voltage at IGBT turn-off.

If the reverse recovery time of the snubber diode is too long, then the power dissipation loss will also be much greater during high frequency switching. If the snubber diode's reverse recovery is too hard, then the IGBT C-E voltage will drastically oscillate.

Select a snubber diode that has a low transient forward voltage, short reverse recovery time and a soft recovery.

5) Snubber circuit wiring precautions

The snubber circuit's wiring inductance is one of the main causes of spike voltage, therefore it is important to design the circuit with the lowest inductance possible.

2.4 Example of characteristic of surge voltage

The characteristic of the surge voltage at the turn-off for the U series IGBT (6MBI450U-120) is shown in Fig. 5-10. The larger the turn-off current the greater the turn-off surge voltage. Fig. 5-11 shows the surge voltage of the FWD for the same U series IGBT in reverse recovery. In general, reverse recovery surge voltage becomes large when collector current is approx. 2 to 20% of maximum Ic rating. Suppress recovery surge within RBSOA.

1300

1200-

1100-

1000

900-

Conditions: Edc=800ViRG=1.1Q VGE=±15ViTi=25^ ls=45nH

Collector current [ A ]

1300

1200-

1100

1000"

Conditions: Edc=800ViR G=3.3Q VGE=±15V ls=45nH

0 100 200 300 400 500

Forward current [ A ]

1300-

1200"

1100

1000-

Igbt With Snubber Diode

Conditions:

RG=1.1Q

Conditions:

RG=1.1Q

600 650 700 750 800

6MBI450U-120 Fig. 5-10 Surge voltage of turn-off

1200

1100

1000

800-

Conditions: IF=20AiEdc=800V VGE=±15ViTj=25C ls=45nH

0 50 100 150

Junction-temperature Tj [ C ]

6MBI450U-120 Fig. 5-11 Surge voltage of reverse recovery

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Responses

  • dorothy
    How to design igbt circuit?
    3 years ago
  • Mike
    How to calculate snubber resistance and inductance of igbt?
    1 month ago

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