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
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
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
'——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
(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.
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.
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-
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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