Chapter 5 Protection Circuit Design
1. Short Circuit (Overcurrent) Protection
Page 5-1
This chapter describes about the protection circuit design.
1.1 Short circuit withstand capability tsc
In the event of a short circuit, the IGBT's collector current Ic will rise, and if it exceeds a certain level, the C-E voltage VCE will increase sharply. Due to this characteristic, the Ic can be kept at or below a certain level during short circuit. However, the IGBT will still continue to be subjected to a heavy load of high voltage and high current. If this abnormal state continues, the IGBT will be destroyed. The time that the IGBT can withstand a short circuit without destruction is specified as short circuit withstand capability tsc. The gate drive circuit must be designed so that the delay time from short circuit detection until the short circuit current cut off is shorter than tsc.
The concept of short-circuit withstand capability for arm short circuit and output short circuit is explained below.
(1) Arm short circuit
Fig. 5-1 shows an arm short circuit test circuit and waveform example. As for the arm short circuit, the Ic rises sharply at the start of the short circuit and drops slightly after saturation. The short circuit (saturation) current value Isc is determined by VGE, device output characteristics, and Tvj, and is almost independent of VDC, RG, and PW. The short circuit withstand capability is expressed by the energization time PW and is specified after specifying the VGE, Tvj, and VDC conditions. Design the protection circuit so that when a short circuit occurs, it will be cut off within the specified short circuit withstand capability.
Fig. 5-1 (a) Arm short circuit test circuit: A circuit diagram showing a power transistor (IGBT) with a freewheeling diode (FWD) connected in parallel. A resistor RG is in series with the gate, and a pulse generator labeled PW is connected to the gate drive. A DC voltage source VDC is connected to the collector, and the emitter is connected to ground. An inductor L is shown in series with the collector.
Fig. 5-1 (b) Arm short circuit waveform example: A graph showing voltage and current over time. The VGE (Gate-Emitter Voltage) starts high, then drops. The IC (Collector Current) rises sharply to a peak value Isc, then drops slightly. The time duration for this event is labeled tsc.
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(2) Output short circuit
Fig. 5-2 shows the output short circuit test circuit and waveform example. In the output short circuit, the short circuit wire has inductance component, thus the current waveform at the start of the short circuit is different from that in the case of the arm short circuit. In this case, the current rise rate di/dt can be expressed as follows:
di/dt = VDC/L (A/sec)
If the time from the start of the short circuit is given as t (sec), Ic can be expressed as follows:
Ic = di/dt * t (A)
The Ic peak value depends on the inductance and the drive circuit (transient VGE rise). After reaching the peak value and saturating, VCE rises sharply. From here, it becomes the same situation with an arm short circuit.
The short circuit withstand capability in the case of output short circuit is shown in Fig. 5-2(b) as (Pw). During Ic rise, VDC is applied to the inductance L, and the voltage across the IGBT is about VCE (sat), thus the load on the IGBT is extremely low compared to the arm short circuit. Therefore, this period is not included in the short circuit withstand capability.
Fig. 5-2 (a) Output short circuit test circuit: A circuit diagram similar to the arm short circuit, but with the inductor L placed in the main output path before the load (represented by Ro). The circuit includes IGBT, FWD, RG, VDC, and PW.
Fig. 5-2 (b) Output short circuit waveform example: A graph showing voltage and current over time. VCE rises sharply after a period labeled PW. IC rises and then saturates, followed by a sharp rise in VCE. The duration PW is indicated.
Short circuit withstand capability depends on conditions such as VCE, VGE, and Tvj. Generally, the higher the VDC and the higher the Tvj, the shorter the short circuit withstand capability.
Also, please note that VGE may rise during short circuit. Please refer to the application manual or technical document for the short circuit capability of each IGBT series.
Page 5-3
1.2 Short circuit modes and causes
Table 5-1 shows the short circuit modes and causes that occur in inverters.
Table 5-1 Short circuit modes and causes:
- Short circuit mode: Arm short circuit
Diagram: A 3-phase inverter bridge with a direct short between the positive DC bus and the negative DC bus, bypassing the inverter legs.
Cause: IGBT or diode destruction. - Short circuit mode: Series arm short circuit
Diagram: A 3-phase inverter bridge with a short across the IGBT and FWD in one leg of the inverter.
Cause: Control circuit / drive circuit failure or malfunction due to noise. - Short circuit mode: Output short circuit
Diagram: A 3-phase inverter bridge with a short between two of the output phases.
Cause: Miswiring or dielectric breakdown of load. - Short circuit mode: Ground fault
Diagram: A 3-phase inverter bridge with a short from one of the output phases to the ground.
Cause: Miswiring or dielectric breakdown of load.
Page 5-4
1.3 Short circuit (overcurrent) detection method
(1) Detection by overcurrent detector
As mentioned, in the event of a short circuit, the IGBT must be turned off as soon as possible. Therefore, the time from short circuit detection to the completion of turn-off must be as short as possible.
Since the IGBT turns off very fast, if the short circuit is turned off with a normal gate drive signal, a large surge voltage will be generated, and the IGBT may be destroyed by overvoltage (RBSOA destruction). Therefore, it is recommended to turn off the IGBT slowly (soft turn-off).
Fig. 5-3 shows the overcurrent detectors position in an inverter circuit, and Table 5-2 shows the features and the types of short circuit that can be detected by each method. Consider what kind of protection is necessary, and select the most appropriate form of detection.
Fig. 5-3 Overcurrent detector position: A schematic of a 3-phase inverter bridge. Four potential overcurrent detector positions are indicated: (1) in series with the smoothing capacitor, (2) at the inverter input, (3) in series with each IGBT, and (4) at the inverter output.
Table 5-2 Overcurrent detector positions and their features
| Overcurrent detector position | Feature | Types of short circuits that can be detected |
|---|---|---|
| In series with smoothing capacitor Fig. 5-3/(1) |
AC current transducer can be used Low detection precision |
Arm short circuit Series arm short circuit Output short circuit Ground fault |
| At inverter input Fig. 5-3/(2) |
DC current transducer is required Low detection precision |
Arm short circuit Series arm short circuit Output short circuit Ground fault |
| In series with each IGBT Fig. 5-3/(3) |
DC current transducer is required High detection precision |
Arm short circuit Series arm short circuit Output short circuit Ground fault |
| At inverter output Fig. 5-3/(4) |
AC current transducer can be used for equipment with high frequency output High detection precision |
Output short circuit Ground fault |
(2) Detection by VCE(sat)
This method can protect against all types of short circuit shown in Table 5-1. Since the operations from overcurrent detection to protection are done on the drive circuit side, this method offers the fastest protection possible. Fig. 5-4 shows an example of short circuit protection circuit using VCE(sat) detection method.
Fig. 5-4 Short-circuit protection circuit using VCE(sat) detection method: A circuit diagram showing an optocoupler, transistors T1-T5, diodes D1-D4, capacitor C1, resistors R1 and RGE, connected between VCC and VEE. The circuit is designed to monitor VCE and control the gate drive of an IGBT.
This circuit uses diode D₁ to constantly monitor the C-E voltage. When the optocoupler is turned on, transistors T2 and T4 are turned on and a positive gate voltage is applied to the IGBT. Also, the capacitor C₁ is charged through the resistor R₁ and diode D4. The operation changes depending on the voltage of capacitor C₁.
Short circuit protection operation
If a short circuit occurs after the IGBT is turned on, the VCE of the IGBT rises. When VCE becomes higher than the voltage of [C₁ - D₁ (VF - VEE)], diode D₁ is turned off and the voltage of capacitor C₁ rises again. When the voltage of capacitor C₁ becomes higher than [VZ of Zener diode D2 + VBE of transistor T₁], short circuit protection operates.
In the short circuit protection operation, a current flows through Zener diode D₂ to the base of transistor T₁, turning it on. When transistor T₁ is turned on, transistors T₂ and T₄ are turned off, and the applied positive gate voltage is cut off. Since the optocoupler is on, the transistor T₃ is on and transistor T₅ is off. Since the transistors T₄ and T₅ are turned off at the same time, the gate accumulated charge is slowly discharged through the RGE. This effect can suppress the generation of excessive surge voltage when the IGBT turns off. Fig. 5-5 shows an example of the short circuit protection waveform.
Normal operation
After the IGBT is turned on, the IGBT is kept on by keeping the voltage of capacitor C₁ below [ VZ of the Zener diode D₂ + VBE of transistor T₁]. When the optocoupler is turned off, the transistors T₂, T₄ turn off, transistor T₃ turns off, and transistor T₅ turns on, applying a negative gate voltage to the IGBT. The charge on capacitor C₁ is discharged through diode D₃ and transistor T₅ and reset to 0V. As can be seen from the above operation sequence, short circuit protection is monitored on each pulse.
Fig. 5-5 Waveforms during short circuit protection: A graph showing VGE, VCE, and IC over time. The parameters are given as: Ed=600V, VGE=+15V/-5V, RG=3.3Ω, Tvj=125°C. Voltage scales are VCE=200V/div, VGE=10V/div. Current scale is IC=250A/div. Time scale is t=2µs/div.
Page 5-7
2. Overvoltage Protection
2.1 Cause of overvoltage and suppression methods
(1) Cause of overvoltage
Due to the high switching speed of IGBTs, during turn-off or FWD reverse recovery, the current change rate di/dt is very high. Therefore, the circuit wiring inductance around the module Ls can generate a high surge voltage VCEP=LS(di/dt).
Fig. 5-6 shows a chopper circuit for measuring the turn-off surge voltage, and Fig. 5-7 shows the switching waveforms.
Fig. 5-6 Chopper circuit: A circuit diagram showing two IGBTs (IGBT1, IGBT2) and their freewheeling diodes (FWD1, FWD2) connected in a half-bridge configuration. A DC supply voltage Ed is connected. A main circuit wiring inductance Ls is shown in series with IGBT1. A load (Lo, Ro) is connected to the output. VGE1 and VCE1 are gate and collector-emitter voltages for IGBT1, while ID2 and VD2 are current and voltage for FWD2.
Fig. 5-7 Switching waveforms: Graphs showing VGE1, VCE1, IC1, and VD2 over time during IGBT turn-on, IGBT turn-off, and FWD reverse recovery. Key voltage levels VCESP1 and VCESP2 are indicated.
The peak value of turn-off surge voltage VCESP can be calculated as follows:
VCESP = Ed + (-Ls * dIc/dt)
Where dIc/dt is the maximum Ic change rate at turn-off.
If VCESP exceeds the VCES rating, the module will be destroyed.
(2) Overvoltage suppression methods
The following methods are available for suppressing turn-off surge voltage:
- a. Suppress the surge voltage by adding a protection circuit such as a snubber circuit to the IGBT. Use a film capacitor and place it as close as possible to the IGBT in order to suppress high frequency surge voltage.
- b. Adjust the -VGE and RG of the drive circuit in order to reduce the di/dt. (For details, refer to Chapter 7, 'Gate Drive Circuit Design')
- c. Place the DC capacitor as close as possible to the IGBT in order to reduce Ls. Use a low impedance type capacitor.
- d. Reduce the Ls of the main circuit and snubber circuit by using thicker and shorter wires. It is also very effective to use laminated bus bars.
- e. Use an active clamp circuit. The surge voltage is suppressed to approximately equal to the Zener voltage of the Zener diode.
2.2 Types of snubber circuits and their features
Snubber circuits can be classified into two types: individual snubber circuit and lump snubber circuit. Individual snubber circuits are connected to each IGBT, while lump snubber circuits are connected between the DC power supply bus and the ground for centralized protection.
(1) Individual snubber circuits
Examples of typical individual snubber circuits are as follows:
- a. RC snubber circuit
- b. Charge-discharge RCD snubber circuit
- c. Discharge-suppressing RCD snubber circuit
Table 5-3 shows the schematic and features of each type of individual snubber circuit.
(2) Lump snubber circuits
Examples of typical lump snubber circuits are as follows:
- a. C snubber circuit
- b. RCD snubber circuit
Lump snubber circuits are becoming increasingly popular due to circuit simplification.
Table 5-4 shows the schematic and features of each type of lump snubber circuit. Table 5-5 shows the guideline for determining lump C snubber capacitance. Fig. 5-8 shows an example of turn-off waveforms of IGBT with lump C snubber circuit.
Table 5-3 Individual snubber circuits
| Snubber circuit schematic | Features (Notes) |
|---|---|
RC snubber circuit |
|
Charge-discharge RCD snubber circuit |
|
Discharge-suppressing RCD snubber circuit |
|
Page 5-10
Table 5-4 Lump snubber circuits
| Snubber circuit schematic | Features (Notes) |
|---|---|
C snubber circuit |
|
RCD snubber circuit |
|
Table 5-5 Guideline for determining lump C snubber capacitance
| Module rating | Gate drive conditions *1 | Main circuit inductance (µH) | Snubber capacitance Cs (µF) | ||
|---|---|---|---|---|---|
| -VGE (V) | RG (Ω) | ||||
| 50A | ≥43 | - | 0.47 | ||
| 75A | ≥30 | - | 0.47 | ||
| 100A | ≥13 | - | 0.47 | ||
| 600V | 150A | ≤15 | ≥9 | ≤0.2 | 1.5 |
| 200A | ≥6.8 | ≤0.16 | 2.2 | ||
| 300A | ≥4.7 | ≤0.1 | 3.3 | ||
| 400A | ≥6 | ≤0.08 | 4.7 | ||
| 50A | ≥22 | - | 0.47 | ||
| 75A | ≥4.7 | - | 0.47 | ||
| 1200V | 100A | ≤15 | ≥2.8 | ≤0.2 | 1.5 |
| 150A | ≥2.4 | ≤0.16 | 2.2 | ||
| 200A | ≥1.4 | ≤0.1 | 3.3 | ||
| 300A | ≥0.93 | ≤0.1 | 3.3 | ||
*1: Standard gate drive conditions of V series IGBT is shown
Page 5-11
Fig. 5-8 Turn-off waveforms of IGBT with lump C snubber circuit: A graph showing VGE, VCE, and IC over time. Parameters are: 2MBI300VN-120-50, VGE=+15V/-15V, VCC=600V, IC=300A, RG=0.93Ω, LS=80nH. Scales are VGE: 20V/div, VCE: 200V/div, IC: 100A/div, Time: 200nsec/div.
Page 5-12
2.3 Discharge-suppressing RCD snubber circuit design
The discharge-suppressing RCD snubber circuit is considered the most suitable snubber circuit for IGBT. The basic design method of this circuit is as follows.
(1) Study of applicability
Fig. 5-9 shows the turn-off locus of IGBT with discharge-suppressing RCD snubber circuit. Fig. 5-10 shows the IGBT turn-off waveform.
Fig. 5-9 Turn-off locus of IGBT: A graph plotting IC (Collector Current) against VCE (Collector-Emitter Voltage). The graph shows the Safe Operating Area (RBSOA) and the turn-off locus of the IGBT, indicating points VCESP, VCEP, and VCES.
Fig. 5-10 Turn-off waveform with discharge-suppressing RCD snubber circuit: A graph showing IC and VCE over time. The waveform illustrates the ideal trajectory (dotted line) and the actual waveform (solid line) during IGBT turn-off, showing surge voltage (VCESP) and peak voltage (VCEP).
The discharge-suppressing RCD snubber circuit operates after VCE of the IGBT exceeds the DC power supply voltage. The ideal operation trajectory is shown by the dotted line. However, in actual equipment, there is surge voltage at turn-off due to the wiring inductance of the snubber circuit and the transient forward voltage of the snubber diode, thus the actual waveform is as shown by the solid line.
The discharge-suppressing RCD snubber circuits applicability is decided by whether the turn-off locus after applying the snubber circuit is within the RBSOA.
The surge voltage at IGBT turn-off is calculated as follows:
VCESP = Ed + VFM + (-Ls * dIc/dt)
Where:
- Ed: DC power supply voltage
- VFM: Transient forward voltage of snubber diode
- Ls: Snubber circuit wiring inductance
- dIc/dt: Maximum Ic change rate at IGBT turn-off
The reference values for VFM are as follows:
- 600V class: 20 to 30V
- 1200V class: 40 to 60V
(2) Calculating the snubber capacitance (Cs)
The capacitance of the snubber capacitor is calculated as follows:
Cs = (Ls * Io²) / (VCEP - Ed)²
Where:
- Ls: Main circuit wiring inductance
- Io: Collector current at IGBT turn-off
- VCEP: Snubber capacitor peak voltage
- Ed: DC power supply voltage
VCEP must be limited to less than VCES of the IGBT. Use a snubber capacitor with good high-frequency characteristics such as a film capacitor.
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(3) Calculating the snubber resistance (Rs)
The function of the snubber resistor is to discharge the accumulated charge in the snubber capacitor before the next IGBT turn-off. To discharge 90% of the accumulated charge by the next IGBT turn-off, the snubber resistance is calculated as follows:
Rs ≤ 1 / (2.3 * Cs * f)
Where:
- Rs: Snubber resistance
- Cs: Snubber capacitance
- 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 as high as possible within the calculated range.
Irrespective of the resistance value, the power dissipation of the snubber resistor P(Rs) is calculated as follows:
P(Rs) = (Ls * Io² * f) / 2
Where:
- P(Rs): Power dissipation of snubber resistor
- Ls: Main circuit wiring inductance
- Io: Collector current at IGBT turn-off
- f: Switching frequency
(4) Snubber diode selection
The transient forward voltage of the snubber diode is one of the cause of surge voltage at IGBT turn-off. If the reverse recovery time of the snubber diode is too long, the power dissipation loss of the snubber diode will also be much higher during high frequency switching. Also, if the reverse recovery of the snubber diode is too hard, then the IGBT C-E voltage will oscillate greatly.
Therefore, select a snubber diode that has a low transient forward voltage, a short reverse recovery time, and a soft reverse recovery.
(5) Snubber circuit wiring precautions
The snubber circuit wiring inductance is one of the main cause of surge voltage, therefore it is important to reduce the wiring inductance, as well as considering the layout of circuit components.
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2.4 Example of surge voltage characteristics
Surge voltage characteristics depend on the operation, drive conditions, circuit conditions, etc. Generally, surge voltage tends to increase when VCE is higher, the circuit inductance is larger, and Ic is larger.
As an example, the current dependency of surge voltage during IGBT turn-off and FWD reverse recovery is shown in Fig. 5-11. As shown in this figure, the surge voltage at IGBT turn-off becomes higher when current is higher, but the surge voltage during FWD reverse recovery tends to increases at the low current region. Generally, the surge voltage during reverse recovery increases at low current that is about 1~10% of the rated current.
The surge voltage shows various characteristics depending on the operation, drive conditions, circuit conditions, etc. Therefore, it is necessary to confirm that the current and voltage are within the RBSOA described in the specification under all operating conditions of the system.
Fig. 5-11 Current dependency of surge voltage during IGBT turn-off and FWD reverse recovery: A graph plotting Spike voltage (V) on the y-axis against Collector current (A) on the x-axis. The graph shows two curves: one for IGBT turn-off (VAKP) and one for FWD reverse recovery (VCEP). Parameters for the test are listed: 2MB1450VN-120-50 (1200V / 450A), VGE=+15V/-15V, VCC=600V, Ic=vari., Rg=0.52 ohm, Ls=60nH, Tj=125deg.C.
Page 5-15
2.5 Overvoltage suppression circuit -example of clamp circuit configuration-
In general, surge voltage can be suppressed by means of decreasing the stray inductance or installing a snubber circuit. However, it may be difficult to suppress the surge voltage under depending on the operating conditions of the equipment. For such cases, it is effective to use active clamp circuits.
Fig. 5-12 shows an example of active clamp circuit. The circuit configuration adds a Zener diode at C-G of the IGBT, and connect a diode in anti-series with the Zener diode.
When voltage exceeding the Zener voltage of the Zener diode is applied on C-E, the Zener diode breakdown and current flows from collector to the IGBT gate. Positive voltage is added to VGE by this current flowing through RG. When VGE exceeds the gate threshold voltage VGE(th), Ic flows through the IGBT, and VCE is clamped to approximately equal to the Zener voltage of the Zener diode. In this way, surge voltage can be suppressed.
On the other hand, since the active clamp circuit turn on the IGBT, the di/dt at turn-off becomes slower than before the addition of the clamp circuit, resulting in a longer turn-off time (refer to Fig. 5-13). As this will increase the switching loss, make sure to apply the clamp circuit after verifying if this has no problem with the design of the equipment.
Fig. 5-12 Active clamp circuit: A circuit diagram showing an IGBT with a Zener diode (ZD) and a diode (D) connected in series between the collector and gate. A freewheeling diode (FWD) is connected in parallel with the IGBT.
Fig. 5-13 Waveform example when active clamp circuit is applied: A graph showing VGE, IC, and VCE over time. Two sets of waveforms are shown: one for operation 'Without active clamp circuit' and one for 'With active clamp circuit'. The active clamp circuit is shown to affect the turn-off characteristics, making VCE rise slower.
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