Fuji Electric REH984f IGBT Module Installation Guide

Fuji Electric REH984f IGBT Module

Fuji IGBT Module Application Manual

March 2023 REH984f

Product Information

This manual contains the product specifications, characteristics, data, materials, and structures as of March 2023. The contents are subject to change without notice for specification changes or other reasons.

The products described in this specification are not designed nor made for being applied to the equipment or systems used under life-threatening situations. When you consider applying the product of this specification to a particular use, such as vehicle-mounted units, shipboard equipment, aerospace equipment, medical devices, atomic control systems, and submarine relaying equipment or systems, Fuji Electric is not responsible for the applicability.

Product Usage Instructions

Chapter 5: Protection Circuit Design

1. Short Circuit (Overcurrent) Protection

This section explains the concept of short-circuit withstand capability for arm short circuits and output short circuits.

Application Manual

Cautions

This manual contains the product specifications, characteristics, data, materials, and structures as of March 2023 The contents are subject to change without notice for specification changes or other reasons When using a product listed in this manual, be sure to obtain the latest specifications
Fuji Electric Co Ltd is constantly making every endeavor to improve product quality and reliability However on rare occasions, semiconductor products may fail or malfunction To prevent accidents causing injury or death damage to property like by fire, and other social damage resulting from a failure or malfunction of the Fuji Electric Co Ltd semiconductor products, take measures to ensure safety such as redundant design fire spread prevention design, and malfunction prevention design
The contents described in this specification never ensure the industrial property and other rights, nor license the enforcement rights
The products described in this specification are not designed nor made for being applied to the equipment or systems used under life-threatening situations When you consider applying the product of this specification to a particular use, such as vehicle-mounted units, shipboard equipment, aerospace equipment, medical devices, atomic control systems and submarine relaying equipment or systems, Fuji Electric is not responsible for the applicability
The data and other information contained in this specification are guaranteed for the product but do not guarantee the characteristics and quality of the equipment applying this product Using this product, please evaluate it in the application in which it will be used, and then judge its applicability at user’s own risk Fuji Electric is not responsible for the applicability

Chapter 5 Protection Circuit Design

1. Short Circuit (Overcurrent) Protection 5-2
2. Overvoltage Protection 5-8

This chapter describes about the protection circuit design.

Short Circuit (Overcurrent) Protection

Short circuit withstands capability etc 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 a 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 circuits and output short circuits is explained below.

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.

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 an 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.

• ??Τ?? = ???Τ? ?Τ???

If the time from the start of the short circuit is given as t (sec), IC can be expressed as follows.

• ??= ??Τ?? ∙ ? ?

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 an 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.

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.

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 (overcurrent) detection method
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 circuits that can be detected by each method. Consider what kind of protection is necessary, and select the most appropriate form of detection.

(1), (2), (3)

| ・Arm short circuit

・Series arm short circuit

・Output short circuit

・Ground fault

(4)

| ・Output short circuit

・Ground fault

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

Detection by VCE(sat)
This method can protect against all types of short circuits 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 the VCE(sat)detection method.

Fig. 5-4 Short-circuit protection circuit using VCE(sat) detection method

This circuit uses diode D 1 to constantly monitor the C E voltage When the optocoupler is turned on, transistors T 2 and T 4 are turned on and a positive gate voltage is applied to the IGBT Also, the capacitor C 1 is charged through the resistor R 1 and diode D 4 The operation changes depending on the voltage of capacitor C 1

【 Short circuit protection operation 】
If a short circuit occurs after the IGBT is turned on, the V CE of the IGBT rises When V CE becomes higher than the voltage of C 1 D 1 V F V EE diode D 1 is turned off and the voltage of capacitor C 1 rises again When the voltage of capacitor C 1 becomes higher than V Z of Zener diode D 2 V BE of transistor T 1 short circuit protection operates In the short circuit protection operation, a current flows through Zener diode D 2 to the base of transistor T 1 turning it on When transistor T 1 is turned on, transistors T 2 and T 4 are turned off, and the applied positive gate voltage is cut off Since the optocoupler is on, the transistor T 3 is on and transistor T 5 is off Since the transistors T 4 and T 5 are turned off at the same time, the gate  accumulated charge is slowly discharged through the R GE This effect can suppress the generation ofexcessive 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 C 1 below V Z of the Zener diode D 2 V BE of transistor T 1 When the optocoupler is turned off, the transistors T 2 T 4 turn off, transistor T 3 turns off and transistor T 5 turns on, applying a negative gate voltage to the IGBT The charge on capacitor C 1 is discharged through diode D 3 and transistor T 5 and reset to 0 V As can be seen from the above operation sequence, short circuit protection is monitored on each pulse

Overvoltage Protection

Cause of overvoltage and suppression methods
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.

The peak value of turn-off surge voltage V CESP can be calculated as follows

?????=??+(−?S∙?????)d I C d t Maximum I C change rate at turn off

If V CESP exceeds the V CES rating, the module will be destroyed

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 V GE and R G of the drive circuit in order to reduce the d i d t ..(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 L S Use a low impedance type capacitor
• d. R educe the L S 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

Types of snubber circuits and their features
Snubber circuits can be classified into two types individual snubber circuits and lump snubber circuits 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

Individual snubber circuits
Examples of typical individual snubber circuits are as follows

• RC snubber circuit
• Charge discharge RCD snubber circuit
• Discharge-suppressing RCD snubber circuit

Table 5 3 shows the schematic and features of each type of individual snubber circuit

Lump snubber circuits
Examples of typical lump snubber circuits are as follows

• C snubber circuit
• 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 guidelines 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.

• When applied to large-capacity IGBTs, the snubber resistance must be low. As a result, the current at turn-on increases and increases the IGBT load.

Charge-discharge RCD snubber circuit

| • Unlike the RC snubber circuit, a snubber diode is added. Thus, snubber resistance can be increased, and decrease the IGBT load at turn-on.

• The power dissipation loss by the snubber resistance of this circuit can be calculated as follows.

_? ? ∙ ?2 ∙ ?      ? ?_ ∙ ?2 ∙ ?

? =          ? +            ?       

2                     2

L S: Wiring inductance of main circuit I O: Collector current at IGBT turn-off _C S: _Capacitance of snubber capacitor E d: DC power supply voltage

f : Switching frequency

Discharge-suppressing RCD snubber circuit

| • Power dissipation loss of the snubber circuit is small.

• The power dissipation loss by the snubber resistance of this circuit can be calculated as follows.

_? ?_ ∙ ?2 ∙ ?

? =          ? __

2

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

Table 5-4 Lump snubber circuits

C snubber circuit

| • This is the simplest snubber circuit.

• The LC resonance circuit, which consists of the main circuit inductance and snubber capacitor, may cause the C-E voltage to oscillate.

RCD snubber circuit

| • If the snubber diode is selected incorrectly, a high surge voltage will be generated or the voltage may oscillate during reverse recovery of the snubber diode.

Table 5-5 Guideline for determining lump C snubber capacitance

Item| Gate drive conditions *1|

Main circuit inductance (μH)

|

Snubber capacitance C S (μF)

---|---|---|---
Module rating| – V GE (V)| R G (Ω)

600V

| 50A|

≦15

| ≧43|

–

|

0.47

75A| ≧30
100A| ≧13
150A| ≧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

1200V

| 50A|

≦15

| ≧22|

–

|

0.47

75A| ≧4.7
100A| ≧2.8
150A| ≧2.4| ≦0.2| 1.5
200A| ≧1.4| ≦0.16| 2.2
300A| ≧0.93| ≦0.1| 3.3

• 1: Standard gate drive conditions of V series IGBT is shown

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
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 the waveform

In the discharge-suppressing RCD snubber circuit operates after the 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 circuit 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.

VCEP must be limited to less than the VCES of the IGBT. Use a snubber capacitor with good high-frequency characteristics such as a film capacitor.

Calculating the snubber resistance R S
The function of the snubber resistor is to discharge the accumulated charge in the snubber capacitor before the next IGBT turn To discharge 90 of the accumulated charge by the next IGBT turn-off, the snubber resistance is calculated as follows

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 R S is calculated as follows

Snubber diode selection
The transient forward voltage of the snubber diode is one of the causes 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
Snubber circuit wiring precautions
The snubber circuit wiring inductance is one of the main causes of surge voltage, therefore it is important to reduce the wiring inductance, as well as consider the layout of circuit components

Example of surge voltage characteristics
Surge voltage characteristics depend on the operation, drive conditions, circuit conditions, etc Generally, surge voltage tends to increase when V CE is higher, the circuit inductance is larger, and I C
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 the 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 is described in the specification under all operating conditions of the system

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 an active clamp circuit. The circuit configuration adds a Zener diode at C-G of the IGBT and connects 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 the 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 thi s 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.

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