REH984e 05
REH984e 05
REH984e 05
CONTENTS Page
5-1
Chapter 5 Protection Circuit Design
5-2
Chapter 5 Protection Circuit Design
5-3
Chapter 5 Protection Circuit Design
Since the IGBT turns off very quickly, if the overcurrent is shut off using an ordinary drive signal, the
collector-emitter voltage will rise due to the inductive kick, and the IGBT may be destroyed by
overvoltage (RBSOA destructions). Therefore, it is recommended that when cutting off the overcurrent
that the IGBT be turned off gently (Soft turn-off).
Figure 5-2 shows the insertion methods for overcurrent detectors, and Table 5-2 lists the features of
the various methods along with their detection possibilities. After determining what kind of protection is
necessary, select the most appropriate form of detection.
5-4
Chapter 5 Protection Circuit Design
D1
VCC
T3 +
T2 RGE
T1 +
D2 VEE
This circuit uses D1 to constantly monitor the collector-emitter voltage, so if during operation the
IGBT’s collector-emitter voltage rises above the limit at D2, then a short-circuit condition will be
detected and T1 will be switched on while T2 and T3 are switched off. At this time, the accumulated
charge at the gate is slowly released through the RGE, so a large voltage spike is prevented when the
IGBT is turned off. Fuji Electric’s gate driver hybrid ICS (model EXB840, 841) have the same kind of
protective circuit built in, thereby simplifying the drive circuit design. For more details, refer to Chapter
7 “Drive Circuit Design”. Fig. 5-4 shows an IGBT waveform during short circuit protection.
5-5
Chapter 5 Protection Circuit Design
2MBI300UD-120
Ed=600V, VGE=+15V, –5V (EXB841), RG=3.3Ω, Tj=125°C
VCE=200V/div, IC=250A, VGE=10V/div, t=2μs/div
Fig. 5-4 Waveforms during short circuit protection
2 Overvoltage protection
2.1 Overvoltage causes and their suppression
1) Overvoltage causes
Due to the high switching speed of IGBTs, at turn-off or during FWD reverse recovery, the current
change rate (di/dt) is very high. Therefore the circuit wiring inductance to the module can cause a high
turn-off surge voltage (V=L(di/dt)).
At an example, using the IGBT’s waveform at turn-off we will introduce the causes and methods of
their suppression, as well as illustrate a concrete example of a circuit (using an IGBT and FWD
together).
To demonstrate the turn-off surge voltage, a simplified chopper circuit is shown in Fig. 5-5, and the
IGBT turn-off voltage and current waveforms are shown in Fig. 5-6.
5-6
Chapter 5 Protection Circuit Design
Ls
IC1
FWD1
IGBT1
VCE1 Load
VGE1
Ed
ID2 L0
(=-IC2)
R0
VD2
(=-VCE2)
IGBT2 FWD2
VGE1
VGE1
0
VCESP1
0
IGBT turn on
VCESP2
If VCESP exceeds the IGBT’s C-E (VCES) rating, then the module will be destroyed.
5-7
Chapter 5 Protection Circuit Design
Table 5-3 shows the schematic of each type of individual snubber circuit, its features, and an outline
of its main uses.
Lump snubber circuits are becoming increasingly popular due to circuit simplification.
Table 5-4 shows the schematic of each type of lump snubber circuit, its features, and an outline of its
main applications. Table 5-5 shows the capacity selection of a C type snubber circuit. Fig. 5-7 shows
the current and voltage turn-off waveforms for an IGBT connected to a lump snubber circuit.
5-8
Chapter 5 Protection Circuit Design
Charge and discharge RCD • The effect on turn-off surge voltage is moderate.
snubber circuit • As opposed to the RC snubber circuit, a snubber diode
P
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 • Io 2 • f C S • Ed 2 • f
P = +
2 2
N
L: Wiring inductance of main circuit,
Io: Collector current at IGBT turn-off,
Cs: Capacitance of snubber capacitor,
Ed: DC supply voltage,
f :Switching frequency
Discharge suppressing RCD • The effect on turn-off surge voltage is small Inverter
snubber circuit • Suitable for high-frequency switching
• Power dissipation loss caused by snubber circuit is
P small.
• The power dissipation loss caused by the resistance of
this circuit can be calculated as follows:
L • Io 2 • f
P =
2
5-9
Chapter 5 Protection Circuit Design
RCD snubber circuit • If the wrong snubber diode is used, a high spike Inverter
voltage will be generated and the output voltage will
P oscillate at the diodes reverse recovery.
5-10
Chapter 5 Protection Circuit Design
2MBI300VN-120-50
VGE=+15V/-15V
Vcc=600V, Ic=300A
Vge =0
Rg=0.93Ω, Ls=80nH
Vge : 20V/div
Vce : 200V/div
Ic : 100A/div
Time : 200nsec/div
Vce,Ic=0
Fig. 5-7 Current and voltage waveforms of IGBT in lump snubber circuit at turn-off
1) Study of applicability IC
VCE
5-11
Chapter 5 Protection Circuit Design
L • Io 2
CS = ·································
(VCEP − Ed )2
VCEP must be limited to less than or equal to the IGBT C-E withstand voltage.
5-12
Chapter 5 Protection Circuit Design
1
RS ≤ ···································
2.3 • C S • f
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:
L • Io 2 • f
P (RS ) = ·······························
2
Select a snubber diode that has a low transient forward voltage, short reverse recovery time and a
soft recovery.
5-13
Chapter 5 Protection Circuit Design
The spike voltage shows various behaviors depending on the operation, drive and circuit conditions.
Generally, the spike voltage becomes higher when the collector voltage is higher, the circuit inductance
is larger, and the collector current is larger. As an example of spike voltage characteristic, the current
dependence of spike voltage at IGBT turn-off and FWD reverse recovery is shown in Figure 5-10.
As this figure shows, the spike voltage at IGBT turn-off becomes higher when the collector current is
higher, but the spike voltage at FWD reverse recovery becomes higher when the current is low.
Generally, the spike voltage during reverse recovery becomes higher when the collector current is in
the low current area that is a fraction of the rated current.
The spike voltage shows various behaviors depending on the operation, drive and circuit conditions.
Therefore, make sure that the current and voltage can be kept within the RBSOA described in the
specification in any expected operating condition of the system.
1600
2MBI450VN-120-50
(1200V / 450A)
1400
VAKP VCEP
1200
Spike voltage (V)
1000
800
Vge=+15V/-15V
Vcc=600V
Ic=vari.
600 Rg=0.52 ohm
Ls=60nH
Tj=125deg.C
400
0 200 400 600 800 1000
Collector current (A)
5-14
Chapter 5 Protection Circuit Design
VGE
IC
VCE
5-15
WARNING
1. This Catalog contains the product specifications, characteristics, data, materials, and structures as of January 2017.
The contents are subject to change without notice for specification changes or other reasons. When using a product listed in this Catalog, be sur to
obtain the latest specifications.
2. All applications described in this Catalog exemplify the use of Fuji's products for your reference only. No right or license, either express or implied,
under any patent, copyright, trade secret or other intellectual property right owned by Fuji Electric Co., Ltd. is (or shall be deemed) granted. Fuji
Electric Co., Ltd. makes no representation or warranty, whether express or implied, relating to the infringement or alleged infringement of other's
intellectual property rights which may arise from the use of the applications described herein.
3. Although Fuji Electric Co., Ltd. is enhancing product quality and reliability, a small percentage of semiconductor products may become faulty. When
using Fuji Electric semiconductor products in your equipment, you are requested to take adequate safety measures to prevent the equipment from
causing a physical injury, fire, or other problem if any of the products become faulty. It is recommended to make your design failsafe, flame retardant,
and free of malfunction.
4. The products introduced in this Catalog are intended for use in the following electronic and electrical equipment which has normal reliability
requirements.
・Computers ・OA equipment ・Communications equipment (terminal devices) ・Measurement equipment
・Machine tools ・Audiovisual equipment ・Electrical home appliances ・Personal equipment ・Industrial robots etc.
5. If you need to use a product in this Catalog for equipment requiring higher reliability than normal, such as for the equipment listed below, it is
imperative to contact Fuji Electric Co., Ltd. to obtain prior approval. When using these products for such equipment, take adequate measures such as
a backup system to prevent the equipment from malfunctioning even if a Fuji's product incorporated in the equipment becomes faulty.
・Transportation equipment (mounted on cars and ships) ・Trunk communications equipment
・Traffic-signal control equipment ・Gas leakage detectors with an auto-shut-off feature
・Emergency equipment for responding to disasters and anti-burglary devices ・Safety devices
・Medical equipment
6. Do not use products in this Catalog for the equipment requiring strict reliability such as the following and equivalents to strategic equipment (without
limitation).
・Space equipment ・Aeronautic equipment ・Nuclear control equipment
・Submarine repeater equipment
8. If you have any question about any portion in this Catalog, ask Fuji Electric Co., Ltd. or its sales agents before using the product.
Neither Fuji Electric Co., Ltd. nor its agents shall be liable for any injury caused by any use of the products not in accordance with instructions set forth
herein.