Download Ringing Phenomenon during Recovery of Power Diodes

Ringing Phenomenon during Recovery of Power Diodes
Ringing is caused by the interaction
between intrinsic behaviour of the diode chip
and the response characteristics of the
external circuit. Fig. 1 shows an example of
experimental waveforms to highlight the
ringing phenomenon. In Fig. 1, the operation
current and voltage waveforms of the diode
during the recovery period where forward
current flows for Δt = 1 are shown. Fig. 2
shows correlation between the peak value of
reverse voltage, VRM, and the time period of
forward current, Δt. The results show the
shorter the flowing period the larger the
peak value of reverse voltage.
The reason why there is correlation
between VRM and Δt can be explained using
Fig. 3 and Fig. 4. During the period from t2
to t3 in Fig.3, holes are injected from the p
region to n region as shown in Fig. 4(a), and
the diode begins conduct. During the period
from t4 to t5, diode current continues flowing
in the reverse direction until holes stored in
n region become extinct by returning to the
p region or by recombining with electrons.
Further reduction of holes forms a depleted
layer in the p-n junction area as shown in
Fig. 4(b), and reverse voltage appears at the
terminals of diode. During the period from t5
to t6, as holes remaining behind are swept
out, current continues flowing and the
depleted layer expands while reverse
voltage increases. The peak value of the
reverse voltage is determined by the product
3000
150℃
25℃
2500
-40℃
Vdc=2200V
VRM (V)
In this document diode ringing
phenomenon is reviewed. The phenomenon
has a tendency to appear under conditions
where the forward current of the diode is
small and the conduction period of the
current is short. When reverse voltage is
applied to the diode after forward current
flow, reverse current flows for an
instantaneous moment. This reverse current
is called recovery current. Depending on
conditions of use, the recovery current may
demonstrate oscillations. This oscillation
may also be referred to as ringing.
2000
1500
1000
Diode Current [A]
600
1
400
Δt
0
IRM
Forward Current
0
3000
2500
2000
1500
1000
500
0
-500
100
Fig.2 Peak value of reverse voltage
Ringing
200
-200
Diode Voltage [V]
10
Δ t (μ s)
Reverse Recovery Current
1
2
3
4
5
6 Time [μs]
Diode
Current
t1
t2
△t
0
Peak Reverse Voltage during Recovery
t3
Vdc
t4
t6
Time
t5
VCEM
VRM
Diode
Voltage
Voltage Oscillation
0
6 Time [μs]
Time
Fig.1 Experimental waveforms for recovery
Fig.3 Recovery phenomenon of diode
0
1
2
3
4
5
12th Feb. ’15 LD-ES-150002
http://www.hitachi-power-semiconductor-device.co.jp/en/index.html
© Hitachi Power Semiconductor Device Ltd. 2015. All rights reserved.
Injection
p region
n region
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●
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●
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●
●
●
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●
●
●
● ●
●
●
●
●
●
●
●
●
●
●
●
VCE
●
●
●
Inductance L
n+ region
IGBT1
●
●
● ●
● ●
●●
●
●
●
●
●
n region
●
Load
n+ region
Vdc
●
●
VR
IL
IGBT2
Depleted
layer
IR
●
(a)(a)Hole
Holedistribution
distributionunder
underforward
forward bias
p region
D1
IF
●
●
D2
Gate
Driver
(b) (b)
Hole
distribution
Hole
distributionunder
underreverse
reverse bias
bias
Fig.4 Distribution of holes
of di/dt as the reverse current returns to zero
and the inductance (L) of the external circuit.
It is important to keep the inductance (L)
small by laminating electric wiring. If forward
current is small or Δt is short, the holes
stored in the n region will be low. In that
case the reverse current after t4 will
becomes small allowing the depleted layer
to expand rapidly. The subsequent di/dt in
the period from t5 to t6 becomes large and a
sudden voltage peak may occur.
Care should be taken during the evaluation
and design cycles to observe if ringing
occurs at the application level. An example
of a one-phase leg of an inverter employing
IGBTs is shown in Fig. 5. Supposing that
forward-current IF flows through upper-arm
free-wheel diode and the lower-arm IGBT
turns on, the free wheel diode begins
reverse recovery and reverse current IR will
flow. Where the current IF flows for a very
short time, the diode recovery may be
instantaneous. This short time recovery
period may lead to the ringing phenomenon
on the free wheel diode. If the load current is
very small, close to zero Amperes (0A), the
phenomenon has a tendency to occur.
12th Feb. ’15 LD-ES-150002
http://www.hitachi-power-semiconductor-device.co.jp/en/index.html
Fig.5 One phase circuit of inverter
During this ringing, the voltage between the
collector and emitter of upper-side IGBT
may not be stable causing the gate voltage
to vary, potentially leading to gate drive
circuit malfunctions depending on the
severity of conditions. As a pre-caution the
behaviour of the diode should be checked
thoroughly during validation and a suitable
gate resistor should be selected to prevent
ringing. Ringing may also occur in any
snubber diode. If used it is necessary to
carefully choose a snubber diode which
generates less ringing.
© Hitachi Power Semiconductor Device Ltd. 2015. All rights reserved.