Datasheet

2
Bootstrap Power Supply Circuit
Figure 2 shows a bootstrapped
power supply circuit. This circuit
includes a bootstrap circuit for pro-
viding output power to the
HCPL-3020 or HCPL-0302 gate drive
optocouplers, thereby eliminating
the need for isolated power sup-
plies or dc-to-dc converters. It can
be modified to suit other Avago Tech-
nologies’ gate drive optocouplers
and current/voltage sensing isola-
tion amplifiers.
In this basic bootstrap topology,
when the lower IGBT, Q
2
in the
bridge is turned on, the emitter
of Q
1
is pulled to -HV. Similarly,
when the upper IGBT, Q
1
is
turned on, the collector of Q
2
is
pulled to +HV. This charges C
BSH
and C
BSL
up to the supply volt-
age V
CC
minus the voltage drop
across the diodes D
H
and D
L
. The
energy stored in C
BSH
and C
BSL
can then be used to turn on Q
1
and Q
2
. 3 mA low supply current
of HCPL-3020 and HCPL-0302
minimizes the value of C
BSH
and
C
BSL
. The V
CC
voltages are
clamped by Zener Diodes, D
ZH
and D
ZL
.
Influence of Gate Resistor
With external gate resistor con-
nection, designers can control
the IGBT gate signal flow, and
have an option to slow down the
device commutation; therefore
reducing the amount of Electro-
Magnetic Interference (EMI)
compared to intelligent power
module (IPM) solution.
Series gate resistor is typically
used for both turn-on and turn-
off of MOS-gated devices. It is
commonly implemented using
only a resistor. Advance gate
control usually realized using
different resistors for turn-on
and turn-off.
A small gate resistor will help in
avoiding cross conduction, limit-
ing IGBT switching losses and
improve di/dt. On the other hand,
large gate resistor can help to
avoid ringing, limiting the free
wheeling diode losses and reverse
recovery voltage. Designers need
to balance the trade-off in select-
ing an optimize gate resistor.
Calculation selection of gate
resistor, based on limiting the
gate current and power dissipa-
tion can be found in application
section of HCPL-3020 and
HCPL-0302 data sheet.
Speed Improvement Circuit
Designers typically focus more
on turn-off process of MOS-gated
devices. This is because the turn-
on speed is generally satisfactory
to drive the MOS-gated devices.
The circumstance is very much
different during turn-off. Theo-
retically, the turn-off speed of
MOS-gated device depends only
on the gate drive circuit. A high
current turn-off circuit can dis-
charge the input capacitors more
rapidly, providing shorter
switching times and as a result
lower switching losses. Higher
discharge current can be accom-
plished by using a low output
impedance gate driver and/or a
negative turn-off voltage.
Low switching losses can be
achieved by having faster switch-
ing, but it comes with a trade-off
of increase ringing in the wave-
forms due to the higher turn-off
di/dt and dv/dt. This must be
taken into consideration in view
of EMI concern.
A. Anti-Parallel Diode Turn-Off Speed
Improvement Circuit
One of the simplest turn-off
speed improvement techniques
is the anti-parallel diode, as
shown in Figure 3. This simple
circuit allows tuning of the IGBT
turn-on speed by varying R
GATE
.
During turn-off, the anti-parallel
diode, D
OFF
shunts out the resis-
tor, R
GATE
. D
OFF
will be forward
biased and starts to conduct only
when the voltage drops across
the gate resistor, is higher than
forward voltage of D
OFF
:
I
G
x R
GATE
> V
FDOFF
Figure 2. Bootstrap Circuit for Power Control System.
1
2
4
3
5
6
7
8
HV+
1
2
4
3
5
6
7
8
HV-
M
D
H
Q
1
Q
2
C
BSH
C
BSL
D
L
V
CCH
V
CCL
V
EEH
V
EEL
D
ZH
D
ZL
1
2
4
3
5
6
7
8
HV+
1
2
4
3
5
6
7
8
HV-
M
1
2
4
3
5
6
7
8
HV+
1
2
4
3
5
6
7
81
2
4
3
5
6
7
8
HV+
1
2
4
3
5
6
7
8
HV-
1
2
4
3
5
6
7
81
2
4
3
5
6
7
8
HV-
MM
D
H
Q
1
Q
2
C
BSH
C
BSL
D
L
V
CCH
V
CCL
V
EEH
V
EEL
D
ZH
D
ZL