Fuji Electric Co., Ltd.
3-9
MT6M12343 Rev.1.0
Dec.-2016
Chapter 3
Detail of Signal Input/Output Terminals
3. Function of Internal BSDs (bootstrap Diodes)
There are several ways in which the VB(*)
*1
floating supply can be generated. Bootstrap method is
described here. The boot strap method is a simple and cheap solution. However, the duty cycle and
on-time are limited by the requirement to refresh the charge in the bootstrap capacitor. As show in Fig.
3-6, Fig. 3-8 and Fig. 3-11, the boot strap circuit consists of bootstrap diode and resistor which are
integrated in the IPM and an external capacitor.
1.Charging and Discharging of Bootstrap Capacitor During Inverter Operation
When lower side IGBT is ON state, the charging voltage
on the bootstrap capacitance
V
C(t1)
is calculated by the
following equations.
V
C(t1)
=
V
CC
–
V
F
–
V
CE(sat)
–
I
b
·
R
…… Transient state
V
C(t1)
V
CC
…… Steady state
V
F
: Forward voltage of Boost strap diode (D)
V
CE(sat)
: Saturation voltage of lower side IGBT
R
: Bootstrap resistance for inrush current limitation (R)
I
b
: Charge current of bootstrap
When lower side IGBT is turned off, then the motor current
flows through the free-wheel path of the upper side FWD.
Once the electric potential of Vs rises near to that of P
terminal, the charging of C is stopped, and the voltage of C
gradually declines due to a current consumed by the drive
circuit.
Gate signal of
Upper side IGBT
Gate signal of
Lower side IGBT
ON
ON
V
cc
Vs
Voltage level of
bootstrap capacitor
Spontaneous discharge of
C
V
b(t1)
Declining due to current consumed
by drive circuit of the upper side IGBT
Fig.3-6 Circuit diagram of charging operation
Fig.3-7 Timing chart of Charging operation
Vcc
<HVIC>
GND
Vs
OUT
VB
IGBT(H)
IGBT(L)
Vcc
C
D
R
OFF
ON
<BSD>
COM
N(U,V,W)
U,V,W
P
VB(U,V,W)
V
CCH
*1 VB(*) : VB(U)-U,VB(V)-V,VB(W)-W
a) Charging operation timing chart of bootstrap capacitor (C)
<Sequence (Fig.3-7) : lower side IGBT is turned on in Fig.3-6>
