<Dual-In-Line Package Intelligent Power Module>
1200V Mini DIPIPM with BSD Series APPLICATION NOTE
Publication Date: September 2015
44
CHAPTER 4 Bootstrap Circuit Operation
4.1 Bootstrap Circuit Operation
For three phase inverter circuit driving, normally four isolated control supplies (three for P-side driving and
one for N-side driving) are necessary. But using floating control supply with bootstrap circuit can reduce the
number of isolated control supplies from four to one (N-side control supply).
Bootstrap circuit consists of a bootstrap diode(BSD), a bootstrap capacitor(BSC) and a current limiting
resistor. (Mini DIPIPM with BSD series integrates BSD and limiting resistor and can make bootstrap circuit by
adding outer BSC only.) It uses the BSC as a control supply for driving P-side IGBT. The BSC supplies gate
charge when P-side IGBT turning ON and circuit current of logic circuit on P-side driving IC. (Fig.4-1-2) Since a
capacitor is used as substitute for isolated supply, its supply capability is limited. This floating supply driving
with bootstrap circuit is suitable for small supply current products like DIPIPM.
Charge consumed by driving circuit is re-charged from N-side 15V control supply to BSC via current limiting
resistor and BSD when voltage of output terminal (U, V or W) goes down to GND potential in inverter operation.
But there is the possibility that enough charge doesn't perform due to the conditions such as switching
sequence, capacitance of BSC and so on. Deficient charge leads to low voltage of BSC and might work under
voltage protection (UV). This situation makes the loss of P-side IGBT increase by low gate voltage or stop
switching. So it is necessary to consider and evaluate enough for designing bootstrap circuit. For more detail
information about driving by the bootstrap circuit, refer the DIPIPM application note "
Bootstrap Circuit Design
Manual
"
The BSD characteristics for Mini DIPIPM with BSD series and the circuit current characteristics in switching
situation of P-side IGBT are described as below.
Fig.4-1-1 Bootstrap Circuit Diagram Fig.4-1-2 Bootstrap Circuit Diagram
N
(GND)
P
(Vcc)
Bootstrap diode
(BSD)
U,V,W
V
D
=15V
Bootstrap capacitor
(BSC)
N-side
IGBT
P-side
IGBT
V
FB
V
FS
HVIC
LVIC
↑
High voltage area
V
N1
V
NC
V
P1
P-side
FWDi
N-side
FWDi
Current limiting
resistor
Level
Shif
t
V
PC
+
BSD
P
(Vcc)
U,V,W
P-side
IGBT
V
FB
V
FS
Voltage of V
FS
that is reference voltage of BSC swings between
VCC and GND level. If voltage of BSC is lower than 15V when
V
FS
becomes to GND potential, BSC is charged from 15V N-side
control supply.
Gate D
rive
Logic & UV
protec
tio
n
Level Shif
t
15V
Low volt
age are
a
V
P1
P-side
FWDi
BSC
V
PC
+
