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DIPIPM+ Series Application note
CHAPTER 4 : Bootstrap Circuit Operation
4.1 Bootstrap Circuit Operation
For three phase inverter circuit driving, it requires four isolated control supplies for driving three P-side ICs and
one N-side IC. But using floating control supply with bootstrap circuit can reduce the number of isolated control
supplies from four to one, it requires N-side control supply only.
Bootstrap circuit consists of a bootstrap diode(BSD), a bootstrap capacitor(BSC) and a current limiting resistor.
DIPIPM+ series integrates BSD and limiting resistor, so it can make bootstrap circuit by adding outer BSC only.
The BSC works 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.
The BSC cannot be charged enough
charge leads to too 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 DIPIPM+ series and the circuit current characteristics in switching situation of
P-side IGBT are described as follows.
Current limiting
Bootstrap diode
resistor
(BSD)
HVIC
VFB
V
P1
V
PC
VFS
↑High voltage area
V
=15V
V
D
N1
LVIC
V
NC
Fig.4-1-1 Bootstrap Circuit Diagram
Publication Date: September 2016
its switching condition, BSC capacitance and so on. Deficient
depending on
Bootstrap capacitor
(BSC)
P(Vcc)
P-side
IGBT
+
P-side
FWDi
U,V,W
N-side
IGBT
N-side
FWDi
N(GND)
BSD
15V
V
P1
V
PC
Voltage of V
that is reference voltage of BSC swings
FS
between VCC and GND level. If voltage of BSC is lower than
15V when V
becomes to GND potential, BSC is charged
FS
from 15V N-side control supply.
Fig.4-1-2 Bootstrap Circuit Diagram
48
BSC
VFB
P(Vcc)
P-side
+
IGBT
P-side
FWDi
V
FS
U,V,W
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