Application Note
34 of 53
V 1.0
2019-04-01
IM393 Application note
IM393 IPM Technical Description
Bootstrap circuit
Table 16
Electrical characteristics of internal bootstrap parameters
Item
Symbol
Value
Unit
Min.
Typ.
Max.
Bootstrap resistance
R
BS
-
200
-
Ω
6.2
Initial charge of bootstrap capacitor
Adequate on-time duration of the low-side IGBT to fully charge the bootstrap capacitor is required for initial
bootstrap charging. The bootstrap capacitor needs to be pre-charged in order to limit peak current and power
through the resistor. The initial charging time (t
charge
) can be calculated with the following equation:
)
V
V
V
V
V
ln(
δ
1
R
C
t
LS
FD
BS(min)
DD
DD
BS
BS
charge
−
−
−
×
×
×
≥
(12)
•
V
FD
= Forward voltage drop across the bootstrap diode
•
V
BS(min)
= The minimum value of the bootstrap capacitor voltage
•
V
LS
= Voltage drop across the low-side IGBT
•
δ = Duty ratio of PWM
15V line
U
VSS
VDD
LI N
LO
HO
VB
VS
N
U, V, W
P
LIN
COM
VDD
VB
VS
LIN
CIPOS
TM
Tiny
C
BS
(a) Bootstrap circuit (b) Timing chart of initial bootstrap charging
Figure 28
Bootstrap circuit operation and initial charging
6.3
Bootstrap capacitor selection
The bootstrap capacitance can be calculated by:
ΔV
Δt
I
C
leak
BS
×
=
(13)
•
Whereby,
•
Δt = maximum ON pulse width of high-side IGBT
•
ΔV = the allowable discharge voltage of the C
BS
.