Application Note
35 of 53
V 1.0
2019-04-01
IM393 Application note
IM393 IPM Technical Description
Bootstrap circuit
•
I
leak
= maximum discharge current of the C
BS
mainly via the following mechanisms:
−
Gate charge for turning on the high-side IGBT
−
Quiescent current to the high-side circuit in the IC
−
Level-shift charge required by level shifters in the IC
−
Leakage current in the bootstrap diode
−
C
BS
capacitor leakage current (ignored for non-electrolytic capacitors)
−
Bootstrap diode reverse-recovery charge
In practice, a leakage current of 1mA is recommended as a calculation basis for IM393-XX. By taking into
consideration dispersion and reliability, the capacitance is generally selected to be 2 to 3 times higher than the
calculated one. The C
BS
is only charged when the high-side IGBT is off, and the VS voltage is pulled down to
ground. Therefore, the on-time of the low-side IGBT must be sufficient to ensure that the charge drawn from
the C
BS
capacitor can be fully replenished. Hence, there is inherently a minimum on-time of the low-side IGBT
(or off-time of the high-side IGBT).
The bootstrap capacitor should always be placed as close to the IM393-XX pins as possible. At least one low ESR
capacitor should be used to provide good local de-coupling. For example, a separate ceramic capacitor close to
IM393-XX is essential, if an electrolytic capacitor is used for the bootstrap capacitor. If the bootstrap capacitor is
either a ceramic or tantalum type, it should be adequate for local decoupling.
6.4
Charging and discharging of the bootstrap capacitor during PWM
inverter operation
The bootstrap capacitor C
BS
charges through the bootstrap MOSFET from the V
DD
supply when the high-side
IGBT is off, and the V
S
voltage is pulled down to ground. It discharges when the high-side IGBT or diode are on.
Example: Selection of the initial charging time
An example of the calculation of the minimum value of the initial charging time
Conditions:
•
C
BS
= 4.7 µF, R
BS
= 200
Ω
, Duty Ratio (δ)= 0.5, V
DD
= 15 V,
•
V
BS (min)
= 12.5 V, V
LS
= 0.1 V
•
In order to ensure safety, it is recommended that the charging time be at least three times longer than the
calculated value.
Example 2: The minimum value of the bootstrap capacitor
Conditions:
ΔV=0.1 V, I
leak
=1 mA
≥
4.7
× 200
Ω
×
1
0.5 × ln
15
15
−
12.5
−
0.1
≅
3.4