Application Note
33 of 53
V 1.0
2019-04-01
IM393 Application note
IM393 IPM Technical Description
Bootstrap circuit
6
Bootstrap circuit
6.1
Bootstrap circuit operation
The V
BS
voltage, which is the voltage difference between V
B (U, V, W)
and V
S (U, V, W)
, provides the supply to the IC
within the IM393-XX. This supply voltage must be in the range of 12.5~17.5 V to ensure that the IC can fully drive
the high side IGBT. IM393-XX includes an under-voltage detection function for the V
BS
to ensure that the IC does
not drive the high-side IGBT if the V
BS
voltage drops below a specified voltage (section 5.3).
Internal bootstrap circuitry is integrated inside the HVIC. It consists of three high-voltage MOSFETs that
eliminate the need for an external circuitry ( resistors). There is one MOSFET for each high-side output
channel, which is connected between the V
DD
supply and its respective floating supply (V
B(U)
, V
B(V)
, V
B(W)
), as
shown in Figure 26. The integrated bootstrap MOSFET is turned ON only when the low-side output (LO) is
“HIGH”, and has a limited source-current due to R
BS
. The V
BS
voltage will be charged each cycle depending on
the time of LO and the value of the C
BS
capacitor, the collector-emitter drop of external IGBT and the low-side
freewheeling diode drop.
The bootstrap MOSFET of each channel follows the state of respective low-side output stage unless the V
B
voltage is higher than approximately 110% of V
DD.
In that case, the bootstrap MOSFET is designed to remain OFF
until V
B
returns below that threshold. This concept is illustrated in Figure 27.
V
DD
V
B(U)
V
B(V)
V
B(W)
Figure 26
Internal bootstrap MOSFET connections
Bootstrap
MOSFET
disabled
V
BS
Time
LO
ON
ON
OFF
Bootstrap
MOSFET
state
Vth ~1.1(V
DD
)
V
DD
=15V
Figure 27
Bootstrap MOSFET state diagram
A bootstrap MOSFET is suitable for most of the PWM modulation schemes, and can be used either in parallel
with an external bootstrap network or as a replacement of it. The use of the integrated bootstrap as a
replacement of the external bootstrap network may have some limitations however. An example of this
limitation may arise when this functionality is used in non-complementary PWM schemes and at very high PWM
duty cycle. In these cases, superior performance can be achieved by using an external bootstrap diode and
resistor in parallel with the internal bootstrap network.