TPD4112K
2006-06-30
12
Description of Bootstrap Capacitor Charging and Its Capacitance
The IC uses bootstrapping for the power supply for high-side drivers.
The bootstrap capacitor is charged by turning on the low-side IGBT of the same arm (approximately 1/5 of PWM
cycle) while the high-side IGBT controlled by PWM is off. (For example, to drive at 20 kHz, it takes approximately
10 ms per cycle to charge the capacitor.) When the VS voltage exceeds 3.8 V (55% duty), the low-side IGBT is
continuously in the off state. This is because when the PWM on-duty becomes larger, the arm is short-circuited
while the low-side IGBT is on. Even in this state, because PWM control is being performed on the high-side IGBT,
the regenerative current of the diode flows to the low-side FRD of the same arm, and the bootstrap capacitor is
charged. Note that when the on-duty is 100%, diode regenerative current does not flow; thus, the bootstrap
capacitor is not charged.
When driving a motor at 100 % duty cycle, take the voltage drop at 100% duty (see the figure below) into
consideration to determine the capacitance of the bootstrap capacitor.
Capacitance of the bootstrap capacitor
= Consumption current (max) of the high-side driver × Maximum drive time
/(VCC VF (BSD) + VF (FRD) 13.5) [F]
VF (BSD) : Bootstrap diode forward voltage
VF (FRD) : Flywheel diode forward voltage
Consideration must be made for aging and temperature change of the capacitor.
VS Range
IGBT Operation
A
Both high- and low-side OFF.
B
Charging range. Low-side IGBT refreshing on the phase the high-side IGBT in PWM.
C
No charging range. High-side at PWM according to the timing chart. Low-side no refreshing.
Safe Operating Area
Note 1: The above safe operating areas are at Tj
= 135°C (Figure 1) and Tc = 95°C (Figure 2). If the temperature
exceeds these, the safe operation areas are reduced.
Note 2: The above safe operating areas include the over-current protection operation area.
1.0
0
400
Peak
winding
current
(A)
Power supply voltage VBB (V)
Figure 1 SOA at Tj
= 135°C
0
1.1
0
400
Peak
winding
current
(A)
Power supply voltage VBB (V)
Figure 2 SOA at Tc
= 95°C
0
Low-side ON
Duty cycle 80%
C
Triangular wave
Duty cycle 100% (VS: 5.4 V)
High-side duty ON
PWM reference voltage
Duty cyle 55% (VS: 3.8 V)
Duty cycle 0% (VS: 2.1 V)
VVsOFF (VS: 1.3 V)
GND
B
A