TPD4152F

2016-03-28

13

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

μs 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 = Current dissipation (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) : First recovery 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: The above safe operating areas are at Tj = 135 °C.

0.7

0

450

Power supply voltage

VBB (V)

Figure 1

SOA at Tj = 135°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 cycle 55 % (VS: 3.8 V)

Duty cycle 0 % (VS: 2.1 V)

VVsOFF (VS: 1.3 V)

GND

B

A