AN4043 datasheet(31/60 Pages) STMICROELECTRONICS

31 / 60 page

AN4043

Electrical characteristics and functions

Doc ID 022726 Rev 2

31/60

a voltage below the UVLO threshold is applied on the bootstrap channel, the IC

disables itself (no output) without any fault signal.

●

the voltage across CBOOT is affected by different components such as drop across the

integrated bootstrap structure, drop across the low-side IGBT, and others.

●

when the high-side IGBT is on, the CBOOT capacitor discharges mainly to provide the

right IGBT gate charge but other phenomena must be considered such as leakage

currents, quiescent current, etc.

2.3.12

Bootstrap capacitor selection

A simple method to properly size the bootstrap capacitor considers only the amount of

charge that is needed when the high voltage side of the driver is floating and the IGBT gate

is driven once. This approach does not take into account either the duty cycle of the PWM,

or the fundamental frequency of the current. Observations on PWM duty cycle, the kind of

modulation (6-step, 12-step and sine-wave) must be considered with their own peculiarity to

achieve the best bootstrap circuit sizing.

During the bootstrap capacitor charging phase, the low-side IGBT is on and the voltage

across CBOOT (VCBOOT) can be calculated as follows:

Equation 7

where:

VCC: supply voltage of gate driver.

VF: bootstrap diode forward voltage drop.

VCE(sat)max: maximum emitter collector voltage drop of low-side IGBT.

VRDS(on): DMOS voltage drop.

The dimension of the bootstrap capacitance CBOOT value is based on the minimum voltage

drop (

ΔVCBOOT) to guarantee when the high-side IGBT is on, and must be:

Equation 8

under the condition:

Equation 9

where:

VGE(min): minimum gate emitter voltage of high-side IGBT.

VBS_thON: bootstrap turn-on undervoltage threshold (maximum value, see datasheet).

Considering the factors contributing to VCBOOT decreasing, the total charge supplied by the

bootstrap capacitor (during high-side ON phase) is:

max

)

sat

(

)

(

RDS

CBOOT

−

max

)

sat

(

(min)

)

(

RDS

CBOOT

−

thON

(min)

CBOOT

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