AN3338 datasheet(35/72 Pages) STMICROELECTRONICS

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Doc ID 18441 Rev 4

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AN3338

Electrical characteristics and functions

The value of the CBOOT capacitor should be calculated according to the application

condition and must take the following into account:

•

voltage across CBOOT must be maintained at a value higher than the undervoltage

lockout level for the IC driver. This enables the high side IGBT to work with a correct

gate voltage (lower dissipation and better overall performances). Bear in mind that if 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.

2.3.13

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 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.

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

across CBOOT (VCBOOT) can be calculated as follows:

Equation 10

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 11

under the condition:

Equation 12

where:

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

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

max

)

sat

(

)

(

RDS

CBOOT

−

max

)

sat

(

(min)

)

(

RDS

CBOOT

−

thON

(min)

CBOOT

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