STK531U394A-E

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11

CB capacitor value calculation for bootstrap circuit

Calculate conditions

Capacitance calculation formula

Thus, the following formula are true

VBS

 CB  QG  IDMAX  TONMAX = UVLO  CB

therefore,

CB = (QG + IDMAX

 TONMAX) / (VBS  UVLO)

The relationship between TONMAX and CB becomes as follows. CB is recommended to be approximately 3 times the value calculated

above. The recommended value of CB is in the range of 1 to 47μF, however, this value needs to be verified prior to production.

Fig. 12 Tonmax - CB characteristic

Parameter

Symbol

Value

Unit

Upper side power supply.

VBS

15

V

Total gate charge of output power IGBT at 15V.

QG

132

nC

Upper limit power supply low voltage protection.

UVLO

12

V

Upper side power dissipation.

IDmax

400

μA

ON time required for CB voltage to fall from 15V to UVLO

TONmax

-

s

0.01

0.1

1

10

100

0.1

1

10

100

1000

Tonmax [ms]

CB vs Tonmax