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 47 μF, 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 15 V 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 15 V to UVLO TONmax - s
0.01
0.1
1
10
100
0.1 1 10 100 1000
Bootstrap Capacitance CB [uF]
Tonmax [ms]
CB vs Tonmax
