AN3338 datasheet(37/72 Pages) STMICROELECTRONICS

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AN3338

Electrical characteristics and functions

Considering the limit cases during the PWM control and further leakages and dispersions in

the board layout, the capacitance value to use in the bootstrap circuit must be selected two

or three times higher than the CBOOT calculated in the graph of Figure 24. The bootstrap

capacitor should be with a low ESR value for a good local decoupling, therefore, in case an

electrolytic capacitor is used, a good quality (low ESR, low ESL) filter capacitor placed

directly on the SLLIMM pins is strictly recommended.

2.3.14

Initial bootstrap capacitor charging

During the startup phase, the bootstrap capacitor must be charged for a suitable time to

complete the initial charging time (tCHARGE), which is, at least, the time VCBOOT needs to

exceed the turn-on undervoltage threshold VBS_thON, as already stated in Equation 12. For

a normal operation, the voltage across the bootstrap capacitor must never drop down to the

turn-off undervoltage threshold VBS_thOFF throughout the working conditions. For the period

of startup, only the low side IGBT is switched on and, just after this phase, the PWM is run,

as shown in the following steps of

Figure 25:

•

t1: the bootstrap capacitor starts to charge through the low side IGBT (LVG)

•

t2: the voltage across the bootstrap capacitor (VCBOOT) reaches its turn-on

undervoltage threshold VBS_thON.

•

t3: the bootstrap capacitor is fully charged, this enables the high side IGBT and the

CBOOT capacitor starts to discharge in order to provide the right IGBT gate charge. The

bootstrap capacitor recharges during the on state of low side IGBT (LVG).

Figure 24. Bootstrap capacitor vs. switching frequency

AM09341v1

fsw (kHz)

STGIPx20K60

δ=50%

ΔV

CBOOT =0.1V

ΔV

CBOOT =0.3V

ΔV

CBOOT =0.5V

AN3338 Datasheet(PDF) 37 Page - STMicroelectronics