AN2785 datasheet(23/51 Pages) STMICROELECTRONICS

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AN2785

Application examples

Doc ID 14785 Rev 1

23/51

6.4

Gate driving: principle of working with inductive loads

This section provides a more detailed description of the power IGBT (or MOSFET) gate

driving with inductive loads. The following explanations and calculations are only intended to

provide a general understanding of the physical principles behind the phenomenon of the

dVOUT/dt control through the gate current limitation performed by the gate resistors. The

purpose is to help the application designer using L6393 ICs to understand the various ways

in which different parameters of the gate driver system act on the power transitions, and

thus identify the main effects on which to focus.

Calculations and formulae should be considered as qualitative indications and not for an

accurate quantitative use since, as explained in the above paragraph, they are the result of a

first approximation study. The experimental verification of the design choices is always

recommended.

The description distinguishes and explores the two main actors of the gate driver system: on

the one hand the inner structure of the gate driver output buffers is described, and on the

other hand the switching mechanism of a generic IGBT (or MOSFET) is detailed.

Figure 16

provides a simplified view of the L6393 gate driver output buffers. Each one can

be considered as a CMOS push-pull stage where a p-channel MOSFET works as source

driver while an N-channel MOSFET works as sink driver. The structure is similar for both the

low side and the high side, and the behavior can be considered the same. In fact, the high-

side driver can be thought of as a floating buffer having as supply the VBOOT voltage and as

reference the OUT pin. The CBOOT capacitor represents the floating supply voltage source

of the high-side driver. During the charge of the power switch gate, each source/sink

MOSFET can be considered (in first approximation, for simplicity) as if it would be in the

ohmic region, so it can be represented as an equivalent resistor with a value equal to its

RDSon. Thanks to this approximation, one can use a simplified equivalent circuit for the turn

ON and the turn OFF commutation (see Figure 16). Regarding the turn ON, this gate charge

circuit has two resistors in series (RGATE_ON and RDSon_SOURCE) and a supply voltage

which is VCC for the low side and Vboot-Vout for the high side. Regarding the turn OFF, the

equivalent circuit is composed of just two resistors (RGATE_OFF and RDSon_SINK) connected

to the source of the power switch.

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