AN2785 datasheet(24/51 Pages) STMICROELECTRONICS

24 / 51 page

Application examples

AN2785

24/51

Doc ID 14785 Rev 1

Figure 16.

Gate driver output buffers: equivalent circuit

This first simplification is related to the inner structure of the gate driver circuitry. The

following section focuses on the side of the power bridge and its commutations.

In a common half-bridge composed of a high-side and low-side switch, the transitions of the

power IGBTs (or MOSFETs) are not all similar, but they can be distinguished into two main

types: soft switching and hard switching (Figure 17). The key element in the dynamics of the

transitions is the direction of the current flowing in the power IGBT (or MOSFET) under

evaluation, which depends on the direction of the load current with respect to the half bridge.

Essentially, if the current goes in the same direction as the power switch, the commutation is

hard. If the current goes in the opposite direction, the commutation is soft.

As shown in Figure 17, hard switching occurs when the high side is commutating and the

load current is outgoing from the bridge or when the low side is commutating and the load

current is ingoing to the bridge. This type of transition is called hard because the power

switch turns ON when the related Vce (Vds) is at the maximum; Vce (Vds) goes back to the

maximum during turn OFF (Vce is close to zero while the switch is steadily ON): the

transition is completely managed by the switch that, during the commutation, dissipates

energy.

Figure 17.

Hard and soft switching

HVG or LVG

OUTPUT DRIVER

HVG/LVG

OUT or GND

BOOT or VCC

OUT or HVbus

HVG or LVG

OUTPUT DRIVER

HVG or LVG

OUT or GND

BOOT or VCC

OUT or HVbus

RGATE

RDSon_source

RDSon_sink

RGATE

RDSon_source

Vge

RGATE

RDSon_sink

Vge

(Vboot-Vout) or VCC

GATE DRIVER OUTPUT: EQUIVALENT CIRCUIT FOR TURN-OFF

GATE DRIVER OUTPUT: EQUIVALENT CIRCUIT FOR TURN-ON

RGATE

Iload

OFF

HARD SWITCHING

CURRENT GOING IN THE BRIDGE

LOW SIDE SWITCHING

OFF

CURRENT GOING OUT FROM THE BRIDGE

HIGH SIDE SWITCHING

OFF

Iload

OFF

SOFT SWITCHING

CURRENT GOING OUT FROM THE BRIDGE

LOW SIDE SWITCHING

OFF

CURRENT GOING IN THE BRIDGE

HIGH SIDE SWITCHING

OFF

Iload

when the power switches ON

the diode is already turned ON

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