AN2738 datasheet(25/54 Pages) STMICROELECTRONICS

25 / 54 page

AN2738

Application example

Doc ID 14589 Rev 2

25/54

Consider that all the following considerations should be considered as approximated

analyses of the gate charge phenomenon, so for a proper sizing of the gate resistor is

always strongly recommended to evaluate the resulting power bridge transitions in bench

analyses.

As shown in Figure 20, the gate line is split into separated paths, one for the turn-on (with, in

the example, a gate resistor of 33

Ω) and the other one for the turn-off, using a small signal

diode as path selector the equivalent turn OFF resistance is in a first approximation the

parallel of the turn OFF and the turn ON resistances (neglecting the diode drop). In the

example the turn-off resistance is set to 0

Ω to provide the lowest resistance path for the

turn-off of the IGBT. In fact, as explained in the two following paragraphs, low impedance on

the gate driver turn-off helps in reducing the induced turn-on phenomena.

Regarding the layout of such gate lines, it is always strongly recommended to place the

power IGBT (or MOSFETs) very close to the gate driver. It is important to reduce as much

as possible the lengths of such line paths as well the areas included in the gate circuits,

because these can act as weak antennas and could catch noise from the surrounding

environment. The larger these areas, the higher the gain of such undesired antenna circuits.

Figure 21.

Layout suggestion for the gate driving circuits

8.10

Gate driving: principle of working with inductive load

In this paragraph a deeper description of the power IGBT (or MOSFET) gate driving, in case

of inductive load, will be provided. The following explanations and calculations are just

intended to provide a general understanding of physical principles lying behind the

phenomenon of the dVOUT/dt control through the gate current limitation performed by the

gate resistors. The target is to help the application designer which uses L6390 ICs to be

aware of the various contributes that different parameters of the gate driver system have on

the power transitions and also to distinguish the main effects on which he should focus on.

HVG

OUT

LVG

GND

MINIMIZE THIS AREA

MINIMIZE THIS LENGHT

AN2738 Datasheet(PDF) 25 Page - STMicroelectronics