AN4191 datasheet(9/20 Pages) STMICROELECTRONICS

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AN4191

Rg impact evaluation by UIS test

Doc ID 023815 Rev 1

9/20

The image shows the electrical connection between the driver and D.U.T. and the current

flowing during switching-on and off.

The “Driver” is composed of two Power MOSFETs in push-pull configuration. The D.U.T.

turn-on is achieved when a positive gate-source voltage (VGS = VS) is applied to the gate,

switching on the upper MOSFET; and vice-versa, the D.U.T. turns off when the lower

MOSFET of the driver is in ON state, pulling down the D.U.T. gate to GND.

The red arrows show the current path charging internal capacitances when D.U.T. is turned

on, and discharging capacitances when it is turned off. The image points out the internal Rg

resistance and Cgs capacitance, since Ton and Toff are affected by those values. Ton and Toff

represent times to complete the D.U.T. turn-on and turn-off.

The combination of Rg and Cgs is a simple RC circuit whose constant time τ =RC affects

charge and discharge time. There are also other parameters that contribute to the turn-

on/off of D.U.T. but it is important to underline that Rg is a key factor. In this case, with

inductive load, the power dissipated by the device with higher Rg is bigger than the standard

Rg device one because the switching slowdown, especially at turn-off, produces an

additional current increase, due to the fact that the device remains on slightly longer, even if

the gate-source voltage is already low.

A similar behavior may be obtained adding an external Rg resistor to a standard device. By

inserting an external resistor of 50

Ω, we have the following waveform:

Figure 7.

High Rg switching waveforms (with 50

Ω additional external resistor)

The curve shows that the external Rg enlarges switching times and the main effect appears

at turn-off. In fact, the Vgs is down and the current increases itself for an additional 1.6 µs.

AM16414V1

AN4191 Arkusz danych(PDF) 9 Page - STMicroelectronics