ZXGD3105N8

9 of 13

www.diodes.com

November 2011

© Diodes Incorporated

A Product Line of

Diodes Incorporated

ZXGD3105N8

Functional descriptions

The operation of the device is described step-by-step with reference to the timing diagram in Figure 2.

1. The detector stage monitors the MOSFET Drain-Source voltage.

2. When, due to transformer action, the MOSFET body diode is forced to conduct there is a negative voltage

on the Drain pin due to the body diode forward voltage.

MOSFET driver stage and current is sourced out of the GATE pin.

4. The controller goes into proportional gate drive control — the GATE output voltage is proportional to the

MOSFET on-resistance-induced Drain-Source voltage. Proportional gate drive ensures that MOSFET

conducts during majority of the conduction cycle to minimize power loss in the body diode.

5. As the Drain current decays linearly toward zero, proportional gate drive control reduces the Gate voltage

so the MOSFET can be turned off rapidly at zero current crossing. The GATE voltage falls to 1V when the

Drain-Source voltage crosses the detection threshold voltage to minimize reverse current flow.

is off.

10%

90%

MOSFET

Drain Voltage

0A

90%

Body Diode

Conduction

10%

MOSFET

Gate Voltage

MOSFET

Drain Current

1

2

3

4

5

6

Figure 2 - Timing diagram for a critical conduction mode Flyback converter