ZXGD3105N8 datasheet(9/13 Pages) DIODES

9 / 13 page

ZXGD3105N8

Document Number DS35101 Rev. 1 – 2

9 of 13

www.diodes.com

November 2011

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.

3. When the negative Drain voltage crosses the turn-off Threshold voltage VT, the detector stage outputs a

positive voltage with respect to ground after the turn-on delay time td(fall). This voltage is then fed to the

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.

6. At zero Drain current, the controller GATE output voltage is pulled low to VG(off) to ensure that the MOSFET

is off.

10%

td(rise)

90%

MOSFET

Drain Voltage

90%

VG(off)

Body Diode

Conduction

td(fall)

10%

MOSFET

Gate Voltage

MOSFET

Drain Current

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

ZXGD3105N8 Datasheet(PDF) 9 Page - Diodes Incorporated