ZXGD3104N8

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November 2011

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ZXGD3104N8

Descriptions of the normal operation

The operation of the controller is described step-by-step with reference to the timing diagram in Figure 1.

1. The controller monitors the MOSFET Drain-Source voltage.

2. When, due to transformer action, the MOSFET body diode is forced to conduct there is approximately -0.8V on the

Drain pin.

3. The detector outputs a positive voltage with respect to ground, 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 on-

resistance-induced Drain-Source voltage drop across the MOSFET. Proportional gate drive ensures that MOSFET

conducts for majority of the conduction cycle and minimizes body diode conduction time.

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 is removed when the Drain-Source

voltage crosses the detection threshold voltage to minimize reverse current flow.

Figure 1. Timing diagram for a critical conduction mode Flyback converter