ZXGD3104EV2

Issue 1 – October, 2011

www.diodes.com

© Diodes Incorporated 2011

4

Waveform and efficiency measurements

The operating waveforms of the controller can be measured using oscilloscope probes. If a current

probe or transformer is also used to measure the MOSFET current, the wire length should be kept

short to avoid excessive loop-inductance which could disturb the controller operation.

Figure 4 shows the operating waveforms in the Flyback converter inside a 19V 90W output power

adaptor. The power adaptor also incorporates a boost power factor correction stage to comply with

IEC61000-3-2.

The controller senses the forward voltage drop of the parasitic diode within the MOSFET, and when

the diode is in conduction, applies a voltage to the MOSFET gate, turning on the MOSFET after an

initial delay time. The gate output voltage of the controller is then proportional to the sensed voltage.

In QR mode, the gate voltage is reduced as the MOSFET’s Drain current decreases. This ensures

that the MOSFET is turned off at the zero current point, with little or no reverse current. Another

advantage is that this technique prevents early termination of the gate voltage at low Drain current.

Early termination of gate voltage can reduce the efficiency due to body-diode conduction loss.

Figure 5 shows that 90% efficiency can be achieved in the 90W adaptor when using the evaluation

board to drive a 10m

100V synchronous MOSFET.

Figure 4: Operating waveforms in QR mode with a 10m

100V synchronous MOSFET

Figure 5: Efficiency measurement in a 19V 90W power adaptor