ZXGD3101EV2

Issue 1 – December 2009

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© Diodes Incorporated 2009

3

Evaluation procedure and operation

To perform a quick functional test of the ZXGD3101, the evaluation board can be used to drive a

MOSFET as a diode replacement in high-side-rectification (see Fig. 3a), as the board can float to any

potential. In practice, the supply voltage could be derived from an auxiliary supply winding across the

transformer secondary. If the board is used for comparison against an existing synchronous

rectification solution, the existing controller must disabled before proceed with the testing.

The recommended device implementation is low side synchronous rectification (Fig. 3b), due to the

ease of acquiring the required supply voltage directly, either from the power supply output post

bleeder resistor, or from the emitter-follower-configured transistor. Before doing this test, it is

important that the existing diode has been removed and/or a short has been applied across its

cathode and anode terminals. The track linking the negative terminal of the converter’s output

capacitor to the transformer secondary-side output should then be cut, and a MOSFET should be

inserted. In general, the MOSFET should be selected to drop between 50 to 150mV at the peak of the

secondary-side current to ensure MOSFET enhancement. The breakdown voltage of the MOSFET

must be higher than the maximum drain-source voltage stress, plus some margin. Designers

interested in squeezing the last percent of efficiency out of the module can place an additional

Schottky or Ultra-fast-recovery diode in parallel with MOSET. The diode prevents body-diode

conduction, so the trace inductance between it and the MOSFET should be kept small to create an

efficient circulating energy flow path.

Figure 3 Test options for ZXGD3101EV1 a) high side and b) low side

(a)

(b)

Figure 3: Test options for ZXGD3101EV2

a) high side and b) low side

To check for functionality, the circuit waveforms should be probed using an oscilloscope probe with a

minimal length for the ground pin, and the probe should be connected directly to the pins of the

device. If a current probe or transformer is used to measure reverse current flow, excessive wire-loop-

inductance and injection of noise, which could disturb normal functioning of the controller, should be

avoided.

At synchronous MOSFET turn-on, current starts to flow through the body-diode after the primary

switch turn-off (see Fig. 4). When this occurs, the drain of the MOSFET will be around -1.25V with

respect to ground, due to body-diode conduction. The detector stage within the ZXGD3101

differential voltage, which, in turn, determines when the current is flowing through the secondary side.

The turn-off phase of the ZXGD3101 happens differently depending on the mode of operation. It

should be noted that the device is most suited to discontinuous and critical conduction mode, however