ZXGD3101EV2

Issue 1 – December 2009

www.diodes.com

© Diodes Incorporated 2009

6

(a)

(b)

Figure 6: Oscillation-induced spurious turn on reduces efficiency of synchronous rectification

(a) Oscillation due to clamp diode recovery (b) Improved clamp circuit

Overcoming premature turn-off

which could make the controller more susceptible to closed-loop instability-induced malfunction under

this circumstance. One phenomenon that could occur particularly with a sub-10m

MOSFET is a

sudden fall of the ZXGD3101’s internal bias transistor voltage gain, which causes the gate drive

voltage to back off, even though the MOSFET is still conducting substantial current. When this

happens it leads to premature fall-off of the controller gate drive voltage (refer to Fig. 7a) which

causes inadequate enhancement of the MOSFET and reduces the effectiveness of synchronous

rectification. To make matters worse, the subsequent ringing incurs additional gate charge loss and

further deteriorates the efficiency.

If the above mentioned problem is observed, the on-board constant bias current source should be

used instead. This could be easily implemented using Q1 alongside R1, R2 and R3 (refer to Fig. 2).