ZXGD3101EV2

Issue 1 – December 2009

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© Diodes Incorporated 2009

4

it can also be used in continuous conduction mode. In discontinuous and critical conduction mode, the

MOSFET current decays linearly and the controller proportionately backs off its gate-drive output

when the on-resistance-induced conduction voltage drop is less than -50mV. Upon the conduction

voltage crossing the turning off threshold, the gate drive is turned off quickly to eliminate any reverse

current flow (see Figure 4a). If the board is evaluated with a converter in continuous conduction mode,

the secondary side circulating current does not decay to zero prior to primary MOSFET turn-on. The

controller then turns off the MOSFET quickly when the primary MOSFET current starts rising in less

than 50ns delay time, as shown in Fig. 4b, so the possibility of cross conduction is minimized. This is

critical because cross conduction due to the primary side MOSFET and secondary rectification

MOSFET conducting simultaneously will degrade efficiency due to the nature of the fast transition.

(a)

(b)

Figure 4: Synchronous rectification operating waveforms (a) Critical conduction mode and (b)

Continuous conduction mode