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AN1645 - APPLICATION NOTE

11/22

7. ANTICIPATION

One of the major problems of synchronous rectification is the generation of proper driving signals for the

mosfets to avoid cross conduction. Contemporary conduction of the two mosfets must be avoided not

only when the two mosfets are both in on state but also when just one is on and the other one is off. In

fact it is possible to have a short circuit loop on the secondary side when the current flows through one

mosfet that is on and the body diode of the other mosfet.

Referring to figure 15, when the primary Mosfet is turned-on, the voltage Vs tends to go positive. This

voltage forward bias the body diode of the Forward mosfet (FR) and, due to some delay in turning-off the

Freewheeling mosfet (FW), an unlimited current can flow in the short circuit loop determined by the FW,

the body-diode of FR and the secondary winding of the isolation transformer. The value of the short

circuit current is only limited by the parasitics of the circuit and eventually by the primary side protection

circuits included in the PWM.

In order to avoid this undesired condition, the freewheeling mosfet must be turned off before the primary

mosfet is turned on; this means that certain ’anticipation’ is needed.

Similar considerations can be made for the transition in which the FR has to be turned-off.

Figure 15: Short circuit in the secondary side

Figure 16 shows the detailed timing of Ck, OUTGate1 and OUTGate2 signals in normal operation. Time

intervals tant1 and tant2 provide the required anticipation to avoid any short circuit condition in the

secondary side. tant1 value is fixed internally while for tant2 it is possible to choose between three

different values using the SETANT2 pin according to the following table.

4

3

Vs

FW

FR

I

FW

t

t

t0 t1

Primary

MOSFET ON

t

V

GS FW

V

S

4

3

Vs

FW

FR

I

FW

t

t

t0 t1

Primary

MOSFET ON

t

V

GS FW

V

S