AN1660 datasheet(3/13 Pages) STMICROELECTRONICS

3 / 13 page

3/13

AN1660 APPLICATION NOTE

t3~t4:

Q1 turns off at t3. Both Q1 and Q2 are off.

The charge stored in the parasitic capacitor CQ2 of Q2 is discharged by means of the resonant current

between L1+L2 and C1+C2. Whereas CQ1 is charged.

t4~t5:

Q1 is off. The resonant current flows through the parasitic diode DQ2 of Q2.

At t4 CQ2 is discharged. The voltage becomes zero. ZVS is achieved. The voltage of C2 increases further.

t5~t6:

Q1 is off and Q2 is on.

The resonant current flows through Q2 and in the opposite direction of t4~t5. The resonant current de-

creases sinusoidally and reaches the minimum value then increases till coincides with the current in L2 at

t6. The difference between resonant current and current in L2 flows through the primary winding N1 of the

transformer. Power is supplied to the load.

t6~t7:

Q1 is off and Q2 is on.

The current -I2 in L1 coincides with the current in L2 at t6. No current flows through the secondary winding

of the transformer. In this mode L1+L2 and C1+C2 resonate.

t7~t8:

Q2 turns off. Q1 and Q2 are off.

The parasitic capacitor CQ2 of Q2 is charged by means of the resonant current between L1+L2 and C1+C2.

Whereas CQ1 is discharged.

Here the circuit returns to the first mode and the cycle is repeated.

A main advantage of this resonant converter is that there are no turn-on switching losses exist in the FET

because its inverse diode carries current and the voltage across the MOSFET is zero before the MOSFET

conducts forward current.

There are still turn-off switching losses. But it can be erased by placing small snubber capacitors directly

across the FET devices. And no discharge resistors are needed. This was because the capacitor is not

discharged by turning the FET on but rather is discharged by turning-off the opposite FET.

Also the switching losses due to Coss and Crss is eliminated by the same reason mentioned before in the

lossless snubbers. The energy stored in any capacitance directly across the device is returned to the DC

source by virtue of the opposite FET turning off.

AN1660 датащи(PDF) 3 Page - STMicroelectronics