AN1660 datasheet(2/13 Pages) STMICROELECTRONICS

2 / 13 page

AN1660 APPLICATION NOTE

2/13

This circuit has three operation.

1. The operation of a resonant circuit between L1 and C1+C2. Which supplies power to the load.

2. The operation of a resonant circuit between L1+L2 and C1+C2. Which does not supply power with the

load.

3. The operation of a resonant circuit between CQ1 + CQ2, L1+L2 and C1+C2. Which achieves the ZVS of

the MOSFET.

Figure 2. Operating Waveforms

t0~t1:

In Q1 reverse current flows through parasitic diode DQ1. Q2 is off.

The initial value of the resonant current between L1 and C1+C2 at t0 is -I2, which coincide with the current

in L2. The current in L2 will increases at the rate of nVout/L2 (n = N1/N2, N2 = N3). At t0 CQ1 is discharged.

The voltage becomes zero. ZVS is achieved. The voltage of C2 decreases further. C2 is discharging.

t1~t2:

Q1 is on and Q2 is off.

The resonant current flows through Q1 and in the opposite direction of t0~t1. The resonant current increas-

es sinusoidally and reaches the maximum value then deceases till coincides with the current in L2 at t2.

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

transformer. Power is supplied to the load.

t2~t3:

Q1 is on and Q2 is off.

The current I1 in L1 coincides with the current in L2 at t2. No current flows through the secondary winding

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

VDQ1

VDQ2

VQ1

VQ2

IQ1

ICQ1

IL1

VC2

ID1

Current in

parasitic

diode (IDQ1)

-I2

Vcmax

Vcmin

∆T

-I1

IL1pk

D03IN1426

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