AN3203 datasheet(12/49 Pages) STMICROELECTRONICS

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Asymmetrical half bridge operation

AN3203

12/49

Doc ID 17402 Rev 2

Asymmetrical half bridge operation

2.1

AHB typical waveforms

In Figure 6 the primary side key waveforms during steady-state operation with full load

applied are shown. Figure 7 shows the detail of the two transitions during one switching

cycle.

The AHB stage has been designed to operate at about 80 kHz with a nominal input voltage

of 400 V (PFC output bus). The transformer design is the result of a trade-off between the

half bridge MOSFETs zero voltage switching (ZVS) operation requirements, the primary rms

current, and duty cycle losses. In fact, ZVS can be achieved by reducing the magnetizing

inductance or increasing the leakage inductance. With the output power of this board, the

first solution implies having very high rms primary current which leads to high losses. The

second solution introduces the so called “duty cycle losses”. When the leakage inductance

is de-magnetizing, the voltages on the secondary side windings are zero and therefore the

output mean value is reduced with respect to the same half bridge duty cycle and negligible

leakage inductance. Duty cycle losses limit the hold-up capability of the power supply

because they increase the minimum input voltage that guarantees output regulation.

In this design the system works with ZVS for both MOSFETs at full load. Because of the

intrinsic asymmetry of the topology the behavior of the two switches is different. When the

load is reduced the low side MOSFET always operates in ZVS while the high side one starts

loosing ZVS. The high side MOSFET never turns on with full bus voltage applied between its

drain and source. As shown in Figure 8, even at 20 % of rated load the Vds at turn-on is

about 100 V, definitely lower compared with the 400 V of a hard switching solution.

This design can therefore meet both efficiency and dynamic requirements.

Figure 6.

AHB primary side key waveforms @ full load

Ch1: LVG pin voltage (yellow)

Ch3: HVG pin voltage (purple)

Ch4: Primary winding current (green)

AN3203 Datasheet(PDF) 12 Page - STMicroelectronics