AN1895

Board description

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Board description

The EVAL6562-375W evaluation board includes a Power Factor Correction (PFC) pre-

regulator for a 300W ATX 12V power supply unit (PSU). It is able to deliver 375W continuous

power on a regulated 400V rail from a wide range of mains voltage. This rail will be the input

for the cascaded isolated DC-DC converter (typically a forward converter) that will provide

the output rails of the silver box. Although the ATX specification envisages air cooling,

typically realized with a fan capable of an airflow in the range of 25-35 CFM, this is not

allowed for in the design of this evaluation board. Enough heat sinking will be provided to

allow full-load operation in still air. With an appropriate airflow and without any change in the

circuit, the evaluation board can easily deliver up to 400-420W.

The L6562 controller chip is designed for Transition-Mode (TM) operation where the boost

inductor works next to the boundary between Continuous (CCM) and Discontinuous

Conduction Mode (DCM). However, with a slightly different usage, the chip can operate so

that the boost inductor works in CCM, hence surpassing the limitations of TM operation in

terms of power handling capability. The gate-drive capability of the L6562 (±0.8A min.) is

also adequate to drive the MOSFETs used at higher power levels.

This approach, which couples the simplicity and cost-effectiveness of TM operation with the

high-current capability of CCM operation, is the Fixed-Off-Time (FOT) control. The control

modulates the ON-time of the power switch, while its OFF-time is kept constant. More

precisely, it will be used the Line-Modulated FOT (LM-FOT) where the OFF-time of the

power switch is not rigorously constant but is modulated by the instantaneous mains

voltage. Please refer to [2] for a detailed description of this technique.

Table 1 summarizes the electrical specification of the application and Table 3 lists

transformer specifications.

The electrical schematic is shown in Figure 1 and the PCB layout in Figure 2.

Appendix A lists the bill of materials.

Table 1.

Electrical specifications

Parameter

Value

Line voltage range

Regulated output voltage

400 V

Rated output power

375 W

20V pk-pk

Hold-up time

17 ms

100 kHz

90%

Maximum ambient temperature

50° C