AN1484 - APPLICATION NOTE

6/11

In these conditions, the VIPer12A is properly

operating. It draws less than 100mW in standby

and the Hiccup mode is safe in short circuit (Figure

5a and Figure 5b).

Figure 5a: Hiccup Mode at 100V

Figure 5b: Hiccup Mode at 380V

2.3 Peak Drain Voltage

This transformer allows the reduction of Drain peak

voltage in any condition.

The gain of this structure is 70V. With the lower

reflected voltage (100V to 50V), the gain is up to

120V. There is a good voltage margin at full load

under 380VDC. This means that a standard 200V

transil clamper will not take any energy in normal

operation.

The clamper is still necessary during start-up and

short circuit, the drain voltage goes above the

730V VIPer12A avalanche voltage.

Table 5: MOSFET Peak Voltage at 380VDC

2.4 EMC Compatibility

Most of the EMC performances are due to the

"floating" voltage of the secondary winding or to

the voltage across C7 EMC capacitor. This

"floating" amplitude is linked to all the parasitic

capacitances along the wire between primary and

secondary

windings.

With

the

optimized

transformer, the EMC performances are degraded.

But it is possible to turn this problem into an

advantage.

The secondary winding is placed between the

primary and the auxiliary ones. The auxiliary

winding is used to compensate the induction from

primary to the secondary. Figure 6 shows this

compensation.

Figure 6: EMC Compensation Technique

- The cold point is wound close to the secondary

winding, limiting the voltage swing of the closest

one.

Conditions

Standard

Transfo

Optimized

Transfo

Full Load Vpeak

750V

630V

Reflected Voltage

100V

50V

Leakage Inductance

105

µH25µH

Primary Capacitance

22pF

26pF

Primary Inductance

3mH

2.6mH

Full Load Ipeak

260mA

275mA

Auxiliary Winding

Primary Winding

R2

Vin

Secondary