AN2528

Experimental results: efficiency and further considerations

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Figure 18.

The importance of having a very good transformer in the turn-on highlight at high input

voltage can be better observed. The collector current leading edge at turn-on is evident in

this condition. The peak amplitude has to be controlled or it can lead to premature turn-off of

the PWM driver with consequent instability. This issue has been deeply discussed in

Section 3.

6

Experimental results: efficiency and further

considerations

Table 2 summarizes the thermal and loss data. All information refers to max load. An

excellent result is the very high efficiency also at the highest input voltage. The case

temperature of ESBT is fine even in the worst case, with consequent low power dissipation

on it.

To get the thermal performances shown, a special feature of the bipolar has been positively

utilized.

In a standard QR mode of operation, when the input voltage rises higher and higher, the

switching losses should increase since the frequency is increasing. For this reason the

L6565 provides the frequency foldback function that limits the minimum OFF-time.

An added feature is given by the storage time of ESBT, that helps to further reduce the

frequency while increasing the Input Voltage and/or reducing the load. This frequency

Table 2.

Power dissipation and efficiency

VinAC (V)

Tc (°C)

Ptot (W)

Efficiency

110 (V)

34

0.3

80%

400 (V)

49

0.8

66%

600 (V)

73

1.6

55%