3/18

AN1537 APPLICATION NOTE

In applications where the switching frequency needs not be tightly fixed for some specific reason there is no

major drawback to this technique. The only point to take care of is that the oscillator frequency be in the audible

range only when the peak current is so low that no sound may come from the transformer, even when it is made

The benefits, on the contrary, are considerable:

1)

Very low switching frequencies are possible which, as already stated, will allow treating the power

throughput as much efficiently as possible: MOSFET's capacitive losses, gate drive consumption and

other parasitic losses will be minimized. See [2] for more details on them.

2)

Since the additional components will be concerned with taking the oscillator frequency to very low

and eliminating the need for a slow feedback to prevent frequency instability. As already said, keep-

3)

As a result of the faster dynamic response, start-up under no-load conditions is possible even with a

minimum pre-load on the output. The dummy load represented by the feedback network, as well as

bleeders, if used, can be minimized. The limit to the dummy load reduction is given by the collapse

that the voltage delivered by the self-supply winding experiences with no load, which must not pull

the supply voltage of the L5991 below the UVLO threshold.

To evaluate how much this function modification improves converter's performance at light or no load, the 45W

wide-range mains AC-DC adapter illustrated in [3] and the 80W power-factor-corrected AC-DC adapter de-

scribed in [4] will be optimized following the guidelines revealed by the above considerations. The "European

Code of Conduct on Efficiency of External Power Supplies", ECC in short, whose limits are summarized in table

1, will be assumed as the reference.

Table 1. Limits envisaged by European Code of Conduct on Efficiency of External Power Supplies

Optimization of a 45W, wide-range mains AC-DC adapter

For reader's convenience, table 2 summarizes the electrical spec of the adapter under consideration. Please

refer to [3] for a detailed description and full evaluation data.

Table 2. 45W, wide-range mains AC-DC adapter: electrical specification of the original design

Rated Input Power

Max. no-load Power Consumption

Phase 1

01.01.2001

Phase 2

01.01.2003

Phase 3

01.01.2005

≥ 0.3W and < 15W

1.0W

0.75W

0.30W

≥ 15W and < 50W

1.0W

0.75W

0.50W

≥ 50W and < 75W

1.0W

0.75W

0.75W

88 to 264 Vac

50/60Hz

45W

Output

Vout = 18V

± 3%

Iout = 2.5A max.

Full load ripple

≤ 2% pk-pk

70kHz typ.

18kHz typ.

Full-load Efficiency (@ Pout =45W, Vin = 88

÷264Vac)

> 80%

Maximum no-load Input Power (Vin = 88

÷264Vac)

< 1W