Circuit operation

AN2544

6/17

Figure 2.

Inductor current: 470 µH VS 1000 µH

The blue trace is the current with 470 µH inductor and the purple trace is the current with a

1000 µH inductor.

On the above scope plot in Figure 2, the trace represents the current going through the

inductor. Current charges up the inductor during the time the MOSFET is on. At this time,

the source pin is the same as the rectified line input and the current is ramping up. At 350

mA output current, the peak of the current is 550 mA for a 470 µH inductor, the blue trace.

The worse case condition for the VIPer Idlim is 560 mA. So therefore we are close to the

pulse by pulse current limit trip point. This is manifested by the output voltage dropping as

the output current is being raised past the limit. 470 µH inductor is the minimum value that

can be used from the calculations for a 350 mA output. A good compromise is a 1000 µH

making the swing less, keeping the peak at 443 mA, away from the 560 mA current limit.

Looking at the purple trace the turn-on losses are increased and the turn-off losses are

decreased in the MOSFET inside the VIPer.

It is best to choose the inductor to give ½ the ripple current between discontinuous to

continuous. This is the best compromise when working close to the maximum current. The

trade-off is a little more heat for the safety margin away from the current trip point.

VIPer temperature rise with two different inductors at 350 mA is:

Table 2.

VIPer temperature rise with different inductors

Inductor

Maximum peak current

VIPer22ADIP-E temperature rise

470 µH

550 mA

34 °C

1000 µH

443 mA

40.5 °C