APPLICATION NOTE

8/9

In the case of an EMC problem, the first solution is

3.2.3. Switching losses

We have evaluated the consequences of poor

behaviour at the turn-off: spike reverse voltage,

possible oscillations and EMC problem. For these

reasons, the designer may wish to use a soft

recovery PN diode, but which, in return, will

increase the switching time and particularly the

switching losses at turn-off.

Switching losses at turn-off due to the diode are

the sum of losses inside the diode and the energy

dissipated in the other elements of the circuit. In

recovery charge, flows through the transformer,

the power MOS transistor and the primary bulk

capacitor. Thus, there are additional losses. The

distribution of power losses at turn-off can be

detailed:

- Losses inside the diode:

P

1

2

tI

V

F

turn off

b

RM

R

diode

−

=⋅ ⋅

⋅

⋅

- Losses due to the energy store in the leakage

inductance:

W

1

2

LI

Lf

f

2

RM

=⋅ ⋅

which is mainly dissipated in the snubber resistor

- Losses due to the eddy current in the transformer

(view AN1262)

- Losses due to the all additional resistor of circuit,

defined by:

PI

R

turn off

2

RMS

R

(IRM)

−

=⋅

∑

As described before (in §1), in a typical converter

working with a switch frequency lower than

100kHz, these different losses can be considered

negligible compared to the conduction losses.

However in applications such as the DC/DC

converter (12V-48V) working with a switching

frequency around 300kHz, these losses can be

predominant, and a 150V Schottky can be very

interesting to reduce the switching losses.

4. RESULT OF EXPERIMENTS

Experimental

measurements

(Fig.

18)

were

carried out in a 45W/24V Flyback converter

working in the following conditions:

These experiments confirm the interest in a 150V

Schottky in comparison with a 200V bipolar diode.

CONCLUSION

We have been able to highlight that when we have

the choice between 150V Schottky and 200V PN

diode, the 150V Schottky is the best choice for the

safety of the component and the environment, the

limitation of parasitic effects and for the efficiency

of the converter.

has a better switching behaviour, due to its

essentially capacitive recovery (less sensibility to

the temperature). We have the advantage of a soft

recovery diode in terms of EMC and the Schottky

is preferable to a fast recovery diode in terms of

losses. The 150V Schottky diode is the better

choice versus the 200V bipolar as for EMC and

losses at turn-off are concerned. Experimental

measurements confirm this.

Moreover, future advancements will mean that this

product will be developed.

In fact, with the arrival of the EN6100-3-2 standard

and the introduction of the PFC, whatever the input

voltage is, there will be a continuous voltage on the

primary. This will lead to a reduction of the

transformation ratio, and in the same time, the

reverse voltage of the diode.

Consequently, a lower breakdown voltage diode

will be needed in the future to replace a 200V PN

diode used today.

Also, the tendency is for the output power of

adaptors to increase. This involves an increase in

the output voltages. The voltage requirements of

the diode in this case will be higher than 100V and

a 150V diode is likely to be the appropriate

component.

η%

∆P(W)

STPR1620CT (2x8A)

84.04

0.41

STPS16150CT (2x8A)

84.4

0.18

STPS20150CT (2x10A)

84.69

Ref

Fig. 18: Experiments of efficiency in a Flyback

converter