November 1993

10

Philips Semiconductors

Product specification

Control circuit for switched mode power supplies

TDA8380A

Demagnetization sense circuit

To enable the SMPS to be kept in the non-continuous mode, an input is available which delays switch-on of the

high-voltage transistor until the transformer currents have decayed to zero. This is an effective way of avoiding

transformer saturation. The waveforms illustrated by Fig.9 show demagnetization with respect to the application diagram

of Fig.13.

output will be prevented from switching on.

Over-current protection

current. This generated voltage is negative-going as the emitter of the high-voltage power transistor is grounded (this

circuit arrangement provides the IC with the best safeguard against a possible collector-emitter short-circuit in the power

The positive current monitor voltage at pin 13 is compared with two voltage levels: the first level = 0.2 V and the second

level = 0 V (see Fig.10).

The first trip level only switches off the high-voltage transistor for a cycle and puts the SMPS in a continuous

cycle-by-cycle current protection mode.

The second trip level is only activated when the primary current rise is very fast which can occur during a short-circuited

output. In this mode the high-voltage transistor is quickly switched off and the slow-start procedure is activated.

Fig.9 Demagnetization function.