Philips Semiconductors

Product specification

NE/SE5560

Switched-mode power supply control circuit

1994 Aug 31

13

δ INCREASES

V6

V15

SET

RESET

CHARGE

.6V

5.6V

SAWTOOTH

1.1V

DISCHARGE

DEAD

TIME

SL00376

Figure 17. Start-Stop Circuit

Remote On/Off Circuit (Pin 10)

In systems where two or more power supplies are used, it is often

necessary to switch these supplies on and off in a sequential way.

Furthermore, there are many applications in which a supply must be

switched by a logical signal. This can be done via the

TTL-compatible remote on/off input on Pin 10. The output pulse is

inhibited for levels below 0.8V. The output of the IC is no longer

blocked when the remote on/off input is left floating or when a

voltage >2V is applied. Start-up occurs via the slow-start circuit.

The Output Stage

The output stage of the NE5560 contains a flip-flop, a push-pull

driven output transistor, and a gate, as indicated in Figure 18. The

flip-flop is set by the flyback of the sawtooth. Resetting occurs by a

signal either from the PWM or the current limit circuit. With this

configuration, it is assured that the output is switched only once per

period, thus prohibiting double pulsing. The collector and emitter of

the output transistor are connected to respectively Pin 15 and Pin

14, allowing for normal or inverted output pulses. An

internally-grounded emitter would cause intolerable voltage spikes

over the bonding wire, especially at high output currents.

≅ 0.4V. An internal clamping diode to the supply voltage protects the

collector against overvoltages. The max. voltage at the emitter (Pin

14) must not exceed +5V. A gate, activated by one of the set or

reset pulses, or by a command from the start-stop circuit will

immediately switch-off the output transistor by short-circuiting its

base. The external inhibitor (Pin 13) operates also via this base.

Demagnetization Sense

As indicated in Figure 18, the output of this NPN comparator will

block the output pulse, when a voltage above 0.6V is applied to Pin

13. A specific application for this function is to prevent saturation of

forward-converter transformers. This is indicated in Figure 19.

Feed-Forward (Pin 16)

The basic formula for a forward converter is

V

n

(n

+ transformer ratio)

cycle

δ must be made inversely proportional to the input voltage. A

pre-regulation (feed-forward) with the function

feedback-loop design.

“I” “I” “n”

NOTES:

The signal V13 can be derived from the demagnetizing winding in

a forward converter as shown below.

FLYBACK

SET

RESET

FROM START STOP

0.6V

13

14

15

+

–

VZ

VZ

V1

B

H

P1

P2

S1

+

–

SL00377

Figure 18. Output Stage

This loop now only has to regulate for load variations which require

only a low feedback gain in the normal operation area. The

transformer of a forward converter must be designed in such a way