Philips Semiconductors

Product specification

NE/SE5560

Switched-mode power supply control circuit

1994 Aug 31

12

V11 (CURRENT LIMITING)

NORMAL OPERATING

POINT

LEVEL 1 LEVEL 2

.48

.60 (V)

OC1

OC2

SL00374

Figure 15. Output Characteristics

Dynamic Current Limit and Current Protection

(Pin 11)

In many applications, it is not necessary to have a real

constant-current output of the SMPS.

Protection of the power transistor will be the prime goal. This can be

realized with the NE5560 in an economical way. A resistor (or a

current transformer) in the emitter of the power transistor gives a

replica of the collector current. This signal must be connected to Pin

11. As can be seen in Figure 14, this input has two comparators with

different reference levels. The output of the comparator with the

lower 0.48V reference is connected to the same gate as the output

of the PWM.

When activated, it will immediately reset the output flip-flop, so

reducing the duty cycle. The effectiveness of this cycle-by-cycle

current limit diminishes at low duty cycle values. When

δ becomes

very small, the storage time of the power transistor becomes

dominant. The current will now increase again, until it surpasses the

reference of the second comparator. The output of this comparator

activates the start-stop circuit and causes an immediate inhibit of the

output pulses. After a certain deadtime, the circuit starts again with

very narrow output pulses. The effect of this two-level current

protection circuit is visualized in Figure 15.

The Start-Stop Circuit

The function of this protection circuit is to stop the output pulses as

soon as a fault occurs and to keep the output stopped for several

periods. After this dead-time, the output starts with a very small,

gradually increasing duty cycle. When the fault is persistent, this will

cause a cyclic switch-off/switch-on condition. This “hiccup” mode

effectively limits the energy during fault conditions. The realization

and the working of the circuit are indicated in Figures 12 and 13.

The dead time and the soft-start are determined by an external

capacitor that is connected to Pin 6 (

An RS flip-flop can be set by three different functions:

1. Remote on/off on Pin 10.

2. Overcurrent protection on Pin 11.

3. Low supply voltage protection (internal).

As soon as one of these functions cause a setting of the flip-flop, the

output pulses are blocked via the output gate. In the same time

transistor Q1 is forward-biased, resulting in a discharge of the

capacitor on Pin 6.

The discharging current is limited by an internal 150

Ω resistor in the

emitter of Q1. The voltage at Pin 6 decreases to below the lower

level of the sawtooth. When V6 has dropped to 0.6V, this will

activate a comparator and the flip-flop is reset. The output stage is

capacitor via R1 to the normal

delivering very narrow pulses as soon as V6 exceeds the lower

sawtooth level. The duty cycle of the output pulse now gradually

increases to a value determined by the feedback on Pin 3, or by the

static

15

14

2

6

12

11

10

LATCH

START/

STOP

SET

RESET

0.6V

0.48V

0.6V

LOW SUPPLY

VOLTAGE

PROTECTION

100

Ω

R1

R2

CSS

Q1

Q2

PWM

VZ

SL00375

Figure 16. Start-Stop Circuit