AN1215 APPLICATION NOTE

4/7

This circuit has been implemented in a 45W, wide range

mains AC-DC Adapter with the L5991 [3] and the results

are shown in fig. 4.

Shutdown will be activated by injecting the current deliv-

ered by the PNP into pin 14 (DIS) so that the voltage at

the pin exceeds 2.5V. The resistor divider R5-R6 refers

to the circuit described in [3] and is part of the latched

OVP circuit. In case of a different value of R5, the collec-

tor resistor and the delay capacitor will be changed con-

sequently.

Besides increasing safety, the advantage of each of

these techniques is that heatsinking of the power parts

needs to be designed neglecting anomalous operating

conditions (overload or short circuit), with a considerable

saving of size and cost.

Conclusions

Pulse-by-pulse and hiccup mode current limitation are the ways ST's advanced PWM controllers L5991 and

L5993 handle overload and short circuit conditions. It has been shown that these two functions cover two differ-

ent fault categories: pulse-by-pulse does not allow the power throughput to rise out of control in case of a load

failure whereas hiccup usually protects against short circuits inside the converter itself (transformer, diodes)

where pulse-by-pulse is not effective.

Unlike hiccup, which dramatically reduces power throughput, pulse-by-pulse limitation little does about that,

thus forcing designers to oversize heatsinking of the critical parts for the sake of safety.

A simple and versatile circuit has been presented that helps overcome this problem and can be used to achieve

two common protection schemes, autorestart and shutdown, resulting in very low power dissipation.

As an appendix, a calculation method that helps evaluate whether a load short circuit can activate hiccup mode

protection or not in a given application, is provided.

REFERENCES

[1] "L5991/A Primary Controller with Standby" Datasheet

[2] "L5993 Constant Power Controller" Datasheet

[3] "45W AC-DC Adapter with Standby Function" (AN1134)

Figure 4. Input power vs. input voltage during

hiccup mode operation

L5991-based 45W AC-DC Adapter

Dead short at output

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Vin [V]

Pin [W]