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AN1317 - APPLICATION NOTE

Fig. 18 represents the output short circuit current. Its duty cycle is about 27% for a peak value of 1.4 A.

This results in an average current of 0.4 A which is perfectly compatible with the type of diodes generally

used for rectifying the output. Actually, these types of converter can withstand the short circuit condition

indefinitely. The temperature elevation of the components is quite moderate.

5. SCHEMATICS IMPROVEMENTS AND VARIANTS

5.1 Non Isolated Buck With Output Overvoltage Protection

The inherent inconvenient of the Buck, already described in paragraph 2.1 and 4.1, is the output voltage

increase, in low load and low input voltage conditions.

On the initial schematic of fig. 1, the zener diode properly clamps the output voltage surges when a

minimum load is guaranteed and if the input voltage rise and fall times between 20V to 50V typical, is

short enough. Otherwise, the output voltage may rise such values that the power dissipation in DZ1

becomes very high, as shown on fig. 19.

Figure 19: Buck non isolated - DZ1 power dissipation in short circuit

The solution implemented on the schematic of fig. 20, allows to drastically improve the output voltage

control, by reducing the nominal switching frequency if the input voltage decreases below a threshold.

This frequency shifter consists of a diode D4 connected on the OSC pin of the VIPer20, and receiving a

fraction of the input voltage through R3 and R4. When the input voltage becomes low, a current is sunk

through D4 from the middle point of the oscillator network R1-C3, thus increasing the charging time of C3

and decreasing the switching frequency. The resistances R3 and R4 are chosen in such a way that the

frequency begins to decrease at 100 Vdc of input bulk voltage, and stops completely the oscillator at

30 Vdc. Fig. 21 and 22 illustrates this behavior for two input voltages, and the final results is shown on

fig. 23: Overvoltages still occur at low input voltage or at low output load conditions, but with a reasonable

amount of power dissipated in the clamping zener diode.

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