AN1842 datasheet(10/17 Pages) STMICROELECTRONICS

10 / 17 page

AN1842 APPLICATION NOTE

10/17

Figure 19. Short circuit on 12V @90 VAC - 60Hz

- full load

Figure 20. Short circuit on 12V @265 VAC - 50Hz

- full load

Short Circuit Protection @ Low Load

After the full load tests, some checks on the short

circuit protection with reduced loads have been

done, as reported here following. The tests have

been done even at minimum and maximum input

voltage with the same results, as the previous

tests.

@Half Load

The waveforms of figure 21 are relevant to a short

circuit test simulating a fault during normal opera-

tions of the equipment, with a consumption halved

with respect to the maximum output power levels

(I+12V = 2A, I+5V = 1A).

– At Vin = 90Vac: shorting each output the over

current protection works correctly, providing for

the hiccup mode.

– At Vin = 265Vac: the circuit behaves correctly.

Figure 21. Short circuit on 5V @265 VAC - 50Hz

- half load

@Reduced Load

This condition simulates a fault during an operat-

ing transition of the equipment, with the backlight

off and a reduced consumption of the other output

(I+12V = 0A, I+5V = 0.5A).

– At Vin = 90Vac: shorting each output the over

current protection works correctly, providing for

the hiccup working mode.

– At Vin = 265Vac: the behaviour of the circuit is

correct like at 90Vac.

–

@Standby

This condition simulates a fault during the standby

operation of the equipment, with the backlight and

the scaler off, and the

µP working with a reduced

consumption (I+12V = 0A, I+5V = 33mA).

– At Vin = 90Vac: shorting each output the over-

current protection works correctly, providing for

the hiccup working mode.

– At Vin = 265Vac: the behaviour of the circuit is

correct like at 115Vac.

CH1:

VQ4 - Drain

CH2:

VPIN8 (Vcc)

CH4:

+12V short circuit current

CH1:

VQ4 - Drain

CH2:

VPIN8 (Vcc)

CH4:

+12V short circuit current

CH1:

VQ4 - Drain

CH2:

VPIN8 (Vcc)

CH4:

+5V short circuit current

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