Electrical characteristics

AN2487

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4.7.4

Transient response

Half load to full load transients were applied to the tested output with 50 V at the input. 16

sweeps were averaged by the oscilloscope to remove noise, which would otherwise obscure

the traces.

●

Channel 1 = 12 V output

●

Channel 2 = 3.3 V output

4.7.5

Overload characteristics

The STHS4257A1 limits the supply input current, thereby limiting the power to all outputs. If

the current drawn by the converter exceeds 350 mA the STHS4257A1 will shut off and

attempt to restart. As it continues to repeatedly recharge the power converter's input

capacitors, it will shut down thermally.

The 12 V output is well protected from overload as it carries the bulk of the output power. In

the short term, if it is sufficiently overloaded, the output voltage will fall on all windings of the

transformer, including the bootstrap voltage for the L6565. If the bootstrap voltage goes low

enough, the L6565 will shut itself off. It will attempt to restart when the bootstrap capacitor

gets recharged to the L6565 startup voltage, and then turn off again.

If the 3.3 V output is shorted, the mechanism above will protect the unit. However, the 3.3 V

output is not well protected against overload because it does not draw a large portion of the

input power. It can supply well over its rated current for short terms, but if an overload

remains in place the 3.3 V Schottky diode, the 3.3 V transformer winding, and the small filter

choke will be overstressed.

4.7.6

Efficiency

●

Full load = 78.2%

●

Half load = 76.5%

Note that these figures include the losses in the LEDs and the input diodes.

Figure 23.

3.3 V half-full transient, 12 V at half

load

Figure 24.

12 V half-full transient, 3.3 V at half

load