Electrical characteristics

AN2487

20/32

The traces in Figure 11 and Figure 12 were taken with 20 ohms in series with the demo

board's input to simulate cable resistance.

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Channel 1: DC input current, 100 mA/division

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Channel 2: 12 V output voltage, 10 V/division

Starting at the first division from the left, note the capacitor charging current, limited by the

STHS4257A1 chip. Until the power converter's input capacitors are charged, this chip does

not permit the converter to start. This prevents the sudden application of both load current

and capacitor charging current at the same time, which would overload the PSE.

The STHS4257A1 chip dissipates considerable heat during the charging of the converter's

input capacitors. It does rather well with the 300 µF capacitance used in the demo board.

Note that 300 µF is a rather high value for the input capacitors, chosen to stress the chips.

However, once the capacitors are charged, (and before the converter starts) the chip turns

on an internal FET switch to reduce losses. Current limiting ceases; the converter is

connected directly to the input power.

After about 3.5 divisions the power converter starts, drawing current from its input capacitors

to charge its output capacitors and to supply the resistive test load. The energy required to

charge the converter's output capacitors can be clearly seen as an overshoot in the input

current (top trace).

Note also that the STHS4257A1 chip must supply input capacitor charging current for a

longer period at 57 V, because it charges the capacitors to a higher voltage at the same

current. The output capacitor charging spike is also shorter at 57 V, because more energy is

stored in the input capacitors. Also note that the converter draws significantly less current at

57 V than it does at 37 V.

4.4.1

Maintain power signature

This design does not meet the DC maintain power signature current of 10 mA minimum.

This could easily be corrected (if necessary) by increasing the LED current or preloading the

outputs, but it is assumed that some load will be present at the converter's output. Input

current with no load was measured over the operating voltage range.

Figure 11.

Full-load startup input current with

cable resistance - 44 V source, 37 V

at PD

Figure 12.

Full-load startup input current with

cable resistance - 57 V source,

input