AN2487 datasheet(7/32 Pages) STMICROELECTRONICS

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AN2487

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It was decided to use the Power Good signal from the STHS4257 to start the converter. This

ensures that the converter's input capacitor is fully charged well before the converter is

allowed to start. If the converter was started from its own input voltage, there would be a

coordination problem to be solved - the converter could start before the input capacitors

were charged if conditions were incorrect. The cost is only a resistor and a PNP transistor,

but the benefits include peace of mind.

Control chip

ST's L6565 quasi-resonant SMPS controller allows easy implementation of the transition

mode peak current control scheme. It was designed for line voltage, but it can operate at

much lower voltages with minor changes in the startup circuitry. The benefits include lower

EMI and much lower turn-on losses than any other scheme.

Operating frequency

Because the L6565 operates in transition mode, it operates at variable frequency. The

frequency is lowest at high output current and minimum input voltage. The converter is

designed to operate at about 30 kHz under full load at 36 V input. The transformer is

designed to go into saturation at a slightly lower frequency, and run at a higher frequency

with a higher input voltage and lighter loads.

If a fixed operating frequency is required, this is NOT the recommended method. In some

applications, the power supply must be synchronized with other circuitry so that its noise

spikes occur between signal sampling times. In this case, another controller should be used.

Noise generated by the transition mode scheme is quite low, however. It is more than

sufficient for the applications best suited for PoE technology.

Physical/mechanical

It was decided early on to keep the converter as small as possible consistent with the power

requirements. SMT was a requirement, as was low profile

Early experiments with small surface-mount power transformers in the 10 W range required

very high operating frequencies, in the 70 to 100 kHz range. The converters worked very

well, but it was very difficult to keep the 2 mm clearance required for the 1500 V isolation

requirement. Safety considerations made it necessary to include two unused pins between

the cable-connected side and the load side of the transformer. The smaller SMT bobbins

available would not support the wire required for 10 W of output. The smallest 12-pin SMT

bobbin available was finally chosen, and the transformer core that fit into it became the core

of choice.

The transformer height became the limiting height for components. Parts selection then

proceeded.

The final height was set at ½”, including the 1/16” thickness of the PC board, but not

including the RJ-45 connectors or their leads, or the terminal strips. These serve

demonstration functions only.

The supply dimensions (with the above exclusions) give a volume of 4" x 1.5" x 0.5", or 3 in3.

At 10 W output, power density is a respectable 3.3 W per in3, including the PoE functions.

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