AN557 datasheet(40/52 Pages) STMICROELECTRONICS

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AN557 APPLICATION NOTE

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Figure 53. Low Cost Application Circuit.

POWER SUPPLY COMPLETE WITH MAINS TRANSFORMER

The fig. 54 shown a power supply with mains 110/220Vac transformer, diode bridge and filter capacitor

with output voltage adjustable between 5.1V. and 24V.

Output capacitors have to be chosen with low ESR in order to reduce the output ripple.Particular care has

to be taken for input filter capacitors, in fact they have to support high current spikes at mains frequency

and at the same time current peak bigger than the output current at the switching frequency.

Therefore they must be chosen with low ESR and able to substain high current ripple in order to guarantee

a good reliability to all the system. The trasformer can be chosen with a single winding and 4 diodes or a

center tap with only 2 diodes with higher reverse voltage.

A cost reduction of the trasformer can be reached using an active power factor corrector.

It work at low voltage and the external components are relatively cheap, more details can be obtained

looking on the power factor corrector application note.

POWER SUPPLY WITH MAINS HIGH FREQUENCY PREREGULATOR.

When it is necessary to eliminate the mains trasformer at 50/60Hz for reasons like weight, dimensions or

cost, a high frequency preregulator can be used.

A ferrite trasformer reduces the rectifier and filtered mains voltage in a convenient voltage to supply direct-

ly the device, providing for the isolation requirements.

Using a free running solution or one of the voltage/current mode controller available, it is possible to com-

pensate the input variation while the output voltage variations due to the load are usually very low. Some

examples regarding how to use this regulator in off-line power supply are now showed:

Flyback Topology

Using a flyback topology with single or double transistors is possible to fix a single output voltage of 35-

40V; it can be a bit increased if using a backup battery of 48 nominal Volts.

From this preregulator (±10%) tolerancevoltage is possible to get one or more indipendent outputs, with

its own current limitation and thermal protection.

Moreover a possibility to syncronize more devices together is available, remembering to fix the master

frequency at least 5% higher than the others device (working as slave) one.

In case of necessity is possible to synronize devices on the trasformer secondary with the swith-cing fre-

quency of the controller (See Fig. 55).

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