LNK302 total power draw from 265VAC mains

Posted by: treez on 08/17/2010

Hello,

What would be the total power draw from the mains for a LNK302 Buck supplying a load of 12V, 12mA, from a 265VAC mains?

I refer to the buck schematic at the top of Table 2, page 6 of the LNK 302-6 datasheet.

Is my calculation of 266mW correct?

My Calculation:-
With 75% efficiency, the load power drawn from the mains is 0.192W.

From page 9 of LNK302 datasheet, I calculate that if there was no bulk capacitance on the DC Bus, then the average voltage there would be 238V (for 265VAC mains) , and with the Drain supply current of 310uA (from page 9) this gives operating power of LNK302 as 74mW

(obviously I realise that a high voltage series Schottky (in series with the LNK302’s FET) would be required for when the rectified mains dips below 12V level every 10ms)

Though is my calculation of 266mW correct?

Supposing I try to use the LNK302 in flyback topology to supply the 12V, 12mA………would it then draw less power from the mains?

If I connect the LNK302 with opto feedback as on the second schematic down on page 6 of the datasheet, would that give lower overall power draw from the mains?

Comments

Hi,

Without knowing the complete specification of your power supply (Min and Max operating voltage range, load variation etc.) it is difficult to provide a specific recommendation.

If what is needed is low no-load/ standby power consumption, I suggest you review our TOP-JX and LNK-CV family parts.

Regards

PI-Sarek

You will want to use a bulk capacitor on the bus for best efficiency for both buck and flyback. A large bus ripple voltage reduces efficiency due to the high drain currents (as the bus approaches zero crossing) and the RDSon of the internal FET.

Your approach for calculating input current is correct for average current. However, the RMS current will be different due to the non linear nature of rectifying and charging the bulk capacitor.

Usually a flyback is more efficient when there is a large difference in input and output voltage. The duty cycle of a buck is very low and the peak currents will be high compared to the flyback.

A word of advice on the direct feedback high side buck. You need a minimum load current to maintain regulation. Using an opto eliminates this problem.

Z. Cochrane

HI,

I thought the LNK302 would cut-off below 50V.

also, surely the LNK302 FET RMS current will be extrmely low for this tiny amount of output power?

I am concerned that if you operate the device without a bulk capacitor, the device will go into a start up mode at every half line cycle. Also you will see considerable output voltage ripple.
This is not the intended mode of operation for the device and it has not been characterized for this kind of operation. It may work in your application, however I can not recommend this application.
Z. Cochrane