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Low efficiency

Posted by: krishnamurthy on 12/03/2010


I assembled a 21V 1.1A power supply unit for a 18W street light (6X3 1W LED). It works fine. But the input power is about 36W when measured. The power efficiency is about only 50%. I expected about 80% or more. The design was done using PI Expert 8, uses TOP 254 IC, Optocoupler and shunt regulater. I dont find any obnormal heating of IC or out put diode. Only the transformer is slightly worm to touch. What could be the problam? Please suggest



Submitted by Ng Kam Fai on 03/05/2011

How did you measure the power?
It should be measure by a precise power meter
Power factor is a concern
P.F=active power / apparent power
I guess you measure the apparent power by VxI
So that the P.F around 0.6 if efficiency about 80% as you saying

Submitted by Adrian A. on 02/18/2012

As you noticed, the temperature of the power supply is small. This is a good indicator of the efficiency. If something really absorb 15-16W, it will become extremely hot in several minutes. As my colleague from previous post told, probably the instrument used to measure is not good enough. If you want to precisely measure the power absorbed using regular multimeter, build an external rectifier - bridge plus capacitor - and measure the DC current furnished from it into your power supply. Measure also the voltage delivered - should be nearby 320Vdc, and multiply current with voltage. Simply.