is it possible to modify the output voltage/current of an smps ??? - Page 1
Forum section: electronics / projects

author: kripton2035
author_rank: Super Contributor
timestamp: August 09, 2012, 09:56:55 pm
content: hi,I bought a 10w led light recently on ebay.I thought it was really not bright compared to other led lights I haveok it was cheap ...so I opened it, and measured 11.2V and 0.58A on the led pins. this makes 6.5W and not 10Wthat may explain the problem.the power supply is a very small smps, after the transformer there is only a diode, a capacitor and the led (and a 20k resistor in // of the led why ?)at the primary there is an oscillator, a small mos and the transformer. no optocoupler.is there a way to slightly increase the voltage and or the current of such supply ?or is it only the transformer that defines themthanks.

author: hlavac
author_rank: Frequent Contributor
timestamp: August 10, 2012, 12:04:45 pm
content: Sounds like a unregulated isolated flyback topology.Is the smps mains powered?Given you don't change the transformer so your inductance is fixedand your input voltage does not change there is only the frequency and duty cycle of the oscillatorleft to play with, and the secondary side diode to improve efficiency.I would first see what the voltage drop across the diode is,the circuit may have been designed for a better one and later a cheaper bad diode was substituted.Maybe replace the diode with one with lower voltage drop at the current it is working on.After that, I would check the current waveform on the transformer for discontinuity.If the off period of the switch is longer than the time it takes to dump the energy into the secondarythere would be room for improvement of power transfer.You would need to tweak the duty cycle of the oscillator to match the off cycle with the dump time as closely as possible.

author: kripton2035
author_rank: Super Contributor
timestamp: August 10, 2012, 02:43:53 pm
content: good point, the diode is a SF24 which has a forward voltage of 0.85V around 600mA ...I have in stock a SB560 which has only 0.45V at 1A... will try itthanks for the tip !

author: kripton2035
author_rank: Super Contributor
timestamp: August 10, 2012, 02:47:11 pm
content: I fed the led directly with a power supply. to get 10W of power, it gives me 12.3V and 800mAcan I go with more voltage on these led or is it enought ?10W leds are often given for a Vf of 9-10Vat what voltage can we go without too much risk to burn the led ?

author: kripton2035
author_rank: Super Contributor
timestamp: August 10, 2012, 02:52:53 pm
content: Quote from: hlavac on August 10, 2012, 12:04:45 pmyes it is a 85-260V mains supply

author: T4P
author_rank: Super Contributor
timestamp: August 10, 2012, 03:36:21 pm
content: Quote from: kripton2035 on August 10, 2012, 02:47:11 pmI'm not sure about that but if it's current limited, well then it's okI'm not entirely sure a higher Vf will result in life shortening, or current limiting will not affect the life of the LED

author: hlavac
author_rank: Frequent Contributor
timestamp: August 10, 2012, 04:31:47 pm
content: Quote from: kripton2035 on August 10, 2012, 02:43:53 pmBe careful about reverse voltage, SF24 has 200V but SB560 only 60V!Flyback smps tend to use 1:1, 1:2 or 1:4 transformers which puts a significant part of mains voltage across secondary during the switch on period!

author: hlavac
author_rank: Frequent Contributor
timestamp: August 10, 2012, 04:39:48 pm
content: Your transformer is probably 1:2 (rectified 260V AC gives about 368V, 400V with 10% margin, original diode is for 200V or half that for a reason)

author: nctnico
author_rank: Super Contributor
timestamp: August 10, 2012, 06:20:11 pm
content: Quote from: hlavac on August 10, 2012, 04:31:47 pmFrom where did you get that? Maybe for higher output voltages like 24V the turns ratio is close to 1:4 but for lower voltages the turns ratio is definitely larger. In any SMPS topology the peak output current reflects back to the switch (mosfet) divided by the turns ratio.