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RE: reduce heat in switching Power supply with half wave

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In Reply to: RE: reduce heat in switching Power supply with half wave posted by wlee on July 05, 2007 at 12:06:36

wlee-

switching power supply is no laughing matter so:

1) negatory. The post rectification part of the L-C filter is carefully calculated to attenuate the HF noise and "hash" with sufficient ripple filtering in mind. Casually manipulating the ultra-low ESR cap with boutique PIO caps (or simply lowering the capacitance) will simply not work (try if if you want)

2) negatory. you will damage the winding and ruin the HF ferrite core transformer.

Bottomline is that your TV switch-mode PSU may be built to a price point, so you may have one of those scenarios where the power transistor/fet does not have a large enough heat sink to handle the heat generated, or you may have a slightly undercapacity PSU to begin with.

For short term resolution, you may try adding a small computer DC fan to give it some fan-forced cooling. For long term solution, perhaps consider getting a better TeeVee unit or see if you can find a higher output switch mode power supply...

Q-TD

Follow Ups:

RE: reduce heat in switching Power supply with half wave - beermanpete@socal.rr.com 20:46:57 07/05/07 (1)
Since it seems that you mainly want to reduce the DC voltage at the input why not just use a step down transformer to step it down the mains to 110 VAC? This is lots easier than going into the TV set and safer as well.
- RE: reduce heat in switching Power supply with half wave - wlee 21:12:58 07/05/07 (0)
  hi Pete,
  
  I don't think the step down will reduce the heat, though it may make the PS a bit more durable. My trying to use half wave is just to reduce the DC voltage before being convert to high freq, and in any case, just to find out if it will work or if the PS will oscilate. Of course, this PS is relatively big, so if it oscilate, there is risks! Once I added a filter coil between the first stage high voltage DC and the wave form converter, the PS oscilated, I need to add another cap after the coild to stop the osc. Well, I know that will happen then. except I did not try enough to know what LC combination i sneed ot avaiod osc.
  
  Best rgds
  
  William
experiment and to find out how LCD TV is beig done... - wlee 13:15:20 07/05/07 (1)
Tks Quest, I meant reducing the input DC voltage in step 1. That is the Dc obtained from the wall 240V AC. in step 2, I hope to reduce the voltage before it came to the regulators, so that the energy would not be wasted. How would they supply voltage to the LCD back lite ? is it high voltage DC? as there is a seperate high voltage coils in the PS board. I thought this is only required in cathode ray tube display?!

BTW I was sort of impulsive buying into this US$ 250 TV. Just to open it up to see how it is being done.... Never open any LCD display before. I found that changing the first set of rectifier that convert street AC to DC, to the FR type, as well as adding teflon and PIO caps usually help the sound in the case of DVD players. That's why I was asking. I hope to remove the grain and the pixle from the TV. It looks ugly displying DVD and cable signal, nothing like the listed 1080 resolution.

best rgds

William Lee
- RE: experiment and to find out how LCD TV is beig done... - Tweekeng 17:38:35 07/05/07 (0)
  First off stop what you are doing unless $250 is what you do not mind paying for a paper weight.
  Because that's what you are going to end up with is you continue the direction you are headed.
  
  It looks ugly displaying DVD and cable signal, nothing like the listed 1080 resolution.
  
  This is a very common complaint for recent TV buyers. So before you go in any try to redesign this TV answer a few questions for me...
  
  Do you have HD service package?
  Do you have an HD cable box?
  Does you cable box have a component video or DVI/HDMI output?
  Are you using the component video or DVI/HDMI output on the cable box and matching input on the TV?
  Is the cable box setup to output 1080i or 720i/p resolution
  
  Standard DVD players are only 420i/p unless you have an upconverting one.
  Are you using the component video output?
And, to a first order - clifff 12:48:10 07/05/07 (1)
the heat loss is not directly proportional to the input voltage as it would be for a linear supply.

Losses are mostly in the sub-optimum switching waveforms not the DC in.
- RE: And, to a first order - wlee 13:17:41 07/05/07 (0)
  I think this section is just analog and video processing, not sure if it drive the LCD back lite. It uses the same usual small switching waveforms geerating IC found in the DVD players...
  
  tks!
  
  William Lee

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