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Old 20th May 2004, 11:52 AM   #1
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Default SMPS efficency issues...

Hi Guys! This is my first post after lurking for a 4 days or so...

I've got some efficency (sp?) issues after building a SMPS. I started building it on monday and I've now tested it and Its taking about 250W (14V @ 18A) input and only putting out 154W (+-34V @ 2.27A) . Its losing about 100W internally. I've used 16*0.4mm2 for the primary at 4 turns and 5*0.4mm2 at 11 turns for the secondary.

The basic layout of the circuit is:
2*470uF caps on the input rails
2*4 turns primary. Center attached to 14V, outside attached to ground alternately. Each side is attached for 37.5% of the time. (This minimises both the 3rd and 5th harmonics)
The core is a ETD49 with 3c90 material
The secondary is 2*11 turns rectified into 4*470uF for each rail, center tapped.

Mosfets for primary: irf3205*2 for each side of primary. They don't even get warm. Turned on/off in about 0.5uS.
Rectification diodes on secondary: BYV28-200 diodes (3.5A, 200V diodes with 25ns turn-off time and 200v reverse voltage). No hotter than can be expected (can touch with fingers without burning sensation setting in at extended time periods).

So can you guys help me out as to where I might be lsing all my power? Faulty meters (I used different meters for the current measurement but not the voltage) or lots of power going into the copper windings? I'm stumped myself - I ran it for 10 minutes today at 250W going in and it was still ok - the output voltage dropped from 68v across both output rails to 67.8v in that time peroid.

Cheers in advance for any helpful hints/pointing out stoopid things I might of done.
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Old 20th May 2004, 03:31 PM   #2
maylar is offline maylar  United States
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If you're really losing 100 watts, something's gotta be getting mighty warm. Are you sure of the measurements?
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Old 20th May 2004, 04:27 PM   #3
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As maylar suggested, try to verify the measurements first. 100 W has to be going somewhere.

What frequency are you switching at? If the windings aren't heating up, the core could be, although you should eventually be able to feel that in the windings. Does the transformer get hot?

Cheers,

bg
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Old 20th May 2004, 05:00 PM   #4
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Default smps efficiency issues

Is this a continuous or discontinuous conduction mode circuit? How large is the inductor, and what is the switching frequency? I would suspect that the primary current measurement may not be correct. If the inductor current is discontinuous, then there will be a large ac ripple current in the primary. Are you using a true rms ammeter? If nothing is getting warm, I doubt that 100 watts is being dissipated, since that is a lot of power. The efficiency computes to be 61.6 percent, low by modern day standards. Are the snubber components getting hot? How about the inductor? If you have a scope, maybe you can observe the primary current waveform using a sensing resistor. The waveform should be a small ac ripple resting on a large dc pedestal Also, power can be lost in the input and/or output filter caps, as well as the inductor. The PWM control IC dissipates the power lost in the gate drive of the MOSFETs. Also, the transformer's core and windings account for some power loss. Lastly, what is the source that is driving the input of the PWM IC? The internal resistance of the source, as well as that of the wiring produces loss. This is the hidden efficiency killer in many cases. My off the cuff guess is that the 18 amp measured value of input current should be re-examined. Best regards.
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Old 20th May 2004, 05:27 PM   #5
IVX is offline IVX  Russian Federation
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IMHO, rated by probability:
A) measured input current isn't average.
B) frequency is too much.
C) incredible bad layout.
Good layout is pretty complex task generally, but for switchmode PS and more for class D amp is especially.
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Old 20th May 2004, 10:03 PM   #6
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I realised that I hadn't mentioned the switching frequency when I first posted it, but since I'm new I couldn't put in changes in. Its switching at 21kHz, with no feed back. The windings do get warm, but can still touch them and the core is still pretty cool to touch.

I think they layout is ok - I've got a two sided PCB with 14V on one side and ground on the other - and similar for the output, and no leads are longer than 40mm.

Claude: Its nothing flash like a boost or cuk converter - its a just a transformer connected one way then the other to 14V, rectified on the otherside - no inductors, just smoothing caps.

As for the measurements - I suppose I'll have to find some beefy low ohm resisters and measure the voltage drop over them with a osiloscope to get a good measurement and to find out if its peak or rms measured current and do the same for the voltage.

I'll get back with the real results.
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Old 20th May 2004, 10:54 PM   #7
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In order to meter the input power you need the average DC-voltage times the average DC-current.
If your input voltage is instable then things are getting more complicated, but probably this is not the issue.

You can measure the average DC input current quite easy, without scope.
One current shunt resistor. Then use a R-C low pass 1kOhm-100uF.
The voltage across the 100uF cap will perfectly reflect the average DC current.

From your description about the ETD 49, I would not expect that you have excessive losses there.
If you still can touch the windings they may be around 60C.
Core is fairly cool....
If this is the situation when the SMPS is in thermal steady state, then
you may have about 5W losses in that transformer.

If there is nothing getting really hot, then I would expect that
you do not have 100W losses.

Cheers
Markus
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Old 20th May 2004, 11:04 PM   #8
Tensop is offline Tensop  Lebanon
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Arent you supposed to compensate for power factor when measuring the current drawn on switch mode power supplies...
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Old 20th May 2004, 11:30 PM   #9
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@tensop:

You may need the powerfactor if you measure the rms-values of voltage and current.

Usually people talk about the power factor in AC applications.
To "compensate" the powerfactor in a proper way may sometimes be quite complicated as the powerfactor is influenced by the the phase shift of current versus voltage (easy to compensate) AND the input current harmonics (difficult to "compensate").

But we are lucky here. We have a fairly stable DC input.
If I understand right our Flying Dutch Man will operate from a
a car battery.
So for power measurement we can work with simple equipment.

Bye
Markus
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Old 21st May 2004, 04:21 AM   #10
IVX is offline IVX  Russian Federation
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Well, if measurement is correct, then we have too small primary induction, and more primary turns or higher frequency (better about 40-50khz) is needed.
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