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Old 24th March 2006, 08:22 PM   #1
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Exclamation Half bridge smps voltage drop problem

Hi,
I'd like to share some of my experiments about smps.

Here is my configuration:

half bridge topolgy
with 1uF 400V in series with primary winding
other point connected to the middle potential created by electrolytics capacitors (2*660uF 200VDC).

SG3525 + IR2113 + 2*IRF740
frequency : 62500Hz
Mains are 230VAC rectified is 320VDC


Core is 3C85 36mm toroid (philips) section is about 1cm2

primary winding : 32 turns (Bmax is 250mT)

secondary winding : 2*8 turns center tapped (secondary ground isolated from primary)

secondary rectifier is 4 TO220 fast diodes (10A 400V)
filtering is 2*1000uF 63V

with no load: secondary voltage is 2*41VDC
With only one secondary loaded by 4.7 oms : 32V (207W)

MY QUESTION IS:
is it a "normal" voltage drop ?
is my core saturating ?
Do i have to take this voltage drop into account and make a voltage regulation (opto...)?

Thanks for your help !
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Old 24th March 2006, 09:47 PM   #2
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Looking at your turns ratio, it looks as if you are running the SMPS UN-regulated now at 50% fixed duty cycle. If you want to fix the output voltage as load varies, you will need to have opto-feedback and design the transformer turns ratio differently, otherwise your supply will sag just like a linear supply.

How much power are you trying to get? With 660uF primary capacitance, I'm not sure you will get much more power. Check to see how much voltage ripple you have on the primary caps. The less capacitance on front end, the more 100Hz ripple you will have passing to the output.

You need to check the primary current waveform to see if the core is saturating. Use a current probe if you have one or measure voltage across a small value, high wattage series resistor (0.1 or 1 ohm 5 or 10 watt) inserted into the primary somewhere. The current should be a nice ramp, like a trapezoid. If the ramp suddenly curves up with a large di/dt at the end of the ramp, you have saturation.

One more thing- a toroid in not the best type of core to use for an offline converter because it becomes trouble when you need to have the required creepage and isolation required from primary to secondary windings. To be completely safe, either triple insulated wire needs to be used on either primary or secondary, or several tape layers need to be used to completely isolate the pri. and sec. I'm not completly sure as I have not personally used a toroid for offline SMPS.

Hope this helps.
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Old 24th March 2006, 11:44 PM   #3
Eva is offline Eva  Spain
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Well, this voltage drop is quite excessive for such a converter while outputting less than 220 watts (32^2/4.7). On the other hand, the voltage is being dropped somewhere in your circuit, so scientifically speaking, the best thing that you can do now is to take an oscilloscope and find out where. Check also primary current waveforms as it has been suggested, and try to post pictures of the results so that we can analyse them.
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Old 25th March 2006, 03:32 AM   #4
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First of all, thanks for your fast reply.

As soon as possible i'll post pictures of the signals (primary current, voltage, secondary voltage...)

may the problem come from the secondary capacitors which are not low esr ones ?
Voltage ripple may occurs there.
I'm gonna check this point first.
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Old 25th March 2006, 05:01 AM   #5
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Default Secondary Loading

Hi Alex,

You and I must've been dreaming of the exact same layout, parts list, topology, and component choice!

I did a half-bridge just like this about 10 years ago. Unfortunately, I cannibalized the parts from it to make other projects. I did use opto feedback and got good regulation. Output was +/- 30V at 4A (240W) and my switching freq was ~35kHz.

I used IRF740s, SG3525, MPIC2113 (Motorola's version of the IR2113), and MOC8102 Optocoupler. The hi-voltage caps I used were Panasonic TSH-series 680mF (105C and low ESR), and the core I used was an Amidon FT-140-77, (a 1.40" toroid made of a very similar material to the 3C85), and MBR10100s for the rectifiers. I chose Schottkies for their lower forward voltage drop. The filter caps on the secondary side were Panasonic HFU 1000mF/50V. I used three layers of Scotch #93 Teflon Tape to insulate the primary and secondaries. It powered 2 50W Amps into 8W.

No noticeable noise at the outputs, and no magic smoke got out, either. In retrospect, I really wish I hadn't taken it part!

I have a couple of questions:

1) Why load only one secondary? Does anything change when you load both secondaries?

2) Is 62.5kHz your oscillator freq. or your switching freq? If it is the oscillator freq, then 32T on the primary is about right for a 31.25kHz switching frequency. But for 62.5kHz, this would be a bit much, and this could be one cause of core saturation. Amen to checking the primary current waveforms- this will tell alot.

Eagerly awaiting waveform pics.


Steve
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Old 25th March 2006, 09:49 AM   #6
mzzj is offline mzzj  Finland
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Default Re: Secondary Loading

Quote:
Originally posted by N-Channel
Hi Alex,

2) Is 62.5kHz your oscillator freq. or your switching freq? If it is the oscillator freq, then 32T on the primary is about right for a 31.25kHz switching frequency. But for 62.5kHz, this would be a bit much, and this could be one cause of core saturation. Amen to checking the primary current waveforms- this will tell alot.

Steve
Umm, did you swap your numbers by accident or..?
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Old 25th March 2006, 09:55 AM   #7
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Hi N-channel,thanks for the interest you're bringing to my project !

1) I've loaded only one secondary because with one 4.7 ohms resitor on each secondary the core produces a HF noise !
kind of overload or great saturation ?

2) 62.5kHz is the transformer operating frequency.
(SG3525 : RT=10k;CT=1nF;Rdisch=100;Css=1uF).
Do you think it is a bit much ?
Around 30kHz should be better with 32 turns on the primary side ?

Could you tell me a little more about your regulation:

1) How many extra primary turns ?
2) Did you implement output inductors ?
3) SG3525 connection and compensation (pin 9) ?
4) opto-coupler wiring (with zener maybe) ?

I post pics as soon as possible.

Thanks for your reply.
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Old 25th March 2006, 11:50 AM   #8
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if i make some maths :

URMS=4.N.f.S.Bmax

where:
4 : for square wave
URMS=160V
N=32
f=62500 Hz
S=1cm2 (cross section area)
that gives Bmax=200mT (2000Gauss)

Do you really think it is too much for a 3C85 core ?

with f=35000 Hz
it gives Bmax=357mT (3570Gauss)

so increassing frequency lowers Bmax

isnt it ?
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Old 25th March 2006, 12:14 PM   #9
mzzj is offline mzzj  Finland
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Quote:
Originally posted by alexclaire


2) 62.5kHz is the transformer operating frequency.
(SG3525 : RT=10k;CT=1nF;Rdisch=100;Css=1uF).
Do you think it is a bit much ?
Around 30kHz should be better with 32 turns on the primary side ?
Better not use 30khz with 32 turns primary, I am affraid that core loss is starting to get too high. 250mW/cm^3 is bit high

Also, problem must be something else than core saturation, half-bridge core doesnt saturate with increased loading.
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Old 25th March 2006, 12:19 PM   #10
mzzj is offline mzzj  Finland
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Quote:
Originally posted by alexclaire
if i make some maths :

URMS=4.N.f.S.Bmax

where:
4 : for square wave
URMS=160V
N=32
f=62500 Hz
S=1cm2 (cross section area)
that gives Bmax=200mT (2000Gauss)

Do you really think it is too much for a 3C85 core ?

with f=35000 Hz
it gives Bmax=357mT (3570Gauss)

so increassing frequency lowers Bmax

isnt it ?
Thats correct. I wouldnt recommend 32 turns primary with 35khz. Something like 50 turns at 35khz sounds better.
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