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Old 29th March 2008, 04:09 PM   #1
Tolik is offline Tolik  Israel
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Default Building my first 12v smps...

Hi all,

After looking at all nice projects here, I started to build my own 12V SMPS for my SKA modules. After prototype was assembled, I
noticed that while SMPS not loaded, primery`s waveforms are pretty rectangular, but some strange forms of voltage on primery windings while simple resestive load connected. The output voltage drop from 60 to 30 V under 200-240W load . This is a kind of rigning. Why it occurs ? Is this caused by flux imbalance, bad pcb layout, or core saturation ?
I captured some screenshots from my scope to illustrate problem.
Please help me to determinate what`s wrong here ?
My setup:
Core - Amidon FT140-A-F (Area of core 0.968cm^2, A window 4.155 cm^2, Mean Lenth 9.252cm)
Primery - 2X5 turns 3X18AWG
Secondary - 2X28 turns 18AWG
F=50000Hz
MOSFET`s - STP80NF06 2+2
Diodes MUR1520G 15A 200V 30ns
Capacitors : input - some cheap caps 6X 4700 uf 16V, output - currently 27000uf.
Attached Files
File Type: zip psu.zip (14.0 KB, 434 views)
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Old 1st April 2008, 02:00 AM   #2
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Can you post a schematic?
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Old 1st April 2008, 08:08 AM   #3
Tolik is offline Tolik  Israel
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First - thank you so much for trying to help !

The shematic attached is for power stage only. Please notify me if you wont to see the cshematic of control stage, or pcb images.
I think about several possible reasons of my problem:
Core too small for 350...400W ?
Area of windings (total cross section of wires) too small ?
Bad quality of main primery capacitors (6X 4700uf 16V are no brand, but 4X 1500uf 16V are the best motherboard caps from Nichicon)
Bad PCB layout ?
MOSFET`s are connected to SG3525 outputs directly ? (now I ordered UCC27324 drivers to improve situation)

What you think about it ? Any you suggestions will be much appreciated.
Attached Files
File Type: zip power_st.zip (16.3 KB, 319 views)
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Old 1st April 2008, 11:57 AM   #4
luka is offline luka  Slovenia
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Hi

I would say your input voltage isn't constant... on your pics I can see it drops a lot, you still have a lot of ringing...
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Old 1st April 2008, 12:49 PM   #5
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Can you post control schematic also?
Are you running a push-pull in voltage mode? If so, that's a no no- how do you handle flux imbalance?
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Old 2nd April 2008, 05:20 AM   #6
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Quote:
Originally posted by switchmodepower
Can you post control schematic also?
Are you running a push-pull in voltage mode? If so, that's a no no- how do you handle flux imbalance?
He's running it un-regulated, so thats not a big deal then.

When you have core saturation, you will see (and maybe hear the transformer) very large spikes on the leading egde of the waveform.

Try a larger value resistor for your mosfet gates 10 -22 ohms and see if that helps
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Old 2nd April 2008, 08:14 AM   #7
Tolik is offline Tolik  Israel
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Default Thank you all for trying to help !

Hello guys,

As ifrythings told, currently control stage is un-regulated for simplifyng of test procedures - I can play around Fsw, dead time, & duty cycle. Generally was SG3525 selected because of low price - I don`t afraid to burn some chips during tests.
In final version of my SMPS I`m planing to use current mode operation. What will be better UC2825 or UC2856N for this purpose ?

Iifrythings,
The transformer not produsing neither noice nor heat, it running at may be 5..10C* above room temperature, but spikes are up to 50v as seen on screenshots.

Switchmodepower,
Absolutley agree with you, disadvantages of voltage mode control for push pull topology are well known. Just I`ll fix the power stage, I`ll start to design current mode control block based on one from the chips specifed above. I wont to make powerful & relable, like military grade SMPS, available for DIY community.

Luka,
Now I using 12V 16A rated output from old 400W PC`s SMPS because I don`t have another power source. Is it able cause such problems ? Is simple mains transformer (250-500W) based power supply will be better here ?

To all,
I`m able to work around this my project at weekends only due to my very busy job- 12..14 hours per day, so results/updates of testing on prototype I`ll be able to post mainly at fridays - saturdays.

Chematic of control attached-
Attached Files
File Type: pdf schematic prints.pdf (11.5 KB, 377 views)
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Old 2nd April 2008, 08:42 AM   #8
luka is offline luka  Slovenia
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Hi

Looking at your gate picture, the one @ 200w load, you have only ~8v on gates. How did that happen? I mean I have supply for SG connected to input of smps, so if input is 12V, gates signal will be just little bit lower, no where few volts lower.

Oscilations that you have on drains @ 200w are I think just after turn ON of fets, leakage inductance creates those spikes, use of filter will make them much more smaller.

You should put filter on seconday too, for even better performance once you even get it. I would use freq. in 20-35kHz range, this will help all spikes, oscilation,.. you have.

Use of unregulated, or regulated, just to keep low and no load voltage to what you want, but unregulated will perfor better then you think...

If you could explain how did you wind your trafo and how many of what wires will maybe help too.


Quote:
Luka, now I using 12V 16A rated output from old 400W PC`s SMPS because I don`t have another power source. Is it able cause such problems ? Is simple mains transformer (250-500W) based power supply will be better here ?
It doesn't matter what you use, but your 12v 16A supply probably isn't enough, since whis is only 192w IF voltage is really 12v then and IF current is really 16A... with this supply you will be able to test probably no more then 170w of load to smps... Check your input voltage and at the same time output...you will see that transformer voltage ratio won't change much
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Old 2nd April 2008, 08:50 AM   #9
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Default Re: Thank you all for trying to help !

Quote:
Originally posted by Tolik


Now I using 12V 16A rated output from old 400W PC`s SMPS because I don`t have another power source. Is it able cause such problems ? Is simple mains transformer (250-500W) based power supply will be better here ?
The problem with PC power supplies is that the 12V line is loosly regulated, the 5V is the main regulated line. I've seen the 12V get pulled to ~10V before the UVLO kicked in.

My main guesses right now would be

1) What is the input voltageage at full load (As Luka stated)
2) Hows your transformer wound (As Luka stated agin)
3) Do you have thick short traces from the mosfets to the transformer?

P.S: try exparimenting with different gate resistor values too.
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Old 2nd April 2008, 09:19 AM   #10
Tolik is offline Tolik  Israel
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Hi ,

I think that input voltage drops to about 8V. I`ll chek it exactly.

Transformer -
primary winded directly to core, 3wires 18AWG (about 1.08 mm diamiter) per each half. Wires not twisted. Both half windings, and all wires are placed wery close one to other.
Secondary plased ABOVE primery such way, that it fills all spaces between primary turns. Some of secondary turns plased above primary wires. Secondary contain 2 wires 18AWG.
Primary - 5+5 turns, secondary 28 turns.
I`ll try to make good quality pics.

PCB - I`ll post the images tomorrow.

I tried to go to 24-30 kHz, with almost same results. I`m preffering higher F sw @ lower Bmax to get more power from the trafo
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