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Old 15th October 2008, 07:28 AM   #1
nitrate is offline nitrate  United Kingdom
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Question Switchmode Transformer

Hi All,

I'm currently working on a new project... A nastily large classD power amp for my car. Everything is going well so far and i'm working on the power supply for the amplifier. The specs i'm aiming for in the psu are +/- 50V capable of 10A constant ( just in case i need it )

The design of the smps uses common logic and op-amps for the gate signal generation and control and is of the variable pulse width push pull topology with voltage feedback. This part of the psu is fine and works very well. The trouble i'm having is with the transformer. I've been winding my own transformers with a ratio of 1:5 to give me approx 60V flat out, regulated by the variable PWM. The switching frequency is 50KHz. The supply runs as expected, regulating to within 0.1V at all times until i run out of power.

This is were the problem lays. I cant seem to get more that 100W out of my design. I also can't find any useful info on the net about how to wind toroids for switch mode use. I tried a small toroid at first ( about 1.5 inch accross by half an inch tall ). I always use multiple fine strands of wire twisted to eliminate the skin effect ( i'm using bunches of .4mm magnet wire ). This toriod must not have had enough inductance as the PSU drew too much idle current and didnt work very well. On my second attempt i used a toroid twice the size ( 1.7" x 1" ). This worked much better and idle current dropped to zero. The turns were 5:25. upon loading however it appeared i could only get 100W out of it. This was with a 4R load on the output..it gave me 5A but always dropped to around 22V. Off load voltage with no or small loads were all fine, 24 to 60V with excellent regulation until i hit 100% pwm. The wired thing is with a 47R load i can't get over 46V. This equates to about half power ( 50W ish ). I didnt expect that!!

O.K i thought, it must be my sloppy windings. I then spent ages re-winding the toroid with lashings of wire. I used 10:45 ratio this time and loads of strands ( 30 strands for the primary and 8 for the secondry ) to allow for 50A in the primary and 10 in the secondry. All 0.4mm wire. It looked the part so i fired it up.

Arrgh... No difference at all! still stuck at a max of 100W!!

How to i get the power up?? Is it that my toroid is too small? too low inductance or not enough windings?? I know very little about ferro-magnetics and i cant seem to locate any info on the web that explains how different permutations of coil parameters effect toroid functioning in regards to smps design. I've looked on here too and ppl seem to be wondering the same as me. Can anybody enlighten me as to were i'm going wrong here as i'd love to know and i'm sure it will be of great help to others too. Do i really need to use a toroid the size of my hand??

Regards
Leigh
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Old 15th October 2008, 08:52 AM   #2
nitrate is offline nitrate  United Kingdom
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I suppose the questions i should really be asking is

1: Assuming the ratio of turns remains the same how does the amount of turns applied to the toroid effect performance e.g does 5:25 turns yield less current than 15:75?

2: What influence on performance does the physical size of the toroid core have? Does it make a difference in how much current one can stuff through it or does it simply limit the amount of wire i can fit through its hole?

3: How critical is core material selection?? E.G is iron powder better or worse than ferrite for this application?


Just a few questions but hopefully the answers should tell me why i can't shift more than 100W through my current toroid.

Cheers
Leigh
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Old 15th October 2008, 11:17 AM   #3
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The material makes a big difference. I generally use P or F ferrite material which have an initial permeability of ~2500-3000. You should use something similar.

I'd use 4+4 on the primary. It's been the best for the supplies I've built. More turns and you get too much loss from the DCR of the wire.

The size of the core is generally determined by the wire that you need to get on it but larger cores will have less loss and will run cooler. You don't want to use anything too large because the coupling between the windings will not be as good (assuming that they're evenly spaced). A 2" (50mm) outer diameter core should be enough. The Magnetics xP-44916TC should work well. The 'x' is the coating material. Z is the coating I prefer but it's not important.

http://specs.mag-inc.com/Ferrite/ZP-44916-TC.pdf

I generally run these at ~30kHz.

How many FETs are you using?

Which FETs are you using?

Does the primary waveform collapse (viewed on an oscilloscope) when you drive it hard?
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Old 15th October 2008, 11:37 AM   #4
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I'm currently using one pair of IRFP260N to drive the primary's. The primary field looks good under all conditions. When up at 100% PWM the signal is still nice and square with little ringing. I don't know why i'm not getting more power out of the circuit. It must be the core material i'm using. I really don't know anything about magnetics and the cores i use are just reclaimed from junk so the material is unknown.

Thanks for the input Perry, at least now i know i don't need to look for a enormous core diameter.

Leigh
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Old 15th October 2008, 11:48 AM   #5
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The two usual problems are layout and leakage inductance. Could you post pictures of your transformer and prototype?
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Old 15th October 2008, 11:57 AM   #6
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Quote:
The two usual problems are layout and leakage inductance. Could you post pictures of your transformer and prototype?
No problem. I'm at work at the mo but as soon as i get home i'll take some piccys so you can get a better look. The proto is still on breadboard but it's fed by a high current battery connection direct to the transformer and fet sources.
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Old 15th October 2008, 04:24 PM   #7
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O.K here is a picture of my coil and driver circuit. Note the leads supplying a second source of high current directly to the coil and fet's as the proto is still on breadboard and i dont want to pull loads of current through the board....
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Old 15th October 2008, 04:25 PM   #8
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Waveform taken directly from one of primary windings at full load.......( or at least 100%ish duty cycle ). Sorry about the blurr...
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Old 15th October 2008, 04:27 PM   #9
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And again at 50% duty. The regulation is working fine at this point as the control circuit is able to adjust things....
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Old 15th October 2008, 04:28 PM   #10
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Again from the primary winding with a very light load.....
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