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Old 7th April 2008, 12:16 AM   #11
zilog is offline zilog  Sweden
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Quote:
Originally posted by switchmodepower
First thing fix those waveforms- they look just as sloppy as your schematic. They look horrible. Get rid of the ringing.

As far as those waveforms not being in sync. Measure across primary fet (as you have) and measure across pins 4-6 of xfmr. If the scope does not like it, measure fronm 4-5.

It's not the ferrite material.

The thing is veroboarded during prototyping - is it possible to even achieve clean waveforms with these currents on veroboard? The thing goes haywire above ~40A peak primary current.

Btw, the picture is for one of the primary fets, and pin 4-6 of the xfmr. The primary side does not have much ringing, but the secondary does. Note however that there is not even any ringing on the secondary during the first part of the on-period.

The schematic looks the way it does since the CAD program is so horrible to use, that I simply cannot stand using it..
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Old 7th April 2008, 12:23 AM   #12
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I wouldn't even attempt to hand wire on veroboard. Go right to a good pwb design- large gnd planes, low inductance traces, etc.. If you have to use flat foil whenever possible to reduce inductances. Component placement is the most important. Do you have a pic of the layout?

Just for a sanity check can you measure across 1-3 and 4-6. They should match up.
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Old 7th April 2008, 12:42 AM   #13
zilog is offline zilog  Sweden
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Quote:
Originally posted by switchmodepower
I wouldn't even attempt to hand wire on veroboard. Go right to a good pwb design- large gnd planes, low inductance traces, etc.. If you have to use flat foil whenever possible to reduce inductances. Component placement is the most important. Do you have a pic of the layout?

Just for a sanity check can you measure across 1-3 and 4-6. They should match up.

I have lots of more things that arent even designed yet, thats why I stay on veroboard. I might not be able to test the high power stuff, but I should at least be able to sort this power level out.

pin 1-3 and 4-6 do NOT match - the primaries have voltage for a heck of a lot more time than the secondaries have, this is what disturbs me.

Well, I have a newly wound transformer sitting on the table waiting for testing, just gotta find time to do it.

As for a picture of the setup, this is the most recent: setup

EDIT: horrible spelling due to epoxy on fingers
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Old 7th April 2008, 02:02 AM   #14
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Well 1 thing I see wrong is your transformer is optimized for high leakage inductance. I assume that large toriod is your transformer? If so you shouldn't have the windings split like that. No wonder why you have all of that ringing.

A toroid is probably not the best xfmr to use but if you want it it must be wound bifilar or layered on top of each other. Also you should really find out what material it is since you get get a 26mu mpp core or a high perm and they look the same.

And for your cs xfmr you should be using a high perm core like 10000u not 3F3- it will add error if you go to current mode.

Here is one mistake I made a long time ago that had me driving crazy. Make sure that the channels on the scope are sync'd- some scopes allow the channels to be offset in time. To test it just put both channels at the same point.
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Old 7th April 2008, 04:42 AM   #15
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yeah optimise the magnetic components for what purpose and frequency you are running. and optimise the winding of the transformers.

If do this it will improve the supply's performance and make it easier to track down problems.
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Old 7th April 2008, 07:30 AM   #16
zilog is offline zilog  Sweden
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Quote:
Originally posted by switchmodepower
Well 1 thing I see wrong is your transformer is optimized for high leakage inductance. I assume that large toriod is your transformer? If so you shouldn't have the windings split like that. No wonder why you have all of that ringing.

A toroid is probably not the best xfmr to use but if you want it it must be wound bifilar or layered on top of each other. Also you should really find out what material it is since you get get a 26mu mpp core or a high perm and they look the same.

And for your cs xfmr you should be using a high perm core like 10000u not 3F3- it will add error if you go to current mode.

Here is one mistake I made a long time ago that had me driving crazy. Make sure that the channels on the scope are sync'd- some scopes allow the channels to be offset in time. To test it just put both channels at the same point.
I just dont see how I should be able to wind the toroid in any other way - the wire is just impossible to bend, that's why I attach turn for turn using epoxy on the prototypes.

I have already made that mistake with the scope once - its checked and not happening now.

The CT:s are running very low volt*time products and inductande is reasonable high, and shouldnt add that much error, I have double checked using 3E25 ones, and no visual difference with the currents I run.
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Old 7th April 2008, 07:43 AM   #17
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just takes a bit of practice.

first transformers i wound were ei core ferrites. quite easy.

toroids are difficult to wind. If you are finding it hard to bend the wire just use a group of thinner strands of wire. It reduces lossess at high frequencies aswell. notably the skin effect...
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Old 7th April 2008, 09:17 AM   #18
zilog is offline zilog  Sweden
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Quote:
Originally posted by guitar_joe
just takes a bit of practice.

first transformers i wound were ei core ferrites. quite easy.

toroids are difficult to wind. If you are finding it hard to bend the wire just use a group of thinner strands of wire. It reduces lossess at high frequencies aswell. notably the skin effect...
I am using 6x0.6mm wires for the prototype. Guess I will aim higher for the "real" one some time in the future. You done have any guidelines on how I should arrange the windings? I see from other people's smps:es that my toroid looks quite different.

Should I space out the 3+3 primary turns to cover more degrees of the toroid, or should I split the primary windings into several 3+3 turns that cover different parts of the toroid, that are coupled in parallell through the PCB?
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Old 7th April 2008, 12:59 PM   #19
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The second sounds like the way to go for sure.
It will decrease esr losses and promote better coupling.

Just as long as it doesn't increase the flux density over using a single 3+3 primary.

That is a question someone else may be able to help you with!
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Old 7th April 2008, 01:52 PM   #20
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The key to reducing leakage and have better coupling is reduce the spacing between primary and secondary. You can add more wire but just use thinner. You can also add more windings in parallel. So instead of winding 1 winding qualfilar you can have several primarys using thinner wire and imbed prim/sec/prim/sec.

However, no matter how you wind do in a toroid, performance will always be better in an EI, EE,PQ, etc core, unless you are designing for resonant mode.
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