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Old 23rd July 2005, 10:27 PM   #11
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Thanks for the warnings. I don't think I'll be attempting an off-line SMPS for a while though.

Is around 30-40VDC input an ok place to start for a SMPS? Or should I go lower than that?
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Old 23rd July 2005, 10:54 PM   #12
Eva is offline Eva  Spain
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If you haven't done it already, you may try classic 12V push-pull car-audio SMPS stuff as an starting point. First you may try a simple unregulated topology. Then you may try to add regulation and mess with filter and transformer design, current and voltage feedback loops, etc.. There is a lot to learn here and this knowledge will be useful later on for off-line designs.

Do you own a car?

Also, don't you have old transformers, heatsinks and filter capacitors scrapped from audio gear? This kind of stuff will be very useful.

Currently I'm experimenting with average current control. It allows for precise current limiting, current sharing between several independent SMPS and very fast transient response. The power switching section of this prototype (full-bridge transformer-coupled buck converter) is mature, reliable and protected by cycle-by-cycle current limiting, so I'm using 200V AC (isolated) for testing (about 280V DC). Actually most of the trouble I'm currently dealing with comes from the control loop and the requirement for an isolated DC current sensing mechanism. I have just discovered zero-flux current transformer operation (invented by someone in 1969). Note that my control circuit is also isolated from the primary switching side, so I have double insulation in my breadboards.
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Old 23rd July 2005, 11:16 PM   #13
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So maybe a car amplifier power supply? Have a car, and I also have a good amount of equipment to get parts from.

Alright, so I'll give an unregulated car SMPS a go. Gonna go find some schematics and PCB layout examples to start, and go from there.

Thanks for your help!
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Old 24th July 2005, 12:07 AM   #14
Eva is offline Eva  Spain
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This is a simple and small unregulated push-pull power supply. It takes 12V DC as input and produces two independent, perfectly isolated and carefully filtered 12V DC 1.5A max. outputs. The layout is somewhat optimized for EMI reduction.

I developed it to break ground loops in multi-media car systems where you have at the same time a DVD player, a TV tuner, one or more TFT screens, etc..., thus removing unwanted audio and video parasitistics caused by badly designed equipment.

Board layout:
Click the image to open in full size.

Three boards just after being etched:
Click the image to open in full size.

More pictures:
Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.

Finished unit:
Click the image to open in full size.
Click the image to open in full size.


Parts list:

1n C9,C10,C17
1N4148 D9,D10,D11,D12
1u 63V C7,C8,C13
2k2 R11,R12,R15
6k8 R10
10n C14,C15
10u 35V C18,C19
10uH L1,L2
11DQ6 D2,D3,D6,D7
11DQ6 D1,D4,D5,D8
22 R5,R6
22k R13,R14
22u 35V C16
47 R7,R8
68 R3,R4
68uH 4.7A L3
100 R9
100p C5,C6
220 R1,R2
470u 35V C1,C2,C3,C4,C11,C12
SG3525 IC1
BC546B Q3
BC556B Q4
IRFZ48V Q1,Q2

switching freq.: 120Khz aprox.

winding notes for magnetic components:

T1 :
NTF 16 (16mm o.d. high permeability ferrite core, probably 3E25 mat.)
pri : 12T + 12T 0,5mm 28cm
sec : 13T + 13T 0,5mm 30cm

T2 y T3 :
NTF 16 (16mm o.d. high permeability ferrite core, probably 3E25 mat.)
20T + 20T 0,6mm 48cm [21T actually]

L3 :
grey-white 16x9x7mm iron-powder core (unknown mat.)
68uH 4.7A 26T 0,6mm 55cm 0,036ohm
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Old 24th July 2005, 12:33 AM   #15
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Thanks Eva. Unfortunately, your pictures don't appear to be coming up for me. Don't know if it's me or the server.

Sounds like a great use for a SMPS though. Still trying to learn, so let me ask a question: you have an extra winding on the secondaries of your transformer. Is that extra winding there so you can still get 12 volts out while leaving some down time between switches to not create a short circuit on the primary side?

Thanks for the part list! I will be referring to it for sure! BTW, where do you typically go to get cores?
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Old 24th July 2005, 12:34 AM   #16
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Update: now the pictures are coming up! Must have been me hitting a wrong button or something.
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Old 24th July 2005, 12:54 AM   #17
Eva is offline Eva  Spain
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The lack of images was my fault.

There are no additional windings, there are just a center-tapped push-pull primary and two independent isolated secondaries.

I get these ring cores in a local store, they are somewhat cheap but their manufacturer and model are unknown so there are almost no specs for them. I had to learn to measure by myself permeability and energy storage for iron powder cores, and volts*seconds/turns product before saturation for ferrite ones, so I can design with them.

In Europe I have also bought magnetic components from Farnell, Conrad Electronic (Germany) and RS-Amidata.
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Old 24th July 2005, 01:46 AM   #18
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Unless I'm mis-reading it (very good possibility FYI), you listed the primaries as having 12 turns, and the secondaries as having 13?

Sounds like learning to measure the permeability of the cores I use is something I'd like to do (especially so I don't design a PSU that saturates the core I'm using, right?).
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Old 24th July 2005, 01:59 AM   #19
Eva is offline Eva  Spain
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Quote:
Originally posted by rjon17469
Unless I'm mis-reading it (very good possibility FYI), you listed the primaries as having 12 turns, and the secondaries as having 13?
This is because the output of each secondary winding has to be rectified and there is 500mV approx. voltage drop on each schottky rectifier under maximum load (1V in total), and this voltage drop has to be compensated in order to get back 12V output.

Quote:
Originally posted by rjon17469
Sounds like learning to measure the permeability of the cores I use is something I'd like to do (especially so I don't design a PSU that saturates the core I'm using, right?).
Right.
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Old 24th July 2005, 02:11 AM   #20
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That makes sense.

Ok, so right now I'm looking at and reading through the car switchmode power supply on the ESP pages. I think I'll give something like that (a lower power/voltage version) a go first.

Thanks for your help!
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