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Switching Power Supply 0-10A 0-25V

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bbjk

New Member
Hey everyone, i need help =(. I've been supplied with a toroidal transformer (18+18V RMS, 4.44/8.88A) and i'm expected to make a variable power supply out it, using a switch to alternate between the turns in series and the parallel, so i figured i'll make two parts.... one 4.44A @ 50V and the other 8.88A @ 25V, ive already got a circuit for the 4.44A @ 50V, just need one for the 8.88A @ 25V..... any tips are welcome too

thx =)
 

smanches

New Member
Post a schematic so we know what you're trying to accomplish. Words are not enough.
 

bbjk

New Member
Well you know how toriods can usually be set up/configured up in two ways... series for max voltage and parallel for max current.... i want my project able to switch between the two... so they will have a common filter stage but different switching regulation curcuits... oh and i need both parts now =(
 

smanches

New Member
You're still missing a lot of details. What's the input? Why do you need to switch from series to parallel? How are you going to get a 10A output from an 8.88A transformer? Why would you use a 36v CT transformer for a SMPS to produce 0-25v?

This is sounding like an ill-concieived class project, as you seem to have a deadline on something you are not quite up to speed on yet. I don't mind helping, but you still have to do the bulk of the work yourself. I think you need to do much studying on SMPS designs.
 
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bbjk

New Member
yeah you're right.... its only a year 11 class project and we don't really know much about it yet... we've only received a couple of notes on linear voltage supplies, about like the different stages of transformation... rectification... filter capactors and finally regulation... our teacher went in depth with everything except the regulation... which i reckon is the most important... i've had a look around for regulation curcuits and found out the switch mode is the way to go, so i googled it and found a couple of guides that start off really simple and then just jumps straight into it.... any ones you can recomend? and also... we're supposed to get the most out of the transformer... so yeah i meant 8.88A... not 10A and the 25 volts comes from 18RMS * 1.414.... same with the 50V (36RMS * 1.414) at 4.44A

thx for helping
 

Leftyretro

New Member
yeah you're right.... its only a year 11 class project and we don't really know much about it yet... we've only received a couple of notes on linear voltage supplies, about like the different stages of transformation... rectification... filter capactors and finally regulation... our teacher went in depth with everything except the regulation... which i reckon is the most important... i've had a look around for regulation curcuits and found out the switch mode is the way to go, so i googled it and found a couple of guides that start off really simple and then just jumps straight into it.... any ones you can recomend? and also... we're supposed to get the most out of the transformer... so yeah i meant 8.88A... not 10A and the 25 volts comes from 18RMS * 1.414.... same with the 50V (36RMS * 1.414) at 4.44A

thx for helping
That part is doable if there are two separate secondary windings (4 wires) each rated at 18vac @ 4.44A. If that is so you will need to switch the secondary windings connections so that they are in series (adding) for the 50V @ 4.44A position and parallel for the 25v @ 8.88A position. You should be able to do that with a DPDT switch. The bridge rectifier and main filter capacitors will have to rated for the max current and voltage respectively.

When connecting winding like this keep in mind there is a winding "polarity" that has to be connected so that the voltages add and not oppose.

Lefty
 
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smanches

New Member
I don't think a switching regulator is going to work in this case. It sounds like you have a power transformer. These will not work with switching regulators which require a very specific inductance of the transformer.

I'm actually not quite sure the best way to handle regulation on a power transformer. Others might have better ideas.
 

tcmtech

Banned
Most Helpful Member
Another alternate set up is to do the two seperate windings as two independent power supplies also.
Then use the series/ parallel control switch after the DC output not before. This could give you two independent 25 volt 4.4 amp sources or one 25 volt 8.8 amp or one 50 volt 4.4 amp with center tap output. This also would make it possible to use 6 amp diodes that do not need heatsinks.
Plus physicaly smaller capacitors could be used as well. There would be more total components but some added functionality.
 

bbjk

New Member
That part is doable if there are two separate secondary windings (4 wires) each rated at 18vac @ 4.44A. If that is so you will need to switch the secondary windings connections so that they are in series (adding) for the 50V @ 4.44A position and parallel for the 25v @ 8.88A position. You should be able to do that with a DPDT switch. The bridge rectifier and main filter capacitors will have to rated for the max current and voltage respectively.

When connecting winding like this keep in mind there is a winding "polarity" that has to be connected so that the voltages add and not oppose.

Lefty
Yep thats exactly what i meant.... but the problem is regulating....
 

bbjk

New Member
I don't think a switching regulator is going to work in this case. It sounds like you have a power transformer. These will not work with switching regulators which require a very specific inductance of the transformer.

I'm actually not quite sure the best way to handle regulation on a power transformer. Others might have better ideas.
Well linear reguation generates too much heat and get really complex for high power applications like these.... i'm looking for something similar to these:
 

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bbjk

New Member
That part is doable if there are two separate secondary windings (4 wires) each rated at 18vac @ 4.44A. If that is so you will need to switch the secondary windings connections so that they are in series (adding) for the 50V @ 4.44A position and parallel for the 25v @ 8.88A position. You should be able to do that with a DPDT switch. The bridge rectifier and main filter capacitors will have to rated for the max current and voltage respectively.

When connecting winding like this keep in mind there is a winding "polarity" that has to be connected so that the voltages add and not oppose.

Lefty
Yep thats exactly what i meant.... but i'm looking for regulation curcuits....
 

smanches

New Member
That transformer is not needed for a SMPS, although maybe it was expected to help protect you from working with mains voltage directly.

In that case, just wire it in parallel to get the current you need at ~25v, then use it as the input to a buck regulator. Which I believe is exactly what the top section of that schematic is. I think the bottom portion is over-current protection, but I'm tired and had beers so I'm not all with it right now. :p
 
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smanches

New Member
The second schematic is a much more "traditional" (and much higher power) buck converter.
 
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bbjk

New Member
ok so obviously i'll need a DPDT switch to switch between two different toroil settings and regulation circuits, how could i adjust circuit 2 to suit my needs? or can someone at least ge me some insight to how it works.... i undersstand the diode bridge and the filter capacitors... but everything afterwards is a blur... i know the concept though of the 555 timer being used to as a pulse with modulator of some sorts to turn the thing on and off at a really high frequency... but thats about it...
 
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smanches

New Member
You should work on the various sub-circuits in this order.

- Variable duty cycle pwm controller using the 555.
- Buck circuit
- Voltage regulation feedback loop
- Over-current protection feedback loop.

Tackling each of them one at a time will be easier than trying to understand it all at once.

For examples and references, just Google. There is a LOT of information out there as these are the most common power supplies in the world by far.

I have to say this is quite a project for 11th year in school. Was this an assigned project that everyone has to do, or did you pick it yourself?
 

bbjk

New Member
thx! thats all i needed, i'll post back as soon as i get stuck. everyone got a transformer of some sorts and they all got given an application for it... i got stuck with the big toroil. A couple of others also had to make a variable power supply, he's only expecting linear though... The teacher isn't all that smart and its a long story but i kinda dislike him at the moment so i really want to prove something... also I thought i might as well make something i could use on all future applications....
 

Hero999

Banned
You can't get 25VDC from an 18VAC transformer, not without a buck-boost or SEPIC converter anyway.

The RMS voltage might be 25.5V but you'll loose some voltage in the rectifier, there'll be some ripple on the output and the voltage regulator will have a certain drop-out voltage.

To get 25V you'll need a switching regulator which can take anywhere between 20V and 30V (the transformer's output voltage will be higher off load) and convert it to 25V. A simple buck converter or boost converter can't do this, you need a SEPIC as mentioned before.

You can't get 8.88A of DC current from a 18V 8.88A transformer.

Once you've added a smoothing capacitor to get the ripple down to say 2Vp-p, the RMS value of the current going into the rectifier will be approximately 1.414 times the current leaving the filter capacitor. This means that the maximum current rating of your transformer plus rectifier is 8.88/1.414 = 6.3A. You need a 12.6A transformer to power your circuit.
 

smanches

New Member
I was trying to keep it easy on him. He said the specs were loose and he just had to get the most out of it. :)

A buck is easy and will still give him the most out of the transformer.
 
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