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Phase Controlled High Current Variable Power Supply

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surfline

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Our current project is to create a variable power supply from 2-30V that can handle up to 15 amps of current. We decided against using a voltage regulator in which the input is 30 Volts (From transformer & Diode bridge) and the output would either stay at 30 volts or drop to the desired voltage through the LM338 voltage regulator. We decided against this because of the excessive power that would be dissipated through that regulator going from 30 to potentially 2 volts at 15 amps of current.

Our alternative method of creating this power supply was to phase control the 110V, 60Hz input, through the use of Triac's, Diac's, and POT's. Essentially we are using a light dimmer circuit in an attempt to reduce the average voltage and power dissipation. We would then send our chopped up AC signal into the transformer -> diode bridge -> smoothing capacitors. Our goal was to be able to control the output voltage through the light dimmer circuit, specifically through the variable resistor (by controlling the average voltage of the AC signal).

The major problem we are having is trying to simulate this through circuit through SPICE programs. We have tried Pspice, hSpice, LTSpice, but our simulation's start getting convergence errors, time step too small errors, and have funky results.

The light dimmer circuit simulated pretty well, however when we connect the transformer as the load instead of a resistor, it doesn't work well. It only phase controls for the first cycle.

Is the concept wrong here or is it just that the software can't handle it? How would you simulate this correctly?

Blue Sin wave = V1 (input)
Red line = voltage at primary winding of transformer
Blue line = voltage at secondary winding of transformer

Thanks.

Surfline

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Phase angle voltage control is very common in welders and other large power control systems. It works very well even into the thousands of amps at thousands of volts power levels. But it needs a feed back circuit for it to work properly.

Your circuit is close but you will need to sense the output voltage and have that control your phase angle system in order to get it to stay stable on its output regardless of the load.

Do a search for Lincoln SP-100 or similar type model of portable welder schematics. They have a simple but very solid phase angle controlled system on the infinitely variable voltage models.

The only schematics I found involved numerous transformer's. Is there one in particular that only uses one transformer ?

You still have the main problem that a light dimmer won't work right with a reactive load like a transformer so you will have to build the mains phase angle control circuit from scratch, then also add an opto isolatror etc to get the feedback like tcmtech said.

I think it would be much easier (and safer) to make a standard type DC power supply, then put a 15 amp buck regulator on the low voltage DC output. A 30 volt 15 amp buck is nothing special with the common 60 amp mosfets you can get anywhere today.

Do you have a recommended schematic or circuit for the 15A buck? I have been looking those up and have found various LTC switching buck regulator IC's that might do the job, but not sure how to variably control the output.

Thanks for all of your input.

What do you think about using the LTC 3713 as the buck regulator? It is adjustable Vout .7 to 36V, supposedly 15 or 20A. https://www.electro-tech-online.com/custompdfs/2009/12/3713fa.pdf

My circuit would be transformer -> diode bridge -> filter capacitor -> LTC 3713.

Do you think this would work? I simulated the LTC 3713 in LTSpice varying the resistors in the voltage divider part to simulate 2~30V. Only problem I saw was the output voltage had a pretty good sized ripple, up to about .7 volts. Attached is an example of what I am talking about.

R4 and R5 are the resistors to adjust the output voltage. Vout=.8(1+R4/R5).

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I was able to find a copy of the welder schematics. Maybe this would help with understanding how a phase controlled voltage regulator works.

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