OK, start with this power supply....

http://www.bgmicro.com/index.asp?PageAction=VIEWPROD&ProdID=12940

24VDC 6.5A SMPS power supply

Basically I have two of these. I'm thinking of making 2 separate regulators (both current and voltage, adjustable) to turn them into variable power supplies. I have an idea how I would do it, but how would you go about it? Basically I'm fishing for ideas to see if something better turns up.

Basically I want a constant voltage / constant current type regulator.

I may go linear, but then again, SMPS is much cooler to run. At 6.5A these puppies will get a bank of series pass transistors very, very hot. I also want them small as I can get by with.

 

I wonder why no one is replying.
Do you like to know what I think on this concept

Rifaa

 

It really depends on the controller they have in there to do the PWM! I would first check (and hope) for a Unitrode (TI) controller like the 384X line. They are relatively easy to understand and pretty reasonable to adjust the output voltage with. Basically they have feedback whose resistors you can adjust to make it variable.

If you adjust the output voltage down I think you'll have to live with the fact that it will likely only supply 6.5A at lower currents. I assume they set the overcurrent protection at the rated voltage and won't give you 156W at all output voltages.

As far as converting to a constant current power supply... that I think would be pretty difficult to modify the internal control for and may have to be external. Do let us know what you find!

 

Next to impossible. ( to CL )
If you could redesign the primary, then it is possible.
For current limiting I would stick to linear regulation with switched secondary to lower power dissipation

Rifaa

 

I'm not interested in modifying the base unit. It provides voltage and current, that's all I want it to do. An external regulator is a pretty easy build, what makes it a challange is if I have a 3V ouput at 6A the regulator will be disappating around 126W, which is a hefty figure. It is one of the reasons I'm thinking SMPS mode regulator. I'll live with the noise.

By going X2 on the build I can have double the voltage and a ± output besides.

The regulator will not have to go to 0V. I figure 1½V is low enough, and 1A as a low end current will work too.

I wonder why no one is replying.
Do you like to know what I think on this concept

Rifaa

Of course! I didn't exclude anyone here.

I don't take it personal. People don't reply when they don't feel comfortable giving an answer. I didn't say this was an easy project.

 

When you're dealing with that much current, might as well look at a switcher solution.

Unless you need another room heater.

Besides, you've been wanting to play with MOSFETs, right?

 

Voltage control in the realm of switching is easy, but constant current seems like it would be a bit harder, since I've never done it. Ideally the control for both would be the same MOSFETs.

Since a lot of the stuff will be audio I'll probably go with something above the range of hearing (22Khz or so).

 

Hey Bill,

I'll take a look at some controller chips today for current regulation. I think there are some that purport to be able to do this. I think it's usually an isolated flyback converter with a third winding on the transformer to be able to sense the output current.

Resonant converters also almost by nature are current source circuits, so that may also be an option. It's trickier to find components for this though!

 

I think everyone agrees that integrating current limit control in a already built SMPS is really tricky.
But I think this can be done if you can find the schema of this supply, after studying it one can only clarify, don't you think?

Rifaa

 

Again, I'm not modifying the existing SMPS, I'm building an external regulator. One with full features. Think of the power supply module as transformer, diodes, capactor, and a preregulator. There is a good chance the modules will be outside the box entirely, like a wall wart.

 

I haven't been able to find any PWM chip reasonably suited to a current source converter. If you build a flyback with a primary, secondary and sense winding I think you may be able to use a UC384x-like chip (love these things) to do it. If you use the sense winding as the input to the error amplifier you should be able to control the average current, it just may not be a quick feedback path, since you'll need to filter the switching frequency out.

If you don't want to use a sense winding or do an isolated converter you can get a somewhat decent "current source" by just using a large output inductor and getting a controller that does peak current control (my old friend the 384x does this). With this you'll just have to deal with a converter that's slow to respond to transients and won't give exactly the same current for all loads.

Either way you go you'll want to provide some open circuit protection. Maybe a ~50V zener connected to a shutdown circuit.