Obtaining 250V, 500mA

[Chris6100]

Hello, I'm attempting to design a physically small power supply (as small as possible) for the power rails of a class D amplifier. This would require around around 250VDC and 500mA from a DC source of between 2 and 24V. Although less current can be tolerated intially - say 60 - 100 mA.

I first thought of generating an AC signal and using a cockroft walton stack but this generates an output voltage which is too high and the parts would have to be too big for a reasonable signal. I also realised that the oscillator and voltage increase can be provided by something similar to a ccfl power supply and then rectified. This is the route I am thinking of using at the moment with a circuit similar to the one on page 34 of this link: http://cds.linear.com/docs/Application Note/an65f.pdf

Again the problem is that the output voltage required for ccfl's is too high ideally and the current requirements are too low. I am wondering if it would be possible to modify the circuit to keep the power roughly the same but lower the voltage and increase the current and if so, how I would go about doing this?

Alternatively, does anybody have any other methods which would fit my requirements? Perhaps just a decent DC-DC converter exists - I'll look into that now. I would ideally like the outcome to fit in a PDA or small laptop to give an idea of size. Smaller would be better and slightly larger is permissable if small is not possible.

 

[BobK]

A power supply of your specs would not be easy. At 2V input you are talking about 62.5 Amps if the efficiency was 100%, more like 75 Amps in the real world. You cannot scale this up from a supply designed to give 5ma, the design criteria are completely different.

Try to find something much closer that that as a starting point. You will want to look at power inverter circuits, they would be more attunted the power needs. But then you are not likely to find one that will run on the wide input range you want.

Edited: It would help if you explained what you are trying to do and why you need the specs you have stated.

Bob

 

[gorgon]

A 150W power supply is never small in my experience, and with these criteria it's bound to be on the upper side of a normal size.

Is there a particular reason that you need to drive the amp from 250V? why not go for a more doable value. 40V should be possible and with 3A you should have the same power. Even that will stretch the input range, but if you raise the lower limit some volts there may be ready chips that can do this.
If you are talking stereo you could split the supply i two and get an even more palatable solution, 40V@ 1.5A should be doable, with a little search effort.

TOK

 

[Chris6100]

Thanks for the replies. The amplifier needs to amplifiy a 5v signal to around 150 V therefore the 250V rails seemed reasonable to work with.

I think I mislead you with the input range. It is more a case that the input voltage is not actually that important, and any value between 2 and 24V would be fine. It can be fixed at one voltage though. Also I fear I did not make myself clear about the power output - since the input voltage does not need to have a wide range, a constant power output is not important - only providing the necessary voltage and ideally the current requirements to the amp.

Unfortunately I cannot explain the final application of the design due to NDA's but I don't think it will be needed.

 

[gorgon]

Shoting from the hip. If this has something to do with a 150V line driven speaker systems for audio sound, the normal way can be to use transformers to get the 150V.
A good torodial transformer will also isolate the driver from the line.

TOK

 

[BobK]

Well, a fixed input voltage makes it a lot easier. I would look up inverter circuits in the power range you are interested in. You will find them that operate off either 12V or 24 volt DC. Tweak the circuit (probably just change the transformer) to get the voltage you want out. Then just use a bridge rectifier and smoothing capacitors to get back to D.C.

Bob

 

[Chris6100]

Thank you - I'll look further into inverters. Are there any particular types which you'd recommend?

The application isn't a line driven speaker system but it is a good analogy and the electrical requirements are pretty similar! Except size is very critical in my application which is proving a bit challenging!

 

[duke37]

A voltage multiplier would not necessarily produce too high a voltage, it depends on the number of stages. It is unlikely to be small and will have difficulty producing a high current.

Inverters have been used for many years to produce anode voltages for valves from a 12V source.
You can make one with a 4047 oscillator, two fat FETs and a high frequency transformer followed by a rectifier. Traditionally a TV line output transformer core was used for the transformer. The higher the frequency the smaller it will be but will be more critical.

You will need to define the input voltage.

I am making a 12v to 300V HV source for a radio 62 set at present. (Photo attached of work in progress)