Voltage Doubler Supply for Power Amp?

[NiccoloDeiConti]

Has anyone out there used a half-wave voltage doubler for a power amp PS?

I have on hand a 1000VA XF with dual 25 V secondaries; and I need to build a +/- 70VDC supply. Rather than spend more money on a dual 50V XF, I had the thought to put the two 25V secondaries in series and then use a voltage doubler to get +/- 50V RMS, or 70V when the caps charge up to the peaks of the sine waves.

Any caveats/precautions, other than using a lot of reservoir capacitance? Reasons not to do it at all?

Thanks in advance.

 

[Joe Berry]

If you're not already planning to do so, you might consider using a bridge amplifier circuit topology, as this will cut your supply rail requirements roughly in half for a given output power. With a bridge circuit, for example, a stiff +/- 35V DC is good for 200W RMS into 8 ohms, or 400W into 4.

 

[djk]

I go along with Joe, build a bridge amp.

A voltage doubler supply has such low efficency your transformer wouldn't be big enough anyway. Plus the filter caps need to be 4X as big, and the diode surge currents too.

 

[Shoog]

Voltage doublers supply through a cap (i think) and so are used where current is not needed. Bad idea all round really.

Shoog

 

[NiccoloDeiConti]

I go along with Joe, build a bridge amp.

No, I'll just buy the "right" XF to supply power for the amp I'm planning to build.

A voltage doubler supply has such low efficency your transformer wouldn't be big enough anyway.

My command of physics is a little weak; could you point me to a source that will better illuminate the reasons for a voltage doubler's low efficiency?

Thanks.

 

[jackinnj]

when you analyze any voltage multiplying circuit -- you are looking at the analysis of a filter -- if you don't have Multisim, go to the Linear Tech website and download their freeware -- you can quickly determine the tradeoffs between voltage multiplying and the expense of a new transformer.

 

[Joe Berry]

[C]ould you point me to a source that will better illuminate the reasons for a voltage doubler's low efficiency?

Voltage multipliers are covered in most books on power supply design, but I don't recall seeing any that discuss efficiency. The reason may be that traditional voltage multiplier applications don't demand high current delivery along with the high voltage.

It's the high current demand of your application that could make a doubler an inefficient solution. To get anything close to double the normal output voltage, you'll need to use caps of roughly the same size as your main filter caps in series with the rectifiers in your doubler circuit, in order to minimize their voltage drops under load.

 

[Joe Berry]

For those who may have seen it, be advised that I have deleted a comment from the message directly above regarding full-wave doublers, which I later realized was inaccurate.

 

[peranders]

Voltage doublers supply through a cap (i think) and so are used where current is not needed. Bad idea all round really.

A voltage doubler is a very heavy load not only for the transformer but also for the poor caps. AVOID! Bad idea

 

[Circlotron]

A voltage doubler supply has such low efficency your transformer wouldn't be big enough anyway. Plus the filter caps need to be 4X as big, and the diode surge currents too.

Voltage doubler efficiency is the same as a normal bridge rectifier setup IME. I'm not sure why people would say otherwise. You just have to use big caps, that's all. Maybe the caps do need to be 4 times as big, but at only *half* the voltage so the energy per cap works out the same anyway. Diode current is no big deal in this day and age.

I say build it, and if you decide it isn't suitable then just add two diodes and a new tranny. Not much to lose there.

 

[peranders]

Voltage doubler efficiency is the same as a normal bridge rectifier setup IME.

I have personal experience from work which proves the opposite. I thought I was smart when I had a voltage doubler, I wasn't! Since I increased the voltage from the transformer instead we got less trouble with heat. It was a small thing with only a 1.5 VA transformer.

 

[paulb]

My command of physics is a little weak; could you point me to a source that will better illuminate the reasons for a voltage doubler's low efficiency?

A great reference:
http://www.onsemi.com/pub/Collateral/HB214-D.PDF

 

[TonyTecson]

a voltage doubler is just one half full wave bridge, the main draw back is poor output voltage regulation, but if you want to use this you certeinly can...

 

[Circlotron]

Let's assume a reasonable sized transformer, say 300VA, not a flea-power one. Now if a voltage doubler is less efficient than a full wave bridge then the wasted power must be going somewhere, i.e. something must be getting very hot. Does it? What about switchmode psu's like in pc's. They usually have a voltage doubler input for 120vac and bridge for 240vac. Doesn't seem to be a problem there.

Q1. Assuming we have 50vac to a bridge and 25vac to a voltage doubler, what is the approximate efficiency of each?

Q2. Have you ever actually built a voltage doubler or are you just repeating what others have said?

BTW, I am talking about a *full-wave* voltage doubler where one end of the tranny connects to the junction of the two electro's and the other end of the tranny connects to the two diodes, the cathode of one going to the free positive end of one electro and the anode of the other diode going to the free negative end of the other electro.

Half-wave voltage doublers?

 

[djk]

I played with this in a power supply program. Even if I made the caps in the doubler 4X as big it still had less voltage. That counts as a loss in my book.

When a commercial design with a doubler comes in, the first thing I check is the doubler.

Actually, I shouldn't gripe. I've made a lot of money fixing the things over the years.