Voltage Multiplier

[Mayur]

Dear Dave I want to know by using the voltage multiplier circuit up to how much voltage we can multiply. Currently I was asked to design a circuit by which I can produce 300 V DC by just giving 20 V ac input so I was looking for such thing and I came to see your multiplier video on the blog. So can I do it using the multiplier circuit, is it possible. I would also like to then which type of capacitor and their rating and range. It would be very helpful from you if you inform me about this

[penfold]

I know you've addressed Dave in your question, but I'll throw a few comments in anyway.

In general, yes you can produce any voltage you want using a voltage multiplier, however, there are practical limitations to what is a sensible option.

The greater number of multiplication stages you add increases the output ripple for a given current draw.  As a general rule, 6 stage stages are quite common with anything above or below being less frequent.  You can reduce the ripple by increasing frequency, however this has practical implications on diodes etc.

Your capacitor will need a relatively low ESL but a further consideration will come from the physical size of the capacitor which meets your requirements for voltage and capacity as this will affect your physical layout which can turn a nice little circuit into a monster.  There will be nice big loops with quite rapid rates of change of voltage and current that are a pig for EMC.

[Mayur]

I am very thankful to you penfold I didnt knew it was an open forum thats why I addressed Dave so can you suggest any alternatives to this. The main complication is that for the AC input I cannot use a transformer so I am using a function generator to give the 20Vp-p square wave If you have any suggestion then I would be more than happy

[penfold]

If you want specific advice I'd recommend you tell us a bit more about your intended application and requirements.

Initially I'd recommend you just build a chain of multipliers that gives the desired voltage then see where the performance is letting you down.

I would generally use a switched mode transformer stepping up no more than 10 times and a chain of multipliers to make up the rest.  That kind of arrangement is relatively forgiving but it does give you a tradeoff of number of multiplier stages vs diode performance (small number of stages with large reverse voltage on diodes or high number of stages with lower voltages).

If you have a specific output current requirement then that will give the final piece of the jigsaw and tell you whether you need more transformer step up from the 20V before the multiplier or more multipliers.

[Mayur]

Actually I had been given a project in which it has been said that i will have to use the multiplier circuit which will give a pulsating DC output by giving a square wave of 20 Vp-p input without using a transformer

[penfold]

Well, that's interesting... an ideal multiplier stage will give you a smooth DC output, a realistic one will give you some ripple on your DC but I wouldn't exactly describe it as being pulsating, you could in theory load the output until it began to 'pulsate' but that voltage of the pulsation would be severely reduced

[dannyf]

I can produce 300 V DC by just giving 20 V ac input

Sounds like a nice project for a transformer.

[Mayur]

Yes there will be ripple and a smooth DC output will not be achieved but I want to chop that DC output and then make It a pulsating signal I tried using a SCR and got for a 4 stage multiplier but the voltage is getting attenuated So the desired voltage is not achieved.


The frequency of the input signal is nearly within a range of 100 to 1000 Hz. And within this frequency range i am getting various outputs for the same range can anyone describe me why this is happening.

I made a 10 stage multiplier and given input is 20 Vp-p and the output is around 80 V dc.

Plz help me my project deadline is coming near so i am worried if unable to complete it

Thanks for your support

[Mayur]

Please help me out 

[BravoV]

I find that its so hard to believe and so amazed that you depend and put all your hope for the help only from internet ?

I'm very serious wishing that I'm wrong though.

[Mayur]

BravoV no I am not totally depending on the internet but I am trying to get help from anywhere I can get

I am also trying out a solution myself but my results are not as i am expecting that's why asking for the help from professionals and hobbyist like you all

[minime72706]

I find your multiple posts hard to piece together. Please explain exactly what you need help with. Also are you sure that "pulsating DC" was meant literally in the assignment?

[minime72706]

Hello?

[Mayur]

minime72706 I am trying to apply 20V AC Square wave input and trying to generate a 300V dc output using the multiplier circuit at the output and then i will chop that DC voltage and convert it to a pulsating DC voltage

thak you for showing interest

[Skimask]

I am also trying out a solution myself but my results are not as i am expecting that's why asking for the help from professionals and hobbyist like you all

Well, let's see what you've built up so far, and let the masses help you figure out what is wrong with your solution.
So far, we've seen exactly ZERO from your end...just a bunch of halfass questions...if you can call them that.

[penfold]

There is at least now a little bit of information to base an answer on.
You will find that the capacitors will charge up almost instantaneously (at 1kHz, pretty much instantaneous) so during the remainder of the period, the output will load the capacitors.
This will produce an average voltage lower than what you are expecting.
There is plenty of information out there on the behavior of Cockcroft and Walton voltage multipliers.  So a quick literature search will mostly likely shine some light on the matter.  There is a rather nice description of it by Cockcroft and Walton themselves out there.  Keywords such as dynamic impedance and frequency dependency should get you a bit further.

Now that is about as far as I can get based on the information you've given so far.  There are million and one methods of reducing the effects of the variation of output voltage with frequency, I doubt anyone will take the time to discuss the merits of each of them with you.  You can help people out by providing some more information such that the list can be whittled down to a number of suggestions that could be useful to you.

If you could perhaps provide a schematic of what you've implemented (with a list of the actual component part numbers you used), a photograph of your constructed circuit, some results you've taken so far, and perhaps even your original briefing (it could be that your interpretation of it has sent you in a direction that's not going to produce a good result).

[minime72706]

I am inclined to believe what you say, but something goes off in my head when you say that the assignment is to chop perfectly good, mostly flat, high-voltage DC after the fact to make it pulsating.

If it were me, I would half-wave rectify the AC signal coming in using a couple of diodes before sending the signal into a multi-stage CW circuit.

BTW you'll need some beefy-ish capacitors if your input signal frequency is below 1kHz. Even if you only try to draw a handful of milliamps from the output, the input signal will fail at replenishing the capacitors fast enough rather quickly.

Is this a school assignment? It helps to know because I can try to get into a professor's head to see what he's trying to accomplish.

[Jibby]

Yes