Electronics > Beginners
Good topology for a capacitor charging power supply
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Gyro:

--- Quote from: soldar on April 07, 2019, 11:23:46 am ---Unless I am missing something this seems extremely simple. By rectifying the 230V mains you can get up to 320 V. Using a voltage doubler up to twice that. So having a capacitor charged to any of those voltages is trivial. That is your starting out DC voltage.

Now from there you want to charge a capacitor to a fraction of that voltage? Just use a resistor and a switch that cuts off when the required voltage is reached.

What am I missing?

--- End quote ---

Given that this is the Beginners section, a mains isolating transformer. However given the hazardous amount of energy being stored in the capacitor, an easy mistake to make.
soldar:

--- Quote from: Gyro on April 07, 2019, 04:30:21 pm ---
--- Quote from: soldar on April 07, 2019, 11:23:46 am ---Unless I am missing something this seems extremely simple. By rectifying the 230V mains you can get up to 320 V. Using a voltage doubler up to twice that. So having a capacitor charged to any of those voltages is trivial. That is your starting out DC voltage.

Now from there you want to charge a capacitor to a fraction of that voltage? Just use a resistor and a switch that cuts off when the required voltage is reached.

What am I missing?

--- End quote ---

Given that this is the Beginners section, a mains isolating transformer. However given the hazardous amount of energy being stored in the capacitor, an easy mistake to make.

--- End quote ---

Yes, I guess I should have said so.

On re-reading I see I also missed the essential spec where he wants to charge from a low voltage battery so what I was suggesting is out.

Here is a post of mine where I included a couple circuits from flash chargers.
David Hess:
Current mode converters are automagically current limiting although you may want to configure a specific current limit.  Many switching battery charger ASICs include a separate current limit and might be amendable to using a flyback transformer reducing the voltage demands on the switch.  I think a SEPIC controller can operate this way and I know some have separate current limiting.

The more powerful configuration that I would use when charging from a low voltage DC source is a current limited buck converter driving a push-pull output transformer.  Some off-line switching regulators work this way.  Output current is continuous and the transformer operating on a current instead of a voltage is inherently balanced.
Buriedcode:
I have built similar things for medium voltage (200-400V) cap charging, and used flyback topology.  It generally lends itself better to cap charging, as its pretty much a current source out of the transformer/coupled inductor.  It can handle shorts on the output, and allows for isolation if you use primary side sensing.  There are many dedicated devices to control it, and you can control the input current, and charge time by adjusting the peak current and off-time.  It scales up nicely by I've only ever built one up to 50W.
Berni:
For capacitor charging flyback topology is usually the best because it works well over a large output voltage range and doesn't mind its output being shorted.

A ZVS oscillator circuit can be used too, but current limiting it can be quite tricky. Softening of its output is likely best done by adding a inductor in series with the transformer secondary, otherwise it will try to draw a massive current if the output is shorted. The troubles of keeping the ZVS cirucit under control are only really worth it if you need a lot of power (over 50W) due to its incredibly low switching losses, or you want the circuit to produce a very clean, noise free output.
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