Rapid charging of a 400V 11000uF capacitor.

[TheForgottenKing]

Hello everyone,

I'm in need of some sort of charge circuit that takes a low-voltage high-current input (think something like a 6S 100C lithium battery, the kind that outputs ~25V at hundreds of amps) and uses this power input to charge a 400V 11000uF capacitor extremely quickly (i.e. under half a second). For safety I'd like to charge it to ~380 V or 800 J of energy.

If my math is correct, a 4A constant-current charge that cuts off at 400 V will be enough to charge one of these in approximately half a second (1600 W into 800 J in half a second). I highly doubt there's an off-the-shelf component that can even remotely do this.

For testing, I bought this DC-DC boost converter, but it never rises above 3A of current even though it should be able to do 20. Not sure why, either I'm doing something wrong or the listing is wrong.

By the looks of it, I'm basically going to be building a DC-DC power supply, but I've never done anything like this. Is this even possible without spending thousands of dollars?

For what it's worth, I'm not opposed to doing anything shady. Ideas include putting a bunch of low-voltage constant current sources in series, or using constant-current AC LED drivers with a rectifier.

[HackedFridgeMagnet]

Maybe a bridge rectifier coming off mains.

Use a variac if you want an adjustable final voltage.

[TheForgottenKing]

Maybe a bridge rectifier coming off mains.

Use a variac if you want an adjustable final voltage.

This is meant to be portable off a DC source, but fear not, I've come up with a terrifying solution, namely reverse engineering one of the linked boost converters, simplifying the design, and then either correctly recreating one to put out 400V or just putting four of them in series.

[ArdWar]

...it never rises above 3A of current even though it should be able to do 20. Not sure why...

With boost converter the rating and the limiting factor is usually for input current. Make sense that a "30A" device doing 25V->100V to output 3A, allowing for 50% derating for what's look like something more like aliexpress specialty crap. Just go get bigger one.

[NiHaoMike]

Take the DC/DC stage from an inverter or UPS and mod it to work in current limit mode. Would take quite a bit of electronics knowledge.

What's the application? I'm thinking a really powerful photography flash or pulsed laser, except such a large flash tube would need quite a bit more than 400V.

[jonpaul]

OP beware of extreme safety risk.

At 400VDC even 1 j or 1 uF can be shocking.

The thought of a 400V 11miliFD is frightning 

Jon

[Someone]

OP beware of extreme safety risk.

At 400VDC even 1 j or 1 uF can be shocking.

The thought of a 400V 11miliFD is frightning 

Jon

Safety standards put 400V 100nF as still dangerous to life!

[Wolfgang]

400V/11mF is a preferred method of seriously shortening your finger if you are silly enough to touch it 

[jonpaul]

Please can the  OP to state their level of experience and actual project end goal?

In 1970s..1990s,  we worked  with flash , radar pulsers and exploding wire HV stored energy.
 if you crowbar a large cap  can blow the tip off a large screwdriver if you crowbar a large cap.

Shock caused muscle contraction leads to jump/falls and  secondary damage.
Besides shock and burns, those who have pacemakers are especially susceptible.

Use the rules of Nikola Tesla (NOT THE CAR!!)  Keep one hand in the pocket, wear thick solde rubber shoes.
Add a bleeder reisistor across any HV cap or alrge lytic > 200V

Jon

[jbb]

At these energy levels, I also recommend safety glasses for flying debris and ear protection. Any shorts etc. will be quite violent.

Also, the thought occurs that normal-rating electrolytic capacitors may not perform well (ie die early and violently) if used for pulse discharge operation.

In terms of topology…
- What’s your minimum expected voltage on the cap bank after firing?
- I would recommend an isolated topology if possible so you don’t accidentally energise the battery pack to 400V
- I suggest you have a look at https://magna-power.com/learn/white-paper/current-fed-power-processing for a nice rugged topology (see Figure 3)