Power Supply for capacitor bank

[Silent Jack]

I have a capacitor bank of 6x 400V 470uF capacitors in series parallel (2 wide, 3 deep) that I'd like to charge from a LIPO battery pack (most likely 12-14 volts).  Can someone help me with the basics of designing a power supply?

 

[pyrohaz1]

Hi Jack, when I was creating my capacitor bank (8000uF @200v) I was looking for a charging circuit and came across this website: http://sites.google.com/site/uzzors2k/boost_converter

It uses some quite simple components (555 timer, power mosfet and inductor) and as it says on the website, can charge his bank up in 30 seconds. Hope it helps

 

[Silent Jack]

Hey pyro, thanks for the link.  It is interesting, and covers some points that I hadn't been able to find good info on so far.  Your bank has some hefty capacitance, what are you using it to drive?

 

[pyrohaz1]

I was originally going to use it to power an ETG gun but after having some extreme switching problems (couldn't find a thyristor or switching device with anywhere near enough current capability) I scrapped the idea and used the capacitors for different projects.

Thanks to one of the users on this website, I picked up a load of them B stock off Rapid, I was really happy with the results and got some MASSIVE bangs out of creating molten flying aluminium sparks

 

[Silent Jack]

Phooey, had to redesign my project a bunch.  Still a lot I'd like to learn about the basics of power supplies, particularly operating off of a battery pack.  Can anybody suggest a good resource for learning/researching about these, or better still anyone with experience that can share?

 

[Silent Jack]

Inductors vs Transformers
I've heard both terms used.  Are they the same thing, different classes of something or simply a vernacular thing?

555 vs other power regulating ICs pro's and cons?

 

[Hero999]

Inductors vs Transformers
I've heard both terms used.  Are they the same thing, different classes of something or simply a vernacular thing?

Look it up on Wikipedia.

555 vs other power regulating ICs pro's and cons?

A 555 is not a regulator, it's an oscillator, you could use any IC which can be configured as an oscillator and will work up to the required frequency. The  op-amp is the part which performs rough regulation by disabling the 555 oscillator when the voltage is too high.

 

[Silent Jack]

I see you are highly rated HERO.  In a supply for a 400V 970uF capacitor bank powered by either a 12V or 14.8V battery, is there a particular IC that you would recommend?  Also, what is the best way to determine what ratings of other components are needed.  I have already obtained a few low level calculators and have been pouring over a host of equations trying to work various conversions backwards to obtain some needed values.  Perhaps your experience might suggest some?

 

[pyrohaz1]

Hey Jack!
I suppose one of the main points on the charge rate would be the battery capacity/chemistry type of the batteries. For example, the internal resistance of standard ni-mh batteries usually limits them (safely) at 3C, 3 times the capacity of the battery. For example, 1.2v at 800mAh would have a maximum output current of 2.4A with a voltage dropping proportionally.

I think that if you try to charge the capacitors at a current of 1A to 1 time constant (63.2% charge) at 1A it would take about 6.4 minutes (T=RC, T(ms) = R(ohms)C(uF) 1A at 400V is a resistance of 400R, 400R x 970uF = 388000ms OR ~6 minutes)

Unless you don't mind the quite long wait (if I worked that out correctly! XD) I think you will be in for designing quite a high current switching circuit. Even at 4A, the charge time will be 1.6 minutes for every time constant.

 

[Hero999]

If it's constant current then the voltage ramps up linearly.

A 970

 

[Silent Jack]

Does it make any difference that they are two 400V 470uF capacitors in parallel?

 

[Hero999]

Does it make any difference that they are two 400V 470uF capacitors in parallel?

I've given you enough information to answer that yourself.

 

[Silent Jack]

I see the 555 has 3 different modes.  Looks like this application calls for it to operate in the Astable mode.  Is the purpose of the oscillation to create a particular wave form or is it for pulse generation?  Also does it inherently create a given frequency in this mode?  One of the calculators I have includes frequency as part of the equation, and you mention it as well.

 

[Hero999]

Yes, the 555 generates when operating in astable mode.

The inductor needs to be pulsed on and off to generate high voltage pulses for the back EMF to be generated. The frequency will depend on the current, the inductance and input and output voltages. You should not think of the frequency but the on and off times.

Have you read the Wikipedia article on boost converters?
http://en.wikipedia.org/wiki/Boost_converter

 

[Silent Jack]

  I'm finally beginning to isolate some of the specific components based on a shared schematic I found online slowly adapting it to my specific application.  As I figure it out, I plan to ask more specific questions in an effort to optimize the design for my application. 
  One question that has definitely come to mind is do any of you know a good IC for switching a device like this via a PC or microcontroller, such as an arduino?

 

[Silent Jack]

What are peoples' feelings on IGBTs vs MOSFETs in SMPS applications?

Also, I figure I can control device activation with a lower voltage/amperage switch in front of the battery, but what about the actual capacitor bank discharge control?  I'd like to be able to control it from a microcontroller or PC.  Any recommendations on this segment.

 

[Hero999]

As far as controlling it from your PC is concerned a parallel port can be used with no MCU - it's fairly easy to set bits high and low.

Unfortunately parallel ports aren't very common on new PCs so you'll need to use USB which requires an MCU but I can't help you with that.

MOSFETs are better at lower voltages, IBGTs are better at higher voltages, of course I'm talking about the voltage actually switched by the transistor, i.e. the input, not the output voltage.
Here's a few articles I've found:
http://www.irf.com/technical-info/whitepaper/choosewisely.pdf
http://en.wikipedia.org/wiki/Power_semiconductor_device
http://electronicdesign.com/article/components/igbts-or-mosfets-which-is-better-for-your-design-7.aspx

 

[Silent Jack]

Is it better to use an overated component or should they match more closely?  (For example: My circuit will likely operate at 400V and 5-6A or so, should I use a 400V 5/6A MOSFET or IGBT, or would it be better to use say a 500V 10A model?)  I prefer to make things as efficient and durable as possible, but must also maintain a low price.

 

[Hero999]

I thought you said it ran from 12V to 14V?

Using a higher voltage rating is fine but you'll find that MOSFETs with a higher voltage rating will have a higher on resistance.