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Boost converter capacitance

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Thomas

Fri Jun 23 2006, 02:43AM

Registered Member #120 Joined: Fri Feb 10 2006, 07:07AM
Location: Westchester New York
Posts: 83

I decided to fine tune my boost converter with a small capacitance, then once it's running between 30 - 400V I could simply connect the right capacitance to the circuit. I tested it with a small small .1uF capacitor, now, however, when I connect my huge 19500uF bank to my circuit it only gives a voltage range of 30 - 100 about. What gives? The formula has nothing to do with capacitance for the output voltage, why is this occurring for me? Do I have to retune the resistance. I'm actually using a voltage comparator to adjust the voltage, so i'd suspect this to be a problem with the voltage reference in the comparator circuit, so I should play around with the resistance there?

-Thanks for your help

EDIT: oh i see, my voltage comparator circuit is good. After monitoring the circuit it works. It just take about two minutes to get to 150Vdc. I don't like this very much. I think I could fix with the duty cycle of my 555 timer circuit right? Where should it be around?
Also with my v-comparator circuit I can never seem to get about 400V. The weather seems to be against this very much. I think I should just have my circuit (555) produce about 420V and make my v-compertaor in the range of 32-420 or so, but never run it at 420? Note that my caps are rated at 400. This wouldn't cause problems would it? thanks again

...

Fri Jun 23 2006, 03:04AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

You post is not really clear as to what exactly you are doing...

I get that you are using a boost converter to charge a a large cap bank to somewhere between 30-400v. It sounds like you have some type of comparator on the output to turn the boost converter off when it the bank reaches the desired voltage.

So my first question is, what were you tuning? The frequency you are running at? The inductance?

If you suspect the comparator is causing it to shut off too early, then why can't you just check the output of the comparator to see if it is enabling the supply?

I would guess (based on other's accounts) that you problem is that your booster just can't charge the bank at any considerable speed, if at all when the leakage resistance of the caps is accounted for. Increasing the cap 200,000x is going to require a lot more out of your booster

Could you try giving it more voltage to see if that helps? It is quite common that boost converters have a hard time charging large banks to voltage above a few hundred volts if the supply voltage to the booster is too low...

EDY19

Fri Jun 23 2006, 03:07AM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408

When talking about boost converters, its always the inductor

No, really, the inductor often is the problem if you aren't getting good charging out if it. Try several different inductors, and you are sure to find one that works better. If the charging completely stops, then its probably the comparator. I believe the higher the duty cycle, the higher the output voltage can go, to a certain limit. Look at this thread,

it will help a lot- heres the equation from that thread
Voutmax < Vin/(1-max duty)
so at 95% duty cycle, (taken from "TheMerovingian")

In voltage Max voltage out
5V -> 100V
6V -> 120V
9V -> 180V
12V -> 240V
15V -> 300V
18V -> 360V
24V -> 480V

Thomas

Fri Jun 23 2006, 03:27AM

Registered Member #120 Joined: Fri Feb 10 2006, 07:07AM
Location: Westchester New York
Posts: 83

The comparator is working fine, that was my fault for incorrectly blaming it. I can get the capacitors to charge, it just takes very long to do so. I'm getting the correct voltages if I wait a while for it to charge completely and the comparator shuts off the 555 timer illuminating a LED. So I don't really think it's my oscillator, its probably my duty cycle? I'm running my circuit at about 28kHz so I can't hear any hums, the inductor is 390uH and lastly the duty is a little under 40% positive.

Wilson

Fri Jun 23 2006, 05:00AM

Registered Member #78 Joined: Thu Feb 09 2006, 11:27AM
Location: Sydney, Australia
Posts: 133

There is a limit to how much power your boost converter will provide you know? Its not unreasonable that in charging up a 1.5kJ bank will take a lot longer than a 0.1uF capacitor!
Giving an optimistic figure for your charger as 50W, it will still take 1500/50 = 30 secs minimum to charge the bank to 400V. Obviously, in reality, there are things like cap leakage and such which will further lengthen the charging process, not to mention the non-linearity of capacitor charging.

Thomas

Fri Jun 23 2006, 05:58AM

Registered Member #120 Joined: Fri Feb 10 2006, 07:07AM
Location: Westchester New York
Posts: 83

My transformer is 50W.

it's a 26V 2A transformer. I'd like to think that about an amp is being used for the SCR triggering isolator, fan, 555 timer, mos driver, plus comparator and a digital voltmeter. So even than it still should be around a 26W boost converter. It shouldn't take as long as it does. I'm going to assume it's the duty cycle, because it's only around 50%. Do you think 90 is too much? I have a nice sized heat sink on my mos right now and it runs very cool at 50%. I guess tomorrow or over the weekend I can fool around with it and get the duty up and still keep it running at 28kHz. Think this is the right path to take?

-Tom

EDY19

Fri Jun 23 2006, 12:31PM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408

I would say bump up the duty cycle a bit and see if it helps.

rupidust

Sat Jun 24 2006, 01:12AM

Banned
Registered Member #110 Joined: Fri Feb 10 2006, 12:23AM
Location: Banned City
Posts: 85

A schematic would be very helpful. Booster power is self limiting if none of the properties change. The frequency of operation and duty will be the first limiting factors.

Questions:
Exactly how long does it take to charge the 1.5kJ bank to 400v?
How much supply voltage is the booster being driven from?
Have you monitored the supply voltage during charging to see the extent of ripple?
How much current do you expect the booster's inductor to consume?

Power:
A 390uH Inductor switched at 28kHZ has an inductive reactance of 2*pi*28,000HZ*.000390H=68.6 Ohms If a 24v supply is used, the booster will consume no more then 24v/68.8 ohms=350mA.

* 24v in / 350mA used = 8.4 Watts consummed - 1/2 Watt from switching = 8 Watt booster.
* 1,500 J bank / 8 Watts = 3 minutes of charge time.

To tell how the booster is performing you can run some timing test. Charge small 100 and 200 J banks to 400v. Record the time it takes to reach 400v. Then take the the bank joules and divide it by the charge time. If the time for the 100J bank is 12.5 seconds and time for 200J bank is 25 seconds, then this assures the booster is in the 8 Watt range.

Answers:
* 28kHz is too high and will not consume enough power to make a higher power booster. You can lower the frequency down to 20khz and still be inaudible. However, if you want real power for realy large banks, drop down to the 3khz-5khz range and furnish the booster with more then a criminal 1A current source.
* Keep the booster duty range from 60% to 85%. I use 70%-75% regardless.
* Yes, duty % less then 50% forces the booster converter to charge slower but does not effect charge voltage. A 12v driven booster at 50% duty will charge past 1200v if inductor has a decent winding count and the transistor, blocking diode, and bank load could match the same 1200v.

kell

Sat Jun 24 2006, 02:52AM

Registered Member #142 Joined: Sat Feb 11 2006, 01:19PM
Location:
Posts: 102

--In this post "on-time" means the amount of time the inductor is energized in each cycle.--

To find out whether you're getting the most out of your inductor, look at the on-time so you can figure out the peak inductor current. You want the inductor to peak at the highest possible current because only then will your booster deliver maximum power.
There are two things that limit how hard you can drive your inductor: saturation and resistive heating. I'll ignore heating. It's less likely to be an issue anyway.
If you know the saturation current (maximum current allowable) of your inductor you can figure out the on-time that will give you that current mathematically. Rather than the rigorously correct exponential model, it's much easier to use an approximation of the inductor behavior as a linear ramp. Voltage divided by inductance gives you the ramping in amps per second. Multiply that by the on-time and you have the peak current (if you get a number significantly less than the inductor's absolute maximum current as defined by its resistance, the approximation will be accurate enough). Then you just have to tweak your frequency or duty cycle to get this on-time.
If you don't know the saturation current of your inductor you can find out maximum on-time easy enough by experimenting. Monitor the current drawn by your booster while changing the duty cycle gradually. When you start increasing the on-time, power consumed should track more or less linearly. As you further increase on-time, you should reach a point where power consumed will start to increase much faster than before (non-linearly), which means your inductor is saturating. Back off a little bit and you have your appropriate on-time.

After you have done this, check your duty cycle. If your duty cycle isn't very high, adjust to a higher frequency and lower duty cycle such that you still have the same on-time.
This way you will have maximum power from each pulse and the least off-time and your booster is tuned to its maximum output.

joshua_

Sat Jun 24 2006, 04:35AM

Registered Member #61 Joined: Thu Feb 09 2006, 05:50AM
Location: Mountain View, CA
Posts: 43

EDY19 wrote ...

Voutmax < Vin/(1-max duty)

Not quite. If you keep pumping, it will keep charging up to infinity. The energy has to go *somewhere*! That equation does hold true, however, for continuous-current mode, the ideal state for a boost converter.

I've once considered building a continuous-current 12v->170v boost converter, then using a H-Bridge and inverting it... But then I decided to just use a 140W radio shack special that is probably much better than I could ever do in my lab, heh. Oh well.

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