boost converter

[scuba14c]

I have been trying to build a boost converter, but am having problems. If you aren't familiar with these, they use an inductor, diode, a semiconducting switch, and a 555 timer. The voltage builds up in the inductor, and is pumped into the capacitors. The diode prevents it from flowing out. I can get it to charge to 45V, and then it won't go any farther. With a 12V input, it does work, but should be able to charge to a much higher voltage. The load on the capacitor doesn't matter either. I have used 100uF and 3600uF banks and both stop at 45V. I have a 100uH inductor and a fast recovery diode. This is the schematic I built it off of...

View attachment 37033

View attachment 37034

 

[scuba14c]

I have changed the duty cycle, but it also changed the frequency. The duty cycle is anywhere from 66.6-99.1%. Because the frequency changed, I can't tell if the duty cycle or the frequency makes the most difference. The timer that works the best is 2186Hz @ 66.66% duty. Is it better to have a larger voltage driving the chip; right now it is a 9V battery. I am also not using the 1200V 20A IGBT, it only charged to 25V or so. I have replaced it with (2) 400V 8A NPN transistors in parallel to reduce the heating of the switch. The NPNs work the best of any switch I have tried.

 

[tavix]

An fast(non-electrolitic) cap must be in parallel with output electrolitic cap for fast response.

 

[EdwardM]

Hi scuba14c

can you describe L1 in more detail or post a photo?

Ed

 

[scuba14c]

L1 is an RF choke form radioshack, 100uH. I have three available. Wah tdo you mean I need another cap. Don't I just have the bank I'm trying to charge in between?

 

[EdwardM]

Sorry scuba14c, I get errors when trying to open your JPG

Ed

 

[scuba14c]

Ok let's try it again.

[attachment deleted by admin]

 

[scuba14c]

On the last post, the first is a pic of two inductors, the second is the diode and inductor. These two are: the whole setup, and the transistors, respectively.

[attachment deleted by admin]

 

[EdwardM]

Hi scuba14c

apart from the HV switch, the inductor design is about the most important part of what you're trying to do, can I suggest you have a look at http://www.semiconductors.philips.com/acrobat_download/applicationnotes/AN96052_1.pdf
and particularly the section on inductor design

Best of Luck

Ed

 

[scuba14c]

I have tried another inductor in parallel and it gave me an increase of about .5V. Is it necessary to put the circuit on the board? I am not quite at the stage of being able to put his on a board. Do you think this is causing the problems?

 

[EdwardM]

Hi scuba14c

the warnings section (page 70) of the app-note I mentioned earlier gives at least a partial answer to your last post

Best of Luck

Ed

 

[tavix]

haw fast is switched off your IGBT transistor?
if is too slow then this is your problem.
or output diode - same question.

 

[scuba14c]

Not sure where to find it. I actually gave up on the IGBT and am using two 400V 8A NPN transistors in parallel because they gave me a higher voltage. If you tell me where it would be, I'll give it to you. I think the switch may be the problem after trying to alter the duty cycles on the timers.

 

[Alun]

scuba14c,
What voltage output do you require, and at what power level?

There are lots of ICs avliable to help you, here are some links:
Google results
http://www.linear.com/pc/productDetail.do?navId=H0,C1,C1003,C1042,C1031,C1061,P2410
http://www.amplicon.co.uk/dr-prod3.cfm/subsecid/10090/secid/6/groupId/12114.htm
http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1545
http://www.chipcatalog.com/National/LM1577.htm
http://www.sparkfun.com/datasheets/IC/MC34063A.pdf
http://www.datasheetcatalog.com/datasheets_pdf/A/D/P/3/ADP3000.shtml

 

[scuba14c]

The capacitor bank I have been using to this point is 330V 1900uF. When I charged this with a photoflash charger, it took 4 minutes. The new bank is 350V @ 56700uF or 700V @ 14400uF. I would like it to be able to charge to 700V, but keep the charge time down to about 30 seconds. The schematic for the boost converter said it would charge a smaller bank in 3 seconds.

 

[Alun]

So basically you want a high voltage at a high energy level, my advice to you would be to use a transformer rather than an inductor, a mains transformer with a 6V secondary (connected to the output transistor) and 240V primary connected to the capacitor bank via a high voltage rectifier would be perfect. You might want to stop the voltage from going too high and destroying the capacitors, you can add an auto switch off circuit for this.

 

[scuba14c]

I was hoping to avoid the using the mains, my parents won't let me. Can't afford a variac, either.

 

[Alun]

A variac is no better than using the mains directly.

I didn't mean directly using the mains anyway. :

I meant using a mains transformer as a step up transformer.

 

[scuba14c]

Still have a problem, don't you need AC to use a transformer?