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Dc/dc Smps Help


acu

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Hi

 

I'm building a simple 555 boost converter (555, IGBT - HGTG12N60A4, 470uH inductor, 350V 22uF capacitor, 1000V, 1A Schottky diode). My Vin is 12V (or lead gel battery pack) and Vout I want is 350V.

 

I want to work out what frequency I should run my timer at (I can see that it will probably be 200kHz going by the datasheet). Next step is I want to find what Pulse-Width to use for getting my output of 350V. I'm not sure what current I want (this is for a small Xenon Stroboscope circuit - flashrate of 0-100Hz). I guess I basically want like most designers to get a reliable output voltage to charge the capacitor fast enough.

 

Does anyone have any pointers, forums, calculation steps I should run though.

 

Thanks,

B

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Hello acu

 

Welcome to the forum

 

The frequency and pulse widths are very much a function of the inductor. If the frequency is too low, or the pulse width too wide, you will saturate the inductor and draw a lot of current which is not what you want.

I wonder why you are using a 555? there are switchmode chips available that are designed for this type of application and have a number of advantages including a current feedback to ensure that you do not fully saturate the inductor.

I would suggest that you research some of the dedicated chips for this type of application. I expect that the result will be simpler and far more reliable. Additionally, there are many application notes about these specialist chips that will answer your questions as well.

 

Best regards,

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Hello acu

 

Welcome to the forum

 

The frequency and pulse widths are very much a function of the inductor. If the frequency is too low, or the pulse width too wide, you will saturate the inductor and draw a lot of current which is not what you want.

I wonder why you are using a 555? there are switchmode chips available that are designed for this type of application and have a number of advantages including a current feedback to ensure that you do not fully saturate the inductor.

I would suggest that you research some of the dedicated chips for this type of application. I expect that the result will be simpler and far more reliable. Additionally, there are many application notes about these specialist chips that will answer your questions as well.

 

Best regards,

 

Hi marke

 

Thats for the welcome. (Fellow kiwi here :))

 

I figured as much. I've ordered a controller chip but it seems to be taking for eternity to arrive and I'm really bolstered for time. I'm doing this for a small project which I'll write up a report later on. It's basically a learning curve. Those sm chips are very handy as you have said. As I'm doing this at my Tech lab room , we have plenty of 555 chips, parts etc. I guess this is by no means a very professional circuit.

 

For now, I plan to just use a Frequency generator to test this little circuit out. I've already blown one IGBT so far. Not to cheap at $8 a pop from Dick Smiths heh.

 

I've got to watch my power I think (how so ?) in this circuit as thier is a Vce (sat) of 2 volts. I did some calculations but I don't think this is correct.

 

What I know:

 

L = 470uH

Vin = 12V

Vout = 350V (that's what we want, so I assume we have to cater the formula to get this output).

ILoad = ?

I (through FET - *is this the same as Iload?*)=?

Frequency =200kHz or greater if possible (I think the temperature comes in to play)

--------------

Vout=Vin/(1-D)

D=0.97

 

So the Duty cycle is quite big, almost on which means the Toff is quite small, and Ton is...

 

D = ton / T

Ton = DT

 

Now with frequency...

 

Try F = 200kHz

Ton= 5us

L=(Vout-Vin + VD)*(1-D) / (min(Iload) * f)

470uH= (350-12)*(1-0.97) / ((I)*200kHz))

I=107mA

 

I was trying to follow the steps in this link http://www.daycounter.com/LabBook/BoostCon...Equations.phtml

 

 

 

 

 

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