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12v to 450v+ Boost Converter

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GeordieBoy

Fri Mar 07 2008, 10:39AM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

The problem with this fixed duty-ratio circuit is that the inductor current can become continuous when the output voltage is low. This means that the inductor current will integrate up higher and higher on successive pulses near the start of the charging cycle until the MOSFET Id rating is exceeded.

Peak current mode control is really the best way to control a boost converter like this. It prevents the inductor current from integrating up to dangerous levels by setting a maximum peak current limit. If it goes from operating in discontinuous mode into continuous current mode, then the peak current will be held at this level.

You should really also put an inrush bypass diode from the 12 volt input to the HV output. Otherwise the huge inrush current to charge the capacitors at power-on flows through the choke, and then the MOSFET when it attempts to turn-on.

Just my thoughts on the matter from an EE viewpoint.

FWIW, The flyback converter is a better choice.

-Richie,

Commanderbob

Mon Mar 10 2008, 05:00AM

Registered Member #1341 Joined: Thu Feb 21 2008, 05:06AM
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Posts: 17

Could using a mosfet and not an IGBT cause this? My wiring seems correct. Any suggestions?
Thanks,
Justin

Commanderbob

Tue Mar 11 2008, 03:58AM

Registered Member #1341 Joined: Thu Feb 21 2008, 05:06AM
Location:
Posts: 17

Sorry, I did not notice there was another page.

Once the capacitors are charged to 12v the current should stop flowing right? The MOSFET only blew after a minute or two of being on. Could this still be the problem you are describing?

I will probably ditch this and go for a flyback, they seem much simpler. Thanks for the suggestion.
Justin

uzzors2k

Tue Mar 11 2008, 09:50PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

My guess is that your mosfet isn't switching. Since toggling the power would charge the capacitor somewhat it seems the inductor/ diode is functioning and that the mosfet was in a semi-closed state (the linear region). This would also explain the slo-mo death. Your 555 should be operating in the audio range so a speaker can be used to test whether it is oscillating, assuming you don't have a scope of course. First get the 555 working, then the rest.

Commanderbob

Thu Mar 13 2008, 03:55AM

Registered Member #1341 Joined: Thu Feb 21 2008, 05:06AM
Location:
Posts: 17

I will remove the dead MOSFET and attach a piezo speaker to the output and see what happens tomorrow (I no longer have a scope). All the flyback circuits I found could not charge the capacitors as fast as this and I don't know were to buy one of those coils, so I will stick with this circuit.

One more thing if the MOSFET does not work. On my robot I need to carry a 9v battery (or boost the voltage high) to trigger the IGBT for the coilgun. The IGBTs that I have have a sat voltage of 2.5v so I could power the 555 with +12v and power the coil with the battery. That way the IGBT will be switched on with +12, but the majority of power will not be changed keeping the efficiency.

Thanks,
Justin

Commanderbob

Thu Mar 13 2008, 11:40PM

Registered Member #1341 Joined: Thu Feb 21 2008, 05:06AM
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Posts: 17

Ok, it works now. I forgot to connect trigger to the capacitor.

There is one problem, it only charges to 600v. At first it very quickly goes to 400v then slows a little at 500v and then at 580v it goes at 1v every two seconds. Is there a way to make it faster or higher max voltage, or should I just use 400v?

Justin

uzzors2k

Fri Mar 14 2008, 04:05PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

You need more power. It'll probably be easier to just wire the bank for 400V though.

Commanderbob

Fri Mar 14 2008, 10:33PM

Registered Member #1341 Joined: Thu Feb 21 2008, 05:06AM
Location:
Posts: 17

Ok it works great with 400v. Charges in seconds not 15 minutes like before. One more question though. The LED that tells when it is done, is it ok to change the resistor that is 18K lower? Say 3K, it is too dim now.
Thanks,
Justin

uzzors2k

Sat Mar 15 2008, 08:43AM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

As long as the LED current doesn't exceed it's max rating. Usually 20mA.

Commanderbob

Sun Mar 16 2008, 04:36PM

Registered Member #1341 Joined: Thu Feb 21 2008, 05:06AM
Location:
Posts: 17

Thanks, I used 2.2k and it is perfect now. I did not know if it was just for the LED or had something to do with the comparator. Thanks for all of your help.
Justin

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