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Registered Member #1525
Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294
I just finished trying to run two boost converters (Uzzors' design) in parallel. The single converter was running really well- it charged my 1.1kJ bank at almost 19W.
Then I added an extra Inductor/Diode/Mosfet train, to be pulsed by the same 555 chip as the other parallel converter. I left all the resistors the same. I have a diagram of this, but it's edited from Uzzor's original so I'm not sure if I'm allowed to post it or not. Anyways, I'm sure you get the idea.
I think it worked, because my 6A fuse blew, meaning it was drawing the extra current. I replaced the 6A with an 8A fuse and it held.
But the weird thing is, the voltage on my caps spikes to 500 almost instantly, which is bad for 400V caps, so I shut it off. As soon as I shut it off though, the voltage goes down to 60. Pulsing it on and off makes the voltage spike to <500-600, then settles to a realistic value once the charger is shut off.
Does anyone have any Idea what might be going on? I'm pretty sure that it's bad for my caps to be overvolted like this.
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
Use separate gate resistors for each mosfet. Capacitors oppose change in voltage, and with 20/40W of charging power I doubt the capacitor voltage would jump around like that. Check for loose connections and make sure your voltmeter is fixed to the capacitor bank, and away from the charger. For your 1.1kJ bank to spike to 500V in an instant would require in excess of 1kW, so it's likely a measurement error.
Registered Member #1525
Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294
Well, adding the second inductor/diode/mosfet has 0 effect. It still takes the same amount of time to charge (and both sets are working, I tested them both individually).
It seems that adding the second inductor/diode/mosfet essentially just adds a second inductor in parallel, which only changes the inductance instead of adding a second boost converter.
The problem is I really don't know how to get around this. Is there away to attach another boost converter without just paralleling another inductor?
Registered Member #1739
Joined: Fri Oct 03 2008, 10:05AM
Location: Moscow, Russia
Posts: 261
Well, in fact paralleling the inductors isn't that bad, you have twice the current and half of inductance, therefore each strobe brings the energy of E=L*I^2/2 being twice as before. However this may also impact the performance of this circuit that isn't very high in fact. You actually should consider a better topology. 1. Push-pull. Probably just Mazzilini circuit as seen on Uzzors' site, or implement a current PWM that will make charging more linear. 2. Flyback. Doesn't differ of the boost variant much, only you need a step-up transformer as the output stage.
As the first one is documented already and we are not going to make a bridge for such a low voltage (doesn't differ of a push-pull version though), I'll tell a bit about flyback method. In fact it is as simple as it feels - you replace the choke with a step-up ferrite transformer (do not take one from a TV though ;)) and move the output diode to the secondary while checking the phase is correct: you only need the spike that forms on the secondary while the MOSFET turns off to be driven into the output.
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Oddity in Parallel Boost Converters
