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Voltage Control On Boost Converter Playing Up

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evilgecko

Sat Jun 03 2006, 05:01AM

Registered Member #288 Joined: Mon Mar 06 2006, 07:52AM
Location: Palmerston North
Posts: 32

I having troubles with the Variable Voltage Boost Converter (the same one as

). The boost converter works fine, I can charge up to 450V no problems. But the voltage regulation is playing up. The circuit uses a LM393 to stop charging the capacitor when it reaches a certain voltage. When the voltage of the caps reaches the set voltage, instead of fully switching of the NE555 and therefore the mosfet (didn't have an IGBT), suddenly the NE555 goes into superfast oscillation and the current jumps from 2.5A (the charging current) to 4.3A. Its werid though because the capacitors stay at the set voltage, the mosfet just absorbs all this energy and heats up real fast and the charged light remains on (although the signal at pin 4 (reset) on the 555 is crazy)

When it all goes haywire if I lower the set voltage for the caps it stops the osscilation and the caps discharge slowly through bleeder until they reach the lower set voltage, then it all goes crazy again.

Help!

Maybe something needs to be grounded and isn't. Or may the superfast osscilaiton is the LM393 keeping the voltage exactly right, and loads of energy is being lost due to the mosfet switching continusely.

HV Enthusiast

Sat Jun 03 2006, 10:34AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

One thing in the design you need is hysteresis on your comparator. This may be why your converter may be oscillating or "chattering." Without it, your caps will charge up to some set voltage, and then it will shut off, but then you have leakage on your capacitors (or load from resistive divider which is almost 1mA) and voltage will droop causing the converter to turn back on instantly.

Hystersis on the comparator prevents this. This is accomplished by use through a positive feedback resistor on the comparator. So, for example, you would set the comparator to turn your converter off at say 600V, but it would not turn back on again until the cap bank voltage falls to 580V.

Search for hysteresis on GOOGLE and you'll find everything you need to know to incorporate this.

Dr. Shark

Sat Jun 03 2006, 10:39AM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

You are using the reset on the 555 as if it was some kind of enable pin, and I am not so sure if it is. I don't know how it is wired internally, but I really don't think you are supposed to switch off the 555 by pulling the reset high. Else it would not be called reset

What I would try is to use a small transistor to short the output of the 555 to ground whenever the comparator goes high.

HV Enthusiast

Sat Jun 03 2006, 03:40PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Posts: 3068

The reset can be used to completely disable the 555 timer. When its pulled low, the 555 timer will stop operating regardless of the state of other inputs on the 555 IC.

The reset is commonly used to gate the 555 timer ON/OFF, so his circuit is perfectly fine in this regards.

I still think the oscillation problem he is seeing is due to oscillation of the comparator circuit, i.e. chattering. There is no hysteresis on the comparator, and as soon as the circuit turns "OFF", the leakage and voltage divider on the output discharges quickly to let the comparator reset and turn the circuit back "ON". This continues the chattering and hence the oscillatory nature of the circuit.

Try this in the short run. Try some resistors with values ranging from 47k to 1Meg from Non-inverting input of the comparator to the ouput of the comparator. This should help the problem. The lower the resistor value, the more hysteresis.

I'm pretty sure this will solve your problem, although you'll need to determine what the correct (desired) value is.

rupidust

Sat Jun 03 2006, 04:05PM

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

evilgecko wrote ...

I having troubles with the Variable Voltage Boost Converter... but the voltage regulation is playing up....

Try here http://www.anothercoilgunsite.com/nf-barriers.htm and click on Charging

evilgecko

Sat Jun 03 2006, 11:11PM

Registered Member #288 Joined: Mon Mar 06 2006, 07:52AM
Location: Palmerston North
Posts: 32

Trying to get my head around hysteresis! The non-inverting gate is just the name for the + gate isn't it? From what I can firgure the LM393 is running in the inverting mode, since when Vin goes high the output goes low. So I would connect the 47K to 1M resister across the + pin and the output?

All the websites about hysteresis were kinda confusing. Thier diagrams showed that you need to use non-inverting hysteresis for a non-inverting comparator and and inverting hysteresis for a inverting comparator. And all the diagrams I saw included 2 extra resistors. The reference voltage come from the pot R3 (10K) and the voltage input from the capacitors (reduced by another voltage divider). Using the attached diagram as reference, I would have to attach R1 between R3 and the + gate, and R2 from the + gate to the output. I'm looking for a hysteresis of about 2V so how do I use those equations in the picture to work out the values of R1 and R2. Do you have to solve them simulataneously since there are two variables?

I looked at the improved version of www.anothercoilgunsite.com boost converter. He just added a 2.2k resister on the output if the comparator and then a 0.01uF cap between pin 4 and ground. I don't see how this fixed the chattering?

Edit: Grr the link isn't working

rupidust

Sun Jun 04 2006, 01:38AM

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

In the improved comparator version, the output of the first comparator is routed to the input of another comparator which is a "filter". Then the output of the second comparator is feed to the 555 pin 4 reset. There are 2 comparators per lm393 package.

evilgecko

Sun Jun 04 2006, 02:43AM

Registered Member #288 Joined: Mon Mar 06 2006, 07:52AM
Location: Palmerston North
Posts: 32

I don't see how two could stop the chattering because the first one would start osscillating and cause the second to do exactly the same, unless that capcitor has something to do with it.

Anyway I've already built the PCB and wiring up the second comparator would be harder than just adding two resistors to provide hysteresis. I'm trying to work out whether to connect the resistor the the plus or negative input.

HV Enthusiast

Sun Jun 04 2006, 02:47AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

the resistor needs to go from non-inverting input to output. You need positive feedback.

evilgecko

Sun Jun 04 2006, 03:16AM

Registered Member #288 Joined: Mon Mar 06 2006, 07:52AM
Location: Palmerston North
Posts: 32

Non-inverting being the input with the + symbol? I know this sounds so stupid!

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