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halfbridge coilgun schematic check

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maruiwlt

Wed Jul 18 2007, 11:50AM

Registered Member #859 Joined: Mon Jun 25 2007, 10:23PM
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Posts: 11

Hi, I just drew up a schematic for a halfbridge coilgun and wanted to check if it was right before I built it. Please disregard IGBT/Diode models for now, im just interested in whether or not i got the basic idea of a halfbridge down... I only drew one stage so assume that all subsequent stages will run off of the same capacitor bank which im not sure is a good idea or not. Im also planning on 4 stages with maybe an SCR prestage.

halfbridge

The driver circuit provides +12V to 0V if thats helpful. And also there is a relay between the inductor and +12, i just couldnt find the relay component in multisim.

Steve Conner

Wed Jul 18 2007, 12:22PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The driver is wrong, you need a device known as a "High side driver" for U3. If you don't understand what a high side driver is or why you need one, you have a learning curve ahead. The topic has been discussed here many times before, so Google and the 4hv archives should turn stuff up.

Marko

Wed Jul 18 2007, 01:21PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Steve: I don't have much experience with coilguns, but what is actually the ''halfbridge'' generally talked about?

This what maruiwlt drew might just work, although it's actually ''forward'' topology. It gives the full supply voltage to load at cost of having two series switches, so I guess it's more or less same.

But high side driver is needed in any case, people prefer things like IR2112 IC's.

Still if you are resourceful you can build your own high side drives considering this is only a pulsed application where pullup heating isn't considerable.

Steve Conner

Wed Jul 18 2007, 01:44PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Firkragg, it's an H-bridge with two of the switching devices replaced by diodes. I think it's also known as an asymmetrical or diagonal halfbridge. The idea is to recover the inductive energy left in the coil after firing, back into the DC bus capacitor. This gives a sharper end to the firing pulse than the usual flywheel diode circuit (which is good as it reduces suck-back of the projectile) not to mention that the flywheel diode circuit dissipates the left-over energy as IR losses in the diode and coil.

The other two switches only need to conduct in the reverse direction, so they can just be diodes.

Marko

Wed Jul 18 2007, 02:21PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

The idea is to recover the inductive energy left in the coil after firing, back into the DC bus capacitor. This gives a sharper end to the firing pulse than the usual flywheel diode circuit

Excuse me, I was well aware of the idea!

For some reason I tought that 'normal' halfbridge with asymmetrical switches is more common, but I was wrong.

maruiwlt

Wed Jul 18 2007, 03:17PM

Registered Member #859 Joined: Mon Jun 25 2007, 10:23PM
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Ive found two high/low side drivers on digikey...

the only difference i see between the two is that 2181 is capable of a high short circuit pulsed current, should this matter for what im using it for? Because ide like to spring for the 21201 which is cheaper :P. Also, im looking at the example circuit in the datasheet and im wondering what the capacitor from Vb to Vs does?

So other than the way im driving it, the halfbridge looks fine?

Thanks for all the help btw!

maruiwlt

Thu Jul 19 2007, 05:16AM

Registered Member #859 Joined: Mon Jun 25 2007, 10:23PM
Location:
Posts: 11

Ive found two high/low side drivers on digikey...

Pelletajuton

Sun Sept 09 2007, 10:02AM

Registered Member #993 Joined: Sun Sept 09 2007, 09:49AM
Location: Southern Finland
Posts: 3

Well, I'm sort of new here, so hello to everyone first!

I was just wondering about why you wish to have a half bridge instead of a .. quarter bridge?

Every transistor connected in series with your system gives additional resistance, and if i'm not entirely off course, you want as little resistance as possible? Also, the half bridge is more complicated and expensive than just a single transistor with driver.. Can't the other leg of the cap be connected to the coil all the time? I don't mean to be mean, just like to see better efficiencies

And then to a dorky question; can i use a low-side non inverting MOSFET driver as a high-side inverting one, taken that Vs is the same for the transistor and the driver, and the driver's GND pin is connected to a voltage 10-12V below Vs?

Zum Beispiel

Sun Sept 09 2007, 10:37AM

Registered Member #514 Joined: Sun Feb 11 2007, 12:27AM
Location: Somewhere in Pirkanmaa, Finland
Posts: 295

The halfbridge is used so that you can recycle the left over energy from the coil back to the caps, instead of burning it as heat. You turn both of the transistors off and the diodes rectify the voltage spike back into the storage caps.

Dr. Dark Current

Sun Sept 09 2007, 11:15AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

maruiwlt wrote ...

Ive found two high/low side drivers on digikey...

The IR21xx drivers are not useful for your application, because they are half-bridge drivers where low side switch's drain is connected directly to high side switch's source. You have the load between these, so it won't work. It won't also work because if you need to turn on the high side switch on, you must first charge the bootstrap cap by turning the low side switch on and as I understand, you need to turn on both switches at the same time.

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