How can I create a "special" waveform for driving my half-bridge? / High Voltage / Forums

Forums

4hv.org :: Forums :: High Voltage

« Previous topic | Next topic »

How can I create a "special" waveform for driving my half-bridge?

1 2 next

Move Thread

LAN_403

Xray

Fri Oct 21 2011, 06:01AM

Registered Member #3429 Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288

This

is the schematic diagram of my plasma speaker (posted elsewhere in this forum). Below is a picture of two waveforms that I drew by hand so that it can help explain what I am looking for. Currently, my 555 timer chip produces a symetrical square wave that drives the mosfet half-bridge. My concern is that, for a very brief period of time, BOTH mosfets are conducting, which causes them to get hot. If the square wave were perfect, and if I were not driving an inductive load (the gate drive transformer) then I would not have this concern. But since there is some ringing on both the leading and trailing edges, I am certain that both fets are ON at the same time which is not good. So, I believe that the bottom waveform in the picture is what I need. In that case, one fet would be completely turned off for a brief time period before the other fet is allowed to turn on. It's a "break before make" scenario. So, does anyone know how to create such a waveform with a 555 timer? If not a 555, then what would it require? Am I correct in my assumption, or is there a better way to handle this problem? THANKS!

Turkey9

Fri Oct 21 2011, 06:33AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

I'm only 90% on this, but I'd bet that shoot through is not your problem. Many many people have used the same circuit without any problems at all.

555 cannot create the waveform you want. You could use the tl494 to do it though. It will create two pulse trains of <50% duty cycle that you can use along with a GDT to get what you want.

Dr. ISOTOP

Fri Oct 21 2011, 11:43AM

Registered Member #2919 Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652

You could put some delays between the 555 and the FET.
But I agree with Turkey9; the TL494 is perfect for this application.

Daedronus

Fri Oct 21 2011, 02:31PM

Registered Member #2329 Joined: Tue Sept 01 2009, 08:25AM
Location:
Posts: 370

If you have a signal that goes from 0-99% duty cycle you can use a J-K flip-flop and 2 and gates to get 2 signals at 1/2 the frequency.
But it's more straight forward to use a tl494.

Xray

Fri Oct 21 2011, 02:58PM

Registered Member #3429 Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288

Many thanks to all who repied! I will check out the tl494 to see if it's something that I want to use here.

Turkey9 -- You are probably correct about the fact that there might be something else going on here. When I scope the signal on the primary of the gate drive transformer, I am seeing ringing on both leading and trailing edges of the squarewave. That ringing could be caused by my circuit layout, and/or I just might need to place a bypass cap somewhere in the circuit to dampen the ringing.

Pinky's Brain

Fri Oct 21 2011, 09:27PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

I posted a circuit in Patrick's thread which is close to what you want ... basically I suggest creating the complement of the PWM signal and delaying the rising edges of the original signal and the complement with LCD circuits, and then cleaning them up with Schmitt triggers ... after that you just drive your half bridge with the two signals.

Xray

Fri Oct 21 2011, 10:07PM

Registered Member #3429 Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288

Pinky's Brain wrote ...

I posted a circuit in Patrick's thread which is close to what you want ... basically I suggest creating the complement of the PWM signal and delaying the rising edges of the original signal and the complement with LCD circuits, and then cleaning them up with Schmitt triggers ... after that you just drive your half bridge with the two signals.

Thanks Pinky's Brain! I see the schematic diagram in the Email alert that I received from your post, but for some reason, the diagram did not show up here on the website! (Did you delete it for some reason after you posted it?)

Anyhow, I will explore it further and decide what (if any) changes I'm going to make to my circuit.

Thanks again!

Pinky's Brain

Fri Oct 21 2011, 10:25PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

The image is still on photobucket AFAICS, when I load the thread in a different browser it shows up fine.

Xray

Fri Oct 21 2011, 11:21PM

Registered Member #3429 Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288

Pinky's Brain wrote ...

The image is still on photobucket AFAICS, when I load the thread in a different browser it shows up fine.

That's strange! I don't even see a link in your post. So how can anyone else view the image if there is no link to it? I thought that whether or not a link shows in the post is a function of the forum software, and not a function of the individual viewer's browser. Please explain.

Pinky's Brain

Sat Oct 22 2011, 10:42AM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

I think we are talking past each other, I didn't post the image in this thread ... only in Patrick's thread :)

Any way, this is the image :
HVblock

PS. a gate drive transformer isn't really an inductive load as such when you use it like you should (high coupling factor transformer with duty cycles near 50%, IRF in their MOSFET appnote suggests not going beyond 35%-65%). It just transforms the load on the other end, which is capacitive.

1 2 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob