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

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Xray

Sat Oct 22 2011, 03:01PM

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

It looks to me like they put an LC wave-shaping network (Basically, a low pass filter) in the input to each op-amp. I suspect that the LC values must be chosen based on the input PRR. My circuit design has a variable frequency oscillator which has a fairly wide range. Therefore, I don't know if this type of circuit would work for my application. As mentioned earlier, the TL494 PWM chip would probably be my best bet.

Patrick

Sat Oct 22 2011, 04:13PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

I thought that looked familiar

, whats PRR mean?

Xray

Sat Oct 22 2011, 04:58PM

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

Patrick wrote ...

I thought that looked familiar

, whats PRR mean?

Pulse Repetition Rate.

Patrick

Sat Oct 22 2011, 05:04PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

ok, yep

Pinky's Brain

Sat Oct 22 2011, 05:12PM

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

Xray wrote ...

It looks to me like they put an LC wave-shaping network (Basically, a low pass filter) in the input to each op-amp.

There is no they, this is just a simplified schematic I hashed together.

The LC is not low pass because of the combined non-linearities of the diode and the Schmitt trigger, the L-C-D circuit and the Schmitt trigger combined act as a delay of the leading edge of the square wave only (the falling edge is passed nearly uninterrupted because of the diode).

As far as I can see the TL494 dead time is not the same as a break before make delay ... you want a half bridge driver with Shoot Through protection like say the MAX5062.

Marko

Sat Oct 22 2011, 05:32PM

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

Hi sir,

If your drive requirements aren't excessive, you can use SG3525 directly like this:

If you find TL494 easier to obtain (dead PC power supplies) you can use it augmented with gate driver IC's or BJT totem poles just as well.

Marko

Xray

Sat Oct 22 2011, 05:47PM

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

Pinky's Brain wrote ...

There is no they, this is just a simplified schematic I hashed together.

Sorry, I didn't know who designed the circuit, and it looked like a typical diagram that you see in a data sheet for an i.c., so I used "they" as an unknown third person. No rudeness intended.

wrote ...

The LC is not low pass because of the combined non-linearities of the diode and the Schmitt trigger, the L-C-D circuit and the Schmitt trigger combined act as a delay of the leading edge of the square wave only (the falling edge is passed nearly uninterrupted because of the diode).

As far as I can see the TL494 dead time is not the same as a break before make delay ... you want a half bridge driver with Shoot Through protection like say the MAX5062.

Yeah, I agree. The TL494 could be used to acheive a similar result as the MAX5062, but only if I were going to actually PWM the fet drive signal. But my plan for a plasma speaker since day-1 has been to amplitude modulate the DC power going to the FETs, because I believe that that would produce the strongest audio output. I could be wrong, but that's my gut feeling which is based on my prior experience with various types of modulation schemes during my earlier days when I worked in US Naval communications.

Anyhow, the more time I put into analyzing my circuit, the more I'm being convinced that my problem of FET heating is with the circuit layout causing some unwanted parasitic noise in the gate drive signal. I believe that if I clean up the layout, and make better use of my bypass caps, I will mitigate the problem. I'll post my results after I improve the circuit.

Xray

Sat Oct 22 2011, 06:03PM

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

Marko wrote ...

Hi sir,

If your drive requirements aren't excessive, you can use SG3525 directly like this:

If you find TL494 easier to obtain (dead PC power supplies) you can use it augmented with gate driver IC's or BJT totem poles just as well.

Marko

Hi Marko -- Thanks for your reply. That circuit you posted has a relatively high, high voltage power supply. My circuit operates with only 30 to 35 volts DC on the FETs that drive the flyback. And since I'm not using PWM, I am not using a SG3525 nor a TL494 PWM i.c. I have no problem obtaining most i.c.s, so that's never been an issue. As I mentioned in my previous post, I believe that I'll be able to mitigate my problem with better parts layout, and with improved bypassing, rather than redesigning the circuit.

Regards,
Xray ("Sir" not necessary here!

)

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