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Sine to square conversion

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cbfull

Sun Jun 11 2006, 03:26AM

Registered Member #187 Joined: Thu Feb 16 2006, 02:54PM
Location: Central Ohio
Posts: 140

Does anyone have any ideas or know of any ICs that will allow me to convert a sinusoidal input to a square wave? It has to be able to closely match phases (zero crossing is a good way to gauge the phase matching).

I want to stay away from clipping an "overvolted" sine wave with zeners, since that would waste a lot of power (and potentially overheat the zeners). I am hoping I can find a way to square-up the feedback signal in my flyback circuits.

HV Enthusiast

Sun Jun 11 2006, 03:37AM

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

Probably a million different ways to do this.

And definitely something a quick GOOGLE search would be able to reveal tons of information on the subject.

Remember the rules . . . do your homework and research first before asking . . . Thanks!

Wilson

Sun Jun 11 2006, 04:28AM

Registered Member #78 Joined: Thu Feb 09 2006, 11:27AM
Location: Sydney, Australia
Posts: 133

Schmitt trigger, op amp wired for max gain, just to name 2

cbfull

Sun Jun 11 2006, 10:41AM

Registered Member #187 Joined: Thu Feb 16 2006, 02:54PM
Location: Central Ohio
Posts: 140

EastVoltResearch wrote ...

Probably a million different ways to do this.

And definitely something a quick GOOGLE search would be able to reveal tons of information on the subject.

Remember the rules . . . do your homework and research first before asking . . . Thanks!

I have already researched the idea via Google on various occasions, but I don't think I am using the right search parameters. Plus, I have a hard time interpreting some of the very technical information when I think I have found the answer, so I thought it was worth asking. Sorry if you disagree.

I was hoping that someone had actually accomplished this before and would chime in. I have followed various flyback circuits that have been posted on this board over the years and I haven't seen anyone attempt to do this outside of the standard back to back zener.

Steve Conner

Sun Jun 11 2006, 04:14PM

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

You want a comparator. Like another poster mentioned, this is basically an op-amp wired up as an amplifier with such a high gain, that it clips the input signal into a square wave. You can use just about any op-amp, but a purpose-designed comparator chip is better.

If you look in the datasheet for the LM339 comparator chip, you'll see a zero crossing detector in the applications section. (the LM339 is fairly slow though.)

All feedback SSTCs convert a sine wave to a square wave. They have to because the feedback signal is a sine wave but the power transistors need a square wave drive to work properly. Steve Ward's circuit uses a simple 74HC14 logic chip as a kind of ghetto comparator to do the conversion. I called it ghetto, but I don't mean to dis it, since it actually works very well and dozens of other people have built it with success. The cheap plasma globes use practically the same circuit but with some sort of 4000 series logic chip instead.

So if you go check out the LM339 datasheet and Steve Ward's Micro-SSTC driver schematic, that should give you some clues. I personally think Ward's circuit would be the direction to explore.

Terry Fritz

Sun Jun 11 2006, 05:14PM

Registered Member #393 Joined: Tue Apr 18 2006, 12:30AM
Location:
Posts: 297

Hi,

I use this circuit:

But I am coming off current transformers with some power in clipping at the IC inputs. Propagation delay is a big factor and I think mine is about 25nS (?). Really fast comparator ICs tend to be noisy and I personally don't like them.

This card:

Uses LM339 for over current setpoint. I forget the delay of the LM339 but it is slow. Also note very severe restrictions about going outside the input voltae range which will mess them all up (guess how I know this :O))

The incoming signals either have to be very clean (filters will ruin the phase) or current looped from a current transfomer, like the above, to keep the signal good. A weak 5V signal will probably not survive near most DRSSTCs without "very good" shielding.

The old LM319 is considered "fast" (80ns), but there are newer exotic ones now.

Also see the note here:

Cheers,

Terry

cbfull

Mon Jun 12 2006, 04:58AM

Registered Member #187 Joined: Thu Feb 16 2006, 02:54PM
Location: Central Ohio
Posts: 140

Thanks a great deal for those suggestions. I will use those as a basis for my search in understanding them and then see if they can accomplish what it is I am after.

You guys are great.

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