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NE555 interrupter and GDT problems

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LAN_403

Steve Conner

Thu Nov 29 2007, 04:20PM

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

If you're using discrete transistors to drive it, do you have anti-parallel diodes across them to let that inductive energy return to the supply? If not, you may break them down in reverse and kill them sooner or later.

If you are using diodes, then the "staircase steps" you see should be about 1.4V in size.

Tonic

Thu Nov 29 2007, 04:53PM

Registered Member #528 Joined: Fri Feb 16 2007, 10:32PM
Location: Warsaw, Poland
Posts: 166

Steve Conner wrote ...

If you're using discrete transistors to drive it, do you have anti-parallel diodes across them to let that inductive energy return to the supply? If not, you may break them down in reverse and kill them sooner or later.

If you are using diodes, then the "staircase steps" you see should be about 1.4V in size.

I've connected like as Richie done with UF4001's (look scheme, few posts above), it's the same as you say? Those steps I have at the moment are around 1.1V.

Next problems - when I lower frequency, tops and bottoms of squarewave are more half-circle shape. As my dad return, I will post pictures of waveforms. Second problem is that transistor is still heating, when it's connected to GDT. When I plug +30V to flyback transformer's primary, the waveform is blurried (it doesn't depend if I draw arc, or not). Perhaps it's caused by field created by flyback transformer and causes oscilloscope disortions? Transformer is 0.5m away from probes. I guess it's hard for you to figure out what's going, so I'll wait unless I do photos and update scheme, including those changes I did for your suggestions, GDT and flyback transformer. I hope it will be clear for you :)

EDIT:

There's updated scheme:

Tonic

Sat Dec 01 2007, 04:18PM

Registered Member #528 Joined: Fri Feb 16 2007, 10:32PM
Location: Warsaw, Poland
Posts: 166

I built new PCB. Now it's less messy. There is his photo:

I played with it a bit and there are my best results as so far (though, they don't differ much from previous).

The wire I used for GDT came from UTP cable:

Wounded GDT:

GDT's dummy loads consisting 2.2nF capacitors and 27ohm resistors:

Two outputs and comparing them:

Separated waveforms:

There are measurements of one output:

Well.. conclusions? GDT has 10 turns, wires are twisted and trifilar, they're tighted on core. No problems with fall and rise times at ~50 kHz, as you can see in photos. No any ringing, but I'm think I'll add more damping resistors in order to decrease overshots, which are ugly for me. I'm still concerned about stair shapes, are they really normal? From measurements I've taken I would say they're bad for transistors. But there I must ask you something important, how can I figure out if bottom part of waveform is below 0V or above? Uzzors, what waveforms you had with this driver?

Next question. Is it worth to buy low-noise resistors? I'm wondering how they will work at high frequencies.

If someone didn't notice, in previous post I gave the updated scheme.

Linas

Sat Dec 01 2007, 04:50PM

Registered Member #1143 Joined: Sun Nov 25 2007, 04:55PM
Location: Vilnius, Lithuania
Posts: 721

here is mine GDT driver with 20nF load and 350KHz :)

Tonic

Sat Dec 01 2007, 05:05PM

Registered Member #528 Joined: Fri Feb 16 2007, 10:32PM
Location: Warsaw, Poland
Posts: 166

Linas wrote ...

here is mine GDT driver with 20nF load and 350KHz :)

How you dare to depress me?

Tell more, do you have any photos of GDT? What core do you use? How many turns, what wires etc.?

And, how do you plug oscilloscope to check gate waveform? Since I do have this toy from not long time, I might be scoping in wrong way.

Linas

Sat Dec 01 2007, 05:12PM

Registered Member #1143 Joined: Sun Nov 25 2007, 04:55PM
Location: Vilnius, Lithuania
Posts: 721

Well, it is driving by using TL494, with TS 555 it will be beter, photos is in my page :),
i connect oscilloscope direct to load (20nF)
16 turns with LAN cable
Mosfet Valdymas 007

Tonic

Sat Dec 01 2007, 05:33PM

Registered Member #528 Joined: Fri Feb 16 2007, 10:32PM
Location: Warsaw, Poland
Posts: 166

Do you use only capacitor for testing? Where do you plug scope's ground end?

Did you test with less than 16 turns on GDT core?

Linas

Sat Dec 01 2007, 06:16PM

Registered Member #1143 Joined: Sun Nov 25 2007, 04:55PM
Location: Vilnius, Lithuania
Posts: 721

no or rise time will be beter?

Tonic

Sat Dec 01 2007, 06:22PM

Registered Member #528 Joined: Fri Feb 16 2007, 10:32PM
Location: Warsaw, Poland
Posts: 166

Theoretically, yes, but practically it might be very small improvement. Maybe with oscilloscope you should reduce turns gradually, until you find a optimal minimum. For more details, check there

and

. These sites provide great advices about GDTs

EDIT: Forgot, would you mind answering to my question about scope's ground lead? :)

uzzors2k

Sat Dec 01 2007, 07:08PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Tonic wrote ...

No any ringing, but I'm think I'll add more damping resistors in order to decrease overshots, which are ugly for me. I'm still concerned about stair shapes, are they really normal? From measurements I've taken I would say they're bad for transistors. But there I must ask you something important, how can I figure out if bottom part of waveform is below 0V or above? Uzzors, what waveforms you had with this driver?

Next question. Is it worth to buy low-noise resistors? I'm wondering how they will work at high frequencies.

The waveforms look great, now you just have to wind your GDT for 1:2:2 because currently your GDT will only give 5V of gate drive. Staircase waveforms are completely normal, and the only reason you would need to worry about them is if they exceed the mosfet's maximum gate voltage. The ground line is where the scope trace is with no voltage applied to the probe. Positive voltage will be above this line, negative below. If you switch to DC coupling on your scope you can see where the bottom part of the waveform is compared to the ground line. Low-noise resistors are of no use at these frequencies.

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