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Registered Member #9039
Joined: Wed Dec 26 2012, 03:31PM
Location: Epping, Victoria, Australia
Posts: 117
Based on Steve Ward's min STTC design:
These are the waveform generated by the two secondary windings of my GDT (1:1:1) with the probe on one end of the primary and the GND clip on the other end.
I have a 6.8uF greencap in series with the primary coil of the GDT.
The inductance of the primary coil and one of the secondary coils is 1.34mH. But for some reason the other secondary has a slightly lower inductance of 1.26mH. It might have something to do with the fact that I used thin hook up wire to wind the GDT and not magnet wire. It seemed more convenient to have the windings colour coded but it is also harder to get the windings tight.
But there are two problems that I can see.
1) There is a lot of ringing going on here. Is that likely to be a problem?
2) My vertical scale is 0.5V and my probe is on x10 so I put the peak voltage at 5V and that is too low. So what is the preferred way to increase the voltage?
a) Increase the value of the series capacitor? b) Reduce the number of windings? c) Increase the number of secondary windings?
Registered Member #33
Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971
Are you using the same gate drivers as in the schematic you linked to? What voltage are they running from? How many turns are there on your GDT? Is there any load on the GDT secondary when you're measuring?
The ringing is likely caused by stray inductance in the GDT circuit. This usually caused by problems with the circuit layout. Can you post a picture of your circuit?
Registered Member #9039
Joined: Wed Dec 26 2012, 03:31PM
Location: Epping, Victoria, Australia
Posts: 117
Wolfram wrote ...
Are you using the same gate drivers as in the schematic you linked to? What voltage are they running from? How many turns are there on your GDT? Is there any load on the GDT secondary when you're measuring?
The ringing is likely caused by stray inductance in the GDT circuit. This usually caused by problems with the circuit layout. Can you post a picture of your circuit?
The primary of the GDT is running off 12V at present but it will be running off 15V.
I found this excellent resource that explains a lot - about the ringing and about the slightly sloped tops of my square wave.
The ringing is occurring because the GDT secondaries are floating at present - I have not yet linked them up to the 5 ohm damping resistor in Steve's design.
And it looks as though I could do with a few more turns on my toroid than Steve specified judging by the slight slope at the top of the square wave - Steve perhaps did not use an N30 type toroid.
But unfortunately it does not really explain the best way to resolve the issue of not getting a high enough peak voltage in secondary windings.
Perhaps the web masters might consider creating a page with links to all the resources I / others have found on GDT design - a hub that folks like em can go straight to rather than asking what I am certain are the same questions over and over.
From this excellent web site: I learned that R3 and R4 are meant to dampen the ringing in the GDT.
The gate of the FETs form a capacitor with source that connects to either GND or the mid point of the half bridge.
Does the combination of the damping resistor and the gate capacitance amount to a low pass filter that attenuates the high frequency ringing?
Also I hooked my GDT up properly to my half bridge today and found that I have this problem:
And according to the above excellent web site, that can be caused by an inadequate gate drive IC. So apparently I have chosen poorly with UCC27425.
But never mind. Due to my so far unsuccessful efforts with a half bridge ignition coil driver I am a great deal more confident with darlington totem pole gate driver, which was working well. Including an inverting totem pole using a 555 based NOT gate on the bases of the darlingtons.
So I will replace my UCC27425 with pair of darlington totem poles - space is not really an issue.
[Edited by moderator Mads Barnkob - merged double post and removed image larger than 600px]
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How can I improve this GDT waveform?
