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NE555 works at 150~ kHz frequency and makes nice square wave, but one thing catched my attention. I marked it with green rectangle:
How it's called and how it can be solved?
Next, the whole config:
It contains main transformer+LM7812+10000uF, NE555 interrupter, two gray blocking capacitors, GDT and a series of capacitors. Question - if I don't use this series of capacitor between GDT and minus, square wave gets completely ruins. Why it's that? The capacitance of this series isn't calculated yet, but it's small, in nanofarads range.
When I connect a GDT without load on secondary (the wire I used came from hard disc drive cable and I twisted, the number of turns is 4, ratio 1:1), I get this wave:
I've tested with load of 2.2nF and 10ohm resistors and with IRFP450 and 10ohm resistor. The results were very similiar, IRFP450 since has 2nF gate capacitance. After scoping across gate and source, I get it: (it's not blurry image)
The another weird thing is that, when I scope across primary, oscilloscope says the voltage is 12V (after setting to 1V/cm, it has 1.2cm height, co 1.2cm x 1 V/cm x 10 (probe divides voltage by 10) = 12V. It though that those complementary BDXXXs will give half of voltage?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Tonic wrote ...
NE555 works at 150~ kHz frequency and makes nice square wave, but one thing catched my attention. I marked it with green rectangle:
How it's called and how it can be solved?
It's called overshoot, can be damped with fast clamping diodes (Schottky preferred) to + and gnd rails.
Tonic wrote ...
Next, the whole config:
It contains main transformer+LM7812+10000uF, NE555 interrupter, two gray blocking capacitors, GDT and a series of capacitors. Question - if I don't use this series of capacitor between GDT and minus, square wave gets completely ruins. Why it's that? The capacitance of this series isn't calculated yet, but it's small, in nanofarads range.
I don't know how exactly are the caps connected so I cannot comment on this.
Tonic wrote ...
The another weird thing is that, when I scope across primary, oscilloscope says the voltage is 12V (after setting to 1V/cm, it has 1.2cm height, co 1.2cm x 1 V/cm x 10 (probe divides voltage by 10) = 12V. It though that those complementary BDXXXs will give half of voltage?
It's actually going from +6V to -6V, so the full voltage swing is 12V.
Btw. I don't know why you call it interrupter, 150kHz is not exactly interrupter frequency
Registered Member #528
Joined: Fri Feb 16 2007, 10:32PM
Location: Warsaw, Poland
Posts: 166
jmartis wrote ...
It's called overshoot, can be damped with fast clamping diodes (Schottky preferred) to + and gnd rails.
Tell me more about it, I'm really green in those pulse things. More than one Shottky? What about parameters? Across where exactly? And how it works?
Tonic wrote ...
I don't know how exactly are the caps connected so I cannot comment on this.
Take a closer look at config photo - you can see that one wire from GDT goes to one end of capacitor chain. Second end of chain goes to rectify bridge, to GND. I've updated scheme, there you can see:
I've calculated also a total capacitance of this chain. It equals to 150 pF and turned out that those 220nF capacitors didn't change much at all, it mainly thanks to 300pF ones.
wrote ...
Btw. I don't know why you call it interrupter, 150kHz is not exactly interrupter frequency
Many SSTC/DRSSTC schemes had 'interrupter' name floating around NE555 chip.. so ;) Well, then what is interrupter?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Tonic wrote ...
jmartis wrote ...
It's called overshoot, can be damped with fast clamping diodes (Schottky preferred) to + and gnd rails.
Tell me more about it, I'm really green in those pulse things. More than one Shottky? What about parameters? Across where exactly? And how it works?
One diode from output to +V rail, anode on output. Second from output to GND, cathode to output. These diodes will clamp the output voltage so it cannot go any higher than supply voltage and any lower than GND. However the overshoot in your scope shot is doesn't seem any harmful.
Tonic wrote ...
Tonic wrote ...
I don't know how exactly are the caps connected so I cannot comment on this.
Take a closer look at config photo - you can see that one wire from GDT goes to one end of capacitor chain. Second end of chain goes to rectify bridge, to GND. I've updated scheme, there you can see:
I've calculated also a total capacitance of this chain. It equals to 150 pF and turned out that those 220nF capacitors didn't change much at all, it mainly thanks to 300pF ones.
I'm surprised that it actually works with the caps, why did you connect them like this? Connect the lower transistor collector to GND directly.
You also should put a ceramic/foil cap at least 100nF (preferrably more) from +V to GND, to kill any HF oscillations occuring on the +V supply. This is called decoupling capacitor and they are critical in any HF/power circuits.
Tonic wrote ...
Many SSTC/DRSSTC schemes had 'interrupter' name floating around NE555 chip.. so ;) Well, then what is interrupter?
Interrupter is a pulse generator that is used in SSTC/DRSSTC to turn the coil on and off. Usually have independently adjustable ON and OFF times. Typically they run at 50-1000 Hz. Their purpose (in SSTC) is to decrease power draw of the coil, while keeping the same spark length.
Registered Member #528
Joined: Fri Feb 16 2007, 10:32PM
Location: Warsaw, Poland
Posts: 166
jmartis wrote ...
I'm surprised that it actually works with the caps, why did you connect them like this? Connect the lower transistor collector to GND directly.
Without them signal gets completely ruined - there's photo:
jmartis wrote ...
You also should put a ceramic/foil cap at least 100nF (preferrably more) from +V to GND, to kill any HF oscillations occuring on the +V supply. This is called decoupling capacitor and they are critical in any HF/power circuits.
I tried with 200nF and noticed a slight frequency increase. Ruined waveform didn't change at all, it's still too far from being a pure square wave.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Tonic wrote ...
jmartis wrote ...
I'm surprised that it actually works with the caps, why did you connect them like this? Connect the lower transistor collector to GND directly.
Without them signal gets completely ruined - there's photo:
hmm, I remember you said something about 4 turns on the GDT- if you are really using just 4, this is way too low, the GDT might be saturating. According to your core dimensions and material, you might need 15 or more turns.
To improve the waveform you could also connect a gate drive chip (TC44xx/UCC3132x) instead of the discrete transistor output stage, but I doubt you have one.
You really should use the diodes as I described earlier, on the output of your transistor driver. This could improve the "ruined" waveform.
Registered Member #528
Joined: Fri Feb 16 2007, 10:32PM
Location: Warsaw, Poland
Posts: 166
Finally, I've managed to remove this chain of capacitors and get a square wave.
It was my error and I'm ashamed of this. The reason why I was getting ruined waveform every time I plug GDT directly to ground, was the fact I forgot about previous DC blocking capacitor. In first revision, I used 220nF capacitor (you can see it on PCB). After some reading, I came to conclusion that is too small capacitance to block DC signals, so I've added two 1uF capacitors (grey ones on photo). I was scatty and forgot about previous capacitor which remained connected in series to those two 1uF's. So, total capacitance was lower! Around 200nF. After I figured it out, I removed old capacitor, chain of capacitors and plugged GDT directly to GND. Voila! It worked :) Duty cycle was out, but I did easily set it to 50%. The waveform without GDT is similiar to one before change, but the major difference is that this new has 500kHz frequency instead of 150kHz:
I think that NE555 driver is working properly and now it's time to work on GDT, where I do still have problems. When I plug IRFP450 and 10ohm resistor to gate, I have this waveform on primary:
On secondary I have sinusoidal.
wrote ... hmm, I remember you said something about 4 turns on the GDT- if you are really using just 4, this is way too low, the GDT might be saturating. According to your core dimensions and material, you might need 15 or more turns.
The GDT core has 4cm of total diameter. The ferrite material is unknown, since I bought it on stock-exchange for 25 eurocents. Okay, I will try with more turns.
wrote ...
To improve the waveform you could also connect a gate drive chip (TC44xx/UCC3132x) instead of the discrete transistor output stage, but I doubt you have one.
Those boosting chips are very hard to buy in my country. And costs a bit. So, it's not a good option since I'm at experiment stage. Maybe in future :)
wrote ...
You really should use the diodes as I described earlier, on the output of your transistor driver. This could improve the "ruined" waveform.
I will try it too, but do those diodes must be rated for 12V?
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
Removing the 10 ohm resistor will help somewhat. Getting good gate drive with BD's at 500kHz is pretty tough, and I wouldn't recommend you bother. Use a smaller gate resistor, 2.2 ohms or so, and drop the frequency.
Registered Member #528
Joined: Fri Feb 16 2007, 10:32PM
Location: Warsaw, Poland
Posts: 166
I've added capacitor to C2, so I'm getting 50kHz frequency. After rewounding bifilarly GDT to 10 turns, things get worsen. Just see two photos:
Rewounded GDT: (the new windings are on left side)
GDT waveform:
Without load, waveforms on primary and secondary are practically same. Adding load doesn't change much.
Now, what to do?
Maybe I'm not right, but using Shottky's diodes or changing gate resistor won't help, since they're just factors that improves driver working, and aren't supposed to decide if anything will work, or not.
Seems a lecture of thedatastream's informations is waiting for me :)
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
By dropping the frequency by a factor of ten, you'll need a lot more turns on the GDT. Otherwise it will saturate and give you poor waveforms. Calculate the number of turns required for 50kHz and you should get proper waveforms.
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NE555 interrupter and GDT problems
