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Registered Member #3704
Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92
I've been thinking guys, should I stick with the dual schmitt-trigger inverter to clean up the feedback, or should I opt for a zero-crossing detector instead? I'm just wondering if there's a way I can improve the signal conditioning section of the circuit.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
The Schmitt trigger inverter is perfect for this job, I'm not aware of any better solution. And you can use only one inverter gate, two of them are not required.
Registered Member #3704
Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92
wrote ...
When the interrupter is disabled, there should be no voltage on the GDT primary. As you pointed out, the drivers by itself are always at opposite logic levels when the inputs are connected, so in this configuration they are unable to turn off the GDT. Also, you did want to power the gate drivers from 5 volts? NO, use 12 or 15 volts.
I see what you mean now. To this point I was using the interrupter to switch the phase, but I see now that that is not what I want to be doing. ONLY the feedback should be changing the phase, correct? That actually explains a lot of the confusion I was having. I was thinking of replacing the TC4420/9 with the UCC27425, which has an inverting and a non-inverting driver, but includes enable pins. I think that would be best in this case, as the interrupter would do its job correctly.
wrote ...
I would use 10 nF but I guess it doesn't matter much. The 10 nF one will quicker block the DC voltage. In this application, you can clamp the CT output to the 5V rail by two 1N4148 diodes. The current here is small. If the current would be larger, you would need to make sure the voltage of the 5V rail does not increase because of the additional rectified "supply current"; in this case it's best to clamp the input to the zener diode instead of the 5V supply.
I'm sure I can get my hands on a 10nF capacitor, so I won't need to use a 100nF one.
wrote ...
You use a CT in a similar configuration like your secondary base current sense, just pass the primary wire through it instead of the secondary ground. The current on the CT output will be increased of course, so the clamping circuit and nearby components would need to be modified. There are different ways of dealing with this.
Sounds good, I'll give that a try!
Dr. Dark Current wrote ...
The Schmitt trigger inverter is perfect for this job, I'm not aware of any better solution. And you can use only one inverter gate, two of them are not required.
Would there be any benefit to using two? Obviously the signal wouldn't be inverted, but I don't know if that's really important. Of course, with two there will be more of a delay. I may just use one after all.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Yes two of them will just increase delay. You can change the phase by the CT connection. IIRC I've had some problems way back with 1 inverter gate producing false pulses, but that was probably just a bad layout. If your frequency is say below 300 kHz, you can use two gates, above that I would use one.
Registered Member #3704
Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92
Hi folks,
Just checking that I've done this right:
I am using a UCC27425 dual MOSFET driver, which contains one inverting and one non-inverting gate, as well as actual enable pins. I could probably still use the TC4420/9 with NAND gates, but I figure the smaller the better. This way I only need a single 8-pin DIP package.
I have added a DC-blocking capacitor in series with the driver output to prevent it from shorting out the chip
I am using a 1:1:1:1:1 GDT instead of a center-tapped one (which I realize was a terrible idea from the start)
Two of the secondaries of the GDT are in-phase with the primary, and two are out of phase. Each secondary is connected (through a 10-ohm resistor to the gate of its corresponding FET (the in-phase windings are connected to the FETs diagonally opposite from each other in the H-bridge)
I have added back-to-back 20V zener diodes between the gate and source of each FET to prevent voltage spikes from damaging them. I may opt for a TVS diode, but either should work ok
I have added a decoupling capacitor between +50V and Ground in the H-bridge
I added a 4.7uF film capacitor in series with the primary to also block DC
Current transformer is simply a ferrite toroid transformer with ~50 windings for the feedback, and the primary coil wire is passed through the center
One side of the CT output is grounded, the other is passed through a 10nF capacitor into a single Schmitt trigger inverter (7414)
Between the capacitor and the schmitt trigger I have opted to use two Schottky diodes to clamp to the 5V rail, which is from a separate supply. It is well-regulated, so I would not worry too much about fluctuation, at least not to an extent that would cause problems
I will see about re-drawing my schematic so that it is more readable. I have been drawing it by hand, and I admit my handwriting is pretty awful!
So far, does everything sound correct? I believe I've followed your suggestions, but feel free to correct me if I misinterpreted them.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
wrote ...
Current transformer is simply a ferrite toroid transformer with ~50 windings for the feedback, and the primary coil wire is passed through the center
One side of the CT output is grounded, the other is passed through a 10nF capacitor into a single Schmitt trigger inverter (7414)
The output current from the CT will be MUCH higher if you use primary feedback. Like 1 amp or more. Simply clamping to the 5V rail is not possible; the regulation of the supply plays no role here, what matters is that the supply cannot take reverse current. Wind more turns on the CT (like at least 100) and clamp the output using 1 amp Schottky diodes (1N5818) to a beefy ("5 watt") zener diode, which should have a voltage rating of about 4-4.7V. Connect a 100 ohm safety resistor in series with the Schmitt inverter's input. Because of the high current involved, increase the DC blocking capacitor size to 100nF (or rather 220nF).
Note: Schottky diodes have a relatively high capacitance. Using them to clamp the high current transformed from the primary winding is OK, but if you used them to clamp the CT sensing the secondary base current, you would again have problems with delays. In the second case, use 1N4148 signal diodes.
Registered Member #3704
Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92
Dr. Dark Current wrote ...
The output current from the CT will be MUCH higher if you use primary feedback. Like 1 amp or more. Simply clamping to the 5V rail is not possible; the regulation of the supply plays no role here, what matters is that the supply cannot take reverse current. Wind more turns on the CT (like at least 100) and clamp the output using 1 amp Schottky diodes (1N5818) to a beefy ("5 watt") zener diode, which should have a voltage rating of about 4-4.7V. Connect a 100 ohm safety resistor in series with the Schmitt inverter's input. Because of the high current involved, increase the DC blocking capacitor size to 100nF (or rather 220nF).
Note: Schottky diodes have a relatively high capacitance. Using them to clamp the high current transformed from the primary winding is OK, but if you used them to clamp the CT sensing the secondary base current, you would again have problems with delays. In the second case, use 1N4148 signal diodes.
The other points seem correct.
Jan
I was wondering how the higher current on the feedback transformer would be handled. I see now what you meant regarding the 5v supply.
Now, when you suggest clamping the output to the zener using the schottkys, I guess I'm still a bit unclear as to how this would be connected...?
I'm not sure if I can get my hands on high-power zeners, but I will see what I can find.
In regards to the current transformer, someone recommended I use a 1:1000 transformer, which is created using two toroid cores, each wrapped with two (twisted) wires connected in such a way as to get 32 windings on each, and 1024 windings total (32*32). I'm wondering if I ought to go this route. What are your thoughts?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Like this
If you use a 1:1000 transformer, the current will be lower and you can then use a "standard 1.3-watt" Zener diode. It doesn't hurt to use your brain to solve such simple problems The current on the sec side is the current on the primary side divided by number of turns. The primary current is your design thing, then from this calculate the clamping circuit. Even with the 1:1000 CT I would still use a Zener diode and not clamp the output to the 5V supply.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Or how about the CT setup I use, a 1:33 (or 1:100 for larger coils) with two UF5408 type diodes for a burden. See top right of this schematic:
This gives about 2V p-p output at the usual primary current. I also use an ordinary burden resistor in series to drive a peak current meter, but this isn't necessary.
Registered Member #3704
Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92
Dr. Dark Current wrote ...
Like this
If you use a 1:1000 transformer, the current will be lower and you can then use a "standard 1.3-watt" Zener diode. It doesn't hurt to use your brain to solve such simple problems The current on the sec side is the current on the primary side divided by number of turns. The primary current is your design thing, then from this calculate the clamping circuit. Even with the 1:1000 CT I would still use a Zener diode and not clamp the output to the 5V supply.
Hmm, guess I was over-thinking the schematic
I knew a 1:1000 transformer would drop the current significantly more, my question was more about if there were any disadvantages compared to a 1:100 transformer.
@Steve Connor, your schematic actually popped up several times in my searches, and is one of the ones I've used for reference.
here is the updated schematic. Please excuse the crudity of this model. I didn't have time to build it to scale or paint it.
It reflects all of the changes you guys have suggested. Once again, feel free to let me know if I've missed something!
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First SSTC design - Need some critique
The current on the sec side is the current on the primary side divided by number of turns. The primary current is your design thing, then from this calculate the clamping circuit. Even with the 1:1000 CT I would still use a Zener diode and not clamp the output to the 5V supply.
