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Registered Member #3964
Joined: Thu Jun 23 2011, 03:23AM
Location: Valenzuela City
Posts: 332
UPDATE:
Hi guys, found the problem. I can also see that waveform (picture posted above) on the GDT secondary is same with the GDT primary, meaning, the driver's voltage profile is sagging dramatically. I've tried to changed my power supply (from 24v/1amp to 33v/3amps) still, voltage waveform dropping around 15vdc, I think I have to revise my driver. maybe the discreet part of my UD is causing these problems. I'm using this circuit(photo below), i checked the waveform of each point from this circuit and found that at POINT A, this "sagging waveform" already exist, boosting it up 'til reach POINT C (one leg of the GDT primary). I also tried to put a local capacitor adjacent to the rail near POINT A. still Voltage is dropping rapidly. Any ideas how can i resolve this other than redesigning my board using mosfet driver ICs? thanks!
NOTE: IRF9530 (must be top mosfet) IRF640N (must be bottom mosfet) - my mistake wrong placement of mosfets in my drawing.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Hi, I can see 2 problems in the schematic you posted: 1) The P- and N-channel MOSFETs are swapped. The P-channel one must have its source connected to the +V, the N-channel one has its source connected to GND. Replace the transistors with new ones and in the correct position. 2) The maximum supply voltage for this circuit is 20 VDC.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
It works good enough with the transistors connected as "inverters". Good decoupling and not-too-low-Rdson transistors are required to decrease supply voltage ripple and shoot-through current. I know of some people who used this circuit with success. However with a slight modification (eg. connecting the gate turn on/off resistors in series with the drains) it would be a cleaner solution.
Registered Member #195
Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111
add 1000 to 2200 uf to your 24v power supply as close as pausible as you can to your driver. also double the size of your GDT so that it won't saturate and then look at it again. You may have more capacitance to the 24v. One danger though pay attention to the temperature of GDT driver fets. the on resistance is 54m ohm and should stay cool
Registered Member #3964
Joined: Thu Jun 23 2011, 03:23AM
Location: Valenzuela City
Posts: 332
Hi, I can see 2 problems in the schematic you posted: 1) The P- and N-channel MOSFETs are swapped. The P-channel one must have its source connected to the +V, the N-channel one has its source connected to GND. Replace the transistors with new ones and in the correct position. 2) The maximum supply voltage for this circuit is 20 VDC.
It works good enough with the transistors connected as "inverters". Good decoupling and not-too-low-Rdson transistors are required to decrease supply voltage ripple and shoot-through current. I know of some people who used this circuit with success. However with a slight modification (eg. connecting the gate turn on/off resistors in series with the drains) it would be a cleaner solution.
Hi Jan,
You are right, the MOSFETS were mistakenly swapped in the drawing, but in my circuit it is correct (e.i. P channel's Source pin connected to V+ and Drain Pin is connected to output/tied w/ N channel's Drain pin & its source pin connected to GND). I used this circuit up to 30volts without any issues. I haven't tried that (gate turn on/off resistors in series with the drains) maybe i'll give some other time to test that too. thank you!
I think the MOSFETs are deliberately used as inverters here, not source followers. Neither way will work that well.
Hi Steve Conner,
you are correct indeed, MOSFETs here used as Current Boosters/inverter (logic). But i think there will be some issues if D & S legs were swapped, though i haven't tested it yet. Thanks!
add 1000 to 2200 uf to your 24v power supply as close as pausible as you can to your driver. also double the size of your GDT so that it won't saturate and then look at it again. You may have more capacitance to the 24v. One danger though pay attention to the temperature of GDT driver fets. the on resistance is 54m ohm and should stay cool
Hi Teravolt,
Yeah, I already done that, putting large capacitance "adjacent" to the 24v rail/of the MOSFETs with some decoupling ones. Still, Voltage drops dramatically to ~ 15 volts. Thank you.
Registered Member #4266
Joined: Fri Dec 16 2011, 03:15AM
Location:
Posts: 874
Hi, I think its 8 times 2volt voltage drop of the diodes, are you using a module or a seprate transitor. They might have one biult in. Hope it helps Andy
Registered Member #3964
Joined: Thu Jun 23 2011, 03:23AM
Location: Valenzuela City
Posts: 332
Andy wrote ...
Hi, I think its 8 times 2volt voltage drop of the diodes, are you using a module or a seprate transitor. They might have one biult in. Hope it helps Andy
Hi Andy,
Thanks for your comment. No there are no 2v drop diode in the circuit, I use 1n5819 schottky diodes so (if there is) the V drop across diode is very little (not reaching 2volts i think). I almost forgot, when I use the circuit above on my previous SSTCs / DRSSTCs I saw no problems on the gates, meaning, there are almost no V drop (all approx 24v )- i think this is because of the very short ON-TIME of about <200 us. Gate waveforms can be seen as almost perfect squares. Not same scenario with the QCWDRSSTC with the on-time reaching extremely to 20ms.
I honestly think that the MOSFET in the level shifting stage (2N7000) with pull up resistor is causing this V drop. today, I'm going to replace it with a new one and then I'll check if it will work. I'll also try changing the value of the 1k resistor but I believe I'm also playing the Rise/Fall time of the level shifting/ and of course the overall switching.
Or maybe, the BC337 NPN is causing this drop due to higher current requirement on its base with the playing frequency of 350~400 khz. I'll try replacing it too, or putting a base resistor in it. I'll post the results sooner. Thanks!
Registered Member #195
Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111
hi Julian, I was thinking is it pausable that your transformer core is saturating or it doesn't have the proper frequency response? If it saturates the transformer would look like a short. would a series resistor with the say 10 ohms help. I don't think there is any thing is wrong with your circuit. Does your 24v sag with your waveform? I wouldn't think driving all 8 gates would drop 1000 uf that badly. If you disconnected a couple gates does your wave form sag less?
Registered Member #3964
Joined: Thu Jun 23 2011, 03:23AM
Location: Valenzuela City
Posts: 332
Hi teravolt,
yes, without the GDT(secondary loaded with 220nf - total to all 4 secondary) the 24v supply rail sags (LESS) means, with or without GDT, 10ms ON time is something that a 24v/1A regulator 7824 cannot supply anymore, I've checked the regulator's input and output, and found that there is still plenty of voltage available in the input but the regulator cannot keep up on the output/and is of course heating badly. I'm currently designing a SMPS to replace the ol' 24v transfomer/24v regulator power supply scheme. hopefully this will solve my problem.
I'll post the results soon but in the mean time, happy new year everybody! Thanks!
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Parallel Full Bridge (8 switches)
