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Registered Member #1694
Joined: Sat Sept 13 2008, 09:13AM
Location: Australia
Posts: 108
Hi,
I'm getting really frustrated with this circuit . . I've tried to make it several times before with no success. Its such a simple circuit but I simply can not get it working.
Here is the components I'm using.
IRFP250s for the fets MR756 diodes (I suspect these might not be fast enough, but I'm not sure.) 2x 3.3uF polypropylene caps in series for 1.65uF @ 500V The transformer is 10 turns with a centre tap, just like i'm sure it should be with roughly 250 turns for the secondary. The inductor is 60 turns on a white and yellow iron powder core. I've also tried 50 turns on a ferrite toroid and got the same results. and yeah, 5watt 470ohm resistors and 1/4watt 10k resistors in parallel with 3 watt 12v zeners.
With power on, it shorts through the inductor, through the transformer center tap and to ground through the fets. Its just not oscillating like it should. Any ideas???
Registered Member #540
Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969
What is your supply voltage? If you are trying to start at too high of a voltage, you may get parasitic oscillations. Could you take a picture of your setup so that we can see what it looks like?
I forgot to mention before that the output from the transformer is being rectified to charge up some capacitors, for a coilgun. The rectifier diodes are 6amp 1N series diodes.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Are you sure you have the diodes "crossed", i.e. from Drain 1 to Gate 2 and vice versa? Are you sure their cathodes are pointing to the drains?
100V diodes are a bit low voltage, try something higher. They don't need to be high current (anything above 0.5A will suffice) but they should be the fast or ultrafast type.
Registered Member #1694
Joined: Sat Sept 13 2008, 09:13AM
Location: Australia
Posts: 108
First of all I tried it loaded, connected to a 2000uF capacitor bank. The voltage of the caps rose to about 10V and stopped, while the inductor & fets got very hot. Once I tried it unloaded, but with the leads from the secondary about 2mm apart to form a spark gap in case something happened. Something did happen . . the windings started arcing together. After that it all it did was get hot. I checked the transformer and there was continuity between the primary and secondary. After rewinding it and testing it at both 24V and 12V, loaded with the 2000uF cap bank, I've still got the same problem. Everything is getting hot, and the caps don't charge.
Registered Member #1694
Joined: Sat Sept 13 2008, 09:13AM
Location: Australia
Posts: 108
Dr. Kilovolt wrote ...
Are you sure you have the diodes "crossed", i.e. from Drain 1 to Gate 2 and vice versa? Are you sure their cathodes are pointing to the drains?
100V diodes are a bit low voltage, try something higher. They don't need to be high current (anything above 0.5A will suffice) but they should be the fast or ultrafast type.
Ooops . . the diodes are actually MR756, 6amp 600V. They are 'fast recovery' diodes, but I'm not sure if they are fast enough. According to the data sheet, their Trr would be between 1 - 30us. And yes the diodes are crossed correctly. I've also checked them with a multimeter and they are still alive and well.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
First, if your construction methods and power supply are good, then this kind of inverter is NOT suitable for charging capacitor banks !
I know several people have used this circuit as a cap bank charger, but it's just WRONG.
This kind of inverter is EXCELLENT for producing constant voltage sinewave power, a discharged cap bank 'looks' like a neat short-circuit as a load.
You should look at constant-current output chargers This is a common topic and looking through this site will reveal many options.
If you power the gate drive resistors directly from the dc supply and apply this power first, With no output load apply dc to the inductor that goes to the primary center-tap (Are you SURE primary phases are correct?). The circuit should be oscillating at full amplitude (peak primay voltage = PI x Vdc) with very little heating - except the resistors. Any heating at this stage must be diagnosed and minimised. Usually it's the dc inductor, For testing add a roll of wire in series. If you use this circuit as a capacitor charger use two inductors in series, the normal hf one plus one as big as you can find/fit.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Sulaiman, if you "integrate" enough inductance into the transformer, it works well for loads such as arcs and cap charging. I don't think it's useful at all as a voltage source because the core gets very hot even with no load (a lot of reactive power).
Camel, what you mean windings started arcing together? Have you correctly identified the HV return (ground) pin?
Registered Member #1694
Joined: Sat Sept 13 2008, 09:13AM
Location: Australia
Posts: 108
Dr. Kilovolt wrote ...
Camel, what you mean windings started arcing together? Have you correctly identified the HV return (ground) pin?
I wound the transformer myself, so yes I have identified it correctly :) I could see sparks arcing in the secondary coil. Also that was the only time I heard it make a wine.
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Mazzilli ZVS flyback circuit
