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Registered Member #952
Joined: Mon Aug 13 2007, 11:07AM
Location: Finland
Posts: 388
Maybe we should get back on the topic. There's no need for another thread complaining 'how bad is the 555+MOSFET driver'.
First, try to reverse your primary phasing (swapping the primary leads.) If you have a DC flyback, that should solve the spark length problem.
Now well, let's have a look at your MOSFET: 2SK2771 Max Vds: 900V, should be fine Max Id: 9A, a bit small, but still fine Rds(on): 1.1 ohms
The high Rds(on), or the ON-state resistance of the MOSFET, may explain your heating problem. As the MOSFET switches ON, it acts as a 1.1 ohm resistor. Let's say there is about 3 amps going into the circuit. So we use I^2*R to determine the resistive losses on the MOSFET: 3^2 * 1.1 = about 10 Watts. That's kinda lot, because at 12V there'll likely be only about 20-50W going to the circuit. So pretty fair amount of the power is lost. There may be other losses, too. However, with your giant heatsink it should be fine to handle.
So two things to try: 1. Swap the primary leads 2. Try a different MOSFET with a lower Rds(on), IRF540 for example.
Also, if you have some pulse-rated .1u >100V capacitors, try placing one of them parallel with your MOSFET.
Registered Member #1576
Joined: Sun Jul 06 2008, 12:48AM
Location:
Posts: 13
With the primary leads the other way around, I get a spark about 1mm long. Also, I forgot to mention that I have a 0.22uF capacitor across the leads of the primary. I'll switch out the mosfet and see what happens.
Edit: the only other mosfet I had was a IRF510, and the sparks that I got were about the same length as the 2SK2771.
Registered Member #834
Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644
0.22 uF is possibly too much. With the normal input winding the correct value is less than 10 nF. With this external winding, that probably has less inductance, it would be larger. Add a potentiometer to control the voltage on pin 5, and you will be able to control the power. A diode across the transistor is important too for the efficiency, if it already doesn't have one.
Registered Member #1576
Joined: Sun Jul 06 2008, 12:48AM
Location:
Posts: 13
I changed the capacitor to a 10nF and added the diodes, and I got sparks that started at 2cm. However, I noticed the primary is sparking between the winds, so I'm going to replace the primary.
Edit: For some reason, switching the primary leads doesn't seem to have any effect on the spark now other than changing the noise.
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flyback driver problem
