only 630V

. zondag 20 februari 2011
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Have been working on the HV supply for the PMT this weekend.
First, using a toroid from a broken computer power supply, I made the transformer, 4wdg primary, 160wdg secondary:

The wire on the primary side is plain copper wire, secondary wire is thin isolated wire, primary is about 80uH, secondary has a resistance of 47.3 ohms.
Next, the low voltage side of the power supply:

I used the SIR-470DP MOSFET from Vishay, it was the only mosfet I had which could handle the current, but it's a bit of an overkill (I'm practising for the 50kV).
I used 0.02 ohm for the sensing resistor (the on resistance of the SIR-470DP is 2.3 mohm, a bit of a waste to use 20 mohm series just for sensing).
Also not in the schematic is a 2200 uF, 10 mohm capacitor across the 12V, and a 4.7nF low impedance capacitor directly next to the LTC1619 also across the power.
For the high voltage diode I used a TVR30.
When I connected the 12V, the output was -630V.
Everything seemed to work OK, but the voltage on the feedback pin of was just 1V (should be 1.24). Analysing the thing with the oscilloscope showed continuous switching at 300kHz, with high positive peaks at the primary side of the transformer, and simultaneous negative peaks at the secondary side. Just not enough power on the secondary side to generate -1000V.
Maybe we are saturating the core (I don't know anything about the magnetic properties of the core), or maybe the primary inductance is too high.
I will try with fewer primary windings, and just to be sure I will replace the 10nF/3kV ceramic capacitor on the high voltage side (got it out of a broken TV, so I don't know how good it is, maybe it leaks).
At least the switching is OK! Just a matter of transforming the transformer to make it work.

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