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I recently made a high frequency transformer which I will be using to drive a large spark gap tesla coil. I machined the bobbin at work from nylon and bought the 60x55x15 cores on Mouser to form a 120x55x15 core which I will expect to handle at least 1kw input and has already been tested to over 4kw input and is holding up very well with no signs of failing. I have also made taps on the primary coil (5,10,15turns) to vary output voltage and power respectively. Now I need an efficient technique to rectify this transformer.

I understand I need avalanche rated diodes. My question is, are ultra fast diodes such as a recovery time of <75ns for example automatically rated as avalanche. I do not fully understand what makes a diode avalanche so I'm struggling to find some from RS Components although I do know they are designed to break down to handle reverse voltages in a controlled fashion.

Switching frequency is 45khz on a full bridge, output voltage on the primary tap of 15turns is roughly 20kv on the secondary

Thanks

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  • \$\begingroup\$ expected current? \$\endgroup\$ – Sunnyskyguy EE75 Jan 4 at 17:13
  • \$\begingroup\$ A maximum of 250ma \$\endgroup\$ – walshy155 Jan 4 at 17:15
  • \$\begingroup\$ I was intending to buy several single diodes from RS Components UK and making a series string, then they will be housed in the oil with the transformer. \$\endgroup\$ – walshy155 Jan 4 at 17:24
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Or use 40 per string for margin. 1N4007 8pF each But verify L and SRF freq. enter image description here

Insert diode PCB in oil.

BEWARE that forward capacitance increases with current and Ri decreases and trr recovery time increases. So if your average current is 250mA yet peak current 10x this you will have a trr problem so use faster diodes such as RS1MWF ( Diodes inc.) SMD.

enter image description here

or Buy from here

Then you need these.

enter image description here

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  • \$\begingroup\$ I was intending to buy several single diodes from RS Components UK and making a series string, then they will be housed in the oil with the transformer. As above thanks \$\endgroup\$ – walshy155 Jan 4 at 17:33
  • \$\begingroup\$ The Oil will add many pF unless you coat the solder joints & leads in Silicone \$\endgroup\$ – Sunnyskyguy EE75 Jan 4 at 17:36
  • \$\begingroup\$ I see, so can I just leave the transformer in oil and the series diode string in air? I see people using UF4007 but I was told to use avalanche rated diodes \$\endgroup\$ – walshy155 Jan 4 at 17:41
  • \$\begingroup\$ 8pF is not bad for each in series. You're going to use > 80 diodes? \$\endgroup\$ – Sunnyskyguy EE75 Jan 4 at 17:43
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    \$\begingroup\$ 1N4007 have perhaps 5-30\$ \mu\$s trr, not at all suitable for 45kHz. They're okay at 60Hz or 400Hz. \$\endgroup\$ – Spehro Pefhany Jan 4 at 18:19

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