Re: Choice of Diode (fwd)

[Steven Roys <sroys@xxxxxxxx>]

---------- Forwarded message ----------
Date: Mon, 5 Apr 2004 09:21:01 +1200
From: Malcolm Watts <m.j.watts@xxxxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: Choice of Diode (fwd)

Hi Matthew,

On 3 Apr 2004, at 20:16, High Voltage list wrote:

> Original poster: Steven Roys <sroys@xxxxxxxx>
>
>
>
> ---------- Forwarded message ----------
> Date: Sat, 03 Apr 2004 20:50:44 +0930
> From: Matthew Smith <matt@xxxxxxxxxx>
> To: High Voltage list <hvlist@xxxxxxxxxx>
> Subject: Choice of Diode
>
> Hi All
>
> Further to the discussion on the Tesla list regarding pulsing car
> (automobile) ignition coils with an HV supply capacitor and gap, I
> have discovered that I am lacking a power supply in the 800-1500V
> range.
>
> Looking at the parts that I have lying around, I have decided to build
> an SMPS (TL494 or similar) with full voltage feedback using a 1:58
> flyback transformer, similar to a TV line-out transformer.  This is
> rated at 1500V output, which would suggest a convenient 25V drive.  (I
> have several matrix printer SMPSs to hand with 5V/24V output.)

What supply topology are you going to use?

> I'll run at somewhere around 40kHz, as I don't know if the ferrite
> core is up to being driven at the 100kHz of some of the other
> switching devices I have lying around.  Switching will be done by an
> IRFZ48V, which has a very low 'on' resistance.  A suitable snubber
> will be provided since this device is only rated to 60V.
>
> On the secondary side, I am unsure what to do.  Do I make a string of,
> say, four standard 1000V Schottky diodes as normally used in SMPS
> duty, or would a microwave oven diode be suitable for operation at
> this frequency?

Schottky diodes aren't made wth PIV more than 80V or so. The recovery
speed is going to depend somewhat on the supply topology. Ultra-fast
recovery is a safe all-encompassing option. Microwave diodes look
like resistors at 40kHz.

 Also, would a microwave oven capacitor be appropriate
> after the diode?  Peak voltage, if run at 25V in, would be just over
> 2100V, so well within range of a cap like this. But what about
> frequency?

It's basically doing a DC filtering job so is probably OK but it
would be as well to bypass it with a polypropylene cap.

Malcolm