Re: Choice of Diode (fwd)

[Steven Roys <sroys@xxxxxxxx>]

---------- Forwarded message ----------
Date: Sun, 04 Apr 2004 14:34:55 -0500
From: John Keith <jskeith@xxxxxxxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: Choice of Diode (fwd)

Hi Matthew,

   I do not think that microwave oven diodes or caps are suitable . They are
designed for 50/60Hz use and would not work well at SMPS frequencies. A
string of 4 fast-recovery diodes, such as the popular UF4007, is the way to
go. For the cap I would use polypropylene HF caps like those used for MMCs,
which are designed for this purpose. They are commonly available up to
3000V.

   I would also recommend a higher-voltage transistor since limiting the
primary voltage to 60V will probably not  efficiently utilize the
transformer. Most flyback circuits use transistors rated at least 400V, and
on-resistance shouldn't be much of a concern as the current is low in a
TV-type flyback.

   Just my opinion, though. Perhaps others with more experience my
clarify...

John Keith



----- Original Message -----
From: "High Voltage list" <hvlist@xxxxxxxxxx>
To: <hvlist@xxxxxxxxxx>
Sent: Saturday, April 03, 2004 10:16 PM
Subject: Choice of Diode (fwd)


> 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.)
>
> 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?  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?
>
> Cheers
>
> M
>
> --
> Matthew Smith
> Kadina Business Consultancy
> South Australia
> http://www.kbc.net.au
>
>