Re: Flyback Converter

From: 	RODERICK MAXWELL[SMTP:tank-at-mail.magnolia-dot-net]
Reply To: 	tank-at-mail.magnolia-dot-net
Sent: 	Friday, January 09, 1998 9:13 PM
To: 	tesla-at-pupman-dot-com
Subject: 	Re: Flyback Converter

> Greetings max
> Message text written by INTERNET:tesla-at-pupman-dot-com
> >   On each gate there will be a 12ohm carbon composition resistor to
> dampen inductive ringing in the gates. Also each gate lead will be
> threaded through a ferrite bead to prevent self-induced ocillations.<
> The resistor is fine, but I'm not sure about the ferrite beads - you're
> shorting the leads to reduce inductance but now with the bead
> you are creating an inductor - it may be counter productive.
> I also have two schottky diodes
> 0v->|---driver output---->|------12v
> because I'm using a driver chip and I don't want ringing taking the
> output negative or too high.
> I'm not sure about calculating the output -
> You can calculate the switching losses and the snubber losses but
>  I think most of the losses are in the output transformer - say 80%
> efficient.
> Have fun
> Alan Sharp (UK)

    Thank you for the information! I did wonder about that. I'm getting
most of these methods from a Motorola Tmos databook. They mention that: 

"mosfets high input impedence and high freq. reponse may cause parasitic
oscillations at freq. greater than 100Mhz when all gates are driven from
a common node. Without individual gate resistances a high-Q network is
established that may cause the device to oscillate when operating in or
switching through the active region. The device transconductance,
gate-to-drain parasitic capacitance, and drain and gate parasitic
inductances have all been shown to influence the stability of the
  "By decoupling the gates of each device with lossy elements such as
resistors or ferrite beads, the Q of the circuit can be sufficiently
degraded to the point that oscillations are no longer possible".
                     Motorola TMOS Power Mosfet Data   page 2-7-11

                                  Frankensteins Helper