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RE: Flyback Converter


From: 	RODERICK MAXWELL[SMTP:tank-at-mail.magnolia-dot-net]
Reply To: 	tank-at-mail.magnolia-dot-net
Sent: 	Wednesday, December 31, 1997 10:22 PM
To: 	Tesla List
Subject: 	Re: Flyback Converter

Tesla List wrote:
> 
> From:   Scott Stephens[SMTP:stephens-at-enteract-dot-com]
> Sent:   Monday, December 29, 1997 12:17 PM
> To:     Tesla List
> Subject:        Re: Flyback Converter
> 
> At 10:44 PM 12/23/97 -0600, you wrote:
> 
> >I have tested The flyback converter
> ...
> >40vdc I am seeing abot 150 volts on the o,scope.
> >I cannot test
> >it at a higher supply voltage yet because the heat sink I have the
> >transistors mounted on is not large enough.
> 
> flyback converters put a high voltage on the transistors, don't they?
> Have you heard of 2ndary breakdown, on transistor data sheets?

 Thats the whole idea! Higher primary voltage/ higher secondary voltage! 
 
  IRF440
  VDS: 500v
  Vdgr: 500v
  ID (continuous): 8amps
  IDM (pulsed drain current): 32amps
  PD: 125 watts
  ILM: 32amps
  
   As long as I keep the drain voltage below 500v, and keep it inside
the maximum safe operating area it should be okay. I have three power
mosfet data books available to me,Harris,Motorola, and Sgs Thomson. Of
the three Motorola has the most application info and circuits, and is
also the most informative.


> >transistors in parrallel. I'm back to using fets because the bipolar
> >blew as soon as I applied power!?
> 
> Fets don't have 2ndary break down, but they do have a RBSOA? curve to check.
> 
> If you keep destroying parts & get discouraged, (as I have in the past)
> get PSPICE from Microsim or Microcap spice or whatever, and a data book.


  I have Electronics Workbench. It gets it in the ballpark as far as
circuit behavior is concerned but it is somewhat inaccurate as far as
the magnitude of the voltage is, and the current draw. 
  My new heat sink came in yesterday. It is in the form of a 4' bar! I
cut off nine inches, drilled the holes for six TO-3 transistors,
deburred, and polished it off with a sos pad. Now I,m waiting for a few
more transistors in the mail. I plan to have six in parrallel.


                               Frankensteins Helper
                                      Max