Re: single mosfet flyback driver

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Greg,

On 10 Oct 2002, at 7:50, Tesla list wrote:

> Original poster: "Gregory Peters by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <s371034-at-student.uq.edu.au>
> 
> Hello all,
> 
> You are probably all familiar with the common single transistor 
> (often a 2N3055) flyback driver which self tunes to the resonant 
> frequency of the flyback. I was wondering if anyone has tried this 
> with a mosfet? Obviously the circuit would need some modification 
> over that of the transistorised version, but the operating principles 
> would be the same. I see that using a mosfet would have two major 
> advantages:
> 
> 1. Faster switching is possible
> 
> 2. Higher voltage/current mosfets are cheaper than transistors.
> 
> 
> Could anyone out there let me know if they have done this. Or else, 
> can someone offer some suggestions on building such a supply?

Firstly, are we talking about a true flyback supply (gapped core and 
energy release at "switch off" - steering diode) or a core-saturating 
inverter (ungapped core)? The latter is fundamentally a forward 
converter. It takes a lot of gapping to allow the core to store any 
amount of energy.

      If the former, I have used both bipolars and MOSFETs. For a 
flyback with a high secondary inductance, device switching speed is 
not worth worrying about since the risetime is slow due to the self-
capacitance of the windings. MOSFETs offer lower conduction losses up 
to some current dependent on the drain current rating, then bipolars 
take over. The drawback with bipolars is the large amount of 
continuous base current required.

      For core-saturating inverters, I've found bipolars are easier 
to drive.

Regards,
Malcolm