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Royer Oscillator as a DC-DC converter

Royer Oscillator as a DC-DC converter

Royer Oscillator as a DC-DC converter

(OP)
We have a relatively old (ca. 1970s) design for a DC-DC converter which centers on a saturable core toroid transformer.  A center tap feedback winding provides the signals to the bases of two complementary NPN transistors; these transistors nominally switch at around 5 kHz.
At room temp and above, the circuit starts up fine.  At cold (below 0°C), the unit sometimes comes up screaming in the hundreds of kHz and stays that way - dissipating lots of power but doing nothing useful.
Total secondary load is around 0.75 watt.
Obviously a driven power supply is the way to go, but our customer still wants this groovy-man era item.
Has anyone had experience with this?

RE: Royer Oscillator as a DC-DC converter

This may be due to the fact that transistor hFE is too low at low temperatures. That prevents the oscillator from starting at the right frequency. Instead it starts at a much higher frequency where the feed-back to the bases is strong enough to keep oscillation going. I would try a few more turns in the base feed-back winding. Or have you done that?
An alternative is to use transistors with a guaranteed hFE.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Royer Oscillator as a DC-DC converter

(OP)
Thanks Skogsgurra.  Transistor hFE definitely is a factor.  Vbe increases at cold, making the situation even worse by reducing the base current.
I've also added a series RC network across the B-C of each transistor as an LPF (adding to the Miller capacitance).  This helps with starting up, but "rounds" the square waves, reducing efficiency.  Frequency response seems to be a double edged sword on this one.

RE: Royer Oscillator as a DC-DC converter

Yes, you are right, Vbe is higher at low temperatures. A few more turns should also help against that.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Royer Oscillator as a DC-DC converter

As a hobby, I occasionally rebuild tube-type mobile transceivers built in the 60's. I've noticed the transistor inverter/toroid type supply don't quickly start at cold temperatures, and I've always assumed that this was because the charcteristics of the germanium power transistors used. There's about a 3 second pause before starting when power is applied at cold temperatures. However, I've never scoped it - you hear the switching audibly. Secondary load on this kind of unit is 20 to 100+ watts.

RE: Royer Oscillator as a DC-DC converter

I assume the guts is similar to this:

http://www.edn.com/archives/1995/021695/graph/04di1fg1.htm

Clearly this is a purely analog circuit, so nobody now knows how to design them!

I suspect a common-mode feedback path is causing the problem (in addition to the low transistor gain at low temperatures). You may be able to suppress this mode with a common-mode choke supplying both transistor bases.

Failing that maybe a base - ground RC snubber on one or both sides will reduce the gain to the common-mode oscillation sufficiently.

RE: Royer Oscillator as a DC-DC converter

The royer convertor is still in use in some modern convertors, it is simple, easy, cheap and reliable.
The bias network should have a forward biased diode in the lower end to compensate for the transistors Vbe change. This is often replaced with a resistor to cheapen the price,  not fitting the diode can make starting a bit hit and miss.

RE: Royer Oscillator as a DC-DC converter

(OP)
Thanks, all.  One issue with the circuit is its symmetry.  A simple circuit analysis is useless in that, with ideal components, the base current splits evenly (like balancing a ball on the end of your finger).  In reality of course the parameters aren't exactly the same and the base current isn't split identically.
That said, a startup circuit to deliberately bias one transistor differently than the other seems to help.  The challenge is to make that part of the circuit "invisible" once it's up and running.

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