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?
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
An alternative is to use transistors with a guaranteed hFE.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Royer Oscillator as a DC-DC converter
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
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
RE: Royer Oscillator as a DC-DC converter
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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 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
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.