High-Frequency Power Supplies

In cathode-ray tubes for oscilloscopes or television receivers, high-voltage low-current sources of d-c power are needed for the accelerating anodes. Voltages range from 1 to 30 kv, and currents are on the order of 1 ma down to 1 μa.

Because of the many turns of wire required in high-voltage 60-cycle transformers, high-frequency power supplies are often used. A tetrode or pentode tube is used in a double-tuned circuit as shown in Fig. 182.

Fig. 182. R-f power supply.

Here the plate circuit is tuned by means of elements L₁ - C₁ and the load circuit by means of L₂ - C₂. The grid winding is coupled from L₁. The coupling between L₁ and L₂ is usually higher than critical, so that changes in load do not affect the resonant frequencies, but instead vary the relative height of the two humps.

In general, the transformer represented by inductances L₁ and L₂ is step-up. Load R_L may be a voltage doubler, tripler, or quadrupler, depending on the voltage needed from the oscillator.⁽¹⁾

Frequencies used in such power supplies range from audio to medium high radio frequency. The oscillator is usually operated class B or C; loaded Q (or ratio of volt-amperes to watts) ranges from 10 to 20. Lower values of Q result in oscillator instability with load changes.

With ferrite cores, departures from the overcoupled case are feasible. Frequency is usually lower than with air-core transformers. The core itself affords a certain amount of load to the circuit and therefore results in better voltage regulation from load-on to load-off conditions. The single-tuned oscillator of Fig. 152 then becomes practical.

(1)	See "Radio-Frequency-Operated High-Voltage Supplies for Cathode-Ray Tubes," by O. H. Schade, Proc. I.R.E., 31, 158 (April, 1943).