One popular approach to producing a variable EHT supply with an output range of a few kilovolts is via control of the input voltage to the EHT transformer’s primary using SCRs or triacs. During the development of such a supply for the thinfilm coating unit, it was observed that the control of the EHT voltage could be done fairly accurately by controlling the angle of firing. However, it was difficult to regulate such a supply over load conditions.
The conventional technique of taking a sample of the high-voltage dc output and converting it to ac was impractical due to the need for an isolation transformer with high insulation. A novel technique was, therefore, developed in which the change in the primary current of the EHT transformer controls the firing angle of the thyristor module.
The increase in the primary current is sensed by a small resistance (about 1 to 2 Ω). Also, the ac voltage, in the form of pulses, is fed to the primary of a miniature step-up transformer (Fig. 1).
The stepped-up output on the secondary of this miniature transformer is then rectified, filtered, and fed to a small resistor (about 100 Ω). The resistor is connected in series with the dc control voltage, which controls the firing angle of the thyristor module. The actual firing angle is determined by comparing a sawtooth wave with a dc control voltage. The load-dependent dc voltage obtained from the feedback circuit is summed with this dc control voltage and the firing angle is advanced by a proportional amount, compensating for the drop in voltage due to load (Fig. 2).
In a typical thyristor-controlled EHT supply (5 kV dc), the output voltage, with no feedback applied, was observed to drop to about 3 kV at a load current of 500 mA (depending upon the triggering angle). When the feedback circuit was included, partial compensation was achieved and the EHT dc output voltage rose to approximately 4 kV at full load. For other supplies
with outputs ranges of 1 to 2 kV, full compensation could be observed up to about 500 mA of load current.
This technique was quite effective at up to about 70% of the rated current of the EHT transformer. But for higher load currents, the circuit was not as effective due to transformer saturation effects. The primary advantage in using this circuit is the elimination of costly isolation transformers having a very high insulation in the feedback path.