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High voltage power supply is made by Cockcroft-Walton circuit

While generating high voltage with the winding ratio of the transformer,
create high voltage while sequentially doubling the voltage with the Cockcroft-Walton booster circuit.

Cockcroft-Walton circuit

Figure 1. High voltage power supply is made by Cockcroft-Walton circuit

 There are several ways to generate high voltage for a high voltage power supplies, but the most commonly used method is the Cockcroft-Walton circuit method. The Cockcroft-Walton circuit consists of a multi-stage combination of capacitors and diodes.  Figure.1-1 is a diagram of the circuit and operation of the Two stage Cockcroft-Walton Multiplier.
 The initial current ID1is charged to C1 through D1. Next, the charge charged to C1 flows the current of ID2 through D2 to C2. And then C2 is charged and the voltage is doubled. By repeating this operation, ID3 as well as ID4 flows and finally, this circuit can boost the secondary side voltage of the transformer to 4 times. This is called Cockcroft-Walton 4-stage boosting.  To create higher voltage, the number of Cockcroft-Walton stages should be increased. However, the double voltage is an even multiple and cannot be an odd multiple. With this method, it is possible to produce up to about 200kV, but it is important to select high-voltage diodes and ceramic capacitors accordingly.

 In order to control the high voltage output at a constant voltage, the input voltage is controlled and the constant voltage control by negative feedback is performed. The transistor on the primary side cannot be PWM controlled. Because the high-voltage transformer has a large number of windings, which drives the capacitance, so the pulse width cannot be controlled.  Then, how much can the winding boost of an isolation transformer be possible? Experience shows that the limit is 1:60 for honeycomb winding. Cockcroft-Walton will be used when the transformer boosting is insufficient.

Figure.1-2 shows a 35kV, 500μA power supply designed with the Cockcroft-Walton circuit.
Figure 1-3 shows an 18kV, 1mA power supply designed with the Cockcroft-Walton circuit.
These are commercialized by filling with insulating material and applying high voltage treatment.

- Written by Shotaro Suzuki