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High Voltages in the range of 10kV to 100kV and at low power finds use in a number of industrial equipment such as ionizers. Special electronic circuits are required to generate this high voltage from the relatively lower and safer to handle voltages available. The most straight forward method would be to use a voltage step-up transformer that would convert the lower voltage alternating current on the primary side to a high voltage alternating current on the secondary side. For a compact and lower cost solution, a multi stage voltage multiplier circuit fed by a high frequency AC voltage is used.Voltage Multipliers
Voltage Multipliers are circuits consisting of a network of capacitors and diode rectifiers. They work on the basic principle of an AC voltage charging sets of capacitors to a peak voltage. The output would be the sum of voltages across a string of such capacitors. The diodes form valves that enable charging in a particular direction and prevent the discharge during the negative of the AC voltage. Voltage multipliers can be used to generate bias voltages of a few hundred volts to millions of volts.
Various types of multiplier circuits, each having a particular advantage, are available. The most simple and common of these is the Half Wave Cockcroft-Walton (CW) Multiplier. The figure below gives the basic schematic for a CW multiplier. This circuit is preferred if the output ripple and voltage drop are not as much critical as the cost and size of the unit.
Basic Circuit for the HV Power Supply
The high voltage generating circuit could have a structure as given in figure below. This is a half wave CW multiplier of ‘n’ stages, powered by a high frequency, high voltage with a peak value Vsec. Voltage Vsec is the output of a step-up transformer. The AC voltage to the primary of the transformer is derived from the 24V DC power supply using an inverter (or oscillator) circuit. High Voltage(105kV negative potential shown here) with respected to the chassis is available at the final stage of the multiplier circuit.
CW Multiplier Design
The basic equation for the output voltage of the power supply would be given by V(out)= V(sec) x 2n
Where V(out) is the output High Voltage V(sec) is the peak of the transformer secondary voltage. This is an alternating voltage. 2n is the number of stages of the CW multiplier. The equation above is valid only if there is no current drawn from the power supply. A voltage is dropped across the multiplier network if a load current is supplied by the circuit. This voltage drop is given by. V(drop)= [I(load) / (Fs x C)] x [ (2/3n^3) + (n^2/2) - (n/6)] Where
V(drop) is the voltage drop across the multiplier circuit.
I(load) is the load current
Fs is the frequency of oscillation of the AC voltage
C is the capacitance of one capacitor on each stage of the multiplier.
n is the number of stages
High Voltage Circuit Design for an Ionizer Power Supply
We consider here a high voltage power supply for a hand held ionizer application. The high voltage design needs to take advantage of the fact that in this application, the output voltage is allowed to drop as the load current is increased. This may mean that the first equation above could be used straight away without considering the drops associated with the load current.
With the source and load parameters usually known, the design narrows down to selecting the following two major parameters. Logic for choosing a particular value is also given.
a)Transformer secondary voltage [V(sec)]:
Due to the overall size restrictions for the example here, V(sec) should be a value that can be achieved and managed in a small transformer of typical EE20 construction. There would be severe restriction on the turns ratio and output high voltage withstand capability of such a transformer. A smaller value of V(sec) would make the transformer construction simple. But the number of stages and therefore the cost and size would go up. A decision based on analytical reasoning and experience needs to be done here. Typically V(sec) would be between 1500V and 7500V.
b)Inverter / Oscillator Frequency [Fs]
The frequency of operation would decide the size and cost of many of the components. The number of turns on the transformer can be reduced with a higher frequency. This would enable higher turns ratio. The capacitance values would come down with higher frequency. At the same time losses associated with higher frequencies would increase specially on the high voltage diodes, transformer and primary side inverter. A frequency between 50 kHz and 100 kHz is practical.
Usually, it is preferred to fix one of the parameter and carry out an iterative design to arrive at an optimum solution for n and C. Practical information about high voltage capacitor and diode voltage ratings will be useful for design. For example, ceramic capacitors of 5kV, 10kV and15kV rating are more common and have multiple sources. It would be better to have the number of stages such that each capacitor sees a voltage that is between 75% and 90% of these rated values.
The iterative design is carried out using a spreadsheet and then fine tuned using a simulation package (eg. pSPICE).
Choice of High Voltage Capacitors
Dry type plastic film or ceramic capacitors are preferred. The exact make and type would depend on the impregnation/coating, size and cost of the capacitors. Ceramic capacitors are normally cheaper and occupy lower volume. However, great care needs to be taken in selecting since many ceramic dielectrics tend have a negative capacitance to voltage curve. Film capacitors with polyester film outer wrapping are not suitable since they are not compatible with some potting methods. Reconstituted mica capacitors are the best in terms of stability and current capacity. For the ionizer application here, ceramic capacitors are sufficient.
Choice of High Voltage Diodes
This would be the most critical choice as they are prone to failure. Almost all high voltage related failures are diode failures. Along with choosing the diode, it is critical to designing the cooling of these semi-conductors. The choosing of the diodes start with the reverse voltage withstand capability of each. Sufficient design margin on this need to be given since high voltage transient phenomenon is destructive, complex and difficult to predict. The design can either use a single high voltage diode of the required inverse voltage or a string of diodes whose sum of inverse voltages add up to the required value. In case it is a string of diodes, voltage equalising networks need to be provided. Else, each diode in the string shall be chosen to have enough avalanche energy capability such that an automatic failure free recovery happens. Practically, for the power supply here, it is preferable to go with a single diode solution. The other parameter of importance is the reverse recovery time. This would be decided based on the inverter frequency and topology.
In order to save space and cost, it is also possible to build the high voltage diodes on a ceramic substrate. For this the raw materials like silicone diode dies, substrate and plating material are purchased from reputed sources. The dies are then mounted to the substrate using glue. The diode dies are then connected in series using a laser etching method.
Choice of Potting Compound
A wide variety of potting material is available in the market. Key parameters for choosing one type are adhesion to components, thermal conductivity and high voltage breakdown strength. Usually a two part semi-viscous urethane or silicone type is preferred. The thermal expansion of the part with respect to the component should also be tested.
The potting process is proprietary and “holds the key” to a successful high voltage product. The process generally involves the following procedures. The electronic circuit assembly is mounted to a suitable mould. The mould gives the shape for the assembly. Further, the choice of the mould material will also determine the overall high voltage capability. This assembly is then cleaned and dried in an oven. Following this, the assembly is coated with a surfactant. This is done to ensure the potting compound adheres to the different parts in the assembly. The assembly is then introduced to a vacuum chamber maintained at a particular temperature. Meanwhile, the potting compound is prepared for the moulding. Generally, the manufacturer recommends a mixing procedure. The potting compound is then poured into the mould in vacuum. The rate of pouring is controlled. Depending on the complexity of the assembly, there may be two or more layers of pouring. Once the pouring is complete, the vacuum is released by introducing an inert gas into the chamber. Following the primary curing time, the assembly is taken out for final curing.
High Voltage Transformer
Since this would be potted module without liquid insulating medium, the high voltage secondary should withstand the voltage by with its insulation alone. Possibly, triple insulated wires may be used as the winding wire on the small transformer. A concept of using cascaded transformers and transformers in series could also be employed.
Inverter /Oscillator
Given the power and primary side voltage levels, it is best to use a single switch converter. This should be configured either in a Flyback or quasi-resonant Forward topology. The transformer parasitics and reflected multiplier capacitance would provide sufficient inductance and capacitance to enable a soft switched converter. A MOSFET could be employed as the primary switch. If a closed loop control is required, a compensated voltage feedback divide is required. Many applications run on open loop with a potentiometer that can be adjusted for manual control of the High Voltage. A self oscillating controller or a simple chip based controller can be used.
Other Considerations
The circuits would be arranged on printed circuit boards of preferably FR4grade laminate. An inrush limiting resistor would need to be added just at the output of the high voltage or at the input to the multiplier to protect the HV diodes or sometimes the load itself. For output current measurement, a current sensing resistor would be placed on the ground return path (see block diagram).