Due to advances in semiconductor technologies, solid-state switches can now substitute thyratrons, ignitrons, spark gaps and electromechanical high voltage relays. TTL control input and low power electronics replace expensive heater supplies and drivers found in older systems. A high voltage switch can be chosen that meets application, system or load type criteria such as voltage, current, frequency and on-time. However, some extra work is required to ensure optimal performance of the switch in the application.
High voltage switches are composed of an array of semiconductors controlled by a sophisticated trigger mechanism. Overvoltages and high peak currents can be destructive so care must be taken when choosing a switch to ensure that specification limits are never exceeded. Also, the high dV/dt and dI/dt created by switching events requires care to be taken with circuit layout, wiring, shielding and grounding so that cross-talk and over-voltages are kept to a minimum.
How is power dissipated?
The equations below show how the power dissipated in the switch is proportional to the resistance R, capacitance CL and frequency f in each case. Typically not much can be done to escape the effects of the voltage but resistance, capacitance and frequency can be minimised to limit power losses.
Power dissipated inside a high voltage switch for frequencies <100Hz (left) and >100Hz (right)
The static on-resistance R will be given on the datasheet for each switch and the load capacitance CL should be known or estimated conservatively. The value for PD should then be compared with the value given on the datasheet.
If the value for power dissipation is greater than that given on the datasheet then additional cooling options may be needed. The power dissipation for a variety of cooling options is usually listed to help identify the correct solution.
Different types of switch
Different switch technologies suit different applications. High voltage switches supplied by PPM use four main technologies to cover 1kV – 140kV and 15A – 16kA:
- MOSFET – (Metal Oxide Semiconductor Field Effect Transistor)
- IGBT (Insulated Gate Bipolar Transistor
- MCT (MOS-controlled Thyristor)
- SCR (Silicon-Controlled Rectifier)
Critical low frequency or resistive load applications may use a low on-resistance switch to help to reduce ohmic losses. Push-pull types have two switches in a half-bridge configuration. This enables the load to be actively discharged to provide a very fast falling edge.
Analysing and modelling the system
The final choice may come down to factors that only become apparent when analysing the entire system. Modelling the entire system using software such as PLECS or SPICE will enable you to optimize parameters and finalise your switch selection. For example, inductive loads or large stray inductance will generate reverse voltages which could damage the switch. Using freewheeling diodes can protect the semiconductors within the switch from these events.
Avoiding damage to the switch
The values specified by the manufacturer should not be exceeded as this may destroy the internal MOSFETs or the trigger circuit. Particular care must be taken with cable routing and shielding as large voltage and current transients on the high voltage circuit can induce significant instantaneous voltages on low level control lines. Subtle overcurrent and overvoltage events should be identified initially at low voltages so as not to create long term reliability issues in the full system at full load. Initial system testing should be done at low voltages to check there are no obvious issues. Overvoltages will scale with voltage – checks should be done to ensure low and high voltage signals remain in the correct limits determined by the switch or other components. Tests should be done at low operating voltages first before increasing to the full high voltage value.
PPM supply fixed on-time switches, variable on-time switches and pulsed power switch assemblies. The full range offered by PPM can be found in the high voltage switches section.
Fixed On-Time Switches
Variable On-Time Switches
Pulsed Power Switch Assemblies
||Current Rise Rate
|Pulsed Power Stacks
||20 – 50 kA
||10 – 30 kA/us