shift from the traditional methods of induction heating power generation. Initially this was limited to the use of thyristors for generating the 'MF range of frequencies using discrete electronic control systems. State of the art units now employ SCR (silicon-controlled rectifier), IGBT or MOSFET technologies for generating the 'MF' and 'RF' current. The modern control system is typically a digital microprocessor based system utilising PIC, PLC (programmable logic controller) technology and surface mount manufacturing techniques for production of the printed circuit boards. Solid state now dominates the market and units from 1 kW to many megawatts in frequencies from 1 kHz to 3 MHz including dual frequency units are now available. A whole range of techniques are employed in the generation of MF and RF power using semiconductors, the actual technique employed depends often on a complex range of factors. The typical generator will employ either a current or a voltage fed topology. The actual approach employed will be a function of the required power, frequency, individual application, the initial cost and subsequent running costs. Irrespective of the approach employed however, all units tend to feature four distinct elements: AC to DC rectifier This takes the mains supply voltage and converts it from the supply frequency of 50 or 60 Hz and also converts it to 'DC'. This can supply a variable DC voltage, a fixed DC voltage or a variable DC current. In the case of a variable systems, they are used to provide overall power control for the system. Fixed voltage rectifiers need to be used in conjunction with an alternative means of power control. This can be done by utilising a switch mode regulator or a by using a variety of control methods within the inverter section. DC to AC inverter The inverter converts the DC supply to a single phase AC output at the relevant frequency. This features the SCR, IGBT or MOSFETS and in most cases is configured as an H-bridge. The H-bridge has four legs each with a switch, the output circuit is connected across the centre of the devices. When the relevant two switches are closed current flows through the load in one direction, these switches then open and the opposing two switches close allowing current to flow in the opposite direction. By precisely timing the opening and closing of the switches, it is possible to sustain oscillations in the load circuit. Output