TPT September 2010

A rticle

each module will be around 30kW to limit their number when the power increases. The working frequency is high, so a comparative evaluation of power switches must also be performed. As well known, MOSFETs and IGBTs are the type of switches available on the market: different types have been compared considering electrical characteristics (voltage, losses, etc) and size in order to get the power with a module of limited dimensions. The component choice is the MOSFET to get the wide frequency range; several devices in parallel are required since the low size standard package switch adopted. The last evaluation regards the topology. Because of the desired extended application, the best solution is the CFI with parallel resonant load. In fact, due to the active input stage, both voltage and current are controllable, thus assuring the process repeatability required in hardening (and not reachable with a VFI where the voltage is uncontrolled and depends on line variation). 3.3 Design Development In the CFI topology, the basic switch is a unidirectional one, so a fast recovery diode is placed in series of the active device to have a one-way current path. The power components are arranged in a specifically designed mechanic, useful both for structural and electrical purposes. A deep layout analysis has been taken to get an equal component

Figure 2 : Current fed inverter with parallel load

2.2 Parallel Resonant Inverter The topology dual with respect to previous one is the parallel resonant inverter, presented in figure 2. A rectifying bridge supplies a CF inverter and a parallel resonant oscillator is considered. A big inductor is inserted between bridge and inverter, required to change the dynamic behaviour of the power supply and make it a current generator. The inductance normally is big and heavy, thus resulting in a non compact design: at least for high power design two cabinets are required, one for the input stage and one for inverter and oscillator. The inductance limits the current in case of short circuit, so the inverter is intrinsically protected. Furthermore, the impedance of generator and load are equivalent, so no transformer is needed and the efficiency can increase. From the control point of view, the input stage (thyristor bridge or chopper) is used to regulate the power by varying the switch conduction time, while the inverter is modulated with a frequency as close as possible to the natural resonance one with duty-cycle 50% and superposition time. 3 Design of the inverter In the following, the design of the inverter will be analysed starting from the choice of the best solution. 3.1 Design Topics The goal is the design of a flexible inverter, to be adopted in different induction heating applications, such as hardening, welding, etc. This means that the working frequency must move from 200kHz up to 500kHz, and the coil output voltage should be higher or lower according to the application. In particular, for hardening normally the maximum voltage is around 400V rms, while for welding the maximum voltage may be higher than 1,000V rms depending on the production line. Furthermore, the power requirements are different: generally lower power is necessary in hardening (maximum 150kW for high frequency applications), while in welding higher levels are needed (up to 600kW or more). 3.2 Preliminary Analysis The analysis of the requirements presented above brings us to the development of a modular solution, where the core will be an elementary “brick” that will be properly composed. The power of

stress. In particular the current of each device has been measured by a small Rogowsky probe and verified by some thermal images. The starting situation is shown in figure 3.

Figure 3 : Current measurement

It can be noted that the temperature is not equally distributed, due to an unbalanced current division among the transistors. Of course, when a number of devices are parallelised, the optimal situation is obtained when the current flowing in each of them is 1/n of the total current. In figure 3, five MOSFETs are visible: an unacceptable current difference (of about 40% between the components) has been verified. To reduce the unbalancing to less than 10% a strong effort has been made to reduce all the parasitic inductances and commands delays through an optimised PCB design. The power connections are integrated in the mechanic, while the communication to the control board is made through optic fiber to avoid possible disturbances. The standard load is a resonant parallel. As discussed above, the optimum exploitation is achieved by working as close as possible to the natural resonance of the load itself. This is performed by a closed loop control made by the digital supervising board shown in figure 6.

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S eptember 2010

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