TPT January 2007

P rogress in W elding T echnology, E quipment & C onsumables

Reliable Mosfet selection for induction welding

GH Group, Spain, offers a wide range of modern high frequency power supplies for induction heating applications, from medium frequencies (IGBT inverters) to high frequencies (Mosfet inverters). This complete range of power supplies allows the use of the most suitable technology in every induction heating application, overtaking obsolete and low efficiency technologies. Concerning tube induction welding, which often requires frequencies up to 500kHz, the most reliable and efficient component is the Mosfet transistor. Claiming to be the first manufacturer in using transistor power supplies for induction heating applications, the GH Group has carried out research and development with the Universitat de Valencia since 1982. This research in transistor inverters has led to the development of advanced and reliable technology in induction welders. It is important to examine the latest developments in improving reliability of induction welders by testing and selecting the most suitable Mosfet for tube welding purposes. The appearance on the market of new semiconductors (Mosfet, Fredfet or CoolMos), with intrinsic diodes to increase robustness, enables these components to be used in the form of inverters. Acting as

a)

a)

b)

resonant circuits, some loads can produce capacitive switching with inverter Mosfets. › Figure 2: a) Inverter bridge topology of the series generator, and b) Simulated output current and voltage This capacitive switching produces a hard turn ‘off’ at the Mosfet intrinsic body diode. During this hard turn ‘off’ the parasitic bipolar transistor of Mosfet turns ‘on’ and the semiconductor fails. The test and study of the failure process led to the choice of the most suitable Mosfet.

b)

suppression of a part or of the complete inductor. This process takes place at the contact of the piece with the inductor or during the deposition of parts (sparks, leftovers etc) in the inductor. It occurs at the instant of › Figure 3: a) Current and voltage of Mosfet M1, and b) Output current and output voltage on the inverter bridge fi Figure 4a (below) current and voltage of Mosfet M1 at capacitive switching, and 4b (bottom) output current (discontinuous line) and gate signal at inverter bridge when capacitive switching occurs

In the area of hard short- circuit conditions, the inverter bridge is a full bridge topology with four high voltage switches. Inverters usually work in an inductive mode. In this mode, switching behaviour is ZVS and losses only occur at the turn ‘off’ stage. Figure 1a shows the current and voltage of Mosfet, while figure 1b illustrates inverter output current and voltage. The voltage is square waveform and the current is sinusoidal. Extreme conditions take place in cases of short- circuits of the heating coil. Short-circuits or short-circuits of the inductor are an abrupt

fi Figure 1: View of a solid state Mosfet inverter

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J ANUARY /F EBRUARY 2007