Early induction heating power supplies were mechanical medium frequency generators, with a low electric efficiency of 70% to 75%, and have gradually been eliminated from the range of induction heating. They have been replaced by thyristor medium frequency power supplies, also known as controllable silicon medium frequency power supplies. The frequency of the thyristor power supply is in the range of 2.5 to 8 kHz, greatly expanding its application range.
Compared to mechanical medium frequency, the advantage is that it has a small volume and light weight; it has no mechanical movement, and the noise is small; it can start and stop instantaneously; and the frequency automatically tracks during the operation of the workpiece.
The disadvantage is that the overload capacity is low, the failure rate is high, and the price is relatively high.
Electronic tube high frequency power supply is easy to tune and use, although its frequency is high, its application range is still relatively wide. The disadvantage is that its electric efficiency is low, about 50%; the operating voltage is too high, and the safety is poor.
Since the 1990s, high frequency power supplies based on transistors began to be developed (SIT high frequency power supply, MOSFET high frequency power supply, IGBT ultrasonic power supply, etc.).
The static induction transistor SIT is actually a junction-type field-effect transistor. The SIT electrostatic induction transistor power supply combines high current, high voltage, and high frequency characteristics, but it is difficult to overcome its disadvantages due to the small power of a single tube. Foreign companies have stopped researching and producing it, and dozens of them have been produced in China. However, due to the lack of spare parts, they have gradually become outdated.
MOSFET ( Metal Oxide Semiconductor Field Effect Transistor) is a voltage-type high-frequency majority carrier device. In China, MOSFET high-frequency power supplies with f=50 to 200 kHz have been produced, with a power of up to 200 kW.
IGBT (Insulated Gate Bipolar Transistor) is a product that combines MOSFET and GTR (Power Transistor). It has the advantages of high input impedance of MOSFET and low on-state voltage drop of GTR.
GTR has a low saturation voltage drop and a large current density, but requires a large driving current; MOSFET requires very little driving power, has a fast switching speed, but has a large on-state voltage drop and a small current density.
IGBT combines the advantages of the above two devices, with small driving power and low saturation voltage drop. In the 1990s, IGBT ultrasonic power supplies were developed successfully in China. Currently, domestically produced IGBT power supplies are very mature, and the frequency can reach several hundred kilohertz.
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