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Posted on 01 June 2019

Pulsed Power Applications

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Direct Light Triggered Thyristors

Direct Light-Triggered Thyristors (LTT) developed by Infineon were designed to withstand the stress of High-Voltage Direct-Current transmission (HVDC) applications. In this kind of application, several LTTs are switched in series connection to ensure a blocking capability for 500 kV DC.

By J. Przybilla and U. Kellner, Werdehausen, Infineon AG Warstein, Max-Planck-Str. 5, D-59581 Warstein, Germany and H.-J. Schulze, and F.-J. Niedernostheide, Infineon AG, Balanstr. 59, D-81541 Munich, Germany

 

The thyristors are able to handle a di/dt of more than 5 kA/μs up to a maximum current of more than 100 kA. The thyristors can be triggered by a 10 μs laser light pulse and the easy way of stacking makes it possible to achieve higher blocking capability and ensures a good performance for many pulsed power applications. All Infineon LTTs are equipped with a Break-over-Diode (BoD). The avalanche current of this diode triggers the thyristor if the voltage drop across the LTT in forward direction is getting too high and prevents destruction of the device.

Direct Light-Triggered Thyristors (LTTs)

The light sensitive area of the LTTs (Figure 1) is triggered by a low-power laser pulse. The light guide, which is in general a glass-fiber cable, ensures electronic isolation between high voltage and control circuit.

Light-triggered thyristor LTT

To achieve a high turn-on di/dt and a high pulse current capability, an optimal gatedesign is required. In the LTT, the low-power laser pulse is converted into an electrical current and amplified by a multi-stage Amplifying Gate (AG) structure. An additional resistor is integrated to limit the load current of the two inner AGs. In this way the most sensitive AGs are protected against destruction.

The design of the AGs and the main cathode area must be adjusted in such a way that the main cathode takes over the load current fast to release the AGs as soon as possible after triggering. After the main thyristor is turned-on, the conductive area spreads out with a velocity of 0.01 mm/μs to 0.1 mm/μs–depending on the current density.

The central area of the thyristor with the integrated Break over Diode (BoD) and the four-stage AG structure is shown schematically in Figure 1. The BoD is located inside the light-sensitive area. If the forward-biased thyristor is illuminated by a short light pulse (typical duration = 10 μs, light power³ 40 mW, wavelength = 900-1000 nm), the generated electron-hole pairs are immediately separated in the space charge region of the p-n- junction formed by the central parts of the p-base and the n--base. The current generated in this way provides the trigger current for the first AG. If a voltage pulse in forward direction with a too high amplitude is applied to the thyristor, the avalanche current of the BoD provides a trigger current to turn the thyristor on. This is achieved by the curved shape of the p-n junction, which ensures that the electric field always has a maximum value at the BoD when a forward voltage is applied to the thyristor. Thus, when the device voltage exceeds a certain threshold, avalanche generation starts in the BoD, providing a trigger current to turn on the thyristor safely by using of the AG structure. Therefore it is guaranteed that the BoD is working over the whole positive temperature range, a wide range of dv/dt and for typical operating frequencies (50 Hz - 60 Hz).

Differences of solid state switches

Figure 2 shows the principle difference in current pulse capability of ETTs derived from GTO technology and LTTs. For longer pulses with lower turn-on di/dt, LTTs are favorable because of their larger active area. In principle it is possible to create LTTs with small emitter structures and a large gate area too, but this presupposes a different design of the whole structure. Unfortunately, the market figures in pulsed power applications are not high enough to justify that development.

Typical di-dt capability for sinusoidal pulse currents of ETTs and LTTs

Pulsed Power Applications

LTTs are most suitable for pulse durations between 10 µs and 100 µs and for high-voltage systems requiring series connections. This makes them favorable for several applications (Figure 3). The square dots in Figure 3 are projects realized with LTT devices.

Typical Pulsed Power Applications

49-MJ Pulsed Power Supply System

This 49-MJ Pulsed Power Supply System (PPS-system) was realized as a power supply for a new pulsed high magnetic field laboratory that was installed at the Forschungszentrum Rossendorf e.V. The PPS-system was developed to generate magnetic fields of the order of 100 T that are needed for basic investigations in the high magnetic field physics. The 49-MJ PPS-system is built up modular. It consists in total of 20 modules. There are 14 modules with 2.88 MJ per module, 5 modules with 1.44 MJ per module (Figure 4) and 1 module with 0.5 MJ. The different types of modules are used to perform different kinds of current waveforms to be flexible in the investigations. All modules are built in the same way: a charging unit with a maximum voltage of 24 kV charges a high density capacitor. A stack of Infineon direct light-triggered thyristors is used as a switch to discharge the stored energy to the load. A stack of diodes is used as a crowbar to avoid the ringing of the current. In Figure 4 a drawing of a 1.44- MJ module is shown. The Pulsed-Power- Supply-System was commissioned in 2005 successfully.

1.44-MJ PPS-Module

Fast medium-voltage semiconductor switches

Because of the advantage of the fast switching capability compared to mechanical switches Infineon delivered thyristor stacks, equipped with LTTs T1503N75T for a short-circuit test field to investigate the behavior of low-voltage switch gears. The thyristor switch supplies the device under test temporary precisely with the surge current and is suitable for 20 kVRMS. The LTTs are bypassed after a running time of about 150 - 200 ms by mechanical switches.

Medium-voltage switch

Replacement of Ignitrons

Ignitrons are subject to aging, like all electron–tube switches. In addition they contain mercury. This is why they are already forbidden in some countries. Interdictions in further countries will follow.

As a replacement for ignitrons, Infineon has supplied stacks provided with LTTs T2563N75T for a power–transfer system and crowbar applications. The stacks are designed for 35 kV blocking voltage and for load currents of 80 kA (700 µs sinusoidal or 25 µs trapezoidal).

Fast Magnetic Forming

Metallic materials are deformed or interconnected by high current pulses. Applying this technique has several advantages. For example, pipes connected in this way do not need any sealing. Figure 6 illustrates the principal electrical circuit of a magnetic forming system.

Block diagram of magnetic deformation

Summary

Direct light-triggered thyristors are a good solution for closing switches in many pulsed power applications because of the easy way of triggering by a low-power laser pulse and their high di/dt capability. Due to the many projects that were realized with LTT devices, Infineon has got good expertise of the switch design in pulsed power applications.

 

 

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