Understanding Solid-State Switching in Electromagnetic Propulsion

Best known in transportation, electromagnetic propulsion has demonstrated positive results as it harnesses magnetic fields to offer a clean, sustainable option to conventional combustion engines. Promising more energy-efficient transport, this propulsion serves as the heartbeat of trains, vehicles, and spacecraft. The center of the electromagnetic propulsion system is solid-state high-voltage switching semiconductors. Unlike conventional mechanical switching, solid-state devices like gate turn-offs (GTOs), thyristors, and insulated gate bipolar transistors (IGBTs) offer precision regarding electromagnetic field modulation. 

This article explores the selection, design, and calculations necessary to understand the application of electromagnetic propulsion.

 

Figure 1. Electromagnetic propulsion systems, Maglev train and rail. Image used courtesy of Pixabay

 

Selecting High-Voltage Switching Devices

Some factors must be considered when selecting a high-voltage switching device to ensure it meets high-voltage switching specifications.

One of the main factors to consider is the switch's voltage-handling capacity. During operation, the voltage range used in the specific electromagnetic propulsion system is assessed to determine this capacity. Based on the assessment results, a voltage-switching device is selected to handle the voltage levels without compromising safety.

Measures like overvoltage protection are also essential in preventing damage and keeping the propulsion system intact. Voltage clamping or surge suppression can handle voltage surges that affect voltage switching.

Another important factor when selecting high-voltage switching devices is switching speed, which must consider the effects of switching transients and propulsion system speed specifications. Due to the precise control needs of electromagnetic propulsion systems, the switching device must operate quickly. However, these fast speeds also have transient effects that must be closely monitored.

The final consideration when selecting a high-voltage switching device is its reliability. This involves assessing the switch's longevity in different operating conditions over time using assessment metrics like mean time between failures (MTBF). Other than durability, fault tolerance should be considered to ensure propulsion system operations are not compromised by faults that could have been prevented. Considering tolerance in terms of temperature and humidity also ensures the switching device offers efficiency and consistency even in harsh environments.

 

GTOs in Electromagnetic Propulsion High-Voltage Switching

To modulate and precisely control the electromagnetic fields in propulsion systems, GTOs offer solid-state high-voltage power switching to create magnetic fields. Working with power inverters and converters, the solid-state switches voltage levels and AC waveforms to adjust the amplitude and frequency according to propulsion system requirements.

When designing a propulsion system using high-power GTOs, optimizing the control signal for faster turn-off and calculating the time off must be implemented for adaptive modulation and dynamic turn-off. To better understand how to do this, let's first understand the design of the gate turn-off thyristors.

Consider Figure 2, which represents a cross-section of a GTO, where A represents the anode connection, K denotes the cathode, and G represents the switching gate signal for the solid-state high-voltage switch. The GTO comprises four layers of semiconductors (N-P-N-P), in which N and P allow for the forward and reverse bias of the power flow in the GTO thyristor for ease of gate turn-off.

 

Figure 2. Gate turn-off thyristor layer cross-section. Image used courtesy of Bob Odhiambo

 

When modulating high-voltage switching speed, the turn-off time is calculated by relating the gate control signals, the time the GTO turns from on-state to off-state, and the collector current in the GTO. This relationship of turn-off time (T_off ) is determined using the formula:

\[t_{off}=t_{r}+t_{f}+t_{d}\]

where the reverse recovery time is t_r, the turn-off initiation delay time is t_d and t_f is the gate voltage’s fall time. 

The accuracy of this turn-off time is essential in ensuring the modulation process is efficiently implemented independently without any overlapping of the turn-on and turn-off actions or events. This is important when switching high voltage that powers magnetic fields in propulsion systems.

 

IGBTs for Dynamic Voltage and Frequency Modulation

The two core structures forming the design of the high-voltage IGBT are the metal-oxide-semiconductor (MOSFET) for voltage control and hybrid versions of the bipolar junction transistor (BJT) that handle high power. Arranged with the collector, emitter, and gate terminals, the semiconductor layers in the IGBT offer precision when it comes to the dynamic control of voltage in gate drive circuitry, ensuring the voltage frequency powering the magnetic field is in the specified range. However, when implementing the circuitry, considering dead time—the time in which the low and high sides are turned off to prevent shoot-through currents that may otherwise result from simultaneous conduction—is important.

Gate signal propagation delay (t_prop) is also a factor affecting the implementation of the gate drive circuitry and can be summed up with the setup time (t_setup) to get the dead time (t_dead).

\[t_{dead}=t_{prop}+t_{setup}\]

 

Impact on Electromagnetic Fields for Propulsion Systems

Solid-state switches allow a mutual or repulsive interaction of magnetic fields through varying frequencies and switching voltages to power the fields to achieve electromagnetic propulsion. The efficiency of the propulsion system is, therefore, affected positively by the variable switching frequencies (f_sw), preventing harmonic resonance with the electromagnetic propulsion system’s natural frequencies and can be determined using: 

\[f_{sw}\neq n\times f_{res}\]

where n is an integer that represents the harmonic order while f_res is the resonance frequency in the propulsion electromagnetic system and can be determined using:

\[f_{res}\frac{1}{2\pi\sqrt{LC}}\]

where the inductance of the electromagnetic system is represented by L while C is the system capacitance.

 

Electromagnetic and Electrodynamic Suspension Applications

In electromagnetic propulsion systems, levitation due to generated magnetic forces, better known as electromagnetic suspension, requires precise control of the power supplied to the magnetic coils used to magnetize levitating systems like Maglev trains. The voltage modulation keeps the magnetic force constant to maintain a desired height. To achieve efficient propulsion, the high-voltage solid state switches also control the power fed in the coils, either increasing or decreasing the intensity of magnetic fields to pull or thrust systems. Electrodynamic suspension is another method in which propulsion can be achieved through levitation and thrust due to the energization of electromagnetic coils. Both methods are often employed in transport systems where speeds are high, offering stable, efficient electromagnetic levitation and propulsion systems.

 

High-Voltage Solid-State Switching in Electromagnetic Propulsion

To best provide a sustainable electromagnetic propulsion system, solid-state switches like IGBTs and GTOs can vary voltage and modulate frequency to precisely control magnetic fields and switching. This adaptive modulation strategy offers flexibility in balancing the power demand of the propulsion systems while adding more responsiveness to the system. 

The shift from traditional methods to high-voltage solid-state switching semiconductors has brought precision and control to electromagnetic propulsion systems like Maglev trains and spacecraft, among other applications. As technology is geared toward providing clean solutions with advanced designs and components, electromagnetic propulsion will keep improving to provide a low-power efficient transport solution.