Power electronics in the PPU: the approach to reducing size and weight

Power electronics are critical to the reliable power supply of all integrated systems within a spacecraft. With this in mind, one of the main objectives of this project is to optimise the power density of the power converters that make up the PPU, with the aim of significantly reducing the overall volume and size of the system. The programme includes a comprehensive study of the relevant technologies currently being used to achieve this miniaturisation of the power converters.

Radio frequency generator (RFG): the architecture

The radio frequency generator is one of the most important basic elements able to supply the radio frequency coil. The coil must be supplied with a high frequency sinusoidal current. It has to deliver a lot of power.

But how is its architecture composed?

• The first stage is a DC/DC converter to provide the galvanic isolation required for the application. The input voltage is regulated at 100V, while the output must be between 15-200V (DC). This regulation is essential to control the power injected into the thruster.
• The second stage involves a high-frequency DC/AC converter, as a high-frequency sinusoidal current needs to be generated.

The project’s key technologies

The first stage: Isolated DC/DC converters

Passive components such as inductors, transformers and capacitors make up the converter. These components dominate the size and weight of power converters, so one of the key design challenges is to reduce the size of the passive components. To do this, it is necessary to modulate and manage the energy supplied through the converter, as energy is ultimately stored in these passive components. This energy is then delivered to the load depending on how the switching devices (semiconductors) are modulated. The ability to switch at higher frequencies avoids the need to store too much energy in the passive components, thus reducing the overall size of the system.

And what is the methodology? The starting point is to design the solution by selecting the appropriate topology. Next, an electrical simulation is performed to verify the correct functioning of the design under all operating conditions. Finally, a model is created to calculate the losses under different operating conditions.

The second stage: the AC/DC converter

In the second phase, the topology of the inverter is quite simple. The electronic architecture consists of two switches and a capacitor that should resonate with the load. The load can be modelled by an inductance and a (non-constant) resistor. The resonant capacitor will resonate with the load inductance. If the switching frequency of the inverter is synchronized with the resonant frequency of the capacitor and the inductance, we can obtain a sinusoidal current circulating in the RFG coil. Depending on the voltage applied to the input of the inverter, we modulate the current to make it higher or lower in order to control the boost.

What really happens to real switches

Although it is possible, as a preliminary assumption, to consider the switches as ideal, in reality the actual behaviour of the switches must be taken into account in the analysis. An ideal transition would mean that when the switches are closed, current should flow through them and when the switch is opened, the current should drop to zero and the voltage should rise. In the real situation, there is an overlap between the current and voltage as they switch, causing losses. Therefore, energy is lost (dissipated power) each time the switch is opened and closed. The highest peak of energy lost during on/off corresponds to the highest frequency. This is a disadvantage because a higher frequency is used to minimize the RFG size.

How this problem can be improved: new technologies

1. WBGs (wide-bandgap semiconductors)

To address the losses energy problem, the project aimed the use of wide-band semiconductors (WBGs) such as silicon carbide(SiC) and gallium nitride (GaN). From a drive technology point of view, these WBGs offer significant improvements. They allow the converter to operate at higher frequencies without increasing losses. Furthermore, by using GaN or SiC instead of silicon (Si) devices, for example, the electron saturation rate is about twice as high, allowing a higher operating frequency. In electric space propulsion systems, the saturation rate has a direct impact on the ability of the device to handle the high voltages and currents required for propulsion, contributing to the overall efficiency and reliability of the system. In addition, the critical electric field of WBGs is ten times higher than that of silicon, meaning that the material can withstand higher electric fields without electrical breakdown, reducing on-resistance.

2. Digital Control technologies

Instead of using an analogue controller, the use of a digital controller can improve the project technology. The benefits are associated with more advanced control solutions and architectures. To optimize the design of the radio frequency generator can be used:

• Matrix transformers. These transformers consist of a matrix of primary and secondary windings that are wound together to create a highly efficient transformation system at high frequencies.
• Resonant converters. Because they operate at resonant frequencies, switching elements are switched when currents or voltages are in phase or ahead, reducing switching losses.
• Zero voltage switching (ZVS). In a ZVS converter, the switching elements change state (open or close) when the voltage across them is close to zero.

Digital control technologies make possible to operate at higher frequencies or to increase the power density of converters. They also allow very complex control algorithms to be optimised. However, the design regarding magnetic components still remains difficult. This kind of development is affected by increasing frequencies, leading to losses in the core and losses in the conductors.

3. Magnectic components: design and technologies

The design of the magnetic components to operate at high frequency is a key issue to make possible the goals of this project. Accurate modelling techniques are very critical to optimize the transformer and inductor designs. The use of advanced winding techniques is of most importance to minimize the size and losses of this components. Finally, the thermal management is also a key design issue.