Patent Description:
Recently, in view of environmental concerns, concepts for zero emission aircraft have been discussed and proposed more sincerely. One particular candidate for realizing this concept is the use of cryogenic liquid hydrogen (LH2) as a fuel. The LH2 is either directly used in adapted jet engines or converted into electrical energy by high-power fuel cells (mostly in the form of solid oxide fuel cells SOFCS). In either case the number of systems on aircraft that are electrical in nature and require specialized power electronics is increasing. Thus there is a need in the industry to improve integration of power electronics and specifically to improve cooling of these systems.

It is the object of the invention to improve cooling capabilities for power electronics in vehicles, preferably in aircraft.

The object is achieved by the subject-matter of the independent claims. Preferred embodiments are subject-matter of the dependent claims.

The invention provides a power electronics system for controlling and supplying an electrical load with electrical energy, the power electronics system comprising:.

Preferably, the controller is configured to control the control valve based on the electrical property of drain-source resistance of each power switching element such that the drain-source resistance is reduced or minimized.

Preferably, the controller is configured to control the control valve based on the electrical properties of drain-source resistance and temperature coefficient behavior of each power switching element such that the drain-source resistance is reduced or minimized.

Preferably, the controller is configured to control the control valve based on the electrical property of power loss caused by each power switching element.

Preferably, the controller is configured to determine the power loss as the sum of on-state and switching losses of each power switching element.

Preferably, the at least two power switching elements are electrically connected in parallel.

Preferably, the cryogenic cooling loop comprises a heat exchanging means that is in thermal contact with the power electronics circuit.

Preferably, the control valve is arranged to control the coolant flow to the heat exchanging means.

Preferably, the cryogenic cooling loop comprises a coolant tank that is configured to store a liquefied coolant at cryogenic temperatures below <NUM>, preferably at temperatures below <NUM>, more preferably at temperatures below <NUM>, and the coolant tank is fluidly connected to the heat exchanging means, preferably via the control valve.

Preferably, the cooling loop comprises a return line that fluidly connects to the heat exchanging means and is configured to transport coolant away therefrom, preferably to the coolant tank.

The invention provides an electrical system for a vehicle, preferably for an aircraft, the system comprising:.

Preferably, the fuel tank is arranged in thermal contact with the cryogenic cooling loop, preferably with the coolant tank, or wherein the fuel tank forms part of the cryogenic cooling loop as its coolant tank.

The invention provides a propulsion system for a vehicle, preferably an aircraft, the system comprising a preferred electrical, and an engine that is arranged for propulsion of the vehicle.

Preferably, the engine is an electrical engine that forms the electrical load.

Preferably, the engine is a jet-type engine that consumes fuel from the fuel tank.

The idea is based in the field of cryogenic power electronics (PE). In this case this means, using power electronic devices at very low temperatures from - <NUM> down to -<NUM> or even less. Due to the fact that some (e.g. GaN) power semiconductors improve their properties (losses are decreasing) when cooled down to very low temperatures, cryogenic PE is one of the enablers of full electric aircraft. This technology is useful for almost every high power system that has to deliver highest power density with lowest weight and volume.

According to one idea the described approach helps to improve efficiency and additionally may allow the use of all semiconductor technologies instead of only those which have a constant PTC behavior over temperature (this is particularly important when multiple semiconductors have to be put in parallel for the same switch).

The idea takes advantage of the temperature dependence of power semiconductors. According to Applicant's investigations the lowest temperature does not have to be automatically the optimal point of operation. Especially for SiC (silicon carbide) MOSFETs the temperature dependency seems to change at temperatures below -<NUM> from a positive temperature coefficient (PTC) to a negative temperature coefficient (NTC). The ideas disclosed herein allows that SiC dies can be put in parallel even below that threshold temperature.

The optimal point of operation can be chosen (also for the complete system approach) and all kinds of semiconductor technologies can be used. Due to this, the overall system can be optimized, power density can be increased and weight and volume may be decreased. These are key factors for the electrification of aircraft.

In one embodiment a power electronics setup with an actively controlled cryogenic cooling loop is proposed. The cooling loop allows to keep the junction of the used power semiconductors at a nearly constant level. Normally, power modules are mounted on some kind of heatsink to get rid of their losses. This approach can also be transferred to the cryogenic sector. By variation of the flow rate in the cooling loop, the heat transfer can be actively controlled. If the heat transfer is reduced, the junction temperature rises and vice versa.

In another embodiment the actual load conditions are considered. To obtain the desired junction temperature, the latter can be measured or calculated in real time based on a model.

The temperature of the dies can be chosen in various ways. In one embodiment, the efficiency of the semiconductors can be improved by controlling the temperature such that a decrease in the drain-source resistance, when switched on (RDS,on) is caused.

In another embodiment the semiconductors are arranged in parallel. The temperature is controlled such that the semiconductors show a PTC behavior. The temperature may simultaneously be controlled to reduce the RDS,on under the precondition that the semiconductors show a PTC behavior.

In another embodiment, the temperature is controlled such that a decrease of the overall losses (sum of on-state and switching losses) is achieved.

In another embodiment the overall system efficiency may be optimized. In another embodiment a tradeoff between lower temperature (RDS,on) and cooling effort may be used, as the cooling effort may strongly increase when the temperature goes down to cryogenic temperatures.

With the discloses ideas, the junction temperature in cryogenic power electronic systems may be actively controlled. This allows for a higher performance. Also the system's weight and volume may be lowered. The reliability of the system may improve, in particular due to a reduction in thermal cycling. The ideas are compatible with a wide variety of semiconductor materials and are also promising for SiC technology. The cooling effort may be reduced due to the cold source being preferably on the circuit board.

Embodiments of the invention are described in more detail with reference to the accompanying schematic drawings that are listed below.

Referring to <FIG> a propulsion system <NUM> for an aircraft is depicted. The propulsion system <NUM> comprises an electrical engine <NUM>. The electrical engine <NUM> drives a fan or a propeller for propelling the aircraft.

The propulsion system <NUM> comprises an electrical system <NUM>. The electrical system <NUM> includes a fuel tank <NUM>. The fuel tank <NUM> is configured for storing liquefied fuel at cryogenic temperatures below <NUM>. The fuel may be liquid hydrogen (LH2), for example. In a variant that is not depicted the electrical engine <NUM> is replaced with a jet-like combustion engine that may use the fuel directly.

The electrical system <NUM> includes a fuel cell apparatus <NUM>. The fuel cell apparatus <NUM> is configured to convert the fuel (e.g. LH2) directly into electrical energy. The fuel cell apparatus <NUM> is fluidly connected to the fuel tank <NUM> by a feed line <NUM>. The fuel cell apparatus <NUM> may include a plurality of fuel cell stacks that are preferably of the solid oxide type.

The electrical system <NUM> includes an electrical load <NUM> that is supplied with the electrical energy that is generated by the fuel cell apparatus <NUM> via a supply line <NUM>. The electrical load <NUM> may include the electrical engine <NUM>. The electrical load <NUM> may include other components such as energy storage, heating apparatus, or generally any other electrical load that warrants the employment of power electronics.

The electrical system <NUM> includes a power electronics system <NUM>. The power electronics system <NUM> comprises a power electronics circuit <NUM>. The power electronics circuit <NUM> is configured to control the electrical load <NUM>. The power electronics circuit <NUM> is configured to supply the electrical load <NUM> with electric energy from an energy source <NUM>. The energy source <NUM> may be formed by the fuel cell apparatus <NUM>, for example.

The power electronics circuit <NUM> includes at least one power switching element. The power switching element may be a MOSFET, preferably a SiC-MOSFET. Depending on the power needs of the electrical load <NUM>, two or more power switching elements may be electrically connected in parallel.

The power electronics system <NUM> includes a cryogenic cooling loop <NUM>. The cryogenic cooling loop <NUM> includes a coolant tank <NUM> that can store cryogenic liquefied coolant, such as LH2 or liquid nitrogen (LN2). The coolant tank <NUM> may be formed by the fuel tank <NUM>.

The cooling loop <NUM> includes a coolant feed line <NUM> that fluidly connects the coolant tank <NUM> to a heat exchanging means <NUM>. The heat exchanging means <NUM> may be configured as a typical heat exchanger. In some embodiments, the power electronics circuit <NUM> especially the power switching elements may be immersed directly in the cryogenic coolant.

The cooling loop <NUM> includes a control valve <NUM>. The control valve <NUM> is arranged in the coolant feed line <NUM> between the coolant tank <NUM> and the heat exchanging means <NUM>. The control valve <NUM> is configured to control the coolant flow from the coolant tank <NUM> to the heat exchanging means <NUM>.

The cooling loop <NUM> includes a coolant return line <NUM> that transports the coolant from the heat exchanging means <NUM> back to the coolant tank <NUM>.

The power electronics system <NUM> includes a controller <NUM>. The controller <NUM> is connected to at least one temperature sensor that is arranged on the power electronics circuit <NUM> for measuring a temperature value that is indicative of the junction temperature of the power switching element(s).

The controller <NUM> may furthermore be configured to measure electrical properties of the power switching elements, such as their drain-source resistance.

The controller <NUM> furthermore is connected to the control valve <NUM> so as to control the coolant flow and keep the junction temperature at a desired value.

The controller <NUM> is configured to maintain the junction temperature at a value that minimizes the drain-source resistance RDS,on of the power switching element. In case of multiple power switching elements, the total or average drain-source resistance of the power switching elements may be minimized.

The controller <NUM> may alternatively or additionally be configured to maintain the junction temperature at a value such that the power switching elements exhibit a PTC behavior. The controller <NUM> may simultaneously minimize the RDS,on with the added condition that the power switching elements exhibit a PTC behavior.

The controller <NUM> may alternatively or additionally be configured to measure electrical properties of the power switching elements that are indicative of the on-state and the switching losses. The controller <NUM> may then maintain the junction temperature at a value that minimizes the total on-state and switching losses.

In this embodiment, the fuel acts simultaneously as the coolant and is fed from the fuel tank <NUM> to the fuel cell apparatus <NUM>. The fuel cell apparatus <NUM> converts the fuel into electrical energy to be supplied to the electrical load <NUM>. The fuel cell apparatus <NUM> and the electrical load <NUM> are controlled with the power electronics system <NUM>.

In addition the power electronics system <NUM> uses the fuel from the fuel tank <NUM> to cool the power electronics circuit <NUM>, specifically the junction temperature of the power switching elements, by controlling the coolant flow with a control valve <NUM>. The junction temperature is determined by the controller <NUM> which in turn controls the control valve <NUM> such that the junction temperature is at a value that is predetermined by one or more goals.

Referring to <FIG> a second embodiment of the propulsion system <NUM> is described only insofar as it differs from the first embodiment.

Here, the fuel tank <NUM> and the coolant tank <NUM> are separate from each other. The fuel tank <NUM> may include LH2, whereas the coolant tank <NUM> includes LN2. The power electronics system <NUM> includes a second heat exchanging means <NUM> that is in thermal contact with the fuel tank <NUM>.

In this embodiment, the fuel and the coolant are in separate fluid lines. The fuel cell apparatus <NUM> converts the fuel from the fuel tank <NUM> into electrical energy to be supplied to the electrical load <NUM>. The fuel cell apparatus <NUM> and the electrical load <NUM> are controlled with the power electronics system <NUM>.

The power electronics system <NUM> uses the coolant from the coolant tank <NUM> to cool the power electronics circuit <NUM>, specifically the junction temperature of the power switching elements, by controlling the coolant flow with a control valve <NUM>. The coolant is kept at a useful temperature by exchanging heat with the fuel in the fuel tank <NUM> via the second heat exchanging means <NUM>. The junction temperature is determined by the controller <NUM> which in turn controls the control valve <NUM> such that the junction temperature is at a value that is predetermined by one or more goals.

Claim 1:
A power electronics system (<NUM>) for controlling and supplying an electrical load (<NUM>) with electrical energy, the power electronics system (<NUM>) comprising:
- a power electronics circuit (<NUM>) configured to control and supply the electrical load (<NUM>) with electrical energy from an energy source, the power electronics circuit (<NUM>) having at least one power switching element;
- a cryogenic cooling loop (<NUM>) configured for cooling the power electronics circuit (<NUM>), the cryogenic cooling loop (<NUM>) having a control valve (<NUM>) configured for controlling coolant flow within the cryogenic cooling loop (<NUM>); and
- a controller (<NUM>) configured for controlling the control valve (<NUM>) based on at least one electrical property of the power switching element and/or based on the way power switching elements are electrically connected together.