Patent Publication Number: US-2023140274-A1

Title: Current limiting diode

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This specification is based upon and claims the benefit of priority from United Kingdom Patent Application No. 2115517.1, filed on 28 Oct. 2021, the entire contents of which are incorporated herein by reference. 
     FIELD OF THE DISCLOSURE 
     The present disclosure relates to a current limiting diode. 
     BACKGROUND 
     Current limiting diodes using both silicon and silicon carbide junction field-effect transistors (JFETs) are widely known. They are configured as 2-terminal devices which “saturate” at a near constant current level and are sometimes referred to as constant-current diodes. As such, they essentially function as highly non-linear resistors which increase in resistance very rapidly in response to a very large increase in an electrical current. The increase in electrical resistance moderates a magnitude of the electrical current while dissipating energy in the form of heat, which increases an internal temperature of the current limiting diode. 
     If a current limiting diode is operated above a threshold temperature for a significant period of time, damage to or loss of function of the current limiting diode may follow as a result. It is therefore advantageous to provide means for reducing an electrical current being passed through a current limiting diode so as to reduce the amount of heat dissipated within the current limiting diode and to therefore reduce a risk of the internal temperature of the current limiting diode rising above the threshold temperature. 
     SUMMARY 
     According to a first aspect there is provided a current limiting diode comprising a gate, a source, a drain electrically connected to the source by an n-channel or p-channel and a thermoelectric generator; wherein the source and the gate are electrically connected by a fill structure, and wherein the thermoelectric generator is configured to: generate a thermoelectric voltage by absorbing heat from the n-channel or p-channel and rejecting heat to a heat sink in a current limiting condition; and apply the thermoelectric voltage between the gate and the source. 
     It may be that the thermoelectric generator is disposed within the fill structure. In addition, it may be that the thermoelectric generator abuts the n-channel or p-channel for heat exchange therebetween. 
     It may also be that the thermoelectric generator comprises a p-type semiconductor and an n-type semiconductor. 
     Further, it may be that the thermoelectric generator comprises: a first metal which abuts the gate; and a second metal which abuts the source; and wherein the first metal has a Seebeck coefficient which differs from a Seebeck coefficient of the second metal. 
     The first metal may abut the n-channel or p-channel and the heat sink. The second metal may abut the n-channel or p-channel and the heat sink. 
     It may be that the drain is electrically connected to the source by an n-channel. 
     According to a second aspect there is provided a bidirectional current limiting device comprising a first current limiting diode and a second current limiting diode, wherein: each current limiting diode is in accordance with the first aspect; and the source of the first current limiting diode is electrically connected to the source of the second current limiting diode. 
     According to a third aspect there is provided an electrical power system comprising: a current limiting diode in accordance with the first aspect or a bidirectional current limiting device in accordance with the second aspect; an electrical power source; and an electrical network, and wherein the current limiting diode or the bidirectional current limiting device is configured to limit a fault current passing between the electrical power source and the electrical load network in a fault condition. 
     The electrical power system may further comprise a controllable circuit interruption device configured to interrupt the fault current in response to a determination that the electrical power system is in the fault condition. The electrical energy source may comprise a battery (e.g. a high-density battery). The electrical power system may further comprise a power electronics converter. 
     According to a fourth aspect, there is provided an aircraft power and propulsion system comprising the current limiting diode in accordance with the first aspect, the bidirectional current limiting device in accordance with the second aspect or the electrical power system of the third aspect. 
     According to a fifth aspect, there is provided an aircraft comprising the current limiting diode in accordance with the first aspect, the bidirectional current limiting device in accordance with the second aspect, the electrical power system of the third aspect or the aircraft power and propulsion system in accordance with the fourth aspect. 
     The skilled person will appreciate that except where mutually exclusive, a feature or parameter described in relation to any one of the above aspects may be applied to any other aspect. Furthermore, except where mutually exclusive, any feature or parameter described herein may be applied to any aspect and/or combined with any other feature or parameter described herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments will now be described by way of example only, with reference to the Figures, in which: 
         FIG.  1 A  is a cross-sectional view of a conventional current limiting diode in a normal operating condition; 
         FIG.  1 B  is a cross-sectional view of the conventional current limiting diode shown in  FIG.  1 A  in a current limiting condition; 
         FIG.  2 A  is a cross-sectional view of a first example current limiting diode in a normal operating condition; 
         FIG.  2 B  is a cross-sectional view of the first example current limiting diode shown in  FIG.  2 A  in a current limiting condition; 
         FIG.  3    is a cross-sectional view of a second example current limiting diode; 
         FIG.  4    is a cross-sectional view of a third example current limiting diode; 
         FIG.  5    is an electrical circuit symbol which represents a bidirectional current limiting device; 
         FIG.  6    is a diagram which shows a first example electrical power system comprising a current limiting diode; 
         FIG.  7    is a diagram which shows a second example electrical power system comprising a bidirectional current limiting device; 
         FIG.  8    is a diagram which shows a third example electrical power system comprising a current limiting diode; 
         FIG.  9    is a diagram which shows a fourth example electrical power system comprising a bidirectional current limiting device. 
     
    
    
     DETAILED DESCRIPTION 
     Aspects and embodiments of the present disclosure will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. 
       FIG.  1 A  shows a cross-sectional view of a conventional current limiting diode  100  in a normal operating condition. The current limiting diode  100  comprises a junction field-effect transistor (JFET) provided with a source  102 , a drain  104  and a gate  106 , with the gate  106  being electrically connected to the source  102  by a fill structure  110  and the drain  104  being electrically connected to the source by an n-channel  120 . Accordingly, the current limiting diode  100  is configured as a 2-terminal device which permits an electric current to be conducted through the n-channel  120  between the source  102  and the drain  104 . In  FIG.  1 A , an electric current being conducted through the n-channel  120  between the source  102  and the drain  104  in the normal operating condition is represented by arrows  130 . 
     The current limiting diode  100  has an electrical resistance which is primarily defined by a size of a depletion region  124  of the n-channel  120 . The depletion region  124  is a region of the n-channel  120  in which there are no free charge carriers. As a result, an electric current cannot be conducted through the depletion region  124 . However, an electric current may be conducted through a current-carrying path  126  through the n-channel  120 . In  FIG.  1 A , a boundary between the depletion region  124  and the current-carrying path  126  is represented by the dashed lines  122 . The size of the depletion region  124  corresponds to a magnitude of an electric current being passed through the n-channel from the drain  104  to the source  102 . That is to say that if the magnitude of the electric current passing through the n-channel from the drain  104  to the source  102  is large, the size of the depletion region  124  will also be large, the size of the current-carrying path  126  will be small and the electrical resistance of the current limiting diode  100  becomes very large. However, the size of the depletion region  124  (and equally the size of the current-carrying path  126 ) does not correspond to a magnitude of an electric current being passed from the source  102  to the drain  104 . 
       FIG.  1 B  shows a cross-sectional view of the conventional current limiting diode  100  shown in  FIG.  1 A  in a current limiting condition. In  FIG.  1 B , an electric current being conducted through the n-channel  120  between the source  102  and the drain  104  in the current limiting condition is represented by a plurality of thin dashed arrows  130 . The electric current being conducted through the n-channel  120  in the current limiting condition may be a fault current which originates from an electrical power system in which the current limiting diode  100  is incorporated. 
     A relationship between the electrical resistance of the current limiting diode  100  and the magnitude of the electric current passing from the drain  104  to the source  102  is highly non-linear. In other words, the current limiting diode  100  operates in a manner which is analogous to a highly non-linear unidirectional resistor in that the electrical resistance of the current limiting diode  100  increases very rapidly as the magnitude of the electric current passing from the drain  104  to the source  102  increases above a saturation threshold. Accordingly, the current limiting diode  100  “saturates” at a near constant current level. 
     In the normal operating condition shown in  FIG.  1 A , a magnitude of an electric current being conducted through the current limiting diode  100  from the drain  104  to the source  102  is relatively low. Accordingly, the size of the depletion region  124  within the n-channel  120  is relatively small, the size of the current-carrying path  126  is relatively large and the electrical resistance of the current limiting diode  100  is also relatively small. Therefore, an amount of resistive heat dissipated by the current limiting diode  100  in the normal operating condition is low. 
     In the current limiting condition shown in  FIG.  1 B , the magnitude of the electric current being conducted through the current limiting diode  100  from the drain  104  to the source  102  is relatively high. Accordingly, the size of the depletion region  124  within the n-channel  120  is relatively large, the size of the current-carrying path  126  is relatively small and the electrical resistance of the current limiting diode  100  is also relatively large. Therefore, an amount of resistive heat dissipated by the current limiting diode  100  in the current limiting condition is high. Because only the current-carrying path  126  permits an electrical current to be conducted through it, the resistive heat is primarily generated within the current-carrying path  126 . In both the normal operating condition and the current limiting condition, the resistive heat generated within the current-carrying path  126  causes an internal temperature profile to develop within the current limiting diode  100 . 
     The current limiting diode  100  is cooled by means of conduction and/or convection to an environment. Consequently, in order to effectively dissipate the resistive heat in the current limiting condition, a maximum internal temperature of the current limiting diode  100  must rise substantially in the current limiting condition. The maximum internal temperature of the current limiting diode  100  corresponds to a maximum of the internal temperature profile. In practice, a resistive heat dissipation rate of the n-channel  120  in the current limiting condition may be sufficiently high such that the maximum internal temperature of the current limiting diode  100  rises above a threshold temperature at which damage to and loss of function of the current limiting diode  100  may occur. 
       FIG.  2 A  shows a cross-sectional view of a first example current limiting diode  200  in a normal operating condition. The current limiting diode  200  comprises a junction field-effect transistor (JFET) provided with a source  202 , a drain  204  and a gate  206 , with the gate  206  being electrically connected to the source  202  by a fill structure  210  and the drain  204  being electrically connected to the source by an n-channel  220 . Accordingly, the current limiting diode  200  is configured as a 2-terminal device which permits an electric current to be conducted through the n-channel  220  between the source  202  and the drain  204 . In  FIG.  2 A , an electric current being conducted through the n-channel  220  between the source  202  and the drain  204  in the normal operating condition is represented by arrows  230 . The current limiting diode  200  further comprises a thermoelectric generator  208 . 
       FIG.  2 B  shows a cross-sectional view of the first example current limiting diode  200  shown in  FIG.  2 A  in a current limiting condition. In  FIG.  2 B , an electric current being conducted through the n-channel  220  between the source  202  and the drain  204  in the current limiting condition is represented by arrows  230 . The electric current being conducted through the n-channel  220  in the current limiting condition may be a fault current which originates from an electrical power system in which the current limiting diode  200  is incorporated. 
     In a similar way to the conventional current limiting diode  100 , the first example current limiting diode  200  has an electrical resistance which is primarily defined by a size of a depletion region  224  (and equally by a size of a current-carrying path  226 ) of the n-channel  220 . The size of the depletion region  224  corresponds to a magnitude of an electric current being passed through the n-channel  200  from the drain  204  to the source  202 . A relationship between the magnitude of the electric current being passed through the n-channel  220  from the drain  204  to the source  202  is very similar to the relationship described with respect to the conventional current limiting diode  100  with reference to  FIGS.  1 A and  1 B . That is to say that the current limiting diode  200  operates in a manner which is analogous to a highly non-linear unidirectional resistor in that the electrical resistance of the current limiting diode  200  increases very rapidly as the magnitude of the electric current passing from the drain  204  to the source  202  increases above a saturation threshold. 
     The thermoelectric generator  208  is configured to generate a thermoelectric voltage by absorbing heat from the n-channel  220  and to reject heat to a heat sink  270  in the current limiting condition by virtue of a temperature differential between the n-channel  220  and the heat sink  270 . The thermoelectric generator  208  is further configured to apply the thermoelectric voltage between the gate  206  and the source  202  of the current limiting diode  200 . A magnitude of the thermoelectric voltage is dependent on a rate of heat absorption by the thermoelectric generator  208  from the n-channel  220 . The rate of heat absorption by the thermoelectric generator  208  is correlated with the resistive heat generated within the current-carrying path  226  in both the normal operating condition and the current limiting condition. Accordingly, the magnitude of the thermoelectric voltage is related to a magnitude of an electric current being conducted through the current limiting diode  200  from the drain  204  to the source  202 . 
     The application of the thermoelectric voltage between the gate  206  and the source  202  has the effect of partially turning the current limiting diode  200  off. When the current limiting diode  200  is partially switched off, the magnitude of an electric current being conducted from the drain  204  to the source  202  is reduced. In turn, the resistive heat generated within the current-carrying path  126  is reduced. As the magnitude of the thermoelectric voltage applied between the gate  206  and the source  202  increases, the current limiting diode  200  is switched off to a greater degree so that the magnitude of an electric current being conducted from the drain  204  to the source  202  is further reduced. In turn, the resistive heat generated within the current-carrying path  126  is further reduced and the magnitude of the thermoelectric voltage generated by the thermoelectric generator  208  is also reduced. The magnitude of the electric current being conducted from the drain  204  to the source  202  then increases, which brings an associated increase in the magnitude of the thermoelectric voltage generated by the thermoelectric generator  208  and so on and so forth. 
     As a result, the magnitude of the thermoelectric voltage and the magnitude of the electric current being conducted through the current limiting diode  200  from the drain  204  to the source  202  are so linked so as to form a negative feedback loop. The magnitude of the thermoelectric voltage and the magnitude of the electric current being conducted from the drain  204  to the source  202  are rapidly driven toward an equilibrium state by the negative feedback loop in which the thermoelectric generator  208  absorbs heat from the n-channel  220  which and applies a thermoelectric voltage across the gate  206  and the source  202 . 
     In the equilibrium state, the magnitude of the electric current being conducted from the drain  204  to the source  202  is lower compared to a conventional current limiting diode  100  (as described with respect to  FIG.  1    above) which is not provided with a thermoelectric generator as described above. Consequently, the resistive heat generated by the current limiting diode  200  in the current limiting condition is lower than the resistive heat generated by the conventional current limiting diode  100  in the current limiting condition. The reduction in the resistive heat dissipation rate of the n-channel  220  in the current limiting condition associated with the provision of the thermoelectric generator  208  reduces a risk of a maximum internal temperature of the current limiting diode  200  rising above a threshold temperature above which damage to and loss of function of the current limiting diode  200  may occur. In addition, this effect is achieved without a need for any additional components such as an external monitoring transducer, a gate drive circuit for a power supply or the like. 
     Otherwise, it may be that the reduction in the resistive heat dissipation rate of the n-channel  220  in the current limiting condition associated with the provision of the thermoelectric generator  208  is unable to prevent the maximum internal temperature rising above the threshold temperature, but nonetheless lengthens a critical time period of the current limiting diode  200 . The critical time period is defined as a duration between a development of a current which places the current limiting diode  200  in the current limiting condition and a time at which the maximum internal temperature of the current limiting diode  200  exceeds the threshold temperature. A lengthened critical time period is associated with an improved durability of the current limiting diode  200  in the current limiting condition, because the current limiting diode  200  can operate in the current limiting condition for a longer period of time before damage to and loss of function of the current limiting diode  200  occurs. 
     In the example of  FIGS.  2 A and  2 B , the thermoelectric generator  208  is shown as being disposed within the fill structure  210  and abutting the n-channel  220  for heat exchange therebetween. The thermoelectric generator  208  may be incorporated into the fill structure  210  by means of, for instance, evaporation deposition or lithography techniques. Nevertheless, it will be appreciated that in other examples the thermoelectric generator  208  may be disposed outside of the fill structure  210  and/or may not abut the n-channel  220 . Examples in which the thermoelectric generator  208  is disposed outside of the fill structure  210  may be referred to as “die-stacked” examples. Similarly, in the example of  FIGS.  2 A and  2 B , the heat sink is shown as abutting the fill structure  210  of the current limiting diode  200 . Nevertheless, it will be appreciated that in other examples, the heat sink  270  may not abut the fill structure  210 . 
     A thermal efficiency of the thermoelectric generator  208  is dependent on, among other things, a heat rejection temperature at which the thermoelectric generator  208  rejects heat to the heat sink  270 , and a heat absorption temperature at which the thermoelectric generator  208  absorbs heat from the n-channel  220 . A higher heat absorption temperature provides an increased thermal efficiency of the thermoelectric generator  208 . Conversely, a lower heat rejection temperature provides an increased thermal efficiency of the thermoelectric generator  208 . 
     If the thermoelectric generator  208  is disposed within the fill structure  210 , as shown in  FIGS.  2 A and  2 B , the heat absorption temperature at which the thermoelectric generator  208  absorbs heat, in use, from the n-channel  220  is increased. In particular, if the thermoelectric generator  208  abuts the n-channel  220 , as shown in  FIGS.  2 A and  2 B , the heat rejection temperature at which the thermoelectric generator  208  absorbs heat, in use, from the n-channel  220  is maximised. Likewise, if the thermoelectric generator  208  is disposed within the fill structure  210  and the heat sink  270  abuts the fill structure  210 , as shown in  FIGS.  2 A and  2 B , the heat rejection temperature at which the thermoelectric generator rejects heat, in use, to the heat sink  270  is minimised. 
       FIG.  3    shows a cross-sectional view of a second example current limiting diode  200 . In the example of  FIG.  3   , the thermoelectric generator  208  comprises an n-type semiconductor  280  and a p-type semiconductor  290 . Each semiconductor is disposed between the n-channel  220  and the heat sink  270 . A direction of electron flow in response to a substantially identical temperature differential across the n-type semiconductor  280  and the p-type semiconductor  290  differs between the n-type semiconductor  280  and the p-type semiconductor  290 . Therefore, the thermoelectric voltage applied between the gate  206  and the source  202  is increased by the inclusion of both the n-type semiconductor  280  and the p-type semiconductor  290  in the thermoelectric generator  208 . 
     In addition, the thermal efficiency of the thermoelectric generator  208  is also dependent on a thermoelectric figure of merit. The thermoelectric figure of merit is a property associated with a material of the thermoelectric generator  208 . A higher thermoelectric figure of merit corresponds to a higher thermal efficiency of the thermoelectric generator  208 . Many known semiconductor materials have a high thermoelectric figure of merit, and therefore provide an increased thermal efficiency of the thermoelectric generator  208 . The thermoelectric figure of merit, zT, is defined by the equation: 
     
       
         
           
             zT 
             = 
             
               
                 σ 
                 ⁢ 
                 
                   S 
                   2 
                 
                 ⁢ 
                 T 
               
               κ 
             
           
         
       
     
     In this equation σ is the material&#39;s electrical conductivity, K is its thermal conductivity, S is its Seebeck coefficient and T is its temperature. 
       FIG.  4    shows a cross-sectional view of a third example current limiting diode  200 . In the example of  FIG.  4   , the thermoelectric generator  208  is disposed within the fill structure  210  and comprises a first metal  285  and a second metal  295 . The first metal  285  abuts the gate  206  and the second metal  295  abuts the source  202 . The first metal  285  has a Seebeck coefficient which differs from a Seebeck coefficient of the second metal  295 . The Seebeck coefficient relates to a thermoelectric response of each metal. Specifically, the Seebeck coefficient relates an electromotive force generated by each metal in response to a temperature differential applied thereto. Since the Seebeck coefficient differs for each metal, the electromotive force generated by the application of a temperature differential to each metal differs accordingly. The thermoelectric voltage produced by the thermoelectric generator  208  and applied between the gate  206  and the source  202  is the result of a difference between the electromotive force generated in each metal. 
     In the example of  FIG.  4   , the first metal  285  and the second metal  295  abut both the n-channel  220  and the heat sink  270 . However, it will be appreciated that in other examples, the first metal  285  and/or the second metal  295  may not abut the n-channel  220  and/or the heat sink  270 . In general, it is desirable to ensure that a larger temperature differential is applied to the first metal  285  and the second metal  295  so as to ensure that the difference between the electromotive force generated in each metal is maximised. In examples in which either the metal  285  and the second metal  295  abut both the n-channel  220  or the heat sink  270 , a larger temperature differential is applied, in use, to the first metal  285  or the second metal  295 . In examples in which both the first metal  285  and the second metal  295  abut both the n-channel  220  and the heat sink  270 , the largest possible temperature differential is applied, in use, to the first metal  285  and the second metal  295 . 
     For better operation of the current limiting diode  200 , the disposition of the thermoelectric generator  208  within the fill structure  210  should not significantly weaken or disrupt the electrical connection between the source  202  and gate  206  provided by the fill structure  210 . In examples in which the thermoelectric generator  208  is disposed within the fill structure  210 , it may be advantageous to configure the thermoelectric generator  208  so as to preserve the electrical connection between the source  202  and the gate  206 . The provision of the first metal  285  and the second metal  295  within the thermoelectric generator  208  better preserves the electrical connection between the source  202  and the gate  206  provided by the fill structure  210 . 
     In all examples described above, the channel that connects the source and drain of each current limiting diode  200  is an n-channel. Those skilled in the art will appreciate that the channel could, in principle, instead be a p-channel, doped such that the majority carriers are ‘holes’ instead of electrons. The mobility of charge carriers in n-channel devices is typically significantly higher than those in p-channel devices, however, such that n-channel devices are generally preferred. 
       FIG.  5    is an electrical circuit symbol which represents a bidirectional current limiting device  200 ′. The bidirectional current limiting device  200 ′ comprises a first current limiting diode  200 A and a second current limiting diode  200 B. Each current limiting diode is in accordance with any of the example current limiting diodes described above with reference to  FIGS.  2 - 4   . 
     In the example of  FIG.  5   , the first current limiting diode  200 A is shown as being provided with a source  202 A, a drain  204 A and a gate  206 A. Likewise, the second current limiting diode  200 B is shown as being provided with a source  202 B, a drain  204 B and a gate  206 B. 
     The source  202 A of the first current limiting diode  200 A and the source  202 B of the second current limiting diode  200 B are electrically connected in series. As such, the first current limiting diode  200 A and the second current limiting diode  200 B are electrically connected so as to form the bidirectional current limiting device  200 ′. Accordingly, the bidirectional current limiting device  200 ′ is configured as a 2-terminal device which permits an electric current to be conducted between the drain  204 A of the first current limiting diode  200 A and the drain  204 B of the second current limiting diode  200 B through the n-channel of each current limiting diode. 
     A relationship between an electrical resistance of the bidirectional current limiting device  200 ′ and a magnitude of an electric current passing from the drain  204 A to the source  202 A of the first current limiting diode  200 A is highly non-linear. Similarly, a relationship between the electrical resistance of the bidirectional current limiting device  200 ′ and a magnitude of an electric current passing from the drain  204 B to the source  202 A of the second current limiting diode  200 B is highly non-linear. 
     In other words, the bidirectional current limiting device  200 ′ operates in a manner which is analogous to a highly non-linear bidirectional resistor in that the electrical resistance of the bidirectional current limiting device  200 ′ increases very rapidly as the magnitude of an electric current passing through the bidirectional current limiting device  200 ′ in either direction increases above a saturation threshold. Accordingly, the bidirectional current limiting device  200 ′ “saturates” at a near constant current level, irrespective of a direction of the electric current passing through the bidirectional current limiting device  200 ′. 
       FIG.  6    is a diagram of a first example electrical power system  300  comprising a current limiting diode  200  in accordance with any of the example current limiting diodes described above with reference to  FIGS.  2 - 4   . The electrical power system  300  further comprises an electrical power source  310  which is configured to supply an electrical power to the electrical power system  300  and an electrical network  330  connected between terminals  332  and  334 . In the example of  FIG.  6   , the electrical network is represented by a load  330 . The electrical network  330  may comprise, for example, at least one electrical load. The electrical network  330  may be a DC electrical network or an AC electrical network. 
     In a normal operating condition of the first electrical power system  300 , a magnitude of an electric current being conducted through the current limiting diode  200  from the drain to the source is relatively low. Accordingly, the size of the depletion region within the current limiting diode  200  is relatively small and the electrical resistance of the current limiting diode  200  is also relatively small. Therefore, an amount of resistive heat dissipated by the current limiting diode  200  when the electrical power system  300  is in the normal operating condition is low. As a result, a power insertion loss associated with the inclusion of the current limiting diode  200  within the electrical power system  300  is minimised in the normal operating condition. 
     On the other hand, in a fault condition of the first electrical power system  300 , a magnitude of an electric current being conducted through the current limiting diode  200  from the drain  204  to the source  202  may become large in a short period of time. Consequently, in the fault condition of the first electrical power system  300 , the current limiting diode  200  may be placed in the current limiting condition. 
     For example, if an internal impedance of the electrical power source  310  is very low and an electrical energy delivery capacity of the electrical power source  310  is very high, a fault in the electrical power system  300  which originates in the electrical network  330  and which presents an effective short circuit across the electrical power source  310  may cause a magnitude of a fault current to be conducted through the electrical power system  300  which reaches an order of thousands of Amperes within a very short period of time. This may be because the magnitude of the fault current is only limited by the internal impedance of the electrical power source  310 . If the magnitude of the fault current were not limited, the magnitude of the fault current could rise beyond a tolerance limit of the electrical power source  310  and/or a component of the electrical network  330 . 
     In this example, the fault originates from the electrical network  330  and the fault current is therefore conducted through the current limiting diode  200  from the drain  204  to the source  202 . If the fault current exceeds the saturation threshold of the current limiting diode  200 , the current limiting diode  200  almost instantaneously limits the magnitude of the fault current by providing a greatly increased resistance through the n-channel  220 . Accordingly, the current limiting diode  200  is configured to limit a fault current passing between the electrical power source  310  and the electrical network  330  in the fault condition. However, in another example, it may be that a fault originates from the electrical power source  310  and a fault current is therefore conducted through the current limiting diode  200  from the source  202  to the drain  204 . 
     As mentioned above, the size of the depletion region of the n-channel  220  does not correspond to a magnitude of an electric current being passed from the source  202  to the drain  204 . Therefore, in the configuration shown in  FIG.  6   , the current limiting diode  200  is not able to limit the magnitude of a fault current caused by a fault originating from the electrical power source  310 . For this reason, an orientation of the current limiting diode  200  within the electrical power system  300  dictates whether the current limiting diode  200  is able to limit a fault current caused by a fault originating from the electrical network  330  or a fault current caused by a fault originating from the electrical power source  310 . It will be appreciated that the current limiting diode  200  may be otherwise orientated within the electrical power system  300  such that the current limiting diode  200  is configured to limit a fault current caused by a fault originating from the electrical storage device  310 . 
     In practice, the current limiting diode  200  is only able to respond to a development of the fault current caused by a fault originating from the electrical network  330  and to limit the fault current within a response time period of the current limiting diode  200 . The response time period of the current limiting diode  200  may be, for example, of an order of a number of nanoseconds. Further, the current limiting diode  200  is configured to ensure that the magnitude of the fault current is maintained below the tolerance limit of the electrical power source  310  and/or a component of the electrical network  330 . 
     In the example of  FIG.  6   , the electrical power source  310  is configured to provide a direct current to the electrical power system  300 . However, it will be appreciated that the electrical power source  310  may equally be configured to provide another type of current to the electrical power system  300 , such as an alternating current. 
       FIG.  7    is a diagram of a second example electrical power system  300 ′ comprising a bidirectional current limiting device  200 ′ in accordance with the example bidirectional current limiting device  200 ′ described above with reference to  FIG.  5   . The electrical power system  300 ′ further comprises an electrical power source  310 ′ which is configured to supply an electrical power to the electrical power system  300 ′ and an electrical network  330 ′ connected between terminals  332 ′ and  334 ′. Many of the components of the second example electrical power system  300 ′ are similar to or identical to the components of the first electrical power system  300 , with like reference numerals indicating similar or identical components. 
     The bidirectional current limiting device  200 ′ is configured to limit the magnitude of a fault current caused by a fault originating from the electrical network  330 ′ and/or to limit the magnitude of a fault current caused by a fault originating from the electrical power source  310 ′ in a fault condition of the second electrical power system  300 ′. In other words, the bidirectional current limiting device  200 ′ is able to limit a fault current caused by a fault originating from the electrical network  330 ′ or a fault current caused by a fault originating from the electrical power source  310 ′ irrespective of an orientation of the bidirectional current limiting device  200 ′ within the electrical power system  300 ′. Accordingly, the bidirectional current limiting device  200 ′ is configured to limit a fault current passing between the electrical power source  310 ′ and the electrical network  330 ′ in the fault condition. 
     In the example of  FIG.  7   , the electrical power source  310 ′ is configured to provide an alternating current to the electrical power system  300 ′. However, it will be appreciated that the electrical power source  310 ′ may equally be configured to provide another type of current to the electrical power system  300 ′, such as a direct current. 
       FIG.  8    is a diagram of a third example electrical power system  400 . Many of the components of the third electrical power system  400  are similar to or identical to the components of the first electrical power system  300 , with like reference numerals indicating similar or identical components. The above description of the features and advantages of the first electrical power system  300  applies, mutatis mutandis, to the features and advantages of the third electrical power system  400 . 
     The electrical power system  400  comprises a controllable circuit interruption device  450 . The controllable circuit interruption device  450  is configured to interrupt the fault current shortly after the development of the fault current in response to a determination that the electrical power system  400  is in a fault condition. The controllable circuit interruption device  450  may monitor an electrical current passing between the electrical power source  310  and the electrical network  330  in order to determine whether the electrical power system  400  is in the fault condition. However, the controllable circuit interruption device  450  is only able to respond to the development of the fault current and interrupt the fault current within a response time period of the controllable circuit interruption device  450 . 
     The response time period of the controllable circuit interruption device  450  is longer than the response time period of the current limiting diode  200 . Accordingly, the current limiting diode  200  and the controllable circuit interruption device  450  are configured to co-operate in use so as to limit and then to interrupt a fault current passing between the electrical power source and the electrical network  330  in the fault condition. The current limiting diode  200  and the controllable circuit interruption device  450  provide the electrical power system  400  with a resettable protection system against faults originating from the electrical network  330  in the fault condition. 
     A safety-time margin of the electrical power system  400  is defined as a difference between the response time period of the controllable circuit interruption device  450  and the critical time period of the current limiting diode  200 . A longer safety-time margin is associated with an improved safety of the electrical power system  400 . As described above, the reduction in the resistive heat dissipation rate of the current limiting diode  200  in the current limiting condition associated with the provision of the thermoelectric generator  208  lengthens the critical time period of the current limiting diode  200  and thereby lengthens the safety-time margin of the electrical power system  400 . 
     Additionally, the provision of the thermoelectric generator  208  within the current limiting diode  200  permits a controllable circuit interruption device having a response period which would not be suitable for use in conjunction with a conventional current limiting diode to be used in conjunction with the current limiting diode  200 . In particular, the controllable circuit interruption device  450  may comprise a controllable contactor (e.g. a DC contactor) which provides a physical circuit break (that is, Galvanic isolation) in an electrical power system but which may have a longer response time period than a critical time period of a conventional current limiting diode. The physical circuit break is associated with an improved safety of the electrical power system  400 . In other examples the controllable circuit interruption device  450  may comprise a solid-state circuit breaker or the like. 
     In the example of  FIG.  8   , an internal impedance of the electrical power source  310  is represented by a resistor  412  and the electrical energy delivery capacity of the electrical power source  310  is represented by a voltage source  414 . The electrical power source  310  may comprise, for example, a battery, a capacitor and/or an ultracapacitor. The electrical power source  310  may comprise a high-density battery. 
     The electrical power system  400  may further comprise a power electronics converter  420 . In the example shown in  FIG.  8   , the power electronics converter  420  comprises a DC to DC converter and the electrical network  330  is a DC electrical network. The DC to DC converter may comprise a plurality of transistors  421 - 424  and an inductor coil  429 . The power electronics converter  420  provides a regulation function to the electrical power system  400 , as described in further detail below. It will be appreciated that the power electronics converter  420  may otherwise comprise a DC to AC to DC converter or the like. In other examples, the power electronics converter  420  comprises an AC to DC converter. In further examples, the power electronics converter  420  comprises a DC to AC converter and the electrical network  330  is an AC electrical network. 
     In the arrangement of  FIG.  8   , the electrical power source  310  is configured to provide a first electrical potential difference to the power electronics converter  420 . The power electronics converter  420  is configured to convert and/or regulate the electrical potential difference provided by the electrical power source  310  and provide a second electrical potential difference to the electrical network  330 . Accordingly, the power electronics converter  420  provides a regulation function to the electrical power system  400 . In particular, the power electronics converter  420  allows the first electrical potential difference provided by the electrical power source  310  to change while maintaining a substantially constant second electrical potential difference provided to the electrical network  330 . The electrical network  330  may comprise at least one load component which benefits from being driven by a substantially constant electrical potential difference. 
     Also in the example shown in  FIG.  8   , the third example electrical power system  400  comprises a plurality of additional components. In  FIG.  4   , the plurality of additional components includes a first DC capacitor  472 , a second DC capacitor  474 , a third DC capacitor  476  and an earth ground  478 . It will be appreciated that these components are shown and described for the purpose of illustration and are not to be construed as essential elements of the third electrical power system  400 . 
       FIG.  9    is a diagram of a fourth example electrical power system  400 ′. Many of the components of the fourth electrical power system  400 ′ are similar to or identical to the components of the second electrical power system  300 ′ and the third electrical power system  400 , with like reference numerals indicating similar or identical components. The above description given with respect to the features and advantages of the second electrical power system  300 ′ and the third electrical power system  400  applies, mutatis mutandis, to the features and advantages of the fourth electrical power system  400 ′. 
     Nevertheless, for the avoidance of doubt, the bidirectional current limiting device  200 ′ and the controllable circuit interruption device  450  are configured to co-operate in use so as to limit and then to interrupt a fault current passing between the electrical power source  310  and the electrical network  330  in a fault condition of the fourth electrical power system  400 ′. In particular, the bidirectional current limiting device  200 ′ is able to limit a fault current caused by a fault originating from the electrical network  330  or a fault current caused by a fault originating from the electrical power source  310 ′ irrespective of an orientation of the bidirectional current limiting device  200 ′ within the electrical power system  400 ′. Consequently, the bidirectional current limiting diode  200 ′ and the controllable circuit interruption device  450  provide the electrical power system  400 ′ with a more versatile resettable protection system against a fault current passing between the electrical power source  310  and the electrical network  330  in the fault condition. 
     It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.