Method of controlling power supply and electrical system

Abstract

The present disclosure relates to a method 300 for controlling power supply from a DC link 210 to a load 260 using a switching system 250, the switching system 250 comprising a switchable current path 201 between the DC link 210 and the load 260, the switchable current path 201 comprising a controllable switch T1, the method 300 comprising performing a conversion process 310, 320, the switching process 310, 320 comprises switching 312, 316, 322, 326 the switchable current path 201 between a non-conducting state and a conducting state by gradually changing a duty cycle of a control signal for the switch T1 between values of 0% and equal to or less than 100%, including end values.

Description

Technical Field

[0001] The present disclosure relates to a method of controlling the supply of power from a direct current (DC) source (eg, a DC link) to a load and also to an electrical system comprising an apparatus adapted to perform such a method. Background Art

[0002] It is known that coupling a relatively large load (e.g., a heater) to a power supply can result in relatively high transient currents, which may be referred to as inrush currents. Such inrush currents can stress the electrical components through which they pass and, therefore, can reduce the long-term reliability of the electrical system of which the electrical components form a part. The present invention was designed with the foregoing in mind. Summary of the Invention

[0003] According to a first aspect, there is provided a method for controlling the supply of power from a DC link to a load using a switching system, the switching system comprising a switchable current path between the DC link and the load, the switchable current path comprising a controllable switch, the method comprising performing a transition process comprising: transitioning the switchable current path between a non-conducting state and a conducting state by gradually changing a duty cycle of a control signal for the switch between 0% and a value equal to or less than 100%, inclusive.

[0004] The switching process may include gradually changing the duty cycle of the control signal for the switch between 0% and 100% (inclusive), thereby switching the switchable current path between a non-conducting state and a conducting state. Gradually changing the duty cycle of the control signal for the controllable switch between 0% and 100% (inclusive) may include gradually changing the duty cycle of the control signal for the switch within a range extending from 1% to 99%. The control signal may include a pulse-width modulated signal. The switch may be a semiconductor-based switch. The switch may be an insulated gate bipolar transistor.

[0005] The switching system may further include a snubber coupled in parallel with the switchable current path between the DC link and the load. The snubber may include (eg, may be) a capacitor.

[0006] It may be that the method comprises transitioning the switchable current path from a non-conductive state to a conductive state by gradually increasing a duty cycle of a control signal for the switch from 0% to 100% inclusive.

[0007] It may be that gradually increasing the duty cycle of the control signal for the controllable switch from 0% to 100% (inclusive) includes gradually increasing the duty cycle of the control signal for the switch within a range extending from 1% to 99%.

[0008] It may be that the method comprises transitioning the primary switchable current path from the conducting state to the non-conducting state by gradually reducing the duty cycle of the control signal for the switch from 100% to 0% inclusive.

[0009] It may be that gradually reducing the duty cycle of the control signal for the controllable switch from 100% to 0% (inclusive) includes gradually reducing the duty cycle of the control signal for the switch within a range extending from 1% to 99%.

[0010] The switchable current path may be a primary switchable current path. The switching system may include a secondary switchable current path coupled in parallel with the primary switchable current path. The transition process may include maintaining the primary switchable current path in a conductive state while transitioning the secondary switchable current path between a non-conductive state and a conductive state.

[0011] The method may include performing a switch-on transition process comprising: maintaining the primary switchable current path in a conduction state while transitioning the secondary switchable current path from a non-conduction state to a conduction state; and subsequently maintaining the secondary switchable current path in a conduction state while transitioning the primary switchable current path from a conduction state to a non-conduction state.

[0012] The method may include performing a turn-off transition process, which includes: maintaining the primary switchable current path in the conduction state while transitioning the secondary switchable current path from the conduction state to the non-conduction state; and then maintaining the secondary switchable current path in the non-conduction state while transitioning the primary switchable current path from the conduction state to the non-conduction state.

[0013] The secondary switchable current path may include a contactor.The method may include transitioning the secondary switchable current path between a non-conducting state and a conducting state by closing or opening the contactor.

[0014] The method may include receiving an input voltage of at least 600 VDC from the DC link at the switching system. The method may include receiving an input voltage of at least 700 VDC from the DC link at the switching system.

[0015] The method may include performing a transition process in response to a demand to change the power consumption of the load.

[0016] The method may comprise performing the or each conversion process in response to a need to change the power consumption of the load from a first non-zero value to a second non-zero value.

[0017] According to a second aspect, an electrical system is provided, comprising a DC link, a load, a controller and a switching system comprising a switchable current path between the DC link and the load, wherein the controller is configured to perform the method according to the first aspect.

[0018] The switchable current path may be a primary switchable current path, and the switching system may include a secondary switchable current path coupled in parallel with the primary switchable current path. The load may be a heating device including one or more heating elements.

[0019] According to a third aspect, there is provided a computer program comprising instructions for causing an electrical system according to the second aspect to perform a method according to the first aspect.

[0020] According to a fourth aspect, there is provided a computer readable medium having stored thereon the computer program according to the third aspect.

[0021] According to a fifth aspect, there is provided a transport refrigeration system comprising the electrical system according to the second aspect. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 A vehicle including a transport refrigeration system is shown;

[0023] Figure 2 Is suitable for Figure 1 A schematic diagram of an example transport refrigeration unit for use with a vehicle comprising an electrical system and a vapor compression refrigeration circuit;

[0024] Figure 3 is shown to be suitable for Figure 2 A circuit diagram of an example electrical system for use with a transport refrigeration unit;

[0025] Figure 4 is a flow chart illustrating an example method of controlling power supply from a DC link to a load; and

[0026] Figure 5 It shows Figure 4 A flow chart of the switch-on process of the illustrated method;

[0027] Figure 6 It shows Figure 4 A flow chart of the shutdown conversion process of the illustrated method;

[0028] Figure 7 is a highly schematic diagram of a machine-readable medium. DETAILED DESCRIPTION

[0029] Figure 1A vehicle 10 is shown including a transport refrigeration system 20. Figure 1 In the example shown, transport refrigeration system 20 forms part of a refrigerated highway semi-trailer and includes a structure 22 that supports (or forms) at least one climate-controlled compartment 24 configured to be cooled and / or heated by TRU 110. Climate-controlled compartment 24 can take the form of multiple compartments or have multiple zones. Structure 22 includes a chassis. Structure 22 supports TRU 110. Vehicle 10 also includes a tractor unit 14 that is removably coupled to the trailer.

[0030] Figure 2 Schematically shows a suitable Figure 1 FIG. 1 is a diagram of an example TRU 110 for use within a vehicle 10 and a transport refrigeration system 20. The TRU 110 includes a vapor compression refrigeration circuit 400 and an electrical system 200.

[0031] The vapor-compression refrigeration circuit 400 includes an evaporator 408 configured to receive heat from the climate-controlled compartment 24 of the transport refrigeration system 20 and a condenser 404 configured to reject heat to a heat sink 44 (e.g., ambient air outside the climate-controlled compartment 24). To this end, the vapor-compression refrigeration circuit 400 also includes a compressor 402 and an expansion valve 406. Thus, the vapor-compression refrigeration circuit 400 can be controlled to remove heat from the climate-controlled compartment 24.

[0032] The electrical system 200 includes a DC link 210 configured to receive power from a DC power source 220, and a heating device 260 (e.g., a heater 260). The present disclosure contemplates that the DC link 210 may not constitute part of the electrical system 200, and that the DC link 210 may instead be coupled to the DC power source 220 and thus configured to receive power from the DC power source 220. It should be understood that the term "DC link" is a term of art and that the DC link may include, for example, an energy storage device such as a DC link capacitor. The DC power source 220 may come from, for example, a feed-in converter, such as the TRU 11 or an on-board charger (OBC) of the vehicle 10.

[0033] exist Figure 2In the example shown, the heating device 260 is positioned near the evaporator 408. Thus, in use, the heating device 260 can be operated to provide heat to the evaporator 408 (e.g., to defrost the evaporator 408). However, the present disclosure contemplates that the heating device 260 can be positioned elsewhere within the TRU 110. Generally speaking, the heating device 260 is configured to provide electrical heating to the TRU 110. The electrical system 200 is configured to control the supply of power from the DC link 210 to the heating device 260, thereby causing the heating device 260 to generate heat for heating the TRU 110. When power is supplied to the heating device 260, the heating device 260 generates heat through resistive (e.g., ohmic) heating.

[0034] Figure 3 is a circuit diagram illustrating an example electrical system 200 suitable for use as a reference in the above Figure 2 1. The electrical system 200 of the TRU 110, wherein like reference numerals indicate similar or common features.

[0035] Electrical system 200 includes a switching system 250. Switching system 250 further includes a half-bridge 252, a snubber 254, and a contactor 256. Switching system 250 is configured to receive power from DC link 210 via input terminals 232 and 234, and to provide power from output terminals 242 and 244. Heating device 260 is configured to receive power from switching system 250 via input terminals 246 and 248. In use, input terminals 232 and 234 of switching system 250 can receive an input voltage of at least 600 VDC from DC link 210, and optionally between 600 VDC and 800 VDC (e.g., 700 VDC).

[0036] A first input connecting rail 231 extends between the DC link 210 and a first input terminal 232 of the switch system 250. Similarly, a second input connecting rail 233 extends between the DC link 210 and a second input terminal 234 of the switch system 250. The input connecting rails 231 and 233 provide electrical connections between the DC link 210 and the heating device 260 via the switch system 250. In use, the first input connecting rail 231 is connected to the positive terminal of the DC power source 220 via the DC link 210, while the second input connecting rail 233 is connected to a reference voltage terminal (e.g., a ground terminal or a negative terminal) of the DC power source 220 via the DC link 210. Therefore, during use, the potential of the first input connecting rail 231 is higher than the potential of the second input connecting rail 233. Therefore, the first connecting rail 231 may be referred to as the positive input connecting rail 231, while the second input connecting rail 233 may be referred to as the negative input connecting rail 233.

[0037] Similarly, first output connecting rail 241 extends between first output terminal 242 of switch system 250 and first input terminal 246 of heating device 260. Similarly, second output connecting rail 243 extends between second output terminal 244 of switch system 250 and second input terminal 248 of heating device 260. Output connecting rails 241 and 243 provide electrical connections between switch system 250 and heating device 260. In use, first output connecting rail 241 is connected to the positive terminal of DC power source 220 via DC link 210 and switch system 250, while second input connecting rail 243 is connected to a reference voltage terminal (e.g., ground or negative terminal) of DC power source 220 via DC link 210 and switch system 250. Therefore, during use, the potential of first output connecting rail 241 is higher than the potential of second output connecting rail 243. Therefore, first output connecting rail 241 can be referred to as positive output connecting rail 241, while second output connecting rail 243 can be referred to as negative output connecting rail 243.

[0038] Half-bridge 252 includes a semiconductor switch T1 (e.g., a switch based on a controllable semiconductor, such as an insulated gate bipolar transistor) and a diode D1. Semiconductor switch T1 and diode D1 are coupled in series between input rails 231 and 234. The cathode of diode D1 is coupled to positive output rail 241 via first output terminal 242, while the anode of diode D1 is coupled to half-bridge junction J1 at the node between diode D1 and semiconductor switch T1. Semiconductor switch T1 is coupled between half-bridge junction J1 and negative input rail 233 via second input terminal 234. Those skilled in the art will appreciate that diode D1 of half-bridge 252 can be replaced with a switch (e.g., another semiconductor switch) as in conventional half-bridge topologies. Half-bridge junction J1 is further coupled to negative output rail 243 via second output terminal 244. Thus, semiconductor switch T1 forms part of (e.g., functionally provides) a switchable current path 201 between second input terminal 234 and second output terminal 244 via half-bridge junction J1.

[0039] Contactor 256 is also coupled between second input terminal 234 and second output terminal 244, but not via half-bridge junction J1. That is, contactor 256 forms part of (e.g., functionally provides) a switchable current path 202 between second input terminal 234 and second output terminal 244 that bypasses half-bridge junction J1. Herein, the switchable current path 202 functionally provided by semiconductor switch T1 may be referred to as a primary switchable current path 201, while the switchable current path 202 functionally provided by contactor 256 may be referred to as a secondary switchable current path 202.

[0040] Thus, each switchable current path 201, 202 extends (partially) between the DC link 210 and the heating device 260. The primary switchable current path 201 and the secondary switchable current path 202 are connected in parallel.

[0041] Buffer 254 includes a capacitor C1 coupled in series between positive input connection terminal 232 and negative input terminal 234. A first terminal of capacitor C1 is coupled to positive input connection rail 231 via first input terminal 232, while a second terminal of capacitor C2 is coupled to negative input connection rail 233 via second input terminal 234. Thus, buffer 254 is coupled in parallel between DC link 210 and heating device 260. Those skilled in the art will appreciate that a buffer typically includes a component having a resistor (e.g., a dedicated resistor). However, in the electrical system according to the present disclosure, buffer 254 does not necessarily need to include a dedicated resistor. Instead, the resistor of heating device 260 can be used for the corresponding purpose.

[0042] The inductance of the input connection rails 231 and 233 is Figure 3 The first inductor L1 is represented in FIG, while the inductance of the output connection rails 241, 243 is represented in FIG. Figure 3 The buffer 254 can advantageously suppress voltage fluctuations (eg, overvoltages) that may occur when performing the following reference to Figures 4 to 6 The buffer 254 may be configured to provide a buffer 254 ...

[0043] Those skilled in the art will also recognize that by varying the duty cycle of the control signal for semiconductor switch T1 when contactor 256 is open, the output voltage supplied to input terminals 246, 248 of heating device 260 can be varied between a value substantially identical to the input voltage supplied from DC link 210 to input terminals 232, 234 of switching system 250 (i.e., when the duty cycle of the control signal for switch T1 is 100%) and a value approximately equal to zero (i.e., when the duty cycle of the control signal for switch T1 is approximately equal to 0%). For example, when the duty cycle of the control signal for switch T1 is 50%, the output voltage supplied to input terminals 246, 248 of heating device 260 will be approximately half the input voltage supplied from DC link 210 to input terminals 232, 234 of switching system 250. In this manner, half-bridge 252, in conjunction with the other features of electrical system 200, functions as a step-down converter (buck converter).

[0044] Heating device 260 includes a first heating element H1 and a second heating element H2. Each heating element has a resistor (for resistive heating), but may also have a capacitor and / or an inductor. Heating device 260 also includes a first switch S1, a second switch S2, and a third switch S3. Heating device 260 as a whole can be considered a load, and / or individual heating elements H1 and H2 can be considered loads. In this example, each heating element has substantially the same resistance, for example, 98 ohms. Each heating element H1 and H2 can have a positive temperature coefficient (PTC) resistor.

[0045] Switches S1, S2, and S3 are controllable to change the operating mode of heating device 260, thereby changing the power consumption of heating device 260. Specifically, in the low-power mode of heating device 260, switch S3 is closed (i.e., conducting), while switches S1 and S2 are open (i.e., non-conducting). Consequently, heating elements H1 and H2 are coupled in series between input terminals 246 and 248 of heating device 260. Consequently, the effective total resistance of heating device 260 can be 196 ohms. In the intermediate-power mode of heating device 260, switches S1 and S3 are open, while switch S2 is closed. Consequently, only first heating element H1 is coupled between input terminals 246 and 248 of heating device 260. Consequently, the effective total resistance of the heating device is 98 ohms. In the high-power mode of heating device 260, switches S1 and S2 are closed, while switch S3 is open. Thus, the heating elements H1, H2 are coupled in parallel with each other between the input terminals 246, 248 of the heating device 260. Consequently, the effective total resistance of the heating device is 49 ohms. Without wishing to be bound by theory, one skilled in the art will appreciate that, with a lower effective total resistance of the heating device 260, the amount of energy lost as heat due to resistive heating in the heating device 260 increases, according to Ohm's law and the definition of electrical power. Specifically, if the potential difference between the input terminals 246, 248 of the heating arrangement is 700 VDC, the power consumption P of the heating arrangement 260 is as follows: In low power mode, P = V 2 / R= 700 2 / 196 = 2.5 kW; in medium power mode, P = 700 2 / 98 = 5 kW; in high power mode, P = 700 2 / 49 = 10 kW.

[0046] The electrical system 200 further includes a controller 290 configured to control the secondary switchable current path 202 (e.g., by controlling the contactor 256) and the primary switchable current path 201 (e.g., by controlling the semiconductor switch T1). The controller 290 may also be configured to change the power consumption of the heating device 260 (e.g., between a low power mode, an intermediate power mode, and a high power mode).

[0047] Figure 4 is a diagram illustrating control of power supply from a DC link to a load according to the present disclosure (e.g., using the above reference Figure 2 and 3 Flowchart of an example method 300 of the electrical system 200 described herein, in particular the switching system 250. The method(s) described herein may be executed by an appropriate data processing device, such as the one described above with respect to Figure 3 In other words, the controller 290 may be configured to perform the method(s) described herein (eg, method 300). Although the description of method 300 continues with direct reference to the above reference Figure 3 The specific example electrical system 200 is described, but it should be understood that the method 300 is more broadly applicable for use with a switching system that includes a primary switchable current path between a DC link (e.g., DC link 210) and a load (e.g., heating device 260) and a secondary switchable current path coupled in parallel with the primary switchable current path.

[0048] Method 300 includes determining, at block 302, whether heating device 260 is on (i.e., generating heat) or off (i.e., not generating heat). In other words, heating device 260 is on when power is supplied to it from DC link 210 and is off when power is not supplied to it from DC link 210. More specifically, heating device 260 is off when both secondary switchable current path 202 and primary switchable current path 201 are in a non-conducting state (e.g., fully deactivated). Therefore, if both secondary switchable current path 202 and primary switchable current path 201 are in a non-conducting state, then heating device 260 may be determined to be off at block 302. On the other hand, if secondary switchable current path 202 is in a conducting state (e.g., fully activated), then heating device 260 may be determined to be on at block 302. The determination at block 302 may also be based on other considerations, such as whether the DC link 210 is supplying power to the input terminals 232, 234 of the switch system 250. If so, the electrical system 200 may be provided with appropriate sensor devices (e.g., transducers) communicatively coupled to the controller 290 for determining whether power is being supplied to the input terminals 232, 234.

[0049] If it is determined at block 302 that the heating device 260 is currently off, the method 300 proceeds to block 304 to determine whether there is a need to turn on the heating device 260. The determination at block 304 may include determining whether a need to turn on the heating device 260 has been received, for example, from a human-machine interface (HMI) such as a graphical user interface (GUI) or from an external data processing device (such as the controller of the TRU 110) via an application program interface (API). Otherwise, the controller 290 executing the method 300 may determine in other ways whether there is a need to turn on the heating device (e.g., due to frost accumulation on the evaporator 408).

[0050] If it is determined at block 304 that there is a need to turn on the heating device 260, the method continues at block 310 by performing a turn-on transition using the switching device 250. Figure 5 The on-transition process is described in further detail. However, after the on-transition process has been performed at block 310, the heating device 260 is turned on, and the method 300 returns to the determination at block 302 of whether the heating device 260 is on or off, and continues with the subsequent process as described herein. Conversely, if it is determined at block 304 that there is no need to turn on the heating device 260, the method 300 returns directly to the determination at block 302 of whether the heating device 260 is on or off, and continues with the subsequent process.

[0051] Alternatively, if it is determined at block 302 that the heating device 260 is currently on, the method 300 proceeds to determine at block 306 whether there is a need to turn off the heating device 260. In a manner similar to the determination at block 304, the determination at block 306 may include determining whether a need to turn off the heating device 260 has been received, for example, from a human machine interface (HMI) such as a graphical user interface (GUI) or from an external data processing device (such as the controller of the TRU 110) via an application program interface (API). Otherwise, the controller 290 executing the method 300 may determine in other ways whether there is a need to turn off the heating device (e.g., due to defrosting of the evaporator 408).

[0052] If it is determined at block 306 that there is a need to shut down the heating device 260, the method continues at block 320 by performing a shut-down transition process using the switching device 250. Figure 6However, after the shutdown transition process has been performed at block 320 , the heating device 260 is turned off, and the method 300 returns to determining whether the heating device 260 is on or off at block 302 and continues as described herein.

[0053] If, on the other hand, it is determined at block 306 that there is no need to turn off the heating device 260, the method 300 proceeds to block 308 to determine whether there is a need to change the mode of the heating device 260 (e.g., change the power consumption). The determination at block 308 may include determining whether a need to change the mode of the heating device 260 to off has been received, for example, from a human machine interface (HMI) such as a graphical user interface (GUI) or from an external data processing device (such as the controller of the TRU 110) via an application program interface (API). Otherwise, the controller 290 executing the method 300 may determine in other ways whether there is a need to change the mode of the heating device 260 (e.g., due to overheating or underheating of the heating device 260).

[0054] If it is determined at block 308 that there is no need to change the mode of the heating device 260, the method 300 returns directly to the determination of whether the heating device 260 is on or off at block 302 and continues as described herein. If it is determined at block 308 that there is a need to change the mode of the heating device 260, the method 300 proceeds to: perform an off transition process at block 320'; change the mode of the heating device 260 at block 330; and then perform an on transition process at block 310'. The off transition process represented by block 320' is generally similar to (e.g., identical to) the off process represented by block 320, and the on transition process represented by block 310' is generally similar to (e.g., identical to) the on process represented by block 310. Changing the mode of the heating device 260, represented by block 330, may include operating switches S1, S2, and S3 of the heating device 260 to transition the heating device 260 between the low power mode, the intermediate power mode, and the high power mode as described above.

[0055] Figure 5 It is shown by Figure 4Flowchart of the switch-on transition process, represented by block 310 in FIG. When the switch-on transition process begins, both the secondary switchable current path 202 and the primary switchable current path 201 are in a non-conducting state. The switch-on transition process includes, at block 312, switching the primary switchable current path 201 from the non-conducting state to the conducting state. This is achieved by gradually increasing the duty cycle of the pulse-width modulated control signal for semiconductor switch T1 from 0% to 100% (inclusive). Consequently, the output voltage supplied to the input terminals 246 and 248 of the heating device 260 gradually increases from approximately zero to the input voltage from the DC link 210. The gradual increase in the duty cycle of the control signal for semiconductor switch T1 can be determined based on / adapted to the characteristics of the heating device 260 (for example, if the heating elements H1 and H2 have PTC-type resistors, the gradual increase in the duty cycle of the heating device 260 can be adapted to account for the fact that the resistance of the heating elements H1 and H2 typically increases over time).

[0056] After the primary switchable current path 201 has been switched to the conducting state as described above, the switch-on transition process includes maintaining the primary switchable current path 201 in the conducting state while switching the secondary switchable current path 202 from the non-conducting state to the conducting state at block 314. This is achieved by maintaining the duty cycle of the pulse width modulated control signal for the semiconductor switch T1 at 100% while closing the contactor 256.

[0057] Once the primary switchable current path 201 has been switched to the conductive state at block 314, the on-switching process includes maintaining the secondary switchable current path 202 in the conductive state while switching the primary switchable current path 201 from the conductive state to the non-conductive state at block 316. This is accomplished by maintaining the contactor 256 closed while decreasing (e.g., gradually decreasing) the duty cycle of the pulse width modulated control signal for the semiconductor switch T1 from 100% to 0% (inclusive).

[0058] After block 316, the switch-on process is complete and the method 300 is as described above with reference to Figure 4 The description continues, at this stage the heating device 260 may receive power from the DC link 210 via the secondary switchable current path 202 rather than via the primary switchable current path 201 .

[0059] The on-transition process, represented by block 310, provides that contactor 256 of secondary switchable current path 202 does not conduct all of the current associated with coupling heating device 260 to DC link 210. Instead, primary switchable current path 201 is operated to gradually increase the voltage applied to heating device 260 before contactor 256 closes and begins conducting current. This limits the magnitude of the inrush current associated with applying voltage to heating elements H1, H2 of heating device 260 during coupling to DC link 210. Because inrush current is associated with stress (e.g., thermal stress) on components such as heating elements H1, H2, the on-transition process reduces the electrical stress on heating device 260, thereby increasing the robustness of electrical system 200 as a whole. Furthermore, because the primary switchable current path 201 conducts a portion of the current supplied from the DC link 210 when the contactor 256 is closed, the risks to the contactor caused by arcing and / or transient effects when the contactor is closed are mitigated by performing the make-on transition process, thereby extending the life of the contactor 256. In other words, the contactor 256 does not carry a significant electrical load when it is closed.

[0060] Figure 6 It shows Figure 4 The flowchart of the turn-off transition process is represented by block 320 in FIG. When the turn-off transition process begins, the secondary switchable current path 202 is in a conducting state, while the primary switchable current path 201 is in a non-conducting state. The turn-off transition process includes, at block 322, switching the primary switchable current path 201 from a non-conducting state to a conducting state. This is achieved by increasing (e.g., gradually increasing) the duty cycle of the pulse-modulated control signal for semiconductor switch T1 from 0% to 100% in a manner similar to the turn-on transition process. However, unlike the turn-on transition process, this does not result in a change in the output voltage supplied to the input terminals 246, 248 of the heating device 260.

[0061] After the primary switchable current path 201 has been transitioned to the conductive state as described above, the off transition process includes maintaining the primary switchable current path 201 in the conductive state while transitioning the secondary switchable current path 202 from the conductive state to the non-conductive state at block 324. This is achieved by maintaining the duty cycle of the pulse width modulated control signal for the semiconductor switch T1 at 100% while opening the contactor 256.

[0062] Once the secondary switchable current path 202 has been transitioned to the non-conducting state at block 324, the turn-off transition process includes maintaining the secondary switchable current path 202 in the non-conducting state while transitioning the primary switchable current path 201 from the conducting state to the non-conducting state at block 326. This is achieved by maintaining the contactor 256 open while gradually increasing the duty cycle of the pulse width modulated control signal for the semiconductor switch T1 from 0% to 100%.

[0063] After block 326, the shutdown transition process is complete and the method 300 is as described above with reference to Figure 4 At this stage, the heating device 260 may not receive power from the DC link 210 via the secondary switchable current path 202 or the primary switchable current path 201 .

[0064] Since the primary switchable current path 201 conducts a portion of the current supplied from the DC link 210 when the contactor 256 is open, the off-transfer process as shown by block 320 enables the life of the contactor 256 to be relatively extended. Therefore, by performing the off-transfer process, the risk to the contactor 256 caused by arcing and / or transient effects when the contactor 256 is open is reduced. That is, the contactor 256 does not carry a significant electrical load when it is open.

[0065] The switching process described herein (i.e., the on- and off-switching processes) provides an efficient and simple means for coupling relatively large loads to a power source. Furthermore, the switching process eliminates the need for semiconductor switch T1 of primary switchable current path 201 to continuously switch between states when heating device 260 is on, thereby increasing the mean time between failures (MTBF) of semiconductor switch T1. This also increases the efficiency of power transmission from DC link 210 to heating device 260. Furthermore, this results in a relatively lower peak temperature of semiconductor switch T1 during use. Consequently, the level of cooling required for semiconductor switch T1 (e.g., the size of the heat sink) can be relatively reduced. This contributes to a simpler and / or less complex electrical system 200. Furthermore, this improves the electromagnetic compatibility (EMC) of electrical system 200.

[0066] Furthermore, the use of two parallel switchable current paths (i.e., the secondary switchable current path 202 and the primary switchable current path 201) introduces a layer of redundancy to the electrical system 200. That is, if the semiconductor switch T1 fails, the heating device 260 can still be switched on and off by simply opening and closing the contactor 256 (although this is generally undesirable for the reasons discussed above).

[0067] Although referenced Figures 3 to 6 While the electrical system 200 is described as including both a primary switchable current path 201 and a secondary switchable current path 202, and the method 300 includes taking actions on both switchable current paths 201 and 202, this need not necessarily be the case. Specifically, the electrical system 200 may include only the primary switchable current path 201 having the controllable switch T1 (which may be more simply referred to as the switchable current path 201), and the method 300 may include taking actions only on the switchable current path 201. If this is the case, the switch-on transition process may simply include switching the primary switchable current path 201 from a non-conducting state to a conducting state, followed by maintaining the primary switchable current path 201 in the conducting state. Conversely, the switch-off transition process may simply include switching the primary switchable current path 201 from a conducting state to a non-conducting state, followed by maintaining the primary switchable current path 201 in the non-conducting state.

[0068] Furthermore, while method 300 has been described as including the act of determining whether heating device 260 is on (i.e., generating heat) or off (i.e., not generating heat) at block 302, this need not necessarily be the case. Thus, if it is determined at block 304 that there is no need to turn heating device 260 on, method 300 proceeds to determine at block 306 whether there is a need to turn heating device 260 off and continues with subsequent processes as described herein. Furthermore, if it is determined at block 308 that there is no need to change the mode of heating device 260, method 300 returns directly to determining at block 304 whether there is a need to turn heating device 260 on. Similarly, after the transition process(es) have been executed at block(s) 310, 310′, 320, method 300 may return to determining at block 304 whether there is a need to turn heating device 260 on.

[0069] Figure 7 A machine-readable medium 600 having stored thereon a computer program 60 comprising instructions which, when executed by a controller 290 provided to a switching system 250 according to the present disclosure (e.g., as described above with reference to FIG. 1 ), is shown highly schematically. Figure 3 The switch system 250 of the electrical system 200 described above is configured so that the controller 290 executes the above reference Figure 4-7 Method 300 is described.

[0070] Unless mutually exclusive, features described with respect to any of the above aspects may be applied, mutatis mutandis, to any of the other aspects. Furthermore, unless mutually exclusive, any feature described herein may be applied to any aspect and / or combined with any other feature described herein. Furthermore, while the present disclosure is presented in the context of transport refrigeration systems and / or vapor compression circuits, it should be understood that the present disclosure has other possible applications in other technical fields.

Claims

1. A method of controlling the supply of power from a DC link to a load using a switching system, the switching system comprising a switchable current path between the DC link and the load, the switchable current path comprising a controllable switch, the method comprising performing a conversion process, the conversion process comprising: The switchable current path is switched between a non-conductive state and a conductive state by gradually changing a duty cycle of a control signal for the switch between 0% and a value equal to or less than 100%, inclusive.

2. The method according to claim 1, wherein The switching system also includes a snubber coupled in parallel between the DC link and the load.

3. The method according to claim 1, comprising: The switchable current path is switched between a non-conducting state and a conducting state by gradually changing a duty cycle of a control signal for the switch between 0% and 100% inclusive.

4. The method according to claim 1, comprising: The switchable current path is converted from a non-conductive state to a conductive state by gradually increasing the duty cycle of the control signal for the switch from 0% to 100%.

5. The method according to claim 1, comprising: The switchable current path is converted from a conducting state to a non-conducting state by gradually reducing the duty cycle of the control signal for the switch from 100% to 0%.

6. The method according to claim 1, wherein The switchable current path is a primary switchable current path, and the switching system includes a secondary switchable current path coupled in parallel with the primary switchable current path, and wherein the conversion process includes: The primary switchable current path is maintained in a conducting state while transitioning the secondary switchable current path between a non-conducting state and a conducting state.

7. The method of claim 6, comprising performing a switch-on transition process, the switch-on transition process comprising: maintaining the primary switchable current path in a conducting state while transitioning the secondary switchable current path from a non-conducting state to a conducting state; and subsequently The secondary switchable current path is maintained in the conducting state while transitioning the primary switchable current path from the conducting state to the non-conducting state.

8. The method of claim 6, comprising performing a shutdown transition process, the shutdown transition process comprising: maintaining the primary switchable current path in a conducting state while transitioning the secondary switchable current path from a conducting state to a non-conducting state; and subsequently The secondary switchable current path is maintained in the non-conducting state while the primary switchable current path is transitioned from the conducting state to the non-conducting state.

9. The method according to claim 6, wherein: The secondary switchable current path comprises a contactor, and wherein the method comprises switching the secondary switchable current path between a non-conducting state and a conducting state by closing or opening the contactor.

10. The method according to claim 1, wherein The method includes performing the conversion process in response to a demand to change the power consumption of the load.

11. The method according to claim 1, wherein The load is a heating device comprising one or more heating elements.

12. An electrical system comprising a DC link, a load, a controller, and a switching system, the switching system comprising a switchable current path between the DC link and the load, wherein: The controller is configured to perform the method according to claim 1 .

13. A computer program comprising instructions causing a controller of an electrical system to perform the method according to claim 1, the electrical system comprising a DC link, a load, the controller, and a switching system comprising a switchable current path between the DC link and the load.

14. A computer-readable medium having stored thereon the computer program according to claim 13.

15. A transport refrigeration system comprising the electrical system of claim 12.

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