Description:FORM 2

THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENT RULES, 2003

COMPLETE SPECIFICATION
(See Section 10 and Rule 13)

Title of invention:
METHOD OF PRE-CHARGING A DC-LINK CAPACITOR

Applicant:
River Mobility Private Limited
A company based in India,
Having its address as:
No. 25/3, KIADB EPIP Zone, Seetharampalya, Hoodi Road, Mahadevapura, Whitefield, Bengaluru, Karnataka, India- 560048

The following specification describes the invention and the manner in which it is to be performed.

PRIORITY INFORMATION
The present application does not claim priority from any other application.

FIELD OF INVENTION
The present invention generally relates to voltage stabilization. More specifically, the present invention is related to pre-charging of DC-link capacitors for voltage stabilization.

BACKGROUND OF THE INVENTION
The subject matter discussed in the background section should not be assumed to be prior art merely as a result of its mention in the background section. Similarly, a problem mentioned in the background section or associated with the subject matter of the background section should not be assumed to have been previously recognized in the prior art. The subject matter in the background section merely represents different approaches, which in and of themselves may also correspond to implementations of the claimed technology.
Electronic systems including capacitive loads usually suffer from high inrush currents during power up operations. A high voltage system with downstream capacitance can be exposed to inrush current when the high voltage system is turned on. If not limited or controlled, the inrush current can cause significant stress or damage to components present in the system.
A pre-charging circuit is used to limit inrush current for slowly providing the charge to a capacitive load. By slowly providing the charge, the pre-charging circuit ensures proper operation and protection of components in high voltage applications. Pre-charging increases lifespan of electric components and reliability of a system as a whole. A pre-charging circuit allows the current to flow in a controlled manner until the voltage level reaches very near to a source voltage. In certain applications, such as electric vehicles, the pre-charging circuit may be used every time the electric vehicles are turned on.
Power electronic devices utilize semiconductor elements such as MOSFETs. Due to rapid switching of power states of these semiconductor elements and due to introduction of transient noise in a circuit, fluctuations arise in input voltage. To get rid of the fluctuations and the transient noise and achieve voltage stabilization, DC-link capacitors are used.
A pre-charging circuit is therefore required in following situations:
i) The load downstream of main contactors include components that may be damaged by the inrush current.
ii) The main fuse or circuit breaker will trip due to flow of the inrush current.
iii) The inrush current will damage the main contactors and/or will cause them to weld.
iv) Battery cells are not rated for the inrush current.
The principle of pre-charging is to limit the inrush current to a safe value when a battery is turned ON and charge a load side DC-link capacitor fully in required time. Fig. 1 illustrates a conventional electrical circuitry for pre-charging a DC-link capacitor 102, in accordance with prior art. Charging of the DC-link capacitor 102 in a required time is achieved by adjusting a value of pre-charging resistor 104 present in a pre-charging path 106. The pre-charging path 106 also includes a pre-charge MOSFET 108 for activating and deactivating the resistor based pre-charging path 106. A main disconnect MOSFET 110 provided in parallel to the pre-charging path 106 is operated either in a cut-off region or a saturation region, hence for pre-charging, the pre-charging path 106 is used to charge the DC-link capacitor 102. After the pre-charging is completed using the pre-charging path 106, the main disconnect MOSFET 110 operates in a saturation region and a battery voltage is successively supplied to an electrical load through the main disconnect MOSFET 110. In such a manner, time taken in charging of the DC-link capacitor 102 is nearly 5RC or more which is significant in terms of user experience.
In view of the above mentioned shortcomings, there arises a need for a method using which pre-charging could be completed quickly.

OBJECTS OF THE INVENTION
A general objective of the invention is to reduce time required for pre-charging a DC-link capacitor.
Another objective of the invention is to provide a mechanism for detection of a short-circuit condition and an open circuit condition in a pre-charging circuit.

SUMMARY OF THE INVENTION
This summary is provided to introduce aspects related to a method of pre-charging a DC-link capacitor, and the aspects are further described below in the detailed description. This summary is not intended to identify essential features of the claimed subject matter nor is it intended for use in determining or limiting the scope of the claimed subject matter.
In one embodiment, a method of pre-charging a DC-link capacitor comprises providing a first pre-charging arm and a second pre-charging arm connected in parallel between a power source for providing an input voltage and a DC-link capacitor. The DC-link capacitor is used for stabilizing the input voltage across an electrical load. The method of pre-charging further includes charging the DC-link capacitor through the first pre-charging arm partly up to a first level. The method of pre-charging further includes charging the DC-link capacitor beyond the first level using both the first pre-charging arm and the second pre-charging arm.
In one aspect, the first level of charging is indicated by a first time interval T1 of an RC circuit including the DC-link capacitor.
In one aspect, the first pre-charging arm and the second pre-charging arm both are used to completely charge the DC-link capacitor during a second time interval.
In one aspect, the first pre-charging arm includes a resistor and a switch.
In one aspect, the second pre-charging arm includes a power Metal–Oxide–Semiconductor Field-Effect Transistor (MOSFET).
In one aspect, the power MOSFET operates in a cut-off region during charging of the DC-link capacitor up to the first level through the first pre-charging arm, the power MOSFET operates in a linear region for an RC delay provided by a gate driver circuit of the power MOSFET during charging of the DC-link capacitor beyond the first level, and the power MOSFET operates in a saturation region when charging of the DC-link capacitor is completed and power is supplied to the electrical load.
In one aspect, the power MOSFET is switched ON by receiving a gate driving signal from a Battery Management System (BMS).
In one aspect, the BMS determines a rate of rise of a voltage across the DC-link capacitor within a predetermined time to detect one of a short-circuit condition and an open circuit condition.
In one aspect, the BMS determines presence of the DC-link capacitor in the short circuit condition when the rate of rise of a voltage across the DC-link capacitor is less than a first predefined value within a predefined time duration.
In one aspect, the BMS determines presence of the DC-link capacitor in the open circuit condition when the rate of rise of the voltage across the DC-link capacitor is higher than a second predefined value.
In one aspect, the second predefined value of the voltage is a battery voltage applied to the DC-link capacitor.
In one aspect, the electrical load is a part of a power electronic device used in an Electric Vehicle or a Switched Mode Power Supply.
In one aspect, the first pre-charging arm is disconnected and the second pre-charging arm is utilized to power the electrical load after the DC-link capacitor is charged completely.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings constitute a part of the description and are used to provide a further understanding of the present invention.
Fig. 1 illustrates a conventional electrical circuitry for pre-charging a DC-link capacitor, in accordance with prior art.
Fig. 2 illustrates an electrical circuitry for pre-charging a DC-link capacitor, in accordance with an embodiment of the present invention.
Fig. 3 illustrates a charge plot of a DC-link capacitor, in accordance with an embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The detailed description set forth below in connection with the appended drawings is intended as a description of various embodiments of the present invention and is not intended to represent the only embodiments in which the present invention may be practiced. Each embodiment described in this disclosure is provided merely as an example or illustration of the present invention, and should not necessarily be construed as preferred or advantageous over other embodiments. The detailed description includes specific details for the purpose of providing a thorough understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced without these specific details.
The present invention pertains to a method of pre-charging a DC-link capacitor. The method includes providing two pre-charging arms connected in parallel with each other and used for charging a DC-link capacitor. Initially, a first pre-charging arm including a resistive element is used for charging the DC-link capacitor up to a first level. Thereafter, the first pre-charging arm along with a second pre-charging arm including a power Metal–Oxide–Semiconductor Field-Effect Transistor (MOSFET) are both used for charging the DC-link capacitor completely.
The present invention provides certain advantages over conventional precharging circuits. For example, in the present invention, MOSFET based precharging circuit is also used along with resistor based precharging circuit. In case of use of only a MOSFET based precharging circuit, the system is prone to cold shorts, that is if the DC link line is already short-circuited in a scenario before the pre-charge is initiated, the pre-charge MOSFET may get damaged. Therefore, in the present invention, the resistor and MOSFET based precharging circuits are used together. Initially, the resistor based precharging is initiated such that the resistor based precharging circuit would ensure the safety of the MOSFET used in the MOSFET based precharging circuit in case of a short circuited condition on DC link line. Therefore, if the rate of rise of voltage is within the expected limits then it signifies that the capacitance load is present on the DC link and pre-charge will enter the next stage of dual pre-charge where both the circuits (resistor based and MOSFET based) would parallelly charge the DC link capacitor.
Fig. 2 illustrates an electrical circuitry for pre-charging a DC-link capacitor, in accordance with an embodiment of the present invention. The electrical circuitry may include a first pre-charging arm 202 and a second pre-charging arm 204 connected in parallel between a power source 206 and a DC-link capacitor 208. The power source 206 may be a battery for powering an electrical load 210. In one implementation, the electrical load 210 may include a motor controller 212, a motor 214, and a DC-DC converter 216. A 60V battery may be used as the power source 206 for powering the motor controller 212, the motor 214, and the DC-DC converter 216 of an Electric Vehicle (EV). The DC-DC converter 216 may be used for powering vehicle peripherals, such as a Vehicle Control Unit (VCU), vehicle body lights, and a telematics unit. The DC-link capacitor 208 present on a DC link line may be pre-charged for filtering transient noises within the electrical circuitry. The electrical load 210 and the DC-link capacitor 208 may be connected with the power source 206 through DC-link harness 218.
The electrical circuitry may further include a power switch 220 connecting the first pre-charging arm 202 and the second pre-charging arm 204 with the power source 206. Pre-charging of the DC-link capacitor 208 may start after the power switch 220 is activated. The power switch 220 may be present in an activated form at all times or may be activated based on a user’s input/action. For example, the user’s input/action may correspond to turning ON of the EV using a physical switch or a touch command.
When the pre-charging is started, the DC-link capacitor 208 is charged through the first pre-charging arm 202 partly up to a first level. The first pre-charging arm 202 may include a resistor 222 and a switch 224. By activating the switch 224, pre-charging through the first pre-charging arm 202 may be initiated. The first level of charging may be indicated by a first time interval T1 indicative of an RC delay of an RC circuit. The RC circuit includes the resistor 222 and the DC-link capacitor 208. Therefore, the first time interval T1 depends on values of the resistor 222 and the DC-link capacitor 208.
After charging the DC-link capacitor 208 up to the first level through the first pre-charging arm 202, the second pre-charging arm 204 gets activated. With this, charging of the DC-link capacitor 208 beyond the first level is performed using both pre-charging arms i.e., the first pre-charging arm 202 and the second pre-charging arm 204. A parallel combination of the first pre-charging arm 202 and the second pre-charging arm 204 is used to completely charge the DC-link capacitor 208 during a second time interval. The second pre-charging arm 204 may include a power MOSFET 226. The power MOSFET 226 operates in a cut-off region during charging of the DC-link capacitor 208 up to the first level through the first pre-charging arm 202. Further, the power MOSFET 226 operates in a linear region and may offer an RC delay provided by a gate driver circuit 228 of the power MOSFET 226 during charging of the DC-link capacitor 208 beyond the first level. The power MOSFET 226 operates in a saturation region when charging of the DC-link capacitor 208 is completed and power is supplied to the electrical load 210. In one implementation, the power MOSFET 226 may be switched ON by receiving a gate driving signal from a Battery Management System (BMS) 230. The first pre-charging arm 202, the second pre-charging arm 204, and the gate driver circuit 228 may be a part of the BMS 230.
The BMS 230 may also determine a rate of rise of a voltage across the DC-link capacitor 208 within a predetermined time to detect one of a short-circuit condition and an open circuit condition. The BMS 230 may determine presence of the DC-link capacitor 208 in the short circuit condition when the rate of rise of the voltage across the DC-link capacitor 208 or the DC-link harness 218 is less than a first predefined value within a predefined time duration. Further, the BMS 230 may determine presence of the DC-link capacitor 208 in the open circuit condition when the rate of rise of the voltage across the DC-link capacitor 208 or the DC-link harness 218 is higher than a second predefined value. The first predefined value and the second predefined value of the voltage may be a function of the battery voltage applied to the DC-link capacitor 208, for example 0.6 times the battery voltage.
After the DC-link capacitor 208 is charged completely, the first pre-charging arm 202 may be disconnected by deactivating the switch 224 and only the second pre-charging arm 204 may be utilized as a least resistance path to power the electrical load 210. In the above described manner, the power MOSFET 226 operates in a saturation region after the DC-link capacitor 208 is completely pre-charged.
Fig. 3 illustrates a charge plot of the DC-link capacitor 208, in accordance with an embodiment of the present invention. A first curve 302 corresponds to conventional method of pre-charging of the DC-link capacitor 208, wherein the DC-link capacitor 208 is completely charged through a resistive element, for example the resistor 222 present in the first pre-charging arm 202. A second curve 304 corresponds to charging of the DC-link capacitor 208 using the method of pre-charging described in the present invention. The second curve 304 represents charging of the DC-link capacitor 208 during the first time interval T1 up to the first level through the resistive element i.e., the resistor 222 present in the first pre-charging arm 202. The second curve 304 also represents complete charging of the DC-link capacitor 208 beyond the first level through a parallel combination of the resistor 222 present in the first pre-charging arm 202 and the power MOSFET 226 present in the second pre-charging arm 204. From Fig. 3, it could be understood that the rate of charging of the DC-link capacitor 208 using the method of pre-charging described in the present invention is very quick compared to the conventional methods which utilizes resistive elements only. Specifically, the DC-link capacitor 208 is charged in less than 2RC time period. Such a time period contributes to a very small turn-on delay, and therefore improves a user’s experience in activation of a circuit or an EV. Further, a small package resistor could be used as the pre-charge resistor i.e., the resistor 222 can be used to lower an overall cost of pre-charging circuit. The pre-charge resistor can be sized such that surface mount resistors can be used instead of through hole resistors.
It must be understood that the above described method of pre-charging a DC-link capacitor could be utilized in a variety of applications, such as an Electric Vehicle (EV) and a Switched Mode Power Supply (SMPS). The SMPS may be present as a DC-DC converter, a DC-AC converter, an AC-DC converter, and an AC-AC converter.
Although implementations of the method of pre-charging a DC-link capacitor has been described in language specific to structural features and/or methods, it is to be understood that the appended claims are not necessarily limited to the specific features or methods described. Rather, the specific features and methods are disclosed as examples of implementations of methods of pre-charging a DC-link capacitor. , Claims:WE CLAIM:
1. A method of pre-charging a DC-link capacitor, the method comprising:
providing a first pre-charging arm (202) and a second pre-charging arm (204) connected in parallel between a power source (206) and a DC-link capacitor (208), wherein the DC-link capacitor (208) is used for stabilizing the input voltage across an electrical load (210);
charging the DC-link capacitor (208) through the first pre-charging arm (202) partly up to a first level; and
charging the DC-link capacitor (208) beyond the first level using the first pre-charging arm (202) and the second pre-charging arm (204).

2. The method as claimed in claim 1, wherein the first level of charging is indicated by a first time interval T1 of an RC circuit including the DC-link capacitor (208).

3. The method as claimed in claim 1, where the first pre-charging arm (202) and the second pre-charging arm (204) both are used to completely charge the DC-link capacitor (208) during a second time interval.

4. The method as claimed in claim 1, wherein the first pre-charging arm (202) includes a resistor (222) and a switch (224).

5. The method as claimed in claim 3, wherein the second pre-charging arm (204) includes a power Metal–Oxide–Semiconductor Field-Effect Transistor (MOSFET) (226).

6. The method as claimed in claim 5, wherein the power MOSFET (226) operates in a cut-off region during charging of the DC-link capacitor (208) up to the first level through the first pre-charging arm (202).

7. The method as claimed in claim 5, wherein the power MOSFET (226) operates in a linear region for an RC delay provided by a gate driver circuit (228) of the power MOSFET (226) during charging of the DC-link capacitor (208) beyond the first level.

8. The method as claimed in claim 5, wherein the power MOSFET (226) operates in a saturation region when charging of the DC-link capacitor (208) is completed and power is supplied to the electrical load (210).

9. The method as claimed in claim 7, wherein the power MOSFET (226) is switched ON by receiving a gate driving signal from a Battery Management System (BMS) (230).

10. The method as claimed in claim 9, wherein the BMS (230) determines a rate of rise of a voltage across the DC-link capacitor (108) within a predetermined time to detect one of a short-circuit condition and an open circuit condition.

11. The method as claimed in claim 10, wherein the BMS (230) determines presence of the DC-link capacitor (208) in a short circuit condition when a rate of rise of a voltage across the DC-link capacitor (208) is less than a first predefined value within a predefined time duration.

12. The method as claimed in claim 10, wherein the BMS (230) determines presence of the DC-link capacitor (208) in an open circuit condition when a rate of rise of a voltage across the DC-link capacitor (208) is higher than a second predefined value.

13. The method as claimed in claim 12, wherein the second predefined value of the voltage is a battery voltage applied to the DC-link capacitor (208).
14. The method as claimed in claim 1, wherein the electrical load (210) is a part of a power electronic device used in an Electric Vehicle or a Switched Mode Power Supply.

15. The method as claimed in claim 1, wherein the first pre-charging arm (202) is disconnected and the second pre-charging arm (204) is utilized to power the electrical load (210) after the DC-link capacitor (208) is pre-charged completely.