Method and apparatus for providing current limiting for an electrical device

Abstract

Method and apparatus for providing current limiting for an electrical device. The apparatus comprises a current mirror having a DC input and establishes a current level for the electrical device. A MOSFET, coupled to the current mirror, limits the current to the established current level.

Description

METHOD AND APPARATUS FOR PROVIDING CURRENT LIMITING FOR AN ELECTRICAL DEVICEBACKGROUNDField

[0001] Embodiments of the present invention generally relate to electrical devices and, in particular, to a method and apparatus for providing current limiting for an electrical device.Description of the Related Art

[0002] Electrical devices such as power conversion units, appliances, motors, air conditioning units, and the like draw huge amounts of current at their startup which can’t be sufficiently supplied by the power supplies (e.g., USB charger, power conditioning units, switching power supplies, power inverters, etc.) which have the tendency to supply a limited amount of current. Hence, the startup scenario requires the supply current to be limited to protect the power source and the connected circuit components from damage.

[0003] A specific example of high efficiency power conversion devices (e.g., microinverters) that may be employed in a solar energy system includes a power conversion device as part of the circuitry for converting DC power to AC power. In some instances, where the power conversion device is used to charge and discharge a battery, it may operate bidirectionally to convert DC power to AC power (battery discharge) and AC power to DC power (battery charge).

[0004] Any electrical device such as a power conversion device (e.g., microinverter) generally includes a large bulk capacitor across the input DC terminals for smoothing the input DC voltage. As soon as such devices are powered, the capacitor with zero initial charge operates as a short circuit or alow impedance path across the solar panel or battery which causes a high inrush current to flow. This may lead to triggering one or more circuit breakers that are used to protect the solar system.

[0005] A negative temperature coefficient (NTC) thermistor in the DC input path may be used to limit the in-rush current. An NTC thermistor has a high resistance when it is at lower temperatures, and the resistance decreases as the thermistor heats up due to the current flow. Consequently, the in-rush current is limited. However, a power conversion device that is used in a solar system is typically powered from a solar panel. As the sun rises, the initial limited amount of solar power is applied to the power conversion device and a startup process is initiated. Due to the high resistance of NTCs at lower temperatures, there is a high voltage drop across the thermistor at device startup which consequently lowers the voltage to operate the power conversion device. The lower applied voltage interferes with the device startup processes. As a result, the use of NTCs in solar power conversion devices is limited.

[0006] Another previous solution incorporates a pre-charge circuit at the DC input to control the flow of current into the capacitor at power conversion device activation. One such pre-charge circuit switches (using relays) a resistor into the DC current path to limit the amount of current flow during a pre-charge period. After the period, the circuit switches (using relays) the resistor out of the input path and directly connects the DC input to the power conversion device’s bulk capacitor. Upon failure of the resistor or the resistor path switching block, a high in-rush current flows until the end of the pre-charge period leading to potential damage to the power conversion device circuitry and / or triggering of circuit breakers.

[0007] Therefore, there is a need for an improved method and apparatus limit current in an electrical device.SUMMARY

[0008] A method and apparatus for providing current limiting for an electrical device is provided substantially as shown in and / or described in connection with at least one of the figures, as set forth more completely in the claims.

[0009] Various features and advantages of the present disclosure may be appreciated from a review of the following detailed description of the present disclosure, along with the accompanying figures in which like reference numerals refer to like parts throughout.BRIEF DESCRIPTION OF THE DRAWINGS

[0010] So that the manner in which the various features of the present invention can be understood in detail, a particular description of the invention, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.

[0011] FIG. 1 depicts a block diagram of an electrical device utilizing a current limiter to limit input current to the electrical device in accordance with at least one embodiment of the invention;

[0012] FIG. 2 depicts a block diagram of an electrical device utilizing a current limiter to limit output current from the electrical device in accordance with at least one embodiment of the invention;

[0013] FIG. 3 depicts a schematic diagram of the current limiter of FIGs. 1 and 2 in accordance with at least one embodiment of the invention; and

[0014] FIG. 4 depicts a current waveform produced by the current limiter in accordance with at least one embodiment of the invention.DETAILED DESCRIPTION

[0015] Embodiments of the present invention comprise apparatus and methods for providing current limiting for an electrical device. The current limiting technique described herein may find its application in any electrical device where current needs to be constrained to a certain level to prevent any detrimental impact to the device or electrical circuits connected to the device. Such current limiting is necessary to limit current drawn by certain loads such as, but not limited to, air conditioners, refrigerators, washers, and other appliances. In other situations, a device may use a bulk capacitor that draws substantial amounts of current at device startup such as, but not limited to, a power conversion device (e.g., microinverter). In a further situation, output current of a power supply such as, but not limited to, a USB charger, LED lighting power source, and the like, may require a current limit to avoid damage to the power supply or to a device being powered. In an LED lighting power source, the current limiter may be used to not only limit the current to an LED array to mitigate over current damage but can also be used to control the current level to adjust LED light brightness.

[0016] One specific embodiment includes a power conversion device having a current limiter and power conversion circuitry comprising a bulk capacitor. The current limiter is coupled between the DC input of the power conversion device and the bulk capacitor. At startup, the current limiter limits the current flow into the capacitor to a defined value such that a DC power source, such as a solar panel, is not damaged during power conversion device startup and / or circuit breakers in the DC path are not triggered.

[0017] FIG. 1 depicts a block diagram of an electrical device 106 (a power sink) utilizing a current limiter 104 to limit input current 102 to the electrical device 106 in accordance with at least one embodiment of the invention. In this embodiment, the electrical device may be any device that is to be protected from excessive current flowing into the device such as devices with bulk capacitors or low resistance at startup.

[0018] FIG. 2 depicts a block diagram of an electrical device 202 (a power source) utilizing the current limiter 104 to limit output current 208 from the electrical device 202 in accordance with at least one embodiment of the invention. In this embodiment, the electrical device 202 may be damaged by supplying too much current or may damage a device being supplied too much current. In either situation, the current limiter 104 limits the amount of current that can be sourced from the electrical device 202. The current limiter may also be used to control the amount of current to a specific value in, for example, an LED light intensity controller (e.g., dimmer control).

[0019] FIG. 3 depicts a schematic diagram of the current limiter 104 of FIGs. 1 and 2 in accordance with at least one embodiment of the invention. The current limiter 104 has a DC input 300 and a DC output 302 (VC). The current limiter 104 comprises a current mirror 304 coupled in series with a MOSFET 306. Transistors Q1 and Q2 (BJT transistors) and resistors R3 and R4 (e.g., 750K ohms) form the current mirror 304. Resistor R1 is the sense resistor that establishes the reference current limit for the current limiter 104. In one embodiment, the resistor R1 (e.g., about 2 ohms) should be able to handle large transient currents such as a Metal Electrode Leadless Face (MELF) resistor. Diode D1 , resistor R2 (e.g., 10K ohms), resistor R5 (e.g., 500K ohms) and resistor R7 (e.g., 100K ohms) establish the reference voltage at node B (the current mirror input). Diode D6, connected across the source and drain terminals, provides overvoltage protection for the MOSFET 306. In the depicted embodiment, the MOSFET 306 is a P-MOSFET, other types of MOSFETs may be used. Circuit component values are exemplary. Other applications of the current limiter 104 will require different component values. The depicted embodiment is designed for limiting current flowing into a bulk capacitor C1 (e.g., 300 pF).

[0020] The P-MOSFET 306 is biased to operate in the triode region of its VI characteristics such that there is a dependance of on-state resistance of the MOSFET with all the three terminals - drain, source and gate.

[0021] The two emitter voltages of the BJT transistors Q1 and Q2 tend to be equal for the current mirror to function.

[0022] Assuming, the DC input voltage is VR and the voltage at node B is VB. Until the voltage at the emitter of Q1 becomes equal to the voltage at node B (VB), the current flows through this sense resistor R1 . By setting the value of this sense current, a reference current is set.

[0023] Initially, the P-MOSFET 306 is completely turned ON as the source voltage is less than node B voltage, therefore no current limit occurs. As the current starts to increase, the source voltage of the MOSFET 306 increases converging to the potential at node B. At that instant, Q2 turns ON causing the gate voltage to rise from ground potential and approach the node B voltage. This potential difference, if lesser than the threshold value, causes the MOSFET 306 to operate in the triode region. As a result, the current drops and thereby, will settle at one value. The output current becomes modulated to provide a controlled average output current.

[0024] FIG. 4 depicts a graph 400 of a current waveform 406 produced by the current limiter 104 in accordance with at least one embodiment of the invention. The vertical axis 402 represents current magnitude and the horizontal axis 404 represents time. The current waveform 406 rises as the MOSFET conducts until the current mirror limits the current flow. The current peaks at a fixed, non-zero DC current level established by the sense resistor R1. As the bulk capacitor C1 charges, the current decreases until the capacitor is fully charged.

[0025] Here multiple examples have been given to illustrate various features and are not intended to be so limiting. Any one or more of the features may not be limited to the particular examples presented herein, regardless of any order, combination, or connections described. In fact, it should be understood that any combination of the features and / or elements described by way of example above are contemplated, including any variation or modification which is not enumerated, but capable of achieving the same. Unless otherwise stated, any one or more of the features may be combined in any order.

[0026] As above, figures are presented herein for illustrative purposes and are not meant to impose any structural limitations, unless otherwise specified. Various modifications to any of the structures shown in the figures are contemplated to be within the scope of the invention presented herein. The invention is not intended to be limited to any scope of claim language.

[0027] Where “coupling” or “connection” is used, unless otherwise specified, no limitation is implied that the coupling or connection be restricted to a physical coupling or connection and, instead, should be read to include communicative couplings, including wireless transmissions and protocols.

[0028] Where conditional language is used, including, but not limited to, “can,” “could,” “may” or “might,” it should be understood that the associated features or elements are not required. As such, where conditional language is used, the elements and / or features should be understood as being optionally present in at least some examples, and not necessarily conditioned upon anything, unless otherwise specified.

[0029] Where lists are enumerated in the alternative or conjunctive (e.g., one or more of A, B, and / or C), unless stated otherwise, it is understood to include one or more of each element, including any one or more combinations of any number of the enumerated elements (e.g., A, AB, AC, ABC, ABB, etc.). When “and / or” is used, it should be understood that the elements may be joined in the alternative or conjunctive.

[0030] While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.

Claims

Claims:1 . Apparatus for providing current limiting for an electrical device comprising: a current mirror having a DC input and adapted to establish a current level for the electrical device; and a MOSFET, coupled to the current mirror, for limiting the current to the established current level.

2. The apparatus of claim 1 wherein the current mirror comprises a pair of BJT transistors.

3. The apparatus of claim 1 further comprising a resistor, coupled to the current mirror, having a resistance value that establishes the current level.

4. The apparatus of claim 1 further comprising a diode coupled from a source to a gate of the MOSFET to provide overvoltage protection.

5. The apparatus of claim 1 wherein the electrical device comprises a bulk capacitor and the MOSFET limits the current flowing into the bulk capacitor.

6. The apparatus of claim 1 wherein the electrical device comprises at least one light emitting diode and the MOSFET limits the current flowing into the at least one light emitting diode.

7. The apparatus of claim 1 wherein the MOSFET, when limiting the current, operates in a triode region.

8. A method of providing current limiting for an electrical device comprising: establishing a current level for the electrical device using a current mirror; and controlling a current at the established current level using a MOSFET.

9. The method of claim 8 wherein the current mirror controls the MOSFET to operate in a triode region.

10. The method of claim 8 wherein the current mirror comprises a pair of BJT transistors.

11. The method of claim 8 further comprising establishing the current level using a resistor, coupled to the current mirror, where a resistance value of the resistor establishes the current level.

12. The method of claim 8 further comprising providing overvoltage protection using a diode coupled from a source to a gate of the MOSFET.

13. The method of claim 8 wherein the electrical device comprises a bulk capacitor and the MOSFET limits the current flowing into the bulk capacitor.

14. The method of claim 8 wherein the electrical device comprises at least one light emitting diode and the MOSFET limits the current flowing into the at least one light emitting diode.

15. The method of claim 8 further comprising operating the MOSFET in a triode region, when the MOSFET is limiting the current.

16. Apparatus for providing current limiting for an electrical device comprising: a current mirror having a DC input and adapted to establish a current level for the electrical device, where the current mirror comprises a pair of BJT transistors; a MOSFET, coupled to the current mirror, for limiting the current to the established current level; a resistor, coupled to the current mirror, having a resistance value that establishes the current level; and a diode coupled from a source to a gate of the MOSFET to provide overvoltage protection.

17. The apparatus of claim 16 wherein the electrical device comprises a bulk capacitor and the MOSFET limits the current flowing into the bulk capacitor.

18. The apparatus of claim 16 wherein the electrical device comprises at least one light emitting diode and the MOSFET limits the current flowing into the at least one light emitting diode.

19. The apparatus of claim 16 wherein the MOSFET, when limiting the current, operates in a triode region.

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