HK1121295B - Supercapacitor backup power supply with bi-directional power flow - Google Patents

Description

Super capacitor backup power supply with bidirectional power flow

Technical Field

The invention relates to power supply technology, in particular to a backup power supply system based on a super capacitor.

Background

Many digital systems require a backup power source in the event that the primary power source is unavailable. This is usually done with a battery, but with the creation of very high capacity capacitors (supercapacitors), it is more often preferable to replace the battery with a capacitor. This is mainly due to maintenance reasons: supercapacitors can withstand more charge/discharge cycles than rechargeable batteries and have a longer service life than batteries thereby reducing the maintenance requirements for a given product requiring a backup mechanism.

Known backup power mechanisms that use super-capacitors to store energy include two separate circuits: a circuit to charge the supercapacitor when the primary power source is available; and a switching power supply to operate the supercapacitor when the primary power supply is unavailable.

A simple example of a backup power mechanism with separate charging and discharging circuits is shown in fig. 1. Vcc is generated by a main power supply (not shown) when available. During this time, switch 102 is closed, allowing supercapacitor 104 to charge via current source 103. The current source 103 includes a resistor, an active current source (active current source), a switching power supply (switching supply), or other mechanisms. Switch 106 is open during charging. The switch 102 is adjusted to maintain a fixed (maximum) voltage across the supercapacitor 104. This is typically performed by a control structure (not shown).

When the main power supply is missing, switch 102 is opened and switch 106 is adjusted to transfer energy from supercapacitor 104 to Vcc via inductor 108 and diode 110. The output filtering is performed by an output capacitor of a main power supply (not shown). Thus, there are separate charging and discharging circuits. The use of separate circuits for charging and discharging requires an additional number of components, thereby increasing cost, layout area and weight of a Printed Circuit Board (PCB).

Higher efficiency can be achieved when the diode 110 has a switch across it to form a synchronous rectifier. A circuit with this additional element is shown in fig. 2. The switch 202 is connected in parallel with the diode 110. However, the circuit of fig. 2 has separate charging and discharging circuits.

The prior art supercapacitor charging solutions have only a simple charging mechanism, where the supercapacitor is placed directly across the voltage, which makes it possible to have a very large current at the start of the charging.

It is therefore desirable to provide an ultracapacitor-based backup power system that minimizes the number of components, provides efficient output voltage generation, and provides controlled (limited instantaneous current demand from the voltage source) and energy efficient charging of the ultracapacitor.

Disclosure of Invention

The present invention relates generally to the charging and discharging of supercapacitors used in the event of a power backup.

It is an object of the present invention to obviate or mitigate at least one disadvantage of prior art circuits for charging and discharging a supercapacitor.

According to an aspect of the present invention, a backup power supply system is provided. The system includes a super capacitor, and a single (single) circuit for charging and discharging the super capacitor. The single circuit includes a path (path) having an inductor for charging when operating in a charging mode and for discharging when operating in a standby mode.

According to another aspect of the present invention, a backup power system is provided. The system includes a supercapacitor, an inductor, a single circuit operating with the inductor for charging and discharging the supercapacitor, and a controller for monitoring and controlling the single circuit.

This summary of the invention does not necessarily describe all features of the invention.

Drawings

These and other features of the present invention will become more apparent from the following description made with reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram showing a prior art supercapacitor-based backup power circuit;

FIG. 2 is a schematic diagram showing another ultracapacitor-based backup power circuit;

FIG. 3 is a schematic diagram illustrating a super capacitor based backup power circuit according to an embodiment of the present invention;

FIG. 4 is a schematic diagram illustrating a super capacitor based backup power circuit according to another embodiment of the present invention;

FIG. 5 is a schematic diagram illustrating a super capacitor based backup power circuit according to yet another embodiment of the present invention;

FIG. 6 is a schematic diagram illustrating an example of a control circuit according to an embodiment of the present invention; and

FIG. 7 is a schematic diagram illustrating a super capacitor based backup power circuit according to yet another embodiment of the present invention.

Detailed Description

Embodiments of the present invention provide a backup power supply that is implemented with a single charge and discharge circuit for a super capacitor. The circuit has a reduced number of components compared to having separate charging and discharging circuits. In the following description, the term "connected" is used to indicate that two or more elements are in direct or indirect contact with each other.

FIG. 3 illustrates a supercapacitor based backup power supply according to an embodiment of the present invention. The backup power supply circuit 300 of fig. 3 includes: switches 302 and 304, diodes 305 and 306, inductor 308, and supercapacitor 310. The switch 302 is connected in parallel with a diode 305. The switch 304 is connected in parallel with a diode 306. Inductor 308 and supercapacitor 310 may be the same as or similar to inductor 108 and supercapacitor 104 of fig. 2, respectively. It should be noted that fig. 3 is somewhat conceptual, and that other circuitry may be included around the circuitry shown in fig. 3.

The diode 306 functions as a so-called freewheeling diode. The combination of switch 302, inductor 308, and diode 306 provides a switching power supply (switching power supply) or so-called buck converter (buck converter) that can be used to charge supercapacitor 310. Since this circuit 300 can be used for charging, the current source and its control switches (103 and 102 of fig. 2) become redundant. Thus, the circuit 300 does not use the current source 103 and its switch 102 of fig. 2. The circuit 300 is used to charge and discharge the supercapacitor 310 without a current source and its switches. In circuit 300, the magnetic element, inductor 308, operates in a bidirectional mode.

When Vcc is generated by a main power supply (not shown), the circuit 300 is in a charging mode. In the charging mode, switch 302 is adjusted to charge supercapacitor 310 to the desired level, i.e., power flows from Vcc to supercapacitor 310. In the charging mode, the switch 304 is normally maintained open at this time. However, to improve efficiency, switch 304 may be closed during the freewheeling time of diode 306. In this case, the switch 304 acts as a synchronous rectifier.

When the absence of a main power source generating Vcc is detected, the circuit 300 is in standby (discharge) mode. In the standby mode, the switch 304 is adjusted so that power flows from the supercapacitor 310 to Vcc. In the standby mode, switch 302 acts as a synchronous rectifier and is closed during the flyback time of inductor 308.

In an embodiment, a controller is provided to the circuit 300 to monitor the main power supply, the supercapacitor voltage, the output voltage (Vcc), the inductor current (if current mode control is to be implemented), or a combination thereof, and then to control the operation of the charging and discharging circuit based on the monitored values (e.g., fig. 4-6).

In one example, the controller monitors the primary power source and enables the charging mechanism of the supercapacitor when the primary power source is available (charging mode). In the charging mode, the controller monitors the voltage across the supercapacitor 310 and operates the switches 302 and 304 along with the inductor 308 to form a buck converter (with synchronous rectifier). In this case, energy flows from Vcc to the supercapacitor.

When the main power supply is absent, then the controller switches to the standby mode. In standby mode, the controller monitors the voltage Vcc and operates switches 302 and 304 along with inductor 308 to form a boost converter (with synchronous rectifier). In this case, energy flows from the supercapacitor to Vcc.

In the charging or standby mode, the controller may implement current mode control. Current mode control limits the peak or average current of inductor 308 using an inner control loop, which will result in a significant shift of the electrode associated with inductor 308 compared to a voltage mode controlled switched mode power supply. The resulting reduced number of stages facilitates a better dynamic response of the power supply and makes compensation of the power supply easier. For such control, the controller includes a mechanism to monitor the current of inductor 308 in a current control mode. The inherent control of the inductor current in current mode control works well under the concept of charging the capacitor at a fixed rate. The circuit 300 may use voltage mode control to control the output voltage.

The circuit 300 is suitable for configurations in which the supply voltage Vcc is greater than or equal to the maximum allowable capacitor voltage. However, it is well understood by those skilled in the art that circuit 300 may be reconfigured to support Vcc less than the maximum supercapacitor voltage. Thus, a boost circuit is provided to charge the supercapacitor, and a buck circuit to supply Vcc in standby, i.e., bi-directional power flow through a common mechanism.

FIG. 4 illustrates a supercapacitor based backup power supply according to another embodiment of the present invention. The super capacitor based backup power circuit 401 of fig. 4 is similar to the circuit 300 of fig. 3. Circuit 401 includes switches 402 and 404, diodes 405 and 406, inductor 408, and supercapacitor 410. Diodes 405 and 406 correspond to diodes 305 and 306 of fig. 3. Inductor 408 may be the same as or similar to inductor 308 of fig. 3. The supercapacitor 410 may be the same as or similar to the supercapacitor 310 of fig. 3. Switches 402 and 404 correspond to switches 302 and 304 of fig. 3. However, in this embodiment, switches 402 and 404 are Metal Oxide Semiconductor Field Effect Transistors (MOSFETs). In this description, the terms "switch 402 (404)" and "MOSFET 402 (404)" are used interchangeably.

In one example, diodes 405 and 406 may be intrinsic diodes of MOSFET 402 and MOSFET 404, respectively. In another example, diodes 405 and 406 may be external schottky diodes connected in parallel with the intrinsic diodes of MOSFET 402 and MOSFET 404, respectively. The schottky diode may provide a current path during the time it takes for the corresponding MOSFET to fully conduct. The schottky diode has a lower forward voltage than the parallel diode inherent in the MOSFET structure and is highly efficient for power rectification applications.

Diodes 405 and 406, switches 402 and 404, and the inductor current sensing mechanism may be integrated into an IC package (integrated circuit) with controller 412. The controller 412 may be implemented in any suitable form. The inductor 408 and the supercapacitor 410 may be located external to any integrated circuit.

In order that the controller 412 may provide the desired functionality, it receives as inputs various signals and is responsive to those signals. These signals are shown in fig. 4 according to an embodiment of the present invention. The controller 412 uses the MODE control signal 414 to provide automatic switching between charging and standby MODEs. In one example, MODE signal 414 is an analog input to a comparator (e.g., 600 of FIG. 6) referenced to a voltage or some other logic level suitable for TTL. This allows MODE 414 to be driven from another circuit or from a scaled version of the main input supply. In the simplest implementation, the resistive divider may scale the main input voltage into the input of the comparator and may scale to less than the minimum input voltage, allowing for redundancy in the event of an unintended loss of power. When the supercapacitor voltage is not sampled, i.e., when in standby mode, the "V _ CAPACITOR" signal 416 is the JFET input (low input current) and the "V _ CAPACITOR _ COMMON" signal 418 is high impedance. The ENABLE signal 424 is a signal that ENABLEs the full functionality of the device.

The I SENSE signal 420 is a single input that allows current input, as needed in current mode control. In this embodiment, the current flowing through inductor 408 is measured at current sense terminal 422. The current can in fact be measured at several points, depending on the topology of the circuit. If current mode control is used, the current sensing mechanism of the controller 412 accepts bidirectional current. In this embodiment, the circuit operates at high frequencies, allowing the use of smaller inductors. To achieve circuit simplicity, the internal reference voltage of controller 412 may be less than Vcc and the maximum voltage of supercapacitor 410.

The circuit 401 is suitable for configurations in which the supply voltage Vcc is greater than or equal to the maximum allowed capacitor voltage. In an alternative embodiment, Vcc is lower than the maximum allowed capacitor voltage. In this case, the topology of the charge and discharge circuit 401 of fig. 4 is reversed, so that the boost circuit charges the capacitor and the buck circuit generates Vcc from the capacitor voltage.

FIG. 5 shows a supercapacitor backup power circuit according to another embodiment of the invention. The configuration shown in fig. 5 is suitable for a supercapacitor backup power supply with bidirectional power flow, where Vcc is greater than the maximum supercapacitor voltage. The controller elements of the circuit include diodes, power switches for the charging and discharging mechanisms, current detectors, voltage detectors, and circuitry to support operation of the dual mode power supply. The controller elements may operate within an integrated circuit (see integrated circuit 502). The supercapacitor 504 and the inductor 506 are located outside the integrated circuit 502.

The supercapacitor 504 may be the same as or similar to the supercapacitor 310 of fig. 3 or the supercapacitor 410 of fig. 4. Inductor 506 may be similar to inductor 308 of fig. 3 or inductor 408 of fig. 4.

The circuit of fig. 5 has a resistor network similar to that of fig. 4. A resistive network having resistors 530 and 532 is disposed between integrated circuit 502 and node 534, where node 534 is the connection node of supercapacitor 504 and inductor 506. A resistor network having resistors 536 and 538 is provided between Vcc and integrated circuit 502.

In fig. 5, only the resistive elements of the feedback path are shown, which set the DC potential. The circuit of fig. 5 includes two feedback paths, one of which is activated depending on whether the supercapacitor 504 is charged (i.e., charging mode) or discharged (i.e., standby mode). Compensation is achieved by adding capacitors to these resistors to provide spectral shaping, thereby achieving stable operation of the circuit in both charging and standby modes. Those skilled in the art will appreciate that more complex feedback mechanisms may be formed depending on the desired operating characteristics of the circuit.

In FIG. 5, the integrated circuit 502 includes a plurality of pins for the INDUCTOR signal 510, the V _ CAPACITOR signal 512, the V _ CAPACITOR _ COMMON signal 514, the ENABLE signal 516, the MODE signal 518, the VCC signal 520, the V _ SENSE signal 522, and the GROUND signal 524. The INDUCTOR signal 510, the V _ CAPACITOR signal 512, the V _ CAPACITOR _ COMMON signal 514, the ENABLE signal 516, the MODE signal 518, and the V _ SENSE signal 522 may be similar to the I _ SENSE signal 420, the V _ CAPACITOR signal 416, the V _ CAPACITOR _ COMMON signal 418, the ENABLE signal 424, the MODE signal 414, and the V0_ SENSE signal, respectively, of FIG. 4.

Fig. 6 shows an example of a control circuit according to an embodiment of the invention. The pin distribution of the circuit of fig. 6 is similar to the controller of fig. 5. In FIG. 6, signals associated with the integrated circuit 502 other than the ENABLE signal 516 are shown as an example. The circuit of fig. 6 is a basic current mode control and, for simplicity, the compensation (feedback) elements of the control loop are not shown.

The MODE input 518 is used to define the MODE of operation (charging or standby) of the circuit and to select the power supply to the voltage error amplifier (i.e., 602 or 604) entering the inner current loop through switch 606. Comparator 600 compares the MODE input 518 to a voltage and operates switch 606. Comparator 608 compares the output of switch 606 with the output of "ISENSE" circuit 616.

Circuit 616 includes resistor 617 and amplitude and level shifting circuit 618. Circuit 616 measures the current flowing through the INDUCTOR connected at input node 510. In this embodiment, the measurement is a high-side measurement, the detection element not being referenced to ground potential. The circuit 616 therefore includes a mechanism to communicate the measurement to the comparator 608, which is referenced to ground potential, to achieve current mode control. The magnitude of the current runs comparator 608. When the inductor current reaches a threshold value, such as the peak current of its current mode, then the current mode is activated.

Latch 610 includes an "S" node connected to clock circuit 612, an "R" node connected to the output of comparator 608, and a "Q" node connected to gate drive circuit 614. The gate drive circuit 614 selects the correct switching operation for the run mode (charging or standby), including the operation of the synchronous rectifiers. In fig. 6, gate drive circuit 614 drives switches 620, 622, and 624.

During the supercapacitor charge mode, switch 620 is conductive. In standby mode, switch 620 is off, so the resistor network with resistors 530 and 532 of fig. 5 does not discharge energy in order to maximize the available standby time. Considering that the released energy tends to be small, switch 620 may be eliminated at the expense of slightly reducing the standby time.

The properties of the ISENSE circuits (616, 618) depend on how the circuits are constructed. If the circuit is built with discrete components, a current transformer is the simplest mechanism. To be implemented in silicon, IC designers need technology that enables high side current measurement.

In the above embodiment, the primary power source has sufficient support time so that the backup power source (i.e., the supercapacitor 310 of fig. 3 or 410 of fig. 4) can detect the missing input power and enter the backup mode from the charging mode.

In another embodiment, the inherent diode of the MOSFET may be used in a synchronous rectifier.

In another embodiment, an additional input is provided to set the peak inductor current for charging and discharging the supercapacitor and compensating for the control loop.

FIG. 7 illustrates a super capacitor based backup power circuit according to another embodiment of the present invention. The standby power supply circuit 700 of fig. 7 is suitable for a configuration in which the power supply voltage Vcc is less than or equal to the maximum allowable capacitor voltage.

Power supply circuit 700 includes switches 702 and 704, diodes 705 and 706, inductor 708, and supercapacitor 710. Switch 702 and diode 705 are connected in parallel. The switch 704 and the diode 706 are connected in parallel. Inductor 708 and supercapacitor 710 may be the same as or similar to inductor 308 and supercapacitor 304 of fig. 3, respectively. In the backup power supply circuit 700, a switch 702 and a diode 705 are provided between an inductor 708 and a supercapacitor 710. Inductor 708 is connected to the Vcc node.

In the charging mode, switch 704 is a power switch for boosting, and switch 702 acts as a synchronous rectifier. In standby mode, switch 702 is a power switch for buck and switch 704 acts as a synchronous rectifier.

Those skilled in the art will appreciate that the topology based on the backup power circuit is not limited to that of fig. 3, 4 and 7, and that other topologies are contemplated.

The invention has been described in connection with one or more embodiments. It will be apparent to those skilled in the art that various changes and modifications may be made without departing from the scope of the invention as defined in the following claims.

Claims (21)

1. A system for use as a backup power source, the system comprising:

a super capacitor; and

a single circuit for charging and discharging the ultracapacitor, the single circuit comprising:

an inductor connected to the supercapacitor and operating in a bidirectional energy flow mode for charging and discharging the supercapacitor, such that energy flows in a first direction in a charging mode and energy flows in a second direction different from the first direction in a discharging mode;

a first switch connected between the inductor and a potential node; and

a second switch connected to the inductor and capable of causing electrical energy to flow from the ultracapacitor to the potential node,

the first switch is modulated to charge the supercapacitor via the inductor when a power source is available at the potential node,

the second switch is modulated to discharge the supercapacitor in conjunction with the inductor when power is lost at the potential node.

2. The system of claim 1, wherein the single circuit comprises:

a first diode connected in parallel with the first switch;

3. the system of claim 1, wherein the single circuit comprises:

a second diode in parallel with the second switch.

4. The system of any of claims 1-3, wherein at least one of the first switch and the second switch is a Metal Oxide Semiconductor Field Effect Transistor (MOSFET).

5. The system of claim 2, wherein the first diode is an intrinsic diode of the Metal Oxide Semiconductor Field Effect Transistor (MOSFET).

6. The system of claim 2, wherein the first diode is a schottky diode.

7. The system of claim 3, wherein the second diode is an intrinsic diode of the Metal Oxide Semiconductor Field Effect Transistor (MOSFET).

8. The system of claim 3, wherein the second diode is a Schottky diode.

9. The system of claim 1, further comprising a controller for controlling the first switch and the second switch.

10. The system of claim 9, wherein the controller operates the first switch and the second switch along with the inductor to form a buck converter with a synchronous rectifier.

11. The system of claim 9, wherein the controller operates the first switch and the second switch along with the inductor to form a boost converter having a synchronous rectifier.

12. The system of claim 9, wherein the controller and at least one of the first switch and the second switch are formed in an integrated circuit.

13. The system of claim 1, wherein the single circuit forms a buck converter or a boost converter depending on whether the single circuit is in a charging mode or a buck mode.

14. The system of claim 13, further comprising a controller for forming the buck converter or the boost converter.

15. The system of claim 9, wherein the system has a plurality of modes including a charging mode for charging and a standby mode for discharging, wherein the controller monitors a primary supply of a potential, the ultracapacitor voltage, the potential, a current of the inductor, or a combination thereof, and controls the mode of the single circuit based on results of one or more of the monitoring.

16. The system of claim 9, wherein the controller comprises a monitor for monitoring current flowing through the inductor.

17. The system of claim 9, wherein the circuitry of the controller is located on an integrated circuit.

18. The system of claim 17, wherein the integrated circuit has as its input a current sense signal.

19. The system of claim 9, wherein the controller has a current control mode, a voltage control mode, or a combination thereof.

20. The system of claim 2, wherein at least one of the first diode, the first switch, and the second switch, and a controller for controlling operation of the single circuit are formed in an integrated circuit.

21. The system of claim 3, wherein at least one of the second diode, the first switch, and the second switch, and a controller for controlling operation of the single circuit are formed in an integrated circuit.