# Patent

## Title
Back-up power supply systems

## Abstract

A back-up power supply system includes, in one embodiment, a threshold detector circuit; a first switching circuit for enabling access to a back-up power source, the first switching circuit comprising at least a first transistor; and an inverting switch coupled between the first switching circuit and the threshold detector circuit, the inverting switch comprising an input and an output, the inverting switch configured to receive a signal at the input, invert the signal, and provide the inverted signal from the output to the first switching circuit, the inverting switch further comprising a delay circuit, the delay circuit configured to provide a substantially-immediate high-output during an off-to-on transition at the output of the inverting switch and a switching delay during an on-to-off transition at the output of the inverting switch, the inverting switch comprising a first resistive branch in parallel to a second branch comprising the delay circuit, the first resistive branch and the second branch connected to respective gates of the first and a second transistor, wherein the threshold detector circuit is configured to cause the first switching circuit to enable access to the back-up power source responsive to a voltage provided by a primary power source dropping below a predetermined threshold.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
This application is a continuation of U.S. utility application entitled, “Systems and Methods for Switching to a Back-Up Power Supply,” having Ser. No. 10/690,946, filed Oct. 22, 2003, which is entirely incorporated herein by reference.


FIELD OF THE INVENTION
This invention relates in general to providing a back-up power supply, and more specifically to systems and methods for switching to a back-up power supply.
DESCRIPTION OF THE RELATED ART
Much of today's electronic equipment needs a constant power source. When a power supply fails, the switch to a backup supply should be instantaneous such that the load voltage does not dip below a set threshold. Typically diodes are used in an “OR” configuration (i.e. either the main supply or the backup supply delivers power to the load). Many applications, however, have tight voltage tolerances, and the loss through a diode is too great. Therefore, there exists a need for systems and methods for addressing these and/or other problems related to providing a back-up power supply.



BRIEF DESCRIPTION OF THE DRAWINGS
The invention can be better understood with reference to the following drawings. The components in the drawings are not necessarily drawn to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. In the drawings, like reference numerals designate corresponding parts throughout the several views.
FIG. 1 is a block diagram depicting a power-switching circuit according to an embodiment of the invention.
FIG. 2A is a block diagram depicting an example of the threshold detector shown in FIG. 1.
FIG. 2B is a graph illustrating a non-limiting example of a transient hysteresis effect within the threshold detector shown in FIG. 2A.
FIG. 3 is a block diagram depicting an example of the inverter shown in FIG. 1.
FIG. 4 is a block diagram depicting an example of an inverting switch shown in FIG. 1.
FIG. 5 is a block diagram depicting an example of a power switch shown in FIG. 1.
FIG. 6 is a block diagram depicting an example of a voltage supply circuit.
FIG. 7 is a flow chart depicting a method according to one embodiment of the invention.



DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
An embodiment of the invention enables sustained power to DC-input end-use electronics. The embodiment is useful in applications that have primary and backup power sources. If the primary source fails, then the backup source is supplied to the load instead of the primary source. Switches used in this embodiment are very low loss and can pass high currents to the load with very little drop in voltage.
A condition for switching between one source and another is a voltage level of the primary source (Vp). If Vp falls below a threshold set by a comparison circuit, then the load is powered by the back-up power supply. Conversely, if Vp rises above the threshold, then the load is powered by the primary power supply.
Low resistance field effect transistors (FETs) may be used as switches, and may be controlled by a threshold detection circuit. Using FETs enables a commercial “off the shelf” power source to be used, without the need to have a higher voltage source to overcome diode losses.
Below is a detailed description of the accompanying 6 figures, which illustrate a preferred embodiment of the present invention: FIG. 1 depicts an embodiment of a power-switching circuit; FIGS. 2-5 depict examples of components of the power-switching circuit; and FIG. 6 depicts an example of a voltage supply circuit. Note, however, that the invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Furthermore, all examples given herein are intended to be non-limiting, and are provided in order to help clarify the description of the invention.
FIG. 1 is a block diagram depicting a power-switching circuit 100 according to an embodiment of the invention. The power-switching circuit 100 may be used in many electronic devices that require a constant DC power source. As a non-limiting example, among others, the power-switching circuit 100 may be used in an up-converter device configured to increase the frequency of a signal.
As shown in FIG. 1, the power-switching circuit 100 includes a threshold detector 102 that is coupled to voltages VB and VP. The threshold detector 102 compares the voltage VB and the voltage VP and is operative to turn on or off an inverting switch 106-1 and an inverting switch 106-2 responsive to whether the voltage VB and the voltage VP are within a predetermined value. The inverting switch 106-1 and the inverting switch 106-2 are configured to turn on and off in a complementary manner. In other words, when the inverting switch 106-1 is turned on, the inverting switch 106-2 is turned off and vice-versa.
The inverter 104 enables the inverting switch 106-1 to act in a complementary manner to the inverting switch 106-2. In an alternative embodiment the inverter 104 may be coupled between the threshold detector 102 and the inverting switch 106-2. In a preferred embodiment, the inverting switch 106-1 and the inverting switch 106-2 are turned off at a time period set by R12 and C2 of FIG. 4, after a corresponding change in the output of the threshold detector 102. Such a time period may vary between a few microseconds to over 100 milliseconds, depending on the values of R12 and C2. In one embodiment, among others, the time period may be 30 milliseconds. This delayed switching is implemented in order to maintain a constant voltage output of the power-switching circuit 100.
The inverting switch 106-1 and the inverting switch 106-2 are coupled to a back-up power switch 108-1 and to a primary power switch 108-2, respectively. The back-up power switch 108-1 and the primary power switch 108-2 may be coupled to the voltage VB and the voltage VP, respectively.
When 100 is in operation, the voltage VO is substantially equal to the voltage VP if the voltage VP is within a certain threshold, otherwise the voltage VO is equal to the voltage VB. In this manner, when a primary power source fails, a backup power source may be provided to a load.
FIG. 2A is a block diagram depicting an embodiment of the threshold detector 102 shown in FIG. 1. The threshold detector 102 receives primary voltage VP and back-up voltage VB as inputs and provides voltage V3 as an output. The threshold detector 102 includes a comparator A1 which receives inputs via the connections 201 and 202, and provides an output via a connection 206. The connection 201 is coupled to nodes 203 and 204. A resistor R2 is coupled between node 203 and ground, while a resistor R1 is coupled between node 203 and back-up voltage VB. The resistors R1 and R2 are configured to provide the connection 201 with a predetermined fraction of the back-up voltage VB.
A resistor R3 is coupled between the connection 202 and the primary voltage Vp. A resistor R4 is coupled in series with capacitor C1 between the nodes 204 and 205. A resistor R5 is coupled between the node 204 and the node 205 (i.e., in parallel with R4 and the capacitor C1). The node 205 is coupled to the connection 206. A resistor R6 is coupled between the connection 206 and the supply voltage VS.
When the threshold detector 102 is in operation, the voltage V3 is “low” if the primary voltage VP is greater than a predetermined fraction of the back-up voltage VB. Conversely, when the primary voltage VP is less than the predetermined fraction of the back-up voltage VB, then the voltage V3 is “high.” This predetermined fraction is based on the relative values of the resistors R1 and R2 as well as the feedback network comprising the resistors R4 and R5, and the capacitor C1. Preferably, the resistor R5 establishes the steady-state component of “hysteresis” while resistor R4 and capacitor C1 create a transient “hysteresis” effect.
FIG. 2B is a graph 210 illustrating a non-limiting example of the transient hysteresis effect created by the resistor R4 and the capacitor C1. Also illustrated are the settled values of the threshold created by resistor R5. The settled values are given as levels 215 and 216. The graph 210 has a time axis 212 and a voltage axis 211. As shown in this example, when the primary voltage VP increases from 0V to its steady state output level 220, the threshold 214 is lowered from level 215 to level 216 after transition period t1. Conversely, as primary voltage VP decreases from steady state output level 220 to 0V, the threshold 214 is increased from level 216 to level 215 after transition period t2 (where t2 is equal to t1). This transient hysteresis (having transition periods t1 and t2) protects against rapid switching between power sources. Such rapid switching may occur when the source load changes from 0% to full load.
FIG. 3 is a block diagram depicting an embodiment of the inverter 104 shown in FIG. 1. The inverter 104 receives voltage V3 and outputs voltage V4. The inverter 104 includes a comparator A2, which receives inputs via connections 301 and 302, and provides an output via connection 303. A resistor R7 is coupled between connection 301 and ground, while a resistor R8 is coupled from connection 301 to Vs. This divides the voltage Vs to a lower value based on the values of resistors R7 and R8. The connection 302 is coupled to the voltage V3. A resistor R9 is used to pull up the voltage at connection 303 to approximately Vs when the voltage at 301 is greater than the voltage at 302. When the inverter 104 is in operation, the voltage V4 is “low” when the voltage V3 is “high” and vice versa.
FIG. 4 is a block diagram depicting an embodiment of an inverting switch 106 (e.g., the inverting switch 106-1 or the inverting switch 106-2) shown in FIG. 1. The inverting switch 106 is coupled to voltage V3 or voltage V4 at connection 401, and outputs voltage V5 at the connection 402.
The inverting switch 106 includes the transistors Q1 and Q2, which are coupled as follows: the emitter of the transistor Q1 is coupled to the collector of the transistor Q2; the collector of the transistor Q1 is coupled to the connection 402; a resistor R10 is coupled between the base of the transistor Q1 and the connection 401; a resistor R12 is coupled between the base of the transistor Q2 and the connection 401; the emitter of the transistor Q2 is coupled to ground; a capacitor C2 is coupled between the base of the transistor Q2 and ground; a resistor R11 is coupled between the collector of the transistor Q1 and the supply voltage Vs. The transistors Q1 and Q2 may be, for example, bipolar npn transistors, among others.
When the inverting switch 106 is in operation, the value of the voltage at the connection 401 determines whether the transistors Q1 and Q2 are on (i.e., conducting between their respective collectors and emitters). The transistors Q1 and Q2 are turned on when the voltage at the connection 401 is “high”, and vice versa. When the transistors Q1 and Q2 are on, the voltage V5 is “low,” and vice versa. The capacitor C2 causes a small delay (for example, among others, 30 milliseconds) between the time that the voltage at the connection 401 transitions from “low” to “high” and the time that the transistor Q2 turns on. A “high” to “low” transition at connection 401 immediately turns off transistor Q1 which causes the voltage V5 to transition “high” regardless of the turn off delay of transistor Q2. This “Instant on-delayed off” switching allows for a more constant voltage output of the power-switching circuit 100 by completely draining the old supply while the new supply is being loaded.
Resistor and capacitor values that may be used in the circuits depicted in FIGS. 2-4 may be, for example, among others, as follows:







TABLE 1





non-limiting examples of component values



















R1
11
kilo-ohms



R2
200
kilo-ohms



R3
1
kilo-ohm



R4
51
kilo-ohms



R5
510
kilo-ohms



R6
4.7
kilo-ohms



R7
15
kilo-ohms



R8
15
kilo-ohms



R9
4.7
kilo-ohms



R10
300
kilo-ohms



R11
4.7
kilo-ohms



R12
300
kilo-ohms



C1
0.1
MF (microfarads)



C2
0.1
MF










Note that many alternative values for the resistors and capacitors referenced in Table 1 may be used, depending on a desired implementation.

FIG. 5 is a block diagram depicting an embodiment of a power switch 108 (e.g., the back-up power switch 108-1 or the primary power switch 108-2) shown in FIG. 1. The power switch 108 is coupled to the voltage VP or the voltage VB at a connection 502, and outputs the voltage V6 at a connection 503.
The power switch 108 includes transistors Q3 and Q4, which are coupled as follows: the gates of the transistors Q3 and Q4 are coupled to the voltage V5; the drains of the transistors Q3 and Q4 are coupled to each other; the source of the transistor Q3 is coupled to the connection 502; the source of the transistor Q4 is coupled to the connection 503.
The power switch 108 is coupled to a corresponding power switch (e.g., the back-up power switch 108-1 (FIG. 1) is coupled to the primary power switch 108-2). When the power switch 108 is in operation, the voltage V5 controls whether the voltage at the connection 502 is equal to the voltage V6 (the voltage at the connection 503). When the voltage V5 is high, the transistors Q3 and Q4 are turned on, and the voltage V6 becomes equal to the voltage at the connection 502. Conversely, when the voltage V5 is low, the transistors Q3 and Q4 are turned off, and the voltage V6 becomes equal to the voltage provided at the connection 504 by the corresponding power switch.
FIG. 6 is a block diagram depicting an embodiment of a voltage supply circuit 600. The voltage supply circuit 600 includes diodes D1 and D2. The diode D1 is coupled between connections 601 and 603, whereas the diode D2 is coupled between connections 602 and 603. The voltages VP and VB are provided as inputs to the voltage supply circuit 600 at the connections 601 and 602, respectively. The voltage supply circuit 600 outputs the voltage Vs at the connection 603. The voltage Vs is equal to the voltage VB or the voltage VP, whichever is higher. Examples of voltage supplies that may be used to provide the voltage VB or the voltage VP include, for example, among others, a battery, an AC to DC converter, or a DC/DC converter.
FIG. 7 is a flow chart depicting a method 700 according to one embodiment of the invention. In step 701, a primary voltage is provided to a load. Then, a drop in the primary voltage below a predetermined threshold is detected, as indicated in step 702. The drop in primary voltage may, for example, be detected using a circuit that is configured in the same or similar manner as the threshold detector 102 (FIG. 2A).
Responsive to the drop in the primary voltage, a first circuit having at least one transistor (e.g., connected in-line) is used to provide a back-up voltage to the load, as indicated in step 703. In addition, a second circuit having at least one transistor is used to disconnect the primary voltage from the load, as indicated in step 704. The first and the second circuits used for implementing steps 703 and 704, respectively, may, for example, each be configured in the same or similar manner as the power switch 108 shown in FIG. 5.
In an alternative implementation, the steps depicted in FIG. 7 may be executed out of order from that shown or discussed, including substantially concurrently or in reverse order, as would be understood by those of ordinary skill in the art. For example, steps 703 and 704 may be executed substantially concurrently. Furthermore, the scope of the invention includes methods having fewer or additional steps than shown in FIG. 7.
It should be emphasized that the above-described embodiments of the present invention are merely possible examples, among others, of the implementations, setting forth a clear understanding of the principles of the invention. Many variations and modifications may be made to the above-described embodiments of the invention without departing substantially from the principles of the invention. All such modifications and variations are intended to be included herein within the scope of the disclosure and present invention and protected by the following claims. In addition, the scope of the present invention includes embodying the functionality of the preferred embodiments of the present invention in logic embodied in hardware and/or software-configured mediums.

## Claims

1. A back-up power supply system, comprising:
a threshold detector circuit;
a first switching circuit for enabling access to a back-up power source, the first switching circuit comprising at least one transistor; and
an inverting switch coupled between the first switching circuit and the threshold detector circuit, the inverting switch comprising an input and an output, the inverting switch configured to receive a signal at the input, invert the signal, and provide the inverted signal from the output to the first switching circuit, the inverting switch further comprising a delay circuit, the delay circuit configured to provide an immediate high-output during an off-to-on transition at the output of the inverting switch and a switching delay during an on-to-off transition at the output of the inverting switch, the inverting switch comprising a first branch in parallel to a second branch comprising the delay circuit, the first branch and the second branch connected to respective gates of a first and a second transistor, the first branch comprising a first resistor and the delay circuit comprising a second resistor and a capacitor, wherein the threshold detector circuit is configured to cause the first switching circuit to enable access to the back-up power source responsive to a voltage provided by a primary power source dropping below a predetermined threshold.

2. The system of claim 1, further comprising a second switching circuit for enabling access to the primary power source, the second switching circuit comprising at least one transistor.

3. The system of claim 2, wherein the threshold detector circuit is configured to cause the second switching circuit to enable access to the primary power source responsive to a voltage provided by the primary power source rising above the predetermined threshold.

4. The system of claim 1, wherein the the emitter of the first transistor is coupled to the collector of the second transistor and the collector of the first transistor is coupled to an output connection with the first switching circuit, and the first and second transistor are configured to produce the immediate high-output during the off-to-on transition at the output when the first transistor turns off, and configured to produce the delayed on-to-off transition at the output when the first and the second transistors are turned on.

5. The system of claim 1, wherein the first switching circuit comprises at least two transistors coupled together.

6. The system of claim 5, wherein a drain of one of the at least two transistors is coupled to a drain of another of the at least two transistors.

7. The system of claim 6, wherein current flow between the at least two transistors terminates access to the back-up power source.

8. The system of claim 6, wherein lack of current flow between the at least two transistors enables access to the back-up power source.

9. A threshold detector circuit;
a first switching circuit for enabling access to a back-up power source, the first switching circuit comprising at least one transistor;
an inverting switch coupled between the first switching circuit and the threshold detector circuit, the inverting switch comprising an input and an output, the inverting switch configured to receive a signal at the input, invert the signal, and provide the inverted signal from the output to the first switching circuit, the inverting switch further comprising a delay circuit, the delay circuit configured to provide an immediate high-output during an off-to-on transition at the output of the inverting switch and a switching delay during an on-to-off transition at the output of the inverting switch, the inverting switch comprising a first resistor in parallel to the delay circuit, the first resistor and the delay circuit connected to respective gates of the first and a second transistor, wherein the threshold detector circuit is configured to cause the first switching circuit to enable access to the back-up power source responsive to a voltage provided by a primary power source dropping below a predetermined threshold; and an inverter coupled between the inverting switch and the threshold detector circuit.

10. The system of claim 9, wherein the inverter comprises a comparator.

11. A back-up power supply system, comprising:
a threshold detector coupled to a backup voltage source and a primary voltage source, the threshold detector configured to provide a first output signal at a first output connection;
an inverter circuit coupled to the first output connection;
a first switch coupled to the inverter circuit, the first switch comprising a second output connection that is configured to provide a low voltage level output at a second output connection when an output of the inverter circuit comprises a high voltage level and a high voltage level output at the second output connection when an. output of the inverter circuit comprises a low voltage level;
a second switch coupled to the first output connection, the second switch comprising a third output connection that is configured to provide an output voltage level that is complementary to the voltage output level at the second output connection;
a backup power switch coupled to the backup voltage source, the second output connection, and a connection to a load; and

a primary power switch coupled to the primary voltage source, the third output connection, and the connection to the load.

12. The system of claim 11, wherein the backup power switch comprises a first transistor and a second transistor, the first and second transistors each comprising a source, gate, and drain, the gate and drain of the first transistor connected to the gate wad drain of the second transistor, respectively, the source of the first transistor connected to the backup voltage source, the source of the second transistor connected to the load and the primary power switch, the gates of the first and second transistors connected to the third output connection.

13. The system of claim 11, wherein the primary power switch comprises a first transistor and a second transistor, the first and second transistors each comprising a source, gate, and drain, the gate and drain of the first transistor connected to the gate and drain of the second transistor, respectively, the source of the first transistor connected to the primary voltage source, the source of the second transistor connected to the load and the backup power switch, the gates of the first and second transistors connected to the second output connection.

14. The system of claim 11, wherein the first switch comprises a first transistor and a second transistor, the first and second transistors each comprising a base, collector, and emitter, the base of the first transistor coupled to the inverter circuit and the collector of the first transistor coupled to the second output connection, the base of the second transistor coupled to the inverter circuit and a resistor-capacitor (RC) network, the emitter of the first transistor coupled to the emitter of the second transistor.

15. The system of claim 11, wherein the second switch comprises a first transistor and a second transistor, the first and second transistors each comprising a base, collector, and emitter, the base of the first transistor coupled to the first output connection and the collector of the first transistor coupled to the third output connection, the base of the second transistor coupled to the first output connection and a resistor-capacitor (RC) network, the emitter of the first transistor coupled to the emitter of the second transistor.

16. The system of claim 11, wherein the threshold detector comprises:
a comparator having a first input connection and a second input connection, the second input connection coupled to the primary voltage source;
a series resistor-capacitor (RC) circuit coupled between the first input connection and the first output connection;
a resistor configured in parallel to the series RC circuit; and

a resistor divider network coupled between the first input connection and the backup voltage source.

17. The system of claim 16, wherein the first output connection is coupled to a voltage supply circuit.

18. The system of claim 16, wherein the series RC circuit is configured to provide a transient hysteresis effect that enables gradual transitions to settled threshold values in response to changes in voltage levels corresponding to the backup and primary voltage sources.

19. The system of claim 18, wherein the threshold values are based on the resistor configured in parallel to the series RC circuit.

20. The system of claim 11, wherein the inverter circuit comprises a comparator having a first input connection and a second input connection, the first input connection coupled to ground and to an output connection of the comparator, the second input connection coupled to the first output connection of the threshold detector, the output connection of the comparator coupled to a voltage supply circuit.