Electrical power system for a self-contained transportable life support system

RELATED APPLICATION

This patent application is a continuation-in-part patent application of U.S. Ser. No. 08/667,693, filed Jun. 21, 1996, now U.S. Pat. No. 5,975,081 and entitled SELF-CONTAINED TRANSPORTABLE LIFE SUPPORT SYSTEM, the contents of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates generally to an electrical power system which provides power to various medical devices, and more particularly to an electrical power system which receives power from an internal and various external power sources and provides uninterrupted power for use in a self-contained transportable life support system, which is utilized in the resuscitation, stabilization, and transport of medical patients such as heart attack, stroke, accident victims and battlefield casualties.

BACKGROUND OF THE INVENTION

The need for simultaneously transporting and providing necessary medical treatment to patients in emergency conditions is well known. Such need can be met with a self-contained transportable life support system which integrates various medical and utility devices into a stretcher upon which a patient is placed.

An example of such system is described in the related patent application Ser. No. 08/667,693 filed on Jun. 21, 1996, entitled SELF-CONTAINED TRANSPORTABLE LIFE SUPPORT SYSTEM.

In order to evacuate a patient from the battlefield to a remote hospital, a number of different military vehicles are commonly used. For example, a HumVee may be used to transport the patient to a helipad, where the patient is subsequently transported via a UH-60 Blackhawk or UH-1 Huey helicopter to an airfield. From the airfield, the patient is then transported typically via a C-130 or C-141 fixed wing aircraft to an airport near the remote hospital. Thus, it is desirable that such self-contained transportable life support system can draw electrical power from various external power sources, such as those which exist in these transport vehicles. It is desirable that the power connection to any external power source is designed to be foolproof, in order to prevent human error. It is also desirable that such system can be powered by a rechargeable internal power source when no external power source is available.

It is imperative that the switching from one type of power source to another does not interrupt the electrical power being delivered to the load of medical and utility devices of the life support system, so that the ongoing medical care is not impeded.

In addition, it is desirable that any failure of one medical or utility device in such life support system will not interfere with the operation of the remaining functioning devices.

The present invention discloses an electrical power system, to be used in a self-contained transportable life support system, which allows such life support system to possess the desired features discussed above.

SUMMARY OF THE INVENTION

A switching control circuit for use in an electrical power system for a transportable life support system is disclosed. The switching control circuit provides uninterrupted power to a load of medical and utility devices by switching between one of a set of converted external power voltages and an internal power voltage outputted by a rechargeable internal power source. The power switching is effected within the time interval during which a residual power voltage still remains on line due to capacitance discharge of the corresponding power converter and capacitance discharge of the load, resulting in uninterrupted electrical power to the load.

The switching control circuit comprises: (a) an input power select circuit for outputting a voltage selected from the group of internal and converted external power voltages; (b) a main bus switch, for controlling the application of the selected voltage to the load; and (c) a main bus for transporting the selected voltage to the load of medical and utility devices.

The input power select circuit monitors the active converted external voltage to determine its magnitude with respect to a threshold voltage. When the converted DC voltage is larger than a threshold voltage, the circuit selects the converted voltage instead of the rechargeable internal power source. When it falls below the threshold voltage, the circuit selects and outputs the rechargeable internal power source. This voltage selection scheme facilitates seamless switching between the rechargeable internal power source and one of the converted external power sources. External power cabling allows for the application of only one external power source at a time. Thus, only one power converter is active at a given time.

The input power select circuit preferably comprises a boost regulator circuit which creates an enhancement voltage that is greater than the selected voltage by a fixed amount of voltage and utilizes the resulting enhanced voltage to control the main bus switch. In the preferred embodiment of the invention, the boost regulator circuit creates an enhanced voltage that exceeds the selected voltage by approximately 12 volts.

The input power select circuit comprises (a) a diode OR gate for preventing contention between the converted external power source and the rechargeable internal power source; (b) a battery control circuit which compares the diode OR gate output described in (a) above with a threshold voltage and enables the rechargeable internal power source to output the internal power voltage to the bus switch when the diode OR gate output is smaller than the threshold voltage.

A converted external power voltage produced by the active power converter is approximately 6 volts greater than the threshold voltage internal to the battery control circuit. Thus, when the converted external power voltage falls below the threshold voltage, this indicates that the corresponding power converter has just been disconnected from an external power source or the power converter has failed. Capacitors residing in the power converter and capacitors residing on the main bus will hold the main bus voltage up briefly while switching to the rechargeable internal power source is accomplished.

In the preferred embodiment of the invention, the threshold voltage is set at approximately 6 volts less than the nominal value of a converted external power voltage. This allows the input power select circuit to select the rechargeable internal power source, during the brief hold up time of the main bus, while the capacitance of the corresponding power converter and the capacitance of the main bus are discharging. This results in uninterrupted electrical power to the load during the voltage switching.

The bus switch is preferably a field-effect transistor (FET) switch.

The switching control circuit also includes a main bus delay circuit for controlling turn ON of the main bus switch. The main bus delay circuit receives the rising edge of the selected voltage from the input power select circuit and controls operation of the bus switch at initial power-up of the transportable life support system. By controlling the bus switch, the main bus delay circuit delays application of the selected voltage to the main bus by a fixed amount of time at initial power-up to prevent fire hazard.

This time delay is about ten seconds. During this time delay, a brushless fan is operated to dissipate any flammable gas build-up in the transportable life support system, before main bus power is applied. This delay prevents potential fire hazard.

An electrical power system for providing uninterrupted power, by switching between different power sources, to a load of medical and utility devices in a transportable life support system, is also disclosed.

The electrical power system comprises: (a) a set of external power sources which provide different external power voltages; (b) a rechargeable internal power source which provides an internal power voltage; (c) a set of different power converters which convert the different external power voltages into converted external power voltages, one of the different power converters providing recharge power to the rechargeable internal power source; (d) a switching control circuit which receives the internal and converted external power voltages, and switches at its output between the internal power voltage and one of the converted external power voltages without power interruption to the load; and (e) a plurality of precision DC-to-DC power supplies (PPS), each of the PPS receiving the output from the switching control circuit (main bus) and outputting a voltage to provide electrical power to the medical or utility devices.

The switching control circuit of the electrical power system is as disclosed above.

The electrical power system is preferably configured such that each of the precision DC-to-DC power supplies provides electrical power to no more than one device of the load, resulting in preventing any failure of one of the devices from affecting the remaining functioning devices.

The electrical power system further comprises: (a) a main power cable for transporting electrical power from one of the external power sources to a corresponding power converter; and (b) a set of different power adapters. Each of the different power adapters couples one of the external power sources to the main power cable. Each of the different power adapters has a unique mating connector which enables connection to only one distinct type of external power sources existing on a distinct type of transport vehicles. This feature allows the power connection to be foolproof.

The different power sources are independently over-current-protected and comprise preferably: (a) main power converter (AC/DC converter) for receiving an input from an external AC power source; (b) a DC-to-DC converter for receiving an input from an external DC power source; and (c) a trickle charger for receiving an input from an external AC or DC power source, and for providing recharge power to the rechargeable internal power source. The rechargeable internal power source is preferably a group of battery cells. The power converters allows utilization of various external power sources, including those having U.S., European or military standards.

These, as well as other advantages of the present invention will be more apparent from the following description and drawings. It is understood that changes in the specific structure shown and described may be made within the scope of the claims without departing from the spirit of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1

illustrates the block diagram of the electrical power system of the present invention in conjunction with the power distribution in a self-contained transportable life support system;

FIG. 2

is a block diagram of the switching control circuit of the present invention; and

FIG. 3

is a schematic diagram of the boost circuit.

DETAILED DESCRIPTION OF THE INVENTION

The detailed description set forth below in connection with the appended drawings is intended as a description of the presently preferred embodiments of the invention, and is not intended to represent the only forms in which the present invention may be constructed or utilized. The description sets forth the functions and the sequence of the steps for constructing and operating the invention in connection with the illustrated embodiments. It is to be understood, however, that the same or equivalent functions may be accomplished by different embodiments that are also intended to be encompassed within the spirit and scope of the invention.

The primary objective of the electrical power system is to supply electrical power to every medical device and to every utility subsystem in a self-contained transportable life support system, such as the one described in the pending patent application Ser. No. 08/667,693, in a safe, precise and efficient manner.

The centralization of power utilities allows all devices to be operated from multiple external power sources and allows communication to the respective interfaces by a single cable.

The rechargeable internal power source provides backup power to all devices in the event of loss of external power. This power switching is effected without power interruption to the load of medical devices and utility subsystems. The rechargeable internal power source supports stand-alone operations, for at least 30 minutes, during movement of a self-contained transportable life support system between locations while external power is unavailable. With the centralized electrical power system, the need to manage the recharge or replacement of batteries of each medical device or utility subsystem is eliminated, and the readiness of all devices is ensured.

In the preferred embodiment of the invention, there is no preference priority which would provide power to one device over another. All power switching is under operator control and hardwired circuit logic.

FIG. 1

illustrates the block diagram of the electrical power system of the present invention in conjunction with the power distribution in a self-contained transportable life support system.

Referring to

FIG. 1

, the electrical power system of the present invention comprises a set of different power adapters

102

, a main power cable

104

, a main power converter (AC/DC converter)

106

, a trickle charger

108

, a DC-to-DC power converter

110

, a group of battery cells

112

, a switching control circuit

114

, and six precision DC-to-DC power supplies (PPS)

116

,

118

,

120

,

122

,

124

,

126

.

The set of different power adapters

102

allows coupling of one of the external power sources

99

to the main power cable

104

. Each of the different power adapters

102

has a unique mating connector enabling connection to one distinct type of external power sources

99

existing on a distinct type of transport vehicles. Thus, there is a unique mating connector for each of the unique combinations of external power sources and transport vehicles, resulting in a foolproof connection of external power to the transportable life support system. An operator cannot make a misconnection because a misconnected adapter will not fit.

The external power sources

99

are over-current protected by thermal circuit breakers, and comprise the following three AC sources (United States, European, and military) and one DC (military) source:

115 Volts AC +/−10% , 60 Hz +/−5 Hz, 1 phase

230 Volts AC +/−10% , 50 Hz +/−3 Hz, 1 phase

108-118/200 Volts AC, 400 Hz +/−7 Hz , any phase

25 Volts DC +/−5 Volts DC

The main power cable

104

provides a hardwired power path from each of the adapters

102

to the main power converter

106

, the trickle charger

108

, or the DC-to-DC power converter

110

, each of which outputs a voltage of approximately 28 volts DC. The main power converter

106

, the trickle charger

108

, and the DC-to-DC power converter

110

have built-in current limiting circuitry which protects the corresponding output from over-current.

The battery group

112

is protected from over-current by thermal circuit breakers. The battery group

112

functions as a primary power source lending portability and autonomy to a self-contained transportable life support system. The rechargeable internal power source

112

supports stand-alone operations, for at least 30 minutes, during movement of the self-contained transportable life support system between locations while external power is unavailable. The rechargeable internal power source

112

also provides backup power to all devices in the event of loss of external power.

The battery group

112

receives a small constant current from the trickle charger

108

to recharge its battery cells, and outputs an internal power voltage 2 of 28 volts DC (nominal) to the automatic switching control circuit

114

. When one of the external power sources

99

is connected to the self-contained transportable life support system, the switching control circuit

114

receives the +28 volt DC (nominal) external power voltages

4

and

6

from the main power converter

106

and the DC-to-DC power converter

110

, respectively, and outputs +28 volts DC (nominal) on its main bus

8

. When there is no external power, the automatic switching control circuit

114

switches to receive the +28 volt DC internal power voltage

2

, and outputs a voltage of +28 volts DC on its main bus

8

. This power switching is effected while a residual voltage still remains on the main bus

8

, resulting in no power interruption to the load of medical devices and utility subsystems. When an external power source

99

is disconnected from the life support system, electrical power is retained on the main bus

8

for several milliseconds due to the discharge of the capacitors residing within the power converter, either

106

or

110

, and due to capacitors residing on the main bus

8

. The automatic switching control circuit

114

is designed to take advantage of this fact, and performs power switching from an external power source to a rechargeable internal power source in less than several milliseconds.

The main bus

8

transports electrical power to the six precision DC-to-DC power supplies (PPS)

116

,

118

,

120

,

122

,

124

,

126

. The PPS

116

outputs a +5 volts DC to sub-bus

11

to power utility subsystems. The PPS

118

outputs a +5 volts DC to sub-bus

13

to power medical subsystems. The PPS

120

outputs a −12 volts DC to sub-bus

15

. The PPS

122

outputs a +12 volts DC to sub-bus

17

. The PPS

124

outputs a +12 volts DC to isolated sub-bus

19

to power the defibrillator

20

. The PPS

126

outputs +16 volts DC to sub-bus

21

. The six PPS

116

,

118

,

120

,

122

,

124

,

126

each have a built-in over-current limiting circuitry to protect the main bus

8

from the sub-buses

11

,

13

,

15

,

17

,

19

,

21

. Each of the main bus and sub-buses is over-current protected from faulty medical devices and utility subsystems by their own internal current limiting.

As depicted in

FIG. 1

, only the defibrillator

20

is connected to a dedicated sub-bus

19

receiving power from a dedicated isolated PPS

124

. However, in the preferred embodiment of the invention, each of the medical devices or utility subsystems is connected to a dedicated sub-bus receiving power from a dedicated isolated PPS. Such configuration results in preventing any failure of one medical device or subsystem from affecting the remaining functioning devices and subsystems.

FIG. 2

shows the block diagram of the automatic switching control circuit

114

of FIG.

1

. The automatic switching control circuit

114

comprises an input power select circuit

200

, a main bus FET Q1, and the main bus

8

. It also includes a main bus delay circuit

300

, and a trickle delay circuit

600

.

In addition to a configuration of diodes and FET switches, the input power select circuit

200

also includes a boost regulator circuit

400

and a battery control circuit (battery charge control block)

500

.

The input power select circuit

200

receives a voltage from the group consisting of the internal and external power voltages. Only one voltage can be selected at a time. The input power select circuit

200

uses a configuration of diodes and FET switches to provide to this voltage a path to the circuit output

201

.

This permits seamless switching between external power and internal battery group

112

. The AC/DC converter

106

and the DC/DC converter

110

are continuously monitored by the input power select circuit

200

via the diode OR gate formed by the diode D

10

and D

11

. The output

206

of the diode OR gate is compared to a threshold voltage internal to the battery charge control block

500

. When the output

206

voltage, representing a converted external power voltage, drops below about +22.0 volts (corresponding to about +21.0V on the main bus

8

, three volts higher than the 18.0V minimum for the PPS), the battery charge control block

500

makes the decision to switch on the FET Q

2

by turning the control FET Q

4

off. This applies +24V+12 from the boost regulator circuit

400

output to the gate of the n-channel FET Q

2

through resistor R

14

. This applies battery group

112

voltage through the 20 amp circuit breaker

208

, through the main power switch S

1

, through the FET Q

2

, through the diode CR

3

, and through the main bus FET Q

1

to the load (medical loads and PPS)

700

. When the FET Q

4

is turned off, the FET Q

5

is also turned off. Simultaneously with the turning on of the FET Q

2

, the battery group

112

configuration relay (RLY+ and RLY−) is de-energized by the turning off of the FET Q

5

. The de-energized relay configuration immediately sets the battery group

112

to its full output voltage mode. Thus, the full battery group

112

voltage rather than the two thirds charging voltage is applied to the load

700

when a power converter (either

106

or

110

) voltage drops below approximately +22.0 volts.

Similarly, if either converter

106

,

110

voltage again rises above approximately +24.0 volts, both FETs Q

4

and Q

5

are switched on by the battery charge control block

500

. This immediately turns off the FET Q

2

by grounding the gate of FET Q

2

through FET Q

4

that has been turned on. This results in one of the two converter

106

,

110

voltages being applied through either the diodes CR

1

or CR

2

and through main bus FET Q

1

to the main bus load

700

. Simultaneously with the turning off of FET Q

2

, the battery group

112

configuration relay is energized through the FET Q

5

, which is ON, switching the battery group

112

from the full output voltage mode to the two-thirds output voltage or charging mode. The constant current trickle charger

108

then begins to charge the battery group

112

by driving a constant current through the diodes D

20

and D

22

into the positive terminal of the battery group

112

. If the battery group

112

is configured for full output voltage mode by its de-energized configuration relay, then D

22

is back biased, thus preventing the battery group

112

from being charged by the trickle charger

108

.

Diodes D

27

and D

30

protect the gate of FET Q

2

by allowing a maximum of 12.7 volts from Q

2

gate to Q

2

source. Similarly, diodes D

9

and D

26

protect the main bus FET Q

1

. Diodes D

30

and D

26

prevent back drive of the Electronic Power System (EPS) through the gate to source Zener protection diodes D

27

and D

9

. The battery charge control block

500

is powered through either diode D

28

or D

29

in a diode OR gate, since either one of the power converters

106

,

110

or the battery group

112

can provide the powering voltage. Similarly, the boost regulator circuit

400

is powered through diodes D

6

, D

7

, and D

8

, since either one of the converters

106

,

110

or the battery group

112

, or the trickle charger

108

can provide the powering voltage.

The boost regulator circuit

400

, shown in more detail at

FIG. 3

, consists of a self tracking switch mode boost regulator that tracks 12 volts higher than the applied voltage. The applied voltage is either the trickle charger

108

voltage, the AC/DC converter

106

voltage, the DC/DC converter

110

voltage, or the battery group

112

voltage applied through the diodes D

6

, D

7

, and D

8

. It is a closed loop servo system that responds within milliseconds to changes in the applied voltage. It is used to enhance the n-channel FETs Q

1

and Q

2

. Sudden step changes of voltage between the converters

106

,

110

, battery group

112

, and trickle charger

108

are quickly compensated for. Main bus

8

voltage can vary anywhere from 18.0 volts to 36.0 volts. The boost regulator circuit

400

must track efficiently. This is especially important, when switching from a low voltage to a high voltage. For example, if the main bus voltage from a low battery group

112

is +18.0 volts, then the boost output is at +30.0 volts (18.0+12.0). If now suddenly a +30.0 volt AC/DC converter

106

is turned on, the enhancement voltage is suddenly 0.0 volts (30.0V—30.0V) and the FET Q

1

begins to turn off, unless the boost regulator circuit

400

quickly responds to the change by going to +42.0 (30.0V +12.0V) volts rapidly. If the boost regulator circuit

400

is slow, a glitch in the power could occur, propagating throughout the life support system.

The diodes CR

1

, CR

2

, and CR

3

form a diode OR gate for the voltages from the two converters

106

,

110

and the battery group

112

. This OR gate isolates the various power sources one from the other. Although each of these diodes consumes approximately 7.5 watts (15A×0.5V), they are needed to protect the life support system. If FET Q

2

fails or switches inadvertently, while either converter

106

,

110

is on, the various power voltages will contend with each other, causing damage to the life support system if it were not for the presence of CR

1

, CR

2

, and CR

3

.

The transportable life support system is vented by running an Environmental Control System (ECS) fan

900

for several seconds before applying power to the main bus load

700

via the main bus FET Q

1

. The transportable life support system is also vented, when only the trickle charger

108

power is first applied. The purpose of this venting is to remove oxygen or other flammable gases from the life support system before power is applied to the main bus load

700

. The ECS fan is designed with a brushless DC motor to prevent fire hazard due to arcing brushes.

The main bus delay circuit

300

and the trickle delay circuit

600

provide the necessary time delays to vent the system of hazardous gases. When either one of the two converters

106

,

110

or the battery group

112

is powered up as sensed by the diode D

21

or D

23

, the main bus delay circuit

300

turns on the FETs Q

7

and Q

6

for a specified period of time, e.g., 10 seconds, determined by an internal RC time constant. When FET Q

7

is turned on the gate of Q

1

is grounded, turning the main bus FET Q

1

off, thus preventing power from being delivered to the main bus load

700

. Simultaneously Q

6

is turned on, applying ground to the negative side of the fan motor

900

. Since the positive side of the fan motor

900

is powered by either the AC/DC converter

106

, the DC/DC converter

110

, the battery group

112

, or the trickle charger

108

through the diode OR gate consisting of D

19

, D

24

, and D

25

, the fan motor

900

spins until the main bus delay circuit

300

times out. After the delay times out, both the FETs Q

6

and Q

7

are turned off. When FET Q

6

turns off, the current flow from the fan motor

900

to ground is interrupted and the fan motor

900

turns off. Simultaneously, when FET Q

7

turns off, +24V+12V enhancement voltage from the boost regulator circuit

400

is applied to the gate of FET Q

1

through resistor R

22

. This turns the main bus FET Q

1

on, applying power to the main bus load

700

.

The main bus delay circuit

300

does not exhibit a constant delay time. The delay is not only a function of the internal RC time constant but also of the magnitude of the voltage applied. Since the voltage applied to the main bus delay circuit

300

can vary anywhere from about 18.0 volts to 36.0 volts, the delay time can also vary significantly. When the voltage is low (e.g., 18.0 volts) the fan

900

runs slow, because of the applied low voltage, but the delay time of the main bus delay circuit

300

is longer. Similarly, when the voltage is high (e.g,. 36.0 volts) the ECS fan runs fast, because of the applied high voltage, however, the delay time of the main bus delay circuit

300

is shorter. The net result is that the volume of air displaced is roughly constant despite the variation in delay time and input voltage. The length of the delay time can be easily changed, by increasing or decreasing the RC time constant in the main bus delay circuit

300

.

The trickle delay circuit

600

starts to time, when the trickle charger

108

is turned on. It turns on FET Q

8

, thus providing a return path to ground for the fan motor

900

, which spins until the trickle delay circuit

600

times out. FET Q

8

then turns off, interrupting current flow to the fan motor

900

. Similar to the main bus delay circuit

300

, the trickle delay circuit

600

is a function of both the internal RC time constant and the magnitude of the applied voltage from the trickle charger

108

. However, the time delay and the fan speed vary in such a way as to keep the volume of air displaced relatively constant, as discussed above.

The switching time through the Battery Charge Control Block

500

, the control FET (Q

4

), and the Batt Bus FET (Q

2

) must be fast enough so that the Main Bus does not glitch, before the Battery group

112

replaces the external power sources. For this reason, high speed N-channel Power MOSFETs are used for both the Batt Bus FET (Q

2

) and the Main Bus FET (Q

1

). At the 20 amp maximum current being drawn from the Main Bus, it is impractical to use hold up capacitors to hold up the Main Bus for slower switching devices such as relays. Such hold up capacitors would be too large. N-channel power MOSFETs on the other hand can switch large amounts of current in a few microseconds with no contact bounce and their channel resistance (a few milliohms) can be made lower than the contact resistance of a power relay (tens of milliohms). Relays have one advantage over MOSFETs in that there is total isolation between the input and output. This is not a problem in the system as long as power grounds and signal grounds are kept separate and not mixed.

To fully enhance most N-channel power MOSFETs requires a gate to source voltage of at least 10.0 volts but not greater than 20.0 volts. This requires a circuit that boosts the MOSFET's gate voltage at least 10.0 volts higher than the selected Main Bus voltage. Since no such voltage exists in the system, it must be created in the Boost Regulator block. The Boost Regulator

500

outputs a signal called +24V+12, which is always 12.7 volts more positive than the selected voltage. This voltage is used to fully enhance the gate of either the Main Bus FET, the Batt Bus FET, or both. The enhancement value of 12.7 volts was selected to guarantee full enhancement under component tolerances, while simultaneously staying as far away as possible from the gate/source breakdown voltage of 20.0 volts. The Batt Bus FET (Q

2

) and the Main Bus FET (Q

1

) have unique gate to source protection circuits. The power MOSFETs must be protected from failure due to overvoltage, because their reliability is essential to the safety of the patient. If something fails in the Boost Regulator, high voltages could be applied to the gates of Q

1

and Q

2

. One way of protecting a MOSFET is by placing a Zener protection diode from gate to source to clamp overvoltages to somewhere between full enhancement (10.0 VDC) and breakdown (20.0 VDC). The problem with this approach in this particular system is that external circuitry can forward bias, such a zener, causing damage to the EPS Board

800

. For this reason signal diodes D

26

and D

30

have been placed in series with zener diodes D

9

and D

27

respectively to prevent back driving the EPS board through the forward biased zeners. For example, when Q

2

is ON, Battery Group

112

voltage is applied to the anode of D

27

(assuming D

30

is a short). The Battery Group

112

could be momentarily shorted when control MOSFET (Q

4

) is turned ON. There is a finite time delay from the time Q

4

is turned ON and Q

2

is turned OFF. During this brief time the Battery Group

112

would be shorted to ground through Q

4

. This could stress Q

4

as well as glitch ground due to the large momentary current spike. The addition of the series signal diode (D

30

) alleviates this problem by preventing the protection zener from ever being forward biased.

Diodes CR

1

, CR

2

, and CR

3

are necessary to prevent the selected power source from back driving or contending with the other two power sources. CR

1

,

2

,

3

are Power Schottky diodes to minimize power consumption. Only one of these 3 diodes carries power at a time depending on the selected power source. There are two halves to each diode and each is capable of carrying the full load current. These halves are paralleled to increase system reliability. If one half of the diode fails, the other half will take over the full load current. Since Power Schottky diodes have a large leakage currents, 1K ohm bleeder resistors are necessary at the outputs of the AC/DC and DC/DC Converters to prevent the Battery voltage from leaking through CR

1

and CR

2

and then back through D

10

and D

11

and inadvertently exceeding the trigger threshold in the Battery Charge Control Block

500

. If this happened, the Battery Charge Control Block could inadvertently turn OFF the Batt Bus FET causing a glitch in Main Bus power. The 1K value of the bleeders is low enough to keep the voltage due to leakage through D

10

and D

11

well below the reference threshold internal to the Battery Charge Control Block

500

.

The EPS board

800

controls charging and discharging of the Battery Group

112

through the Battery Charge Control Block

500

. When the AC/DC or DC/DC Converter output voltages are not present or dip below a reference threshold internal to the Battery Charge Control Block

500

, control MOSFETs Q

4

and Q

5

are turned OFF. Q

4

, being OFF, applies +24V+12 from the Boost Regulator

400

through R

14

to fully enhance the gate of the Batt Bus FET, Q

2

. This applies the full Battery voltage through the 20 amp circuit breaker, through the Main Power Switch (S

1

,) through Batt Bus FET (Q

2

), through CR

3

, and through the normally ON Main Bus FET (Q

1

) to the Main Bus. Q

5

, being OFF, removes the ground return for the configuration relays internal to the Battery. With no current in these relays, the Battery Group

112

is configured to full output. The whole process is fast enough to prevent glitching of the Main Bus

8

during power transfers without the need for massive hold up capacitors.

When either the AC/DC or DC/DC Converter,

106

and

110

, rises above the reference threshold internal to the Battery Charge Control Block

500

, control MOSFETs Q

4

and Q

5

are turned ON. Q

4

, being ON, turns OFF the Batt Bus FET thus removing the Battery Group

112

from the Main Bus

8

. Q

5

, being ON, allows current to be driven from the Trickle Charger

108

, through D

20

, through the internal configuration relays of the Battery and down through Q

5

to ground. This actuates the Battery relays and configures the Battery Group

112

in the two-thirds of full output or charge mode. The Trickle Charger

108

then drives a constant current through D

20

and D

22

into the positive terminal of the Battery Group

112

and out the negative ;terminal of the Battery Group

112

to ground.

The Boost Regulator

400

consists of a self tracking flyback regulator. The schematic for the Boost Regulator is shown in FIG.

3

. The input to the Boost Regulator is +24V from either the AC/DC Converter

106

or the DC/DC Converter

110

, the Battery Group

112

, or the Trickle Charger

108

. The +24V input is derived from the diode OR of D

6

, D

7

, and D

8

shown in FIG.

2

. The +24V input is zenered down to +12V by D

3

, where it is used to power the NE555 timer (U

1

). U

1

is configured for astable operation. The timer output (U

1

-

3

) is triggered to an active high level (12 volts), when the trigger input (U

1

-

2

) dips below one third of V+ (U

1

-

8

) (4 volts). The timer output is triggered to an active low level (0.1 volts), when the threshold input (U

1

-

6

) rises above two thirds of V+(8 volts). At power on, C

4

is initially uncharged, so that trigger input (U

1

-

2

) is sitting at a low. This triggers the output of the timer IC to an active high. This active high is applied to the base of chopper transistor Q

3

, through the parallel combination of C

3

and R

3

, causing Q

3

to turn ON. Energy is stored in L

1

, when Q

3

is turned ON according to the formula ½LI

2

. Q

3

remains ON until C

4

is charged above 8 volts (two-thirds of V+) through resistor R

4

. U

1

's output then falls low, discharging C

4

through R

4

, while turning Q

3

OFF. When Q

3

turns OFF, L

1

releases its stored energy to C

19

and the load, through the forward biased diode, D

5

. Eventually, the energy from L

1

will charge C

19

to a voltage large enough to breakdown the 12V zener, D

1

, and the base emitter junction of Q

2

. When this happens, Q

2

turns ON and charges C

4

more rapidly than would have occurred had it been solely charged through R

4

alone. This rapidly terminates the energy transfer from L

1

to C

19

by turning Q

3

ON and begins anew the process of energy storage in L

1

. Thus the +24V+12 output of the Boost Regulator always tracks about 12.7 volts (one 12V zener drop plus one 0.7V base/emitter junction drop) above the +24V input. As the load at the output of the Boost Regulator increases, charge is pulled more rapidly from C

19

, requiring a more rapid replenishment of energy from L

1

. This causes the frequency of the U

1

to increase as the load is increased.

Q

1

provides overcurrent protection for the Boost Regulator

400

. If for any reason inductor L

1

saturates, high currents could flow through Q

3

and L

1

causing damage. If the current through L

1

becomes too high, the voltage drop across R

6

becomes high enough to breakdown the base emitter junction of Q

1

. This turns ON Q

1

, providing a current path through R

5

to rapidly charge C

4

above the 8.0V threshold, which drives U

1

's output low and thus turns Q

3

OFF. This mechanism protects the Boost Regulator from inadvertent high currents by quickly terminating the ON time of Q

3

. D

31

is a 12V zener diode that protects U

1

in case the voltage at C

4

rises above the input rail at V+(12.0V). D

31

clamps the threshold and trigger inputs of U

1

to 12V.

It is understood that the exemplary switching control circuit and electrical power system described herein and shown in the drawings represent only presently preferred embodiments of the invention. Indeed, various modifications and additions may be made to such embodiments without departing from the spirit and scope of the invention. Those skilled in the art will recognize that various other configurations are equivalent and therefore likewise suitable. Thus, these and other modifications and additions may be obvious to those skilled in the art and may be implemented to adapt the present invention for use in a variety of different applications.