Power switching circuit

Claim 2

A semiconductor integrated circuit device formed on a common semiconductor substrate, comprising: an insulated gate field-effect transistor, having a gate, source and drain, and wherein the semiconductor substrate functions as a drain region; and a bipolar transistor which is formed on said semiconductor substrate, said bipolar transistor having an emitter, base and collector and wherein the base is connected to the source of said insulated gate field-effect transistor, the emitter is connected to the gate of said insulated gate field-effect transistor, and the collector is connected to the gate or the drain of said insulated gate field effect transistor.

Claim 3

A semiconductor integrated circuit device according to claim 1, wherein said semiconductor substrate is of n-type conductivity, and said bipolar transistor is formed in an isolation region of p-type conductivity that is formed in said semiconductor substrate of n-type conductivity.

Claim 4

A semiconductor integrated circuit device according to claim 2, wherein said source is adapted for connection with one end of a load having the other end grounded, and said drain is adapted for connection with a positive power source voltage.

Claim 5

A semiconductor integrated circuit device according to claim 3, wherein said insulated gate field-effect transistor is of an N-channel conductivity type, and wherein said bipolar transistor is an NPN type bipolar transistor.

Claim 6

A semiconductor integrated circuit device according to claim 4, wherein the collector of said bipolar transistor is connected to the gate of said insulated gate field effect transistor, said insulated gate field effect transistor being a MOSFET.

Claim 7

A semiconductor integrated circuit device according to claim 1, wherein the collector of said bipolar transistor is connected to the gate of said insulated gate field effect transistor, said insulated gate field effect transistor being a MOSFET.

Claim 8

A semiconductor integrated circuit device according to claim 4, wherein the collector of said bipolar transistor is connected to the drain of said insulator gate field effect transistor, said insulated gate field effect transistor being a MOSFET.

Claim 9

A semiconductor integrated circuit device according to claim 1, wherein the collector of said bipolar transistor is connected to the drain of said insulated gate field effect transistor, said insulated gate field effect transistor being a MOSFET.

Claim 10

A power switching circuit according to claim 12, wherein the power output element, driver means and said circuit are formed on a common semiconductor substrate.

Claim 11

A power switching circuit according to claim 9, wherein the power output element is a power MOSFET of N-channel conductivity type.

Claim 12

A power switching circuit according to claim 10, further comprising: a power supply terminal to which a power supply voltage is supplied; and wherein the power MOSFET has a source-drain path coupled between the power supply terminal and the output terminal and a gate coupled to receive the output signal of the driver means.

Claim 13

A power switching circuit, comprising: a power output element coupled to a load element and providing a driving current to said load element; driver means coupled to the power output element and providing an output signal to drive said power output element in response to an input signal; and a circuit for detecting a current flowing into the power output element and controlling operation of the driver means in accordance with a magnitude of the current detected so that an operating period of the power output element will vary in inverse proportion to the magnitude of the detected current when the magnitude of the detected current is greater than predetermined magnitude, including: a first sense circuit for detecting a first current flowing into the power output element when voltage on an output terminal is lower than a predetermined voltage, said power output element having an output end coupled to the output terminal; a second sense circuit for detecting a second current flowing into the power output element when the voltage on the output terminal is higher than the predetermined voltage; a selecting circuit for selecting that one of said first and second currents which has the greater magnitude; current-voltage converting means for converting the one of said first and second currents selected, in accordance with the magnitude thereof, into a voltage signal; a saw-tooth wave generator for forming a saw-tooth voltage waveform in response to the input signal; a comparator circuit for comparing the converted voltage signal with the saw-tooth voltage waveform and for providing an indication signal which indicates that the converted voltage signal is of a voltage magnitude greater than or equal to that of the saw-tooth voltage waveform; and a flip-flop which is set by the indication signal and which is reset by each start of the operation of the saw-tooth wave generator, said flip-flop having an output being supplied to an input of said driver means so as to control the operation of the driver means.

Claim 14

A power switching circuit, comprising: a power output element coupled to a load element and providing a driving current to said load element; driver means coupled to the power output element and providing an output signal to drive said power output element in response to an input signal; and a circuit for detecting a current flowing into the power output element and controlling operation of the driver means in accordance with a magnitude of the current detected so that an operating period of the power output element will vary in inverse proportion to the magnitude of the detected current when the magnitude of the detected current is greater than predetermined magnitude, including: a first sense circuit for detecting a first current flowing into the power output element when voltage on an output terminal is lower than a predetermined voltage, said power output element having an output end coupled to the output terminal; a second sense circuit for detecting a second current flowing into the power output element when the voltage on the output terminal is higher than the predetermined voltage; a selecting circuit for selecting that one of said first and second currents which has the greater magnitude; current-voltage converting means for converting the one of said first and second currents selected, in accordance with the magnitude thereof, into a voltage signal; a saw-tooth wave generator for forming a saw-tooth voltage waveform in response to the input signal; a comparator circuit for comparing the converted voltage signal with the saw-tooth voltage waveform and for providing an indication signal which indicates that the converted voltage signal is of a voltage magnitude greater than or equal to that of the saw-tooth voltage waveform; and a flip-flop which is set by the indication signal and which is reset by each start of the operation of the saw-tooth wave generator, said flip-flop having an output being supplied to an input of said driver means so as to control the operation of the driver means.

Claim 15

A power switching circuit according to claim 11, wherein the driver means includes: a booster circuit for boosting said power supply voltage to a voltage level of said power supply voltage plus the threshold voltage of said power MOSFET, wherein said boosted voltage is supplied to the gate of the power MOSFET, a first control MOSFET having a gate coupled to receive the input signal and a source-drain path coupled between the gate of the power MOSFET and a ground potential terminal of the circuitry, and a second control MOSFET having a gate coupled to receive the output of the flip-flop and a source-drain path coupled between the gate of the power MOSFET and the ground potential terminal.

Claim 16

A power switching circuit, comprising: a power output element coupled to a load element and providing a driving current to the load element; driver means coupled to the power output element and providing an output signal to drive the power output element in response to an input signal; and a circuit which includes means for detecting a current flowing into the power output element and means for controlling operation of the driver means in accordance with a magnitude of the current detected by the detecting means so that an operating period of the power output element will vary in inverse proportion to the magnitude of the detected current when the magnitude of the detected current is greater than a predetermined magnitude, wherein the controlling means includes a flip-flop circuit coupled to the driver means and having first and second logic states, the first logic state thereof occurring when the driving current is being provided to the load element, and the second logic state thereof occurring when a supply of the driving current to the load element is being inhibited, wherein the load element to be coupled has a resistance characteristic in which the resistance thereof is very small at a moment when the load element is first driven, during start-up of operation of the load element, and that the resistance thereof increases with time as heat is being generated therefrom, wherein the magnitude of the driving current is inversely responsive to the amount of heat being generated from the load element, the driving current during a normal operating condition of the load element is in the form of intermittent pulse signals during an initial part of the operation of the load element, respectively of progressively decreasing magnitudes and increasing pulse-widths, beginning with an initial driving current pulse signal of a maximum magnitude and minimum pulse-width when the power element is first driven, until a steady-state load element operating condition is effected at which time the drive current remains constant for the duration of the input signal, and wherein in response to detection of an overcurrent condition at the power output element, of a magnitude substantially greater than the predetermined magnitude, the operating period of the power output element is limited to provide a series of current pulses as the driving current to the load element, for the duration of the input signal, thereby to prevent breakdown of the power output element.

Claim 17

A power switching circuit, comprising: a power output element coupled to a load element and providing a driving current to the load element; driver means coupled to the power output element and providing an output signal to drive the power output element in response to an input signal; and a circuit which includes means for detecting a current flowing into the power output element and means for controlling operation of the driver means in accordance with a magnitude of the current detected by the detecting means so that an operating period of the power output element will vary in inverse proportion to the magnitude of the detected current when the magnitude of the detected current is greater than a predetermined magnitude, wherein the controlling means includes a flip-flop circuit coupled to the driver means and having first and second logic states, the first logic state thereof occurring when the driving current is being provided to the load element, and the second logic state thereof occurring when a supply of the driving current to the load element is being inhibited, wherein the load element to be coupled has a resistance characteristic in which the resistance thereof is very small at a moment when the load element is first driven, during start-up of operation of the load element, and that the resistance thereof increases with time as heat is being generated therefrom, wherein the magnitude of the driving current is inversely responsive to the amount of heat being generated from the load element, the driving current during a normal operating condition of the load element is in the form of intermittent pulse signals during an initial part of the operation of the load element, respectively of progressively decreasing magnitudes and increasing pulse-widths, beginning with an initial driving current pulse signal of a maximum magnitude and minimum pulse-width when the power element is first driven, until a steady-state load element operating condition is effected at which time the drive current remains constant for the duration of the input signal, and wherein in response to detection of an overcurrent condition at the power output element, of a magnitude substantially greater than the predetermined magnitude, the operating period of the power output element is limited to provide a series of current pulses as the driving current to the load element, for the duration of the input signal, thereby to prevent breakdown of the power output element.

Claim 18

A power switching circuit according to claim 14, wherein the load element is a lamp with a filament.

Claim 19

A semiconductor integrated circuit for driving a lamp mounted on a vehicle, comprising: a power supply terminal to which a power supply voltage is supplied; an output terminal coupled to the lamp; an input terminal coupled so as to receive an input signal; a power MOSFET having a source-drain path coupled between the power supply terminal and the output terminal, said power MOSFET providing via the source-drain path thereof a driving current for turning the lamp on in response to a driving signal corresponding to a voltage level of said power supply voltage plus at least the threshold voltage of said power MOSFET and which is applied to a gate of the power MOSFET; driver means responsive to the input signal and providing the driving signal to the gate of the power MOSFET to turn the power MOSFET on, the driver means including means for selectively turning the power MOSFET off in response to a control signal; detecting means coupled to the power MOSFET and providing an output in accordance with a magnitude of the driving current detected; and control means for controlling operation of the means for selectively turning the power MOSFET off in response to the output of the detecting means, wherein the control means includes a bistable circuit coupled to the means for selectively turning the power MOSFET off and providing the control signal when the magnitude of the driving current is greater than a predetermined value so that lighting periods of the lamp become less regular, wherein the control signal is a pulse width-modulated signal effecting an operating period of the power MOSFET which varies inversely to the magnitude of the detected driving current when the driving current is greater than the predetermined value, wherein the magnitude of the driving current is inversely responsive to the amount of heat being generated by the lamp, the driving current during a normal operation condition of the lamp is in the form of intermittent pulse signals during an initial part of the operation of the lamp, respectively of progressively decreasing magnitudes and increasing pulse-widths, beginning with an initial driving current pulse signal of a maximum magnitude and minimum pulse-width when the power element is first driven, until a steady-state operating condition of the lamp is effected at which time the driving current remains constant for the duration of the input signal, and wherein the response to detection of an overcurrent condition at the power MOSFET, of a magnitude substantially greater than the predetermined magnitude, the operating period of the power MOSFET is limited to provide a series of current pulses as the driving current to the lamp, for the duration of the input signal, thereby to prevent breakdown of the power MOSFET.

Claim 20

A semiconductor integrated circuit for driving a lamp mounted on a vehicle, comprising: a power supply terminal to which a power supply voltage is supplied; an output terminal coupled to the lamp; an input terminal coupled so as to receive an input signal; a power MOSFET having a source-drain path coupled between the power supply terminal and the output terminal, said power MOSFET providing via the source-drain path thereof a driving current for turning the lamp on in response to a driving signal corresponding to a voltage level of said power supply voltage plus at least the threshold voltage of said power MOSFET and which is applied to a gate of the power MOSFET; driver means responsive to the input signal and providing the driving signal to the gate of the power MOSFET to turn the power MOSFET on, the driver means including means for selectively turning the power MOSFET off in response to a control signal; detecting means coupled to the power MOSFET and providing an output in accordance with a magnitude of the driving current detected; and control means for controlling operation of the means for selectively turning the power MOSFET off in response to the output of the detecting means, wherein the control means includes a bistable circuit coupled to the means for selectively turning the power MOSFET off and providing the control signal when the magnitude of the driving current is greater than a predetermined value so that lighting periods of the lamp become less regular, wherein the control signal is a pulse width-modulated signal effecting an operating period of the power MOSFET which varies inversely to the magnitude of the detected driving current when the driving current is greater than the predetermined value, wherein the magnitude of the driving current is inversely responsive to the amount of heat being generated by the lamp, the driving current during a normal operation condition of the lamp is in the form of intermittent pulse signals during an initial part of the operation of the lamp, respectively of progressively decreasing magnitudes and increasing pulse-widths, beginning with an initial driving current pulse signal of a maximum magnitude and minimum pulse-width when the power element is first driven, until a steady-state operating condition of the lamp is effected at which time the driving current remains constant for the duration of the input signal, and wherein the response to detection of an overcurrent condition at the power MOSFET, of a magnitude substantially greater than the predetermined magnitude, the operating period of the power MOSFET is limited to provide a series of current pulses as the driving current to the lamp, for the duration of the input signal, thereby to prevent breakdown of the power MOSFET.

Claim 21

A semiconductor integrated circuit according to claim 16, wherein the power MOSFET is of an N-channel conductivity type.

Claim 24

A method of lighting a lamp according to claim 18, wherein said step (d) further includes: converting the detected magnitude of the current into a voltage signal, generating a saw-tooth voltage by a saw-tooth generator, and comparing said voltage signal with said saw-tooth voltage to determine a period when the saw-tooth voltage is greater than the voltage signal; and wherein said step (e) further includes: controlling the operating period of the power MOSFET for lighting the lamp in accordance with the comparison of said voltage signal and said saw-tooth voltage thereby controlling the set and reset states of the bistable circuit.

Claim 25

A power switching circuit according to claim 14, wherein the power output element, driver means and said circuit are formed on a common semiconductor substrate.

Claim 26

A power switching circuit according to claim 20, wherein the power output element is a power MOSFET.

Claim 27

A power switching circuit according to claim 21, further comprising: a power supply terminal to which a power supply voltage is supplied; and an output terminal, wherein the power MOSFET has a source-drain path coupled between the power supply terminal and the output terminal and a gate coupled to receive the output signal of the driver means.

Claim 28

A power switching circuit according to claim 22, wherein said circuit includes: a first sense circuit for detecting a first current flowing into the power MOSFET when voltage on the output terminal is lower than a predetermined voltage; a second sense circuit for detecting a second current flowing into the power MOSFET when the voltage on the output terminal is higher than the predetermined voltage; a selecting circuit for selecting that one of said first and second currents which has the greater magnitude; current-voltage converting means for converting the one of said first and second currents selected, in accordance with the magnitude thereof, into a voltage signal; a saw-tooth wave generator for forming a saw-tooth voltage waveform including a series of saw-tooth waves in response to the input signal; and a comparator circuit for comparing the converted voltage signal with the saw-tooth voltage waveform and for providing an indication signal which indicates when the amplitude of the converted voltage signal is greater than or equal to that of the saw-tooth voltage waveform with respect to each occurrence of a saw-tooth wave, wherein the flip-flop is set to the first state by each start of a saw-tooth wave of the saw-tooth wave generator and is set to the second state by the indication signal, said flip-flop having an output being supplied to an input of said driver means so as to control the operation of the driver means.

Claim 29

A power switching circuit according to claim 23, wherein the driver means includes: a booster circuit for boosting said power supply voltage to a voltage level of said power supply voltage plus the threshold voltage of said power MOSFET, wherein said boosted voltage is supplied to the gate of the power MOSFET, a first control MOSFET having a gate coupled to receive the input signal and a source-drain path coupled between the gate of the power MOSFET and a ground potential terminal of the circuitry, and a second control MOSFET having a gate coupled to receive the output of the flip-flop and a source-drain path coupled between the gate of the power MOSFET and the ground potential terminal.

Claim 30

A power switching circuit according to claim 24, wherein the load element is a lamp with a filament.

Claim 31

A power switching circuit according to claim 25, wherein the power MOSFET is of an N-channel conductivity type.

Claim 32

A power switching circuit according to claim 10, wherein the load element to be coupled has a resistance characteristic and wherein the resistance thereof is very small at a moment when the load element is driven and that the resistance thereof increases with time as heat is being generated therefrom.

Claim 33

A power switching circuit according to claim 27, wherein the load element is a lamp with a filament.

Claim 34

A semiconductor integrated circuit according to claim 17, wherein said bistable circuit includes a flip-flop.

Claim 35

A semiconductor integrated circuit according to claim 16, wherein said bistable circuit includes a flip-flop.

Claim 36

A method of lighting a lamp according to claim 19, wherein said bistable circuit includes a flip-flop.

Claim 37

A method of lighting a lamp according to claim 18, wherein said bistable circuit includes a flip-flop.

Claim 38

A power switching circuit according to claim 15, wherein the overcurrent condition is effected by a short-circuit condition of the lamp.

Claim 39

A power switching circuit according to claim 23, wherein the overcurrent condition is effected by a short-circuit condition of the lamp in which the operating period of the power MOSFET is time-wise limited to a small and constant portion of each of the saw-tooth waves thereby limiting the driving current for the load element to narrow pulses provided at regular intervals for the duration of the input signal.

Claim 40

A semiconductor integrated circuit according to claim 16, wherein upon detection of the overcurrent condition effected from a short-circuit condition of the lamp, the control means provides a periodic control signal to effect a driving current for the lamp comprised of narrow pulses provided at regular intervals for the duration of the input signal.

Claim 41

A method of lighting a lamp according to claim 18, wherein the overcurrent condition results from short-circuit breakdown of the lamp.

Claim 42

A power switching circuit, comprising: a power MOSFET having a source-drain path coupled between a terminal for receiving a power supply potential and a load element and providing a driving current to said load element; driver means coupled to the power MOSFET and providing an output signal to drive said power MOSFET in response to an input signal; and a circuit for detecting a current flowing through the power MOSFET and for controlling operation of the driver means in accordance with a magnitude of the current detected so that an operating period of the power MOSFET will vary in inverse proportion to the magnitude of the detected current when the magnitude of the detected current is greater than a predetermined magnitude, including: a bistable circuit coupled to the driver means and having first and second logic states, the first logic state thereof occurring when the driving current is being provided to the load element, and the second logic state thereof occurring when a supply of the driving current to the load element is being inhibited; a first sense circuit for providing a first current indicative of detection of a first current magnitude flowing through the power MOSFET when voltage on an output terminal is lower than a predetermined voltage, the output terminal being effected at the coupling of the power MOSFET to the load element; a second sense circuit for providing a second current indicative of detection of a second current magnitude through the power MOSFET when the voltage on the output terminal is higher than the predetermined voltage; a selecting circuit for selecting that one of said first and second currents which has the greater magnitude; current-voltage converting means for converting the one of said first and second currents selected, in accordance with the magnitude thereof, into a voltage value at each occurrence of an operating period of the power MOSFET; a saw-tooth wave generator for forming a saw-tooth voltage waveform including a series of saw-tooth waves, each of a time-wise decreasing magnitude, in response to an input signal; and a comparator circuit for comparing each converted voltage value with a respective saw-tooth wave and for providing an indication signal indicative of when the saw-tooth wave declines to a voltage value that is equal to or less than the converted voltage value, wherein the bistable circuit is set to the first state by each start of a saw-tooth wave of the saw-tooth wave generator and is set to the second state by the indication signal, said flip-flop having an output being supplied to an input of said driver means so as to control the operation of the driver means, and wherein said load element has a resistance characteristic in which the resistance thereof is very small at a moment when the load element is driven and that the resistance thereof increases with time as heat is being generated therefrom.

Claim 43

A power switching circuit, comprising: a power MOSFET having a source-drain path coupled between a terminal for receiving a power supply potential and a load element and providing a driving current to said load element; driver means coupled to the power MOSFET and providing an output signal to drive said power MOSFET in response to an input signal; and a circuit for detecting a current flowing through the power MOSFET and for controlling operation of the driver means in accordance with a magnitude of the current detected so that an operating period of the power MOSFET will vary in inverse proportion to the magnitude of the detected current when the magnitude of the detected current is greater than a predetermined magnitude, including: a bistable circuit coupled to the driver means and having first and second logic states, the first logic state thereof occurring when the driving current is being provided to the load element, and the second logic state thereof occurring when a supply of the driving current to the load element is being inhibited; a first sense circuit for providing a first current indicative of detection of a first current magnitude flowing through the power MOSFET when voltage on an output terminal is lower than a predetermined voltage, the output terminal being effected at the coupling of the power MOSFET to the load element; a second sense circuit for providing a second current indicative of detection of a second current magnitude through the power MOSFET when the voltage on the output terminal is higher than the predetermined voltage; a selecting circuit for selecting that one of said first and second currents which has the greater magnitude; current-voltage converting means for converting the one of said first and second currents selected, in accordance with the magnitude thereof, into a voltage value at each occurrence of an operating period of the power MOSFET; a saw-tooth wave generator for forming a saw-tooth voltage waveform including a series of saw-tooth waves, each of a time-wise decreasing magnitude, in response to an input signal; and a comparator circuit for comparing each converted voltage value with a respective saw-tooth wave and for providing an indication signal indicative of when the saw-tooth wave declines to a voltage value that is equal to or less than the converted voltage value, wherein the bistable circuit is set to the first state by each start of a saw-tooth wave of the saw-tooth wave generator and is set to the second state by the indication signal, said flip-flop having an output being supplied to an input of said driver means so as to control the operation of the driver means, and wherein said load element has a resistance characteristic in which the resistance thereof is very small at a moment when the load element is driven and that the resistance thereof increases with time as heat is being generated therefrom.

Claim 44

A power switching circuit according to claim 37, wherein the power MOSFET, the driver means and the circuit are formed on a common semiconductor substrate.

Claim 45

A power switching circuit according to claim 37, wherein the bistable circuit is an RS flip-flop.

Claim 46

A power switching circuit according to claim 39, wherein the first and second states are the reset and set operating states of the RS flip-flop.

Claim 47

A power switching circuit according to claim 40, wherein the magnitude of the driving current is inversely responsive to the amount of heat being generated from the load element.

Claim 48

A power switching circuit according to claim 41, wherein the load element is a lamp with a filament.

Claim 49

A power switching circuit according to claim 37, wherein the magnitude of the driving current is inversely responsive to the amount of heat being generated from the load element.

Claim 50

A power switching circuit according to claim 43, wherein the load element is a lamp with a filament.

Claim 51

A power switching circuit, comprising: a power output element coupled to a load element and providing a driving current to the load element; driver means coupled to the power output element and providing an output signal to drive the power output element in response to an input signal; and a circuit which includes means for detecting a current flowing into the power output element and means for controlling operation of the driver means in accordance with a magnitude of the current detected by the detecting means so that an operating period of the power output element will vary in inverse proportion to the magnitude of the detected current when the magnitude of the detected current is greater than a predetermined magnitude, wherein the controlling means includes a flip-flop circuit coupled to the driver means and having first and second logic states, the first logic state thereof occurring when the driving current is being provided to the load element, and the second logic state thereof occurring when a supply of the driving current to the load element is being inhibited, wherein the load element to be coupled has a directly varying resistance-heat characteristic relationship during normal operation thereof in which the resistance thereof is very small at a moment when the load element is first driven and that the resistance thereof increases with time as heat is being generated therefrom, wherein upon start-up of operation of the power element, in response to said input signal, the driving current is provided as intermittent pulse signals to operate the load element, the pulse signals are provided as long as the magnitude of the detected current is greater than the predetermined magnitude, and wherein the magnitude of each additional driving current pulse signal, when the load element is operating normally, is progressively decreased and the pulse-width thereof is increased from that of the preceding pulse signal until a steady-state load element operating condition is effected at which time the driving current remains constant for the duration of the input signal, the decrease in magnitude of each additional pulse signal, starting from an initial driving current pulse signal of maximum magnitude and minimum pulse-width, is in response to a corresponding increase in resistance of the load element.

Claim 52

A power switching circuit, comprising: a power output element coupled to a load element and providing a driving current to the load element; driver means coupled to the power output element and providing an output signal to drive the power output element in response to an input signal; and a circuit which includes means for detecting a current flowing into the power output element and means for controlling operation of the driver means in accordance with a magnitude of the current detected by the detecting means so that an operating period of the power output element will vary in inverse proportion to the magnitude of the detected current when the magnitude of the detected current is greater than a predetermined magnitude, wherein the controlling means includes a flip-flop circuit coupled to the driver means and having first and second logic states, the first logic state thereof occurring when the driving current is being provided to the load element, and the second logic state thereof occurring when a supply of the driving current to the load element is being inhibited, wherein the load element to be coupled has a directly varying resistance-heat characteristic relationship during normal operation thereof in which the resistance thereof is very small at a moment when the load element is first driven and that the resistance thereof increases with time as heat is being generated therefrom, wherein upon start-up of operation of the power element, in response to said input signal, the driving current is provided as intermittent pulse signals to operate the load element, the pulse signals are provided as long as the magnitude of the detected current is greater than the predetermined magnitude, and wherein the magnitude of each additional driving current pulse signal, when the load element is operating normally, is progressively decreased and the pulse-width thereof is increased from that of the preceding pulse signal until a steady-state load element operating condition is effected at which time the driving current remains constant for the duration of the input signal, the decrease in magnitude of each additional pulse signal, starting from an initial driving current pulse signal of maximum magnitude and minimum pulse-width, is in response to a corresponding increase in resistance of the load element.

Claim 53

A power switching circuit according to claim 45, wherein in response to detection of an overcurrent condition at the power output element resulting from a malfunction of the load element, the operation period of the power output element is limited to provide a series of current pulses of substantially constant magnitude and pulse-width as the driving current to the load element, for the duration of the input signal, thereby to prevent breakdown of the power output element.

Claim 54

A power switching circuit according to claim 46, wherein the load element is a lamp with a filament.