Current-limiting circuit for LED power supply
A regulator circuit and driver including the regulator circuit use a variable-resistance element, such as a transistor, to limit the current in the circuit when the current is above a defined current threshold. The circuit may be comprised of a first amplifier circuit that senses a current in the circuit and generates a voltage signal in proportion thereto. The circuit also includes a mechanism for controlling a voltage applied to the first circuit element to limit the current flow. That may be a second amplifier circuit that takes the voltage signal from the first op amp and a reference voltage and outputs a control voltage for the variable-resistance element, or it may be a digital computing device that outputs a control voltage that causes the variable-resistance element to stop current flow for some defined period of time around current peaks.
1 . A regulator circuit, comprising:
a first circuit element having a variable, controllable resistance and a resistance control terminal, the first element adapted to be disposed in either a voltage-out line or a minus-return line of a power supply, the power supply producing a cyclically time-varying current;
a first amplifier circuit coupled to the voltage-out line or the minus-return line of the power supply to detect the cyclically time-varying current and to generate an amplified voltage signal in proportion thereto, the first amplifier circuit including a first op amp configured as a non-inverting amplifier with inverting and non-inverting inputs connected across a resistor disposed in the voltage-out line or the minus-return line; and
means for controlling a voltage applied to the resistance control terminal of the first circuit element to limit the current flow proximate to peaks in the cyclically time-varying current, the means for controlling the voltage applied to the resistance control terminal including a second op amp configured as a differential amplifier, the second op amp receiving an output of the first op amp and a reference voltage from a reference voltage source, an output of the second op amp connected to the resistance control terminal of the first circuit element.
2 . The regulator circuit of claim 1 , further comprising:
a third op amp receiving a remainder voltage at a first input thereof and a second reference voltage at a second input thereof and generating at an output thereof an adjustment voltage, the output of the third op amp being connected to the reference voltage source such that the adjustment voltage alters the reference voltage when the remainder voltage is higher than a predefined remainder voltage threshold;
wherein the altered reference voltage causes a change in a voltage applied to the resistance control terminal of the first circuit element and a consequent reduction in power in the first circuit element.
3 . The regulator circuit of claim 1 , wherein the first circuit element comprises a transistor and the resistance control terminal comprises a gate of the transistor.
4 . A driver, comprising:
a power-line transformer receiving high-voltage, alternating current (AC) power;
a full-bridge rectifier connected to an output of the power-line transformer;
a voltage-out line connected at one end to an output of the full-bridge rectifier;
a voltage-out terminal connected to the voltage out line;
a return line;
a minus-return terminal connected to the return line; and
a regulator circuit according to claim 1 .
5 . A regulator circuit, comprising:
a transistor having a gate that defines, at least in part, a variable, controllable resistance of the transistor, the transistor adapted to be disposed in either a voltage-out line or a minus-return line of a power supply, the power supply producing a cyclically time-varying current;
a first amplifier circuit coupled to the voltage-out line or the minus-return line of the power supply to detect the cyclically time-varying current and to generate an amplified voltage signal in proportion thereto;
a reference voltage source generating a predefined reference voltage;
a second amplifier circuit having an output connected to the gate of the transistor and receiving the amplified voltage signal from the first amplifier circuit, the second amplifier circuit generating, at the output, a control voltage that causes the transistor to increase in resistance to limit the current flow when the current flow, as represented by the amplified voltage signal of the first amplifier circuit, is greater than a predefined threshold; and
a third amplifier circuit receiving a remainder voltage at a first input thereof and a second predefined reference voltage at a second input thereof and generating at an output thereof an adjustment voltage, the output of the third amplifier circuit being connected to the reference voltage source such that the adjustment voltage alters the reference voltage when the remainder voltage is higher than a predefined remainder voltage threshold, such that the altered reference voltage causes a change in the control voltage and a consequent reduction in power in the transistor;
wherein the predefined threshold is predefined such that it is exceeded proximate to peaks in the cyclically time-varying current.
6 . The regulator circuit of claim 5 , wherein the transistor is an n-channel field-effect transistor (FET) disposed in the minus-return line.
7 . The regulator circuit of claim 5 , wherein the regulator circuit is without a multiplier.
8 . The regulator circuit of claim 5 , wherein the first amplifier circuit, the second amplifier circuit, and the third amplifier circuit comprise first, second, and third op amps, respectively.