Active preload for TRIAC dimmers

1. An active preload circuit comprising:

a rectifier configured to receive an AC input from a triode for alternating current (TRIAC) dimmer and to generate DC power;

a preload; and

a phase angle control circuit configured to selectively couple the preload to the DC power, via a switch, when a phase angle of the AC input is below a level, the phase angle control circuit comprising:

a first resistor, a second resistor, and a third resistor coupled together between a positive terminal and a negative terminal of the rectifier, the third resistor being a variable resistor configured to adjust the level at which the phase angle control circuit couples the preload to the DC power;

a zener diode coupled between a gate of the switch and a negative terminal of the rectifier;

a bipolar transistor having a source coupled to the zener diode; and

a fourth resistor coupled between the source of the bipolar transistor and a positive terminal of the rectifier.

2. The active preload circuit of claim 1 , wherein the phase angle control circuit comprises:

a first resistor coupled between a positive terminal of the rectifier and a gate of the bipolar transistor;

a second resistor coupled between the gate of the bipolar transistor and a third resistor;

the third resistor coupled between the second resistor and a negative terminal of the rectifier, the third resistor being the variable resistor; and

a capacitor coupled between the gate of the bipolar transistor and the negative terminal of the rectifier,

wherein the switch is coupled between a terminal of the preload and the negative terminal of the rectifier, the gate of the switch being coupled to the source of the bipolar transistor, the preload being coupled between the switch and the positive terminal of the rectifier, a drain of the bipolar transistor being coupled to the negative terminal of the rectifier, and

wherein the zener diode is coupled between a source of the bipolar transistor and the negative terminal of the rectifier.

3. The active preload circuit of claim 2 , wherein a wiper terminal of the third resistor is coupled to the negative terminal of the rectifier.

4. The active preload circuit of claim 3 , wherein a position of a wiper of the third resistor is configured to be adjustable.

5. The active preload circuit of claim 1 , wherein the preload is a resistive load.

6. An active preload circuit comprising:

a rectifier configured to receive an AC input from a triode for alternating current (TRIAC) dimmer and to generate DC power;

a preload; and

a phase angle control circuit configured to selectively couple the preload to the DC power when a phase angle of the AC input is below a level,

wherein the phase angle control circuit comprises:

an operational amplifier;

a first resistor coupled between a positive terminal of the rectifier and an inverting input of the operational amplifier;

a reference voltage coupled to a non-inverting input of the operational amplifier;

a supply voltage coupled to a positive supply voltage input of the operational amplifier;

a second resistor coupled between the inverting input of the operational amplifier and a third resistor;

the third resistor coupled between the second resistor and a negative terminal of the rectifier, the third resistor being a variable resistor;

a capacitor coupled between the inverting input of the operational amplifier and the negative terminal of the rectifier;

a zener diode coupled between an output of the operational amplifier and the negative terminal of the rectifier;

a fourth resistor coupled between the output of the operational amplifier and the positive terminal of the rectifier; and

a switch coupled between a terminal of the preload and the negative terminal of the rectifier, a gate of the switch being coupled to the output of the operational amplifier, the preload being coupled between the switch and the positive terminal of the rectifier.

7. The active preload circuit of claim 6 , wherein the reference voltage is configured to be adjustable.

8. The active preload circuit of claim 6 , wherein a wiper terminal of the third resistor is coupled to the negative terminal of the rectifier.

9. The active preload circuit of claim 8 , wherein a position of a wiper of the third resistor is configured to be adjustable.

10. An LED lighting system comprising:

a phase cut AC input;

an LED driver configured to receive an AC phase cut signal from the phase cut AC input and to generate a DC supply voltage to provide a driving current to an LED lamp; and

an active preload circuit comprising a rectifier and being configured to receive the AC phase cut signal from the phase cut AC input and to selectively couple a preload to the phase cut AC input, via a switch, when a phase angle of the phase cut AC input is below a level, the active preload circuit comprising:

a first resistor, a second resistor, and a third resistor coupled together between a positive terminal and a negative terminal of the rectifier, the third resistor being a variable resistor configured to adjust the level at which the active preload circuit couples the preload to the phase cut AC input;

a zener diode coupled between a gate of the switch and a negative terminal of the rectifier;

an operational amplifier having an output coupled to the zener diode; and

a fourth resistor coupled between the output of the operational amplifier and a positive terminal of the rectifier.

11. The LED lighting system of claim 10 , further comprising a TRIAC dimmer, wherein the TRIAC dimmer receives an AC input and generates the AC phase cut signal at the phase cut AC input.

12. The LED lighting system of claim 10 , wherein the active preload circuit comprises the switch, the preload, and a phase angle control circuit configured to couple the preload to the phase cut AC input by controlling the switch.

13. The LED lighting system of claim 12 , wherein the phase angle control circuit comprises:

a first resistor coupled between a positive terminal of the rectifier and an inverting input of the operational amplifier;

a reference voltage coupled to a non-inverting input of the operational amplifier;

a supply voltage coupled to a positive supply voltage input of the operational amplifier;

a second resistor coupled between the inverting input of the operational amplifier and a third resistor;

the third resistor coupled between the second resistor and a negative terminal of the rectifier, the third resistor being the variable resistor;

a capacitor coupled between the inverting input of the operational amplifier and the negative terminal of the rectifier;

wherein the switch is coupled between a terminal of the preload and the negative terminal of the rectifier, a gate of the switch being coupled to the output of the operational amplifier, the preload being coupled between the switch and the positive terminal of the rectifier, and

wherein the zener diode is coupled between an output of the operational amplifier and the negative terminal of the rectifier.

14. The LED lighting system of claim 13 , wherein the reference voltage is configured to be adjustable.

15. The LED lighting system of claim 13 , wherein a wiper terminal of the third resistor is coupled to the negative terminal of the rectifier.

16. The LED lighting system of claim 15 , wherein a position of a wiper of the third resistor is configured to be adjustable.

17. The LED lighting system of claim 10 , wherein the preload is a resistive load.

18. An LED lighting system comprising:

a phase cut AC input;

an LED driver configured to receive an AC phase cut signal from the phase cut AC input and to generate a DC supply voltage to provide a driving current to an LED lamp; and

an active preload circuit comprising a rectifier and being configured to receive the AC phase cut signal from the phase cut AC input and to selectively couple a preload to the phase cut AC input, via a switch, when a phase angle of the phase cut AC input is below a level, the active preload circuit comprising:

a variable resistor configured to adjust the level at which the active preload circuit couples the preload to the phase cut AC input; and

a zener diode coupled between a gate of the switch and a negative terminal of the rectifier,

wherein the active preload circuit comprises the switch, the preload, and a phase angle control circuit configured to couple the preload to the phase cut AC input by controlling the switch, and

wherein the phase angle control circuit comprises:

a bipolar transistor;

a first resistor coupled between a positive terminal of the rectifier and a gate of the bipolar transistor;

a second resistor coupled between the gate of the bipolar transistor and a third resistor;

the third resistor coupled between the second resistor and a negative terminal of the rectifier, the third resistor being the variable resistor;

a capacitor coupled between the gate of the bipolar transistor and the negative terminal of the rectifier; and

a fourth resistor coupled between a source of the bipolar transistor and the positive terminal of the rectifier;

wherein the switch is coupled between a terminal of the preload and the negative terminal of the rectifier, a gate of the switch being coupled to the source of the bipolar transistor, the preload being coupled between the switch and the positive terminal of the rectifier, a drain of the bipolar transistor being coupled to the negative terminal of the rectifier, and

wherein the zener diode is coupled between a source of the bipolar transistor and the negative terminal of the rectifier.

19. The LED lighting system of claim 18 , wherein a wiper terminal of the third resistor is coupled to the negative terminal of the rectifier.

20. The LED lighting system of claim 19 , wherein a position of a wiper of the third resistor is configured to be adjustable.