High power factor primary regulated offline LED driver
A fly-back type switched current regulator includes a primary transformer winding coupled to receive a rectified DC signal derived from an AC signal. The drain of a power transistor is coupled to the primary winding, with the source of the power transistor coupled to an input of a comparison circuit and a primary transformer winding sense resistor. A control terminal of the power transistor is coupled to an output of the comparison circuit. A capacitor stores a variable reference signal for application at a first capacitor terminal to another input of the differential circuit. The variable reference signal is compared to a winding current signal generated by the sense resistor by the comparison circuit. An injection circuit applies an AC signal derived from the rectified DC signal to a second terminal of the capacitor so as to modulate the stored variable reference signal. The regulator is coupled to drive LEDs.
1. A circuit, comprising:
a transformer including a primary winding having a first terminal configured to receive a rectified DC signal derived from an AC signal, and having a second terminal;
a comparison circuit having a first input, a second input and an output;
a power transistor having a first conduction terminal coupled to the second terminal of the primary winding, and having a second conduction terminal coupled to the first input of the comparison circuit, and further having a control terminal coupled to the output of the comparison circuit;
a first capacitor having a first terminal coupled to the second input of the comparison circuit and having a second terminal;
a second capacitor having a first terminal coupled to the second terminal of the first capacitor and having a second terminal coupled to a reference supply node; and
an injection circuit configured to apply an AC signal derived from the rectified DC signal directly to the second terminal of the first capacitor.
2. The circuit of claim 1 , further comprising a PWM logic circuit coupled between the output of the comparison circuit and the control terminal of the power transistor.
3. The circuit of claim 1 , further comprising:
a current sourcing circuit coupled to the second input of the comparison circuit; and
a current sinking circuit coupled to the second input of the comparison circuit.
4. The circuit of claim 3 , further comprising a control circuit configured to selectively actuate the current sinking circuit.
5. The circuit of claim 4 , wherein the transformer further comprises a secondary winding and wherein the circuit further comprises a sensing circuit configured to sense current delivered by the secondary winding to a load.
6. The circuit of claim 5 , wherein the sensing circuit comprises an auxiliary winding on the transformer.
7. The circuit of claim 5 , wherein the control circuit responds to sensing circuit sensing of current delivery by the secondary winding to the load by actuating the current sinking circuit.
8. The circuit of claim 7 , wherein the control circuit responds to sensing circuit failure to sense current delivery by the secondary winding to the load by deactuating the current sinking circuit.
9. The circuit of claim 1 , further comprising a feedback resistor coupled between the second conduction terminal of the power transistor and the first input of the comparison circuit.
10. The circuit of claim 1 , wherein the comparison circuit comprises a comparator.
11. A circuit, comprising:
a first winding coupled to receive a rectified DC signal derived from an AC signal;
a current sensing resistor coupled in series with the first winding and configured to generate a first signal indicative of sensed current in the first winding;
a comparison circuit configured to compare the first signal to a second signal presenting a variable reference value;
a first capacitor configured to store the variable reference value, the first capacitor having a first terminal coupled to the comparison circuit and having a second terminal;
a second capacitor having a first terminal coupled to the second terminal of the first capacitor and having a second terminal coupled to a reference supply node; and
an injection circuit configured to apply an AC signal derived from the rectified DC signal directly to the second terminal of the first capacitor storing the variable reference value.
12. The circuit of claim 11 , further comprising a switching circuit coupled in series with the first winding and current sensing resistor, wherein activation of the switching circuit is controlled in response to a signal output from the comparison circuit.
13. The circuit of claim 11 , further comprising:
a current sourcing circuit coupled to the first terminal of the first capacitor; and
a current sinking circuit coupled to the first terminal of the first capacitor.
14. The circuit of claim 13 , further comprising a control circuit configured to selectively actuate the current sinking circuit.
15. The circuit of claim 14 , further comprising a second winding magnetically coupled to the first winding and wherein the circuit further comprises a sensing circuit configured to sense current delivered by the second winding to a load.
16. The circuit of claim 15 , wherein the sensing circuit comprises a third winding magnetically coupled to the first winding.
17. The circuit of claim 15 , wherein the control circuit responds to sensing circuit sensing of current delivery by the second winding to the load by actuating the current sinking circuit.
18. A circuit, comprising:
a first winding coupled to receive a rectified DC signal derived from an AC signal;
a current sensing resistor coupled in series with the first winding and configured to generate a first signal indicative of sensed current in the first winding;
a comparison circuit having a first input configured to receive the first signal and having a second input;
a first capacitor having a first terminal coupled to second input and a second terminal coupled to a first node;
a second capacitor having a first terminal coupled to the first node and a second terminal coupled to a reference supply node;
a resistive divider coupled between the rectified DC signal and the reference supply node, the resistive divider having a second node configured to generate an AC signal derived from the rectified DC signal that is applied to the first node.
19. The circuit of claim 18 , further comprising a switching circuit coupled in series with the first winding and current sensing resistor, wherein activation of the switching circuit is controlled in response to a signal output from the comparison circuit.
20. The circuit of claim 18 , further comprising:
a current sourcing circuit coupled to the first terminal of the capacitor; and
a current sinking circuit coupled to the first terminal of the capacitor.
21. The circuit of claim 20 , further comprising a control circuit configured to selectively actuate the current sinking circuit.
22. The circuit of claim 21 , further comprising a second winding magnetically coupled to the first winding and wherein the circuit further comprises a sensing circuit configured to sense current delivered by the second winding to a load.
23. The circuit of claim 22 , wherein the control circuit responds to sensing circuit sensing of current delivery by the second winding to the load by actuating the current sinking circuit.