High power-factor control circuit and method for switched mode power supply
A switch mode power supply (SMPS) system includes a rectifying circuit for coupling to an AC input voltage and a transformer having a primary winding for coupling to the rectifying circuit and a secondary winding coupled to the primary winding. The system also has a power switch coupled to the primary winding and a control circuit coupled to the power switch. The control circuit is configured to control current flow in the primary winding such that an envelope waveform formed by peak points of current pulses is in phase with the magnitude of the AC input voltage. Moreover, the SMPS system is configured to provide a constant average output current.
1. A switch mode power supply (SMPS) system, comprising:
a rectifying circuit for coupling to an AC input voltage for providing a rectified AC voltage having a magnitude that varies in phase with the magnitude of the AC input voltage;
a transformer having a primary winding for coupling to the rectifying circuit and a secondary winding coupled to the primary winding;
a power switch coupled to the primary winding; and
a control circuit coupled to the power switch,
wherein:
the control circuit is configured to provide a reference signal that is in phase with the rectified AC voltage,
the control circuit is configured to control current pulses in the primary winding such that an envelope waveform formed by peak points of the current pulses is in phase with the AC input voltage
wherein each current pulse in the primary winding is triggered by the power switch being turned on for a period of time by the control signal, and each current pulse in the primary winding induces a current flow in the secondary winding for a time period Tons, followed by a time period Toffs during which the current flow in the secondary winding is off;
wherein the control circuit is configured to determine the time period Toffs and the time period Tons; and
wherein the control circuit is configured to maintain a constant ratio of Tons/Toffs, and the control circuit is configured to cause a sum of Tons and Toffs to vary with the reference signal such that the magnitude of the sum is high when the reference signal is high and the magnitude of the sum is low when the reference signal is low, thereby causing the SMPS to provide a constant average output current.
2. The power supply system of claim 1 , wherein the control circuit is configured to provide a control signal to control the power switch, the control signal having a frequency that varies with the reference signal, thereby causing the SMPS to provide a constant average output current.
3. The power supply system of claim 1 , wherein the control circuit further comprises a control pulse generation circuit configured to:
charge a capacitor using a first current source during the time period Toffs; and
discharge the capacitor using a second current source during the time period Tons;
wherein and the constant ratio of Tons/Toffs is based on the ratio of the first current source and the second current source.
4. The power supply system of claim 1 , wherein the control circuit is configured to turn off a current flow in the SMPS, when a voltage signal associated with the current in the primary winding reaches the reference signal.
5. The power supply system of claim 1 , wherein the reference signal is a sinusoidal voltage signal |VrefA|*|sin(2πft)| (V) characterized by:
a frequency 2πf matching the frequency of the AC input voltage; and
a magnitude |VrefA| (V) selected to produce a desired output current.
6. The power supply system of claim 1 , wherein the SMPS system is configured to provide a constant average output current substantially independent of a magnitude of the peak voltage of the AC input voltage.
7. The power supply system of claim 1 , wherein the SMPS system is configured to provide a constant average output current to an LED lighting apparatus, wherein the average output current is output current averaged over a time period of 10 milliseconds or longer, and
wherein, in a time scale of less than 10 milliseconds, the average output current varies with time, and the magnitude of the varying current is characterized by an envelope waveform that is in phase with the rectified input AC voltage.
8. The power supply system of claim 1 , wherein the rectifying circuit comprises a rectifying capacitor characterized by a capacitance that allows an output voltage of the rectifying circuit to retain phase information of the AC input voltage.
9. The power supply system of claim 1 , wherein the control circuit comprises a phase detection circuit and a reference signal generation circuit.
10. The power supply system of claim 1 , further comprising a dimmer circuit that turns off the AC input voltage during a portion of each AC cycle, wherein the control circuit comprises a dimmer phase detection circuit and is configured to stop energy transfer to the secondary winding in said portion of the AC cycle.
11. A control circuit for a switch mode power supply (SMPS), comprising:
a first input terminal for coupling to a periodic input voltage for receiving a rectified AC voltage;
a second input terminal for sensing a current flow in a primary winding of the SMPS;
a third input terminal for receiving a feedback signal associated with a current flow in a secondary winding of the SMPS;
an output terminal for providing a control signal to a power switch for regulating the current flow in the SMPS; and
a phase detection circuit and a reference signal generation circuit configured to provide a reference signal that is in phase with the rectified AC voltage and has a magnitude selected to produce a desired output;
wherein the control circuit is configured to control current pulses in the primary winding of the SMPS such that an envelope formed by peak points of the current pulses is in phase with the periodic input voltage, wherein each current pulse in the primary winding is triggered by the power switch being turned on for a period of time by the control signal, and each current pulse in the primary winding induces a current flow in the secondary winding for a time period Tons, followed by a time period Toffs during which the current flow in the secondary winding is off;
wherein the control circuit is configured to determine the time period Toffs and the time period Tons; and
wherein the control circuit is configured to maintain a constant ratio of Tons/Toffs and the control circuit is configured to cause a sum of Tons and Toffs to vary with the reference signal such that the magnitude of the sum is high when the reference signal is high and the magnitude of the sum is low when the reference signal is low, thereby causing the SMPS to provide a constant average output current.
12. The power supply system of claim 11 , further comprising a control pulse generation circuit configured to:
charge a capacitor using a first current source during the time period Toffs; and
discharge the capacitor using a second current source during the time period Tons;
wherein the constant ratio of Tons/Toffs is based on the ratio of the first current source and the second current source.
13. The power supply system of claim 11 , wherein the control circuit is configured to turn off a current flow in the SMPS when a voltage signal received at the second input terminal reaches the reference signal.
14. The power supply system of claim 13 , wherein the reference signal is a sinusoidal voltage signal |VrefA|*|sin(2πft)| (V) characterized by:
a frequency 2πft matching the frequency of the AC input voltage; and
a magnitude |VrefA| (V) selected to produce a desired output current.
15. A control circuit for a switch mode power supply (SMPS), comprising:
a first input terminal for coupling to a periodic input voltage for receiving a rectified AC voltage;
a second input terminal for sensing a current-sense signal representing a current flow in a primary winding of the SMPS;
a third input terminal for receiving a feedback signal associated with a current flow in a secondary winding of the SMPS; and
an output terminal for providing a control signal to a power switch for regulating the current flow in the SMPS;
wherein the control circuit is configured to provide a reference signal that is in phase with the rectified AC voltage, and to turn off a current flow in the SMPS, when the current-sense signal received at the second input terminal reaches the reference signal;
wherein the control circuit is configured to provide a control signal having a frequency that varies inversely with the reference signal in response to the current-sense signal associated with the current flow in the primary winding of the SMPS, thereby causing the SMPS to provide a constant average output current.
16. The control circuit of claim 15 , wherein the control circuit is configured to determine a time period Toffs, when the feedback signal indicates that the current flow in the secondary winding is off, and a time period Tons, when the feedback signal indicates that the current flow in the secondary winding is on;
wherein the control circuit is configured to maintain a constant ratio of Tons/Toffs, the on-time and off-time of the secondary current, respectively, thereby causing the SMPS to provide a constant average output current.
17. The control circuit of claim 15 , further comprising a phase detection circuit and a reference signal generation circuit configured to provide a reference signal that is in phase with the rectified AC voltage and has a magnitude selected to produce a desired output.
18. The control circuit of claim 15 , further comprising a control pulse generation circuit configured to:
charge a capacitor using a first current source during the time period Toffs; and
discharge the capacitor using a second current source during the time period Tons;
wherein and the constant ratio of Tons/Toffs is based on the ratio of the first current source and the second current source.
19. The control circuit of claim 15 , wherein the reference signal comprises a sinusoidal voltage signal |VrefA|*|sin(2πft)| (V) characterized by:
a frequency 2πft matching the frequency of the AC input voltage; and
a magnitude |VrefA| (V) selected to produce a desired output current.
20. The control circuit of claim 15 , further comprising a dimmer phase detection circuit configured to stop energy transfer to the secondary winding when the AC voltage is turned off during either an early portion or a latter portion of each AC cycle.