Efficiency improvement in power factor correction
An power converter that is operable to convert AC power into DC power that may be delivered to a load. The power converter includes a transformer and a controllable switch. The switching frequency of the power converter is configured to be dependent on the level of the AC voltage of an AC power source. The switching frequency may be proportional to the AC voltage to provide a constant magnetic flux density swing for the transformer in the power converter. The switching frequency may be controlled by using a circuit that converts the AC voltage from the AC power source into a frequency signal that is proportional to the AC voltage.
1. A switching power converter for converting power from an AC input power source to an output at a DC output voltage, comprising:
a transformer having a primary winding coupled to the AC input power source;
an output capacitor coupled to a secondary winding of the transformer;
a switch element coupled to the primary winding of the transformer, the switch drawing input current from the AC power source through the primary winding of the transformer when the switch is closed and interrupting the input current from the AC power source when the switch is open; and
a switch control circuit that is operable to cycle the switch element at a frequency that is proportional to the RMS voltage of the AC input power source,
wherein an envelope of the input current pulsed waveform of the power converter is a sinusoidal waveform that follows an AC voltage of the AC input power source.
2. The power converter of claim 1 , wherein the switch control circuit is operable to cycle the switch element at a frequency such that the RMS value of the flux density of the transformer is substantially constant for a range of magnitudes of AC voltage levels of the AC power source.
3. The power converter of claim 2 , wherein the range of magnitudes of AC voltage levels of the AC power source is approximately between 80 and 400 volts RMS.
4. The power converter of claim 1 , wherein the switch control circuit includes a voltage-to-frequency converter that is operable to cycle the switch element at a frequency that is dependent upon the magnitude of the AC voltage.
5. The power converter of claim 4 , further comprising a rectifier operable to convert the AC input voltage into a rectified voltage, wherein the voltage-to-frequency converter includes:
a filter having an input terminal coupled to the rectifier, and an output terminal, the filter operable to convert the rectified voltage into a DC voltage;
a voltage-to-current converter that is coupled to the output terminal of the filter, the voltage-to-current converter being operable to convert the DC voltage to a current that is dependent upon the DC voltage; and
a current-to-frequency converter that is coupled to the voltage-to-current converter, the current-to-frequency converter being operable to convert the current into a drive signal that has a frequency that is dependent upon the current.
6. The power converter of claim 5 , wherein the filter is a passive filter that includes a capacitor and a resistor.
7. The power converter of claim 5 , wherein the current-to-frequency converter includes a comparator.
8. The power converter of claim 5 , wherein the voltage-to-current converter includes a current mirror.
9. The power converter of claim 5 , wherein the voltage-to-frequency converter includes an integrated circuit.
10. The power converter of claim 1 , wherein the power converter is a flyback type converter.
11. The power converter of claim 1 , wherein the power converter is a forward type converter.
12. The power converter of claim 1 , wherein the power converter is a double-switch forward type converter, a boost converter, or a SEPIC converter.
13. A method for use in a power converter, the power converter being operable to receive AC power from an AC power source on an input terminal and deliver DC power to a load on an output terminal, the AC power source having an input AC voltage, the method comprising:
supplying the input AC voltage to a converter stage that includes a controllable switch that is operable to open and close in response to a drive signal, and a transformer that is coupled to the controllable switch, the switch drawing input current from the AC power source through a primary winding of the transformer when the switch is closed and interrupting the input current from the AC power source when the switch is open; and
controlling the drive signal, wherein the drive signal switches at a frequency that is proportional to a substantially constant DC voltage dependent upon the magnitude of the input AC voltage of the AC power source, and wherein an envelope of the input current pulsed waveform of the power converter is a sinusoidal waveform that follows the AC voltage of the AC input power source.
14. The method of claim 13 , wherein the controlling includes switching the drive signal at a frequency such that the RMS value of the flux density of the transformer is substantially constant for a range of magnitudes of AC voltage levels of the AC power source.
15. The method of claim 14 , wherein the range of magnitudes of AC voltage levels of the AC power source is approximately between 80 and 400 volts RMS.
16. The method of claim 13 , wherein the controlling includes:
measuring the magnitude of the AC voltage of the AC power source;
converting the magnitude of the AC voltage to a frequency that is dependent upon the magnitude of the AC voltage; and
switching the drive signal at the frequency.
17. The method of claim 16 , wherein the converting includes:
rectifying the AC input voltage into a rectified voltage;
filtering the rectified voltage to a substantially constant DC voltage;
first generating a current that is dependent upon the filtered DC voltage; and
second generating a frequency that is dependent upon the current.
18. The power converter of claim 17 , wherein the filtering includes providing a capacitor and a resistor.
19. The method of claim 17 , wherein the second generating includes providing a comparator.
20. The method of claim 17 , wherein the converting includes providing an integrated circuit.
21. The method of claim 13 , wherein the power converter is a flyback type converter.
22. The method of claim 13 , wherein the power converter is a forward type converter.
23. The method of claim 13 , wherein the power converter is a double-switch forward type converter, a boost converter, or a SEPIC converter.
24. A switching power converter for converting power from an AC input power source to an output at a DC output voltage, the switching power converter comprising:
a transformer having a primary winding coupled to the AC input power source;
an output capacitor coupled to a secondary winding of the transformer;
a switch element coupled to the transformer, the switch drawing input current from the AC power source through the primary winding of the transformer when the switch is closed and interrupting the input current from the AC power source when the switch is open; and
a switch control element that is operable to cycle the switch element at a frequency that is proportional to the RMS voltage of the AC input power source and at a duty cycle that relates to the difference between a reference voltage and the DC output voltage.