Controller for a switched mode power converter, a switched mode power converter and method of controlling the same
A method of controlling a switched mode converter is disclosed in which the switching frequency varies in proportion to the square of the sine of the phase of the input AC supply. Thus the switching frequency is a maximum, and the respective on period of the switch is a minimum, when the mains voltage is a maximum. Conversely, the switching frequency is reduced, and the respective on time of the switch is increased, when the mains voltage is reduced. Such a switching method provides for a high power factor. Implementation by means of a phase locked loop and a comparator may prevent the need for complex circuitry, and may provide for direct use of a digital controller or digital signal processing through a counter output in the phase locked loop. A controller configured to operate such a method, together with an AC/DC converter embodying such a controller are also disclosed.
1. A controller for use in an AC-DC switched mode power converter for converting an alternating current supply having a phase Φ relative to a zero-crossing of the alternating currently supply and having a switch operable at a switching frequency fs, configured to control the switching frequency such that it is proportional to the square of the sine of the phase, over at least a part of an alternating current half-cycle, which half-cycle commences at a zero-crossing of the alternating current supply, with a proportionality constant Fmax, according to:
fs (Φ)= F max*sin 2 (Φ).
2. A controller according to claim 1 , comprising a phase locked loop configured to determine the phase.
3. A controller according to claim 2 , further comprising a comparator for determining a zero-crossing of the phase.
4. A controller according to any preceding claim, in which the controller is a digital controller.
5. A controller according to claim 1 , wherein the switch is configured to be switched on just until a primary current reaches a predetermined peak value Ipeak.
6. A controller according to claim 1 , configured to ensure that the switching frequency does not fall below a limit frequency Fmin, according to the relationship:
fs (Φ)=MAX{ F max*sin 2 (Φ), F min}.
7. An AC-DC power supply comprising a controller as claimed in claim 1 .
8. A method of controlling an AC-DC switched mode power converter, the switched mode power converter being for converting an alternating current supply having a phase Φ relative to a zero-crossing of the alternating currently supply and having a switch operable at a switching frequency fs the method comprising:
determining the phase Φ by means of a phase locked loop;
controlling the switching frequency over at least a half of an alternating current half-cycle according to
fs (Φ)= F max*sin 2 (Φ),
where Fmax is a maximum switching frequency.
9. The method of claim 8 , further comprising
controlling the switching frequency to not fall below a limit frequency Fmin.
10. The method of claim 8 , wherein the control is digital.