Method of operating a resonant power converter and a controller therefor
A method of operating a resonant power converter( 1, 2 ), having a high side switch( 3 ) and a low side switch( 4 ), is disclosed in which the switching is controlled to allow for improved operation at low power levels. The method involved an interruption to the part of the switching cycle in which the low side switch ( 4 ) is normally closed, by opening the switch at a particular moment in the cycle which allows the energy to be store in the resonance capacitor ( 5 ). Since, as a result, the energy is largely not resonating but stored in a single component, the time quantization of the mode of operation is significantly reduced or eliminated.
1. A method of controlling a resonant power converter,
the resonant power converter having a switching cycle period and comprising a primary side circuit comprising a first switch and a second switch connected in series and having a half-bridge node therebetween, the primary side circuit being connectable between a supply voltage and a ground,
a resonant capacitor forming part of a resonance circuit, and
an inductor forming another part of the resonance circuit,
the method comprising the steps of:
at the start of the switching cycle period closing the first switch for a first period,
opening the first switch at the end of the first period and thereafter closing the second switch for a second period,
opening the second switch at the end of the second period and thereafter closing the first switch to conclude the switching cycle period,
characterised in that the method includes the step of
interrupting the second period by
opening the second switch at the end of an initial part of the second period,
storing energy in the resonant capacitor whilst the second switch is open, and
closing the second switch for the remainder of the second period.
2. A method according to claim 1 wherein the second switch is opened to interrupt the second period at a moment when there is substantially no current in the primary side circuit.
3. A method according to claim 1 wherein the second switch is opened to interrupt the second period at a moment when the current in the primary side circuit is approximately zero.
4. A method according to claim 3 wherein the second switch is opened to interrupt the second period within 0.5 microseconds of a moment when there is no current in the primary side circuit.
5. A method according to claim 1 , further comprising a delay between opening the first switch and closing the second switch such that zero voltage switching of the second switch occurs.
6. A method according to claim 1 , further comprising a second delay between opening the second switch and closing the first switch such that zero voltage switching of the first switch occurs.
7. A method according to claim 1 , wherein the initial part of the second period has a duration which is longer than a resonance period of the resonance circuit, the resonance period being associated with the power converter being in a state wherein a secondary diode in a secondary side circuit of the resonant power converter is conducting.
8. A method according to claim 1 , wherein at least the duration of the initial part of the second period is adapted for use with a resonant power converter in which the resonant capacitor is connected between the half bridge node and a Vcap node.
9. A method according to claim 1 , wherein at least the duration of the initial part of the second period is adapted for use with a resonant power converter in which the resonant capacitor is connected to the second switch and connectable in use to ground.
10. A method according to claim 8 , wherein the duration of the first period and the duration of the second period are each controlled in dependence on the voltage at the Vcap node, to achieve a desired energy transfer per cycle.
11. A method according to claim 10 wherein the duration of the first period and the duration of the second period are each further controlled to achieve a desired switching cycle period.
12. A method according to claim 1 , wherein the resonant power converter comprises a series resonant power converter.
13. A method according to claim 1 , wherein the resonant power converter comprises a parallel resonant power converter.
14. A method according to claim 1 , including a further step of within the initial part of the second period, opening the second switch thereby starting an energy dump interval, and closing the second switch thereby defining the end of the energy dump interval.
15. A method according to claim 14 , wherein the second switch is closed at the end of the energy dump interval at a moment when the primary current reverses polarity.
16. A method according to claim 14 , wherein the second switch is closed at the end of the energy dump interval at a moment which is after the primary current reverses polarity and which is when the voltage at the half-bridge node is at a negative maximum.
17. A method according to claim 14 , wherein the second switch is opened to start the energy dump interval at a moment at which a magnetizing current in the inductor is at a negative maximum.
18. A method according to claim 14 , wherein the second switch is opened to start the energy dump interval at a moment when the modulus of the voltage across the resonant capacitor is equal to the modulus of the voltage of the resonant capacitor at the start of the second period.
19. A method as claimed in claim 1 , wherein the step of interrupting the second period further comprises an energy dump interval.
20. A controller for a resonant power converter, the resonant power converter having a switching cycle period and comprising a primary side circuit comprising a first switch and a second switch connected in series and having a half-bridge node therebetween, the primary side circuit being connectable between a supply voltage and a ground, the resonant power converter further having a resonant capacitor forming part of a resonance circuit, the controller being adapted to control the operation of the first switch and the second switch in accordance with the method of claim 1 .
21. A controller as claimed in claim 20 , further comprising a respective drive circuit a drive circuit for driving each of the first and the second switches.
22. A controller as claimed in claim 20 , wherein the controller including the respective drive circuits is packaged in a single package.
23. A controller as claimed in claim 22 , wherein the controller including the respective drive circuits is implemented on a single semiconductor chip.