Bulk capacitor switching for power converters
In one example, a circuit includes a voltage rail, a reference node, a first capacitor, and a capacitor module. The first capacitor is coupled to the voltage rail and to the reference node. The capacitor module includes a second capacitor and a switching unit. The switching unit is configured to operate in a closed state and an open state. The switching unit couples the second capacitor in parallel with the first capacitor in the closed state. The switching unit decouples the second capacitor from the first capacitor in the open state.
1. A circuit comprising:
a voltage rail;
a reference node;
a first capacitor coupled to the voltage rail and to the reference node;
a first capacitor module comprising:
a second capacitor; and
a first switching unit configured to operate in a closed state when a voltage between the voltage rail and the reference node does not exceed a first voltage threshold,
wherein the first switching unit couples the second capacitor in parallel with the first capacitor in the closed state of the first switching unit, and
wherein the first switching unit decouples the second capacitor from the first capacitor in an open state of the first switching unit; and
a second capacitor module comprising at least:
a third capacitor; and
a second switching unit configured to operate in a closed state when the voltage between the voltage rail and the reference node does not exceed a second voltage threshold,
wherein the second switching unit couples the third capacitor in parallel with the first capacitor in the closed state of the second switching unit, and
wherein the second switching unit decouples the third capacitor from the first capacitor in an open state of the second switching unit.
2. The circuit according to claim 1 , further comprising:
a voltage source coupled to the voltage rail and the reference node.
3. The circuit according to claim 1 , wherein:
the first switching unit comprises a voltage controlled circuit element including a first node coupled to the voltage rail, a second node, and a control node; and
the second capacitor includes a first node coupled to the second node of the voltage controlled circuit element of the first switching unit and a second node coupled to the reference node.
4. The circuit according to claim 3 , further comprising:
a first resistive element including a first node and a second node, the first node of the first resistive element being coupled to the second node of the voltage controlled circuit element of the first switching unit and a first node of the second capacitor; and
a second resistive element including a first node coupled to the second node of the first resistive element and to the control node of the voltage controlled circuit element of the first switching unit and a second node coupled to the reference node.
5. The circuit according to claim 3 , further comprising:
a first resistive element including a first node and a second node, the first node of the first resistive element being coupled to the voltage rail;
a second resistive element including a first node coupled to the second node of the first resistive element and a second node coupled to the reference node; and
a comparator including an output, a first input coupled to a reference voltage, and a second input coupled to the second node of the first resistive element,
wherein the first switching unit selectively couples the second capacitor to the voltage rail and the reference node based on a signal generated on the output of the comparator.
6. The circuit according to claim 3 , further comprising:
a controller including a first input, a second input, and an output;
a transformer of a converter, the transformer comprising at least a first winding and a second winding, the first winding of the transformer including a first node and a second node and the second winding of the transformer being coupled to the output of the converter;
a first resistive element including a first node coupled to the first node of the first winding and a second node coupled to the first input of the controller; and
a second resistive element including a first node coupled to the first input of the controller and a second node coupled to the second input of the controller and the second node of the first winding,
wherein the first switching unit selectively couples the second capacitor to the voltage rail and the reference node based on a signal generated on the output of the controller.
7. The circuit according to claim 1 , wherein the first capacitor module includes a startup unit, the startup unit comprising:
a voltage controlled circuit element including a first node coupled to the voltage rail, a second node coupled to a first node of the second capacitor, and a control node;
a first resistive element including a first node coupled to the second node of the voltage controlled circuit element of the startup unit and a second node coupled to the control node of the voltage controlled circuit element of the startup unit; and
a second resistive element including a first node coupled to the control node of the voltage controlled circuit element of the startup unit and a second node coupled to the reference node.
8. The circuit according to claim 1 , wherein:
the first capacitor has a lower capacitance than the second capacitor; and
the first capacitor has a higher voltage rating than the second capacitor.
9. The circuit according to claim 1 , wherein:
the first capacitor is a parasitic capacitance of the circuit.
10. A method comprising:
estimating, by a circuit, a voltage between a voltage rail and a reference node;
determining, by the circuit, whether the voltage between the voltage rail and the reference node exceeds a first voltage threshold in response to estimating the voltage between the voltage rail and the reference node;
increasing, by the circuit, a capacitance between the voltage rail and the reference node by selectively coupling, by the circuit, a first capacitor to the voltage rail and to the reference node in response to determining that the voltage between the voltage rail and the reference node does not exceed the first voltage threshold;
determining, by the circuit, whether the voltage between the voltage rail and the reference node exceeds a second voltage threshold in response to estimating the voltage between the voltage rail and the reference node; and
increasing, by the circuit, the capacitance between the voltage rail and the reference node by selectively coupling, by the circuit, a second capacitor to the voltage rail and to the reference node in response to determining, by the circuit, that the voltage between the voltage rail and the reference node does not exceed the second voltage threshold.
11. The method according to claim 10 , comprising:
decoupling, by the circuit, the capacitor from the voltage rail and the reference node in response to determining that the voltage between the voltage rail and the reference node exceeds a third voltage threshold.
12. The method according to claim 10 , wherein:
estimating the voltage between the voltage rail and the reference node comprises estimating, by the circuit, a voltage of the capacitor.
13. The method according to claim 10 , wherein:
estimating the voltage between the voltage rail and the reference node comprises estimating, by the circuit, a voltage on a first winding of a transformer, and
the transformer includes a second winding coupled to an output of a switched-mode power converter that converts the voltage between the voltage rail and the reference node to another voltage.
14. The method according to claim 10 , wherein:
the first voltage threshold is a portion of a voltage rating of the first capacitor.
15. The method according to claim 10 , further comprising:
coupling, by a startup unit of the circuit, the first capacitor to the voltage rail and to the reference node;
estimating, by the startup unit, a voltage of the first capacitor in response to coupling the capacitor to the voltage rail and to the reference node; and
selectively coupling, by the startup unit, the first capacitor to the voltage rail and to the reference node based on the estimated voltage of the first capacitor.
16. A system comprising:
a rectifier configured to supply a rectified AC voltage to a voltage rail and a reference node;
a first capacitor configured to reduce an alternating current (AC) voltage ripple of the rectified AC voltage, the first capacitor being coupled to the voltage rail and to the reference node;
a first capacitor module configured to reduce the AC voltage ripple of the rectified AC voltage, the first capacitor module comprising:
a second capacitor; and
a first switching unit configured to operate in a closed state when a voltage between the voltage rail and the reference node does not exceed a first voltage threshold,
wherein the first switching unit couples the second capacitor in parallel with the first capacitor in the closed state of the first switching unit, and
wherein the first switching unit decouples the second capacitor from the first capacitor in an open state of the first switching unit;
a second capacitor module configured to reduce the AC voltage ripple of the rectified AC voltage, the second capacitor module comprising:
a third capacitor; and
a second switching unit configured to operate in a closed state when the voltage between the voltage rail and the reference node does not exceed a second voltage threshold,
wherein the second switching unit couples the third capacitor in parallel with the first capacitor in the closed state of the second switching unit, and
wherein the second switching unit decouples the third capacitor from the first capacitor in an open state of the second switching unit; and
a converter coupled to the voltage rail and to the reference node.
17. The system according to claim 16 , wherein the converter comprises:
a switched-mode power converter configured to receive the rectified AC voltage at a first voltage and output a direct current (DC) voltage at a second voltage.
18. The system according to claim 17 , further comprising:
a load coupled to the output of the switched-mode power converter, the load including an electronic device configured to operate at the second voltage.