SWITCHED MODE POWER SUPPLY

• US 20120327691A1
• Filed: 05/29/2012
• Published: 12/27/2012
• Est. Priority Date: 06/22/2011
• Status: Active Grant

• Current Assignee: NXP B.V. (NXP Semiconductors NV)
• Original Assignee: NXP B.V. (NXP Semiconductors NV)
• Inventors: STRIJKER, Joan Wichard
• Granted Patent: US 9,263,954 B2
• Time in Patent Office: Days
• US Class Current: 363/21.01
• CPC Class Codes: H02M 1/0025 Arrangements for modifying reference values, feedback values or error values in the control loop of a converter
• CPC Class Codes: H02M 1/32 Means for protecting converters other than automatic disconnection
• CPC Class Codes: H02M 3/33507 with automatic control of the output voltage or current, e.g. flyback converters H02M3/33561, H02M3/33569 take precedence

Abstract

A power supply has over power protection based on the same signal as used by the feedback path for controlling the switching of the power supply. This means that no additional signal needs to be supplied to the control circuit to implement mains over protection.

Claims

1. A switched mode power supply comprising:

an input for connection to a rectified mains supply,

a transformer having a primary winding on a mains side and a secondary winding on a dc output side;

a switching element for controlling current flow in the primary winding of the transformer;

a sense resistor (Rsense) in series with the switching element;

a feedback control system for controlling the switching element in dependence on a sense voltage which is dependent on the sense resistor,

wherein the feedback control system is part of an integrated circuit, and the transformer is outside the integrated circuit, and

wherein the integrated circuit further comprises a detection arrangement for converting a rate of change of the sense voltage into a voltage level, and thereby derive a voltage signal representing the mains voltage.

2. A power supply as claimed in claim 1, wherein the switching element and the sense resistor are outside the integrated circuit and the sense voltage is provided to a sense pin of the integrated circuit.

3. A power supply as claimed in claim 1, wherein the feedback control system comprises a comparator for comparing the sense voltage with a reference voltage and controlling a set reset latch in dependence on the comparison.

4. A power supply as claimed in claim 1, wherein the detection arrangement comprises a capacitor connected to the sense voltage, and a current mirror circuit which copies the current through the capacitor to a detection resistor, wherein the voltage across the detection resistor comprises a detector voltage which is used as a measure of the mains voltage.

5. A power supply as claimed in claim 4, wherein the comparator reference voltage is derived from the detector voltage.

6. A power supply as claimed in claim 5, further comprising a selection unit which selects, as the comparator reference voltage, one of the detector voltage and a voltage corresponding to the normal regulation current level.

7. A power supply as claimed in claim 1, further comprising a compensation resistor between a terminal of the sense resistor and the sense voltage.

8. A power supply as claimed in claim 7, wherein the detection arrangement comprises a capacitor connected to the sense voltage, a sample and hold circuit for sampling a voltage dependent on a current through the capacitor, and a voltage to current converter circuit for driving a current through the compensation resistor.

9. A power supply as claimed in claim 8, wherein the feedback control system comprises a second comparator, wherein the comparator and the second comparator are for comparing the sense voltage with respective reference voltages, wherein a set-reset latch is provided which receives the two comparator outputs and which generates the sample signal for the sample and hold circuit.

10. A method of controlling a switched mode power supply, comprising:

transforming a mains supply input to a dc output;

controlling coupling of a current flow in a primary winding of the transformer to a sense resistor using a switching element;

controlling the switching element in dependence on the current flowing using a feedback control system which is part of an integrated circuit and which receives a sense resistor voltage, and

converting a rate of change of the sense voltage into a voltage level, and thereby deriving a voltage signal representing the mains voltage.

11. A method as claimed in claim 10, further comprising comparing the sense voltage with a reference voltage and controlling a set reset latch in dependence on the comparison, the set reset latch driving the switching element.

12. A method as claimed in claim 10, wherein the step of converting the rate of change of the sense voltage comprises providing the sense voltage to a capacitor and copying the current through the capacitor to a detection resistor to derive a detector voltage which is used as a measure of the mains voltage.

13. A method as claimed in claim 12, further comprising selecting, as the comparator reference voltage, one of the detector voltage and a voltage corresponding to the normal regulation current level.

14. A method as claimed in claim 13, wherein the step of converting the rate of change of the sense voltage comprises providing the sense voltage to a capacitor and sampling a voltage dependent on a current through the capacitor, and driving a current through a compensation resistor based on the sampled voltage, thereby controlling a voltage drop between a terminal of the sense resistor and the sense pin.

15. A method as claimed in claim 10, further comprising using the voltage signal representing the mains voltage to provide over power protection.