Over current protection in a variable output power supply
In accordance with embodiments of the present disclosure, a switch mode power supply may include a transformer, a controller configured to generate a periodic switching signal, a switching transistor coupled between a sense node and the transformer, primarily configured to prevent or permit current flow in the transformer primary winding in accordance with the switching signal, a sense resistor coupled to the switching transistor at a sense node, and a limit circuit configured to obtain a voltage of the sense node at a particular point of the switching period as an indicator of the primary winding current, detect a duration of a demagnetizing interval, and generate a limit signal, based on the sense node voltage and the demagnetizing interval, indicating a power supply output current exceeding a limit threshold.
1. An information handling system, comprising:
a processor;
a memory device coupled to the processor;
a bus controller coupled to the processor and configured to support communication with a peripheral device via a peripheral bus; and
a switch mode power supply coupled to the bus controller, wherein the switch mode power supply includes:
a controller configured to generate a switching signal having a switching period comprising an on phase and an off phase;
a switching transistor including:
a control electrode configured to receive the switching signal; and
first and second output electrodes configured to form a current path, in accordance with the control electrode, between a transformer primary winding and a sense node;
a sense resistor coupled to the sense node; and
a limit circuit configured to:
close a charging switch to couple the sense node to a first node of a sense capacitor during a first on phase;
sample a sense node voltage, comprising a voltage of the sense node, at a midpoint of the first on phase; and
close a discharging switch to couple a discharging current source to the sense capacitor during a first half of a second on phase.
2. The information handling system of claim 1 , wherein the limit circuit includes a midpoint circuit configured to open the discharging switch during the first on phase and open the charging switch during the second on phase and wherein a magnitude of a charging current is one half of a magnitude of a discharging current.
3. The information handling system of claim 2 , wherein the midpoint circuit includes a latch configured to:
generate a discharge cycle signal indicative of the sense capacitor voltage exceeding a particular threshold at a switching signal transition corresponding to a beginning of a switching period; and
generate a charging cycle signal indicative of the sense capacitor voltage below a particular threshold at the switching signal transition.
4. The information handling system of claim 1 , wherein a midpoint circuit is configured to close the charging switch during the second on phase and wherein a magnitude of a charging current is one third of a magnitude of a discharging current.
5. The information handling system of claim 1 , wherein the limit circuit is configured to apply the sense node voltage to an input of a voltage-controlled current source.
6. The information handling system of claim 1 , wherein the limit circuit includes a demagnetizing circuit configured to signal a beginning of a demagnetizing interval responsive to detecting a transition of the switching signal at an end of the first on phase.
7. The information handling system of claim 6 , wherein the demagnetizing circuit is configured to signal an end of the demagnetizing interval corresponding to a first occurring of two events, the two events comprising:
a discontinuous current mode end event comprising a negative transition of a switching transistor control electrode voltage; and
a continuous current mode end event comprising a transition of the switching signal at an end of an off phase.
8. The information handling system of claim 1 , further comprising:
a rectification stage configured to:
receive an AC voltage; and
produce a rectified voltage; and
a DC stage coupled between the rectification stage and the primary winding of the transformer.
9. The information handling system of claim 1 , wherein the controller comprises a pulse width modulator controller configured to control a duty cycle of the switching signal.
10. The information handling system of claim 1 , further comprising:
a flyback circuit configured to couple a signal indicative of an output voltage to the controller.
11. A switch mode power supply, comprising:
a controller configured to generate a switching signal having a switching period comprising an on phase and an off phase;
a switching transistor including:
a control electrode configured to receive the switching signal; and
first and second output electrodes configured to form a current path, in accordance with the control electrode, between a transformer primary winding and a sense node;
a sense resistor coupled to the sense node; and
a limit circuit configured to:
close a charging switch to couple the sense node to a first node of a sense capacitor during a first on phase;
sample a sense node voltage, comprising a voltage of the sense node, at a midpoint of the first on phase; and
close a discharging switch to couple a discharging current source to the sense capacitor during a first half of a second on phase.
12. The switch mode power supply of claim 11 , wherein the limit circuit includes a midpoint circuit configured to open the discharging switch during the first on phase and open the charging switch during the second on phase and wherein a magnitude of the charging current is one half of a magnitude of the discharging current.
13. The switch mode power supply of claim 12 , wherein the midpoint circuit includes a latch configured to:
generate a discharge cycle signal indicative of a sense capacitor voltage exceeding a particular threshold at a switching signal transition corresponding to a beginning of a switching period; and
generate a charging cycle signal indicative of the sense capacitor voltage below a particular threshold at the switching signal transition.
14. The switch mode power supply of claim 11 , wherein the midpoint circuit is configured to close the charging switch during the second on phase and wherein a magnitude of the charging current is one third of a magnitude of the discharging current.
15. The switch mode power supply of claim 11 , wherein the limit circuit is configured to apply the sense node voltage to an input of a voltage-controlled current source.
16. The switch mode power supply of claim 11 , wherein the limit circuit includes a demagnetizing circuit configured to signal a beginning of a demagnetizing interval responsive to detecting a transition of the switching signal at an end of the first on phase.
17. The switch mode power supply of claim 16 , wherein a demagnetizing circuit is configured to signal an end of the demagnetizing interval corresponding to a first occurring of two events, the two events comprising:
a discontinuous current mode end event comprising a negative transition of a switching transistor control electrode voltage; and
a continuous current mode end event comprising a transition of the switching signal at an end of the off phase.
18. The switch mode power supply of claim 11 , further comprising:
a rectification stage configured to:
receive an AC voltage; and
produce a rectified voltage; and
a DC stage coupled between the rectification stage and the primary winding of the transformer.
19. The switch mode power supply of claim 11 , wherein the controller comprises a pulse width modulator controller configured to control a duty cycle of the switching signal.
20. The switch mode power supply of claim 11 , further comprising:
a flyback circuit configured to couple a signal indicative of an output voltage to the controller.