Control circuit for switched mode power supply unit

• US 20050254268A1
• Filed: 02/11/2005
• Published: 11/17/2005
• Est. Priority Date: 03/10/2003
• Status: Active Grant

• Current Assignee: MyPAQ Holdings Ltd. (Transpacific IP Group Limited)
• Original Assignee: Friwo Mobile Power GmbH
• Inventors: Berghegger Schroder, Ralf Gennant, Malsch, Rudiger, Wegener, Armin, Reinhard, Arno
• Granted Patent: US 7,102,899 B2
• Time in Patent Office: Days
• US Class Current: 363/20
• 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

The present invention relates to a control circuit for controlling the output power of a primary-controlled switched mode power supply unit, which comprises a primary switch and a transformer with an auxiliary winding. Within the auxiliary winding, voltage pulses are induced by primary-sided switching operations, which can be taken into account when controlling the output power. In order to provide an improved control and an enhanced flexibility with respect to the operational parameters, the sampling instance is determined based on the duration of the voltage pulse at the auxiliary winding during a previous switching cycle. In order to detect a defective contact between the auxiliary winding and the control circuit in a particularly simple and secure manner, the negative voltage pulse at the auxiliary winding occurring when the switch is closed may be used for confirming a proper connection. In order to optimize the switching off process of the switch and to reduce power losses, the output power may be controlled by comparing the current through the primary winding with two threshold values.

Claims

1. Control circuit for controlling the output power of a primary-control switched mode power supply unit, said switched mode power supply unit comprising a primary-side switch and a transformer with an auxiliary winding, wherein after the opening of the primary-side switch a voltage pulse is induced,

wherein said control circuit comprises a circuit arrangement for generating a sampling signal which defines a sampling instance with respect to the instance of the opening of the primary-side switch,

a sample and hold device for sampling and storing a height of the voltage pulse in response to the sampling signal for generating a controlled variable, and

a control unit for comparing the controlled variable with a reference value and for adjusting the output power dependent on the result of this comparison,

wherein said circuit arrangement is adapted to determine the sampling instance based on the duration of said voltage pulse.

2. Control circuit according to claim 1, wherein said circuit arrangement is interconnected in a way, that it receives a control signal for closing the primary-side switch and a demagnetization signal indicating whether the transformer is without energy, in order to determine therefrom the duration of the voltage pulse.

3. Control circuit according to claim 1, wherein the circuit arrangement comprises the following elements:

a constant current source,

a first capacitor to be charged by means of the constant current source,

a second capacitor to be connected in parallel to the first capacitor,

a comparator which is connected to one terminal of each of the two capacitors and is operable to output the sampling signal, if the voltages at the two capacitors are equal.

4. Control circuit according to claim 3, wherein the circuit arrangement comprises a plurality of switches in order to coordinate the timing of the charging and discharging of the first and the second capacitor.

5. Control circuit according to claim 4, wherein between the constant current source and the first capacitor a first switch is arranged,

that a second switch for short-circuiting the second capacitor is arranged in parallel to the second capacitor, and that the second switch is controllable by the sampling signal,

that the first and the second capacitor are connected with one terminal each to a common ground potential and that between the respective other terminals a third switch is arranged, and

that a fourth switch for short-circuiting by the first capacitor is arranged in parallel to the first capacitor.

6. Control circuit according to claim 2, wherein the control signal for the primary-side switch is on high potential, when the switch is closed, and on low potential, when the switch is opened, and that the demagnetization signal is on high potential, when the voltage at the auxiliary winding is below a predetermined threshold value, and is on low potential, when the voltage at the auxiliary winding is above a predetermined threshold value.

7. Control circuit according to claim 4, wherein the circuit arrangement further comprises a control device, which comprises at least one output for controlling the plurality of switches dependent on the duration of the voltage pulse.

8. Control circuit according to claim 7, wherein the control device comprises a first and a second RS flip-flop,

wherein the first RS flip-flop is connected with its setting input to the negated switching-on signal and with its reset input to the demagnetization signal, and wherein the second RS flip-flop is connected with its setting input to the demagnetization signal and to its reset input with the switching on signal,

wherein the output of the first RS flip-flop controls the first switch, and is further connected with a NOR-Gate for controlling the third switch and is connected with a negated input of an AND-gate which controls with its output the fourth switch,

wherein an output of the second RS flip-flop is connected with a second input of the NOR-Gate and further with the second input of the AND-gate.

9. Primary-controlled switched mode power supply unit with a control circuit according to claim 1, wherein the primary-side switch is an electronic switch, preferably a power bipolar transistor.

10. Circuit arrangement for generating a sampling signal that determines the instance of sampling a controlled variable for controlling the output power of a primary-controlled switched mode power supply unit, wherein said switched mode power supply unit comprises a primary-side switch and a transformer with an auxiliary winding, wherein after the opening of the primary-side switch a voltage pulse is induced and wherein the circuit arrangement comprises the following elements:

a constant current source,

a first capacitor to be charged by means of the constant current source,

a second capacitor to be connected in parallel to the first capacitor,

a comparator which is connected to one terminal of each of the two capacitors and is operable to output the sampling signal, if the voltages at the two capacitors are equal.

11. Circuit arrangement according to claim 10, wherein the circuit arrangement comprises a plurality of switches in order to coordinate the timing of the charging and discharging of the first and the second capacitor.

12. Circuit arrangement according to claim 11, wherein between the constant current source and the first capacitor a first switch is arranged,

that a second switch for short-circuiting the second capacitor is arranged in parallel to the second capacitor, and that the second switch is controllable by the sampling signal,

that the first and the second capacitor are connected with one terminal each to a common ground potential and that between the respective other terminals a third switch is arranged, and

that a fourth switch for short-circuiting the first capacitor is arranged in parallel to the first capacitor.

13. Circuit arrangement according to claim 11, further comprising a control device which comprises at least one output for controlling the plurality of switches in response to at least one input signal that contains information about the duration of the voltage pulse.

14. Circuit arrangement according to claim 13, wherein the input signal is formed by a control signal for the primary-side switch and a demagnetization signal, which is at high potential, when the voltage at the auxiliary winding is below a predetermined threshold value, and is at low potential, when the voltage at the auxiliary winding is above a predetermined threshold value.

15. Circuit arrangement according to claim 14, wherein the control device comprises a first and a second RS flip-flop,

wherein the first RS flip-flop is connected with its setting input to the negated switching on signal and with its reset input to the demagnetization signal, and wherein the second RS flip-flop is connected with its setting input to the demagnetization signal and with its reset input to the switching on signal,

wherein the output of the first RS flip-flop controls the first switch, and is further connected with a NOR-Gate for controlling the third switch and is connected with a negated input of an AND-gate which controls with its output the fourth switch,

wherein an output of the second RS flip-flop is connected with a second input of the NOR-Gate and further with the second input of the AND-gate.

16. Method for controlling the power of a primary-controlled switched mode power supply unit with a transformer and a primary-side switch, wherein the transformer comprises an auxiliary winding wherein after the opening of the primary-side switch a voltage pulse is induced, and wherein the method comprises the following steps:

during one switching cycle, sampling and storing a height of the voltage pulse in response to a sampling signal which defines a sampling instance after the opening of the switch for generating a controlled variable,

comparing the controlled variable with a reference value and adjusting the output power dependent on the result of said comparison,

wherein the sampling instance is determined based on the duration of the voltage pulse at the auxiliary winding during a previous switching cycle.

17. Method according to claim 16, wherein a further sampling signal is generated for a following switching cycle.

18. Method according to claim 16, wherein the sampling instance is determined with respect to the instance when the primary switch opens at a fixed fraction, preferably two thirds, of the duration of the voltage pulse.

19. Method according to claim 16, wherein the step of generating the sampling signal comprises the following steps:

charging the first capacitor after the switch has opened,

comparing the voltage at the first capacitor with a voltage at a second capacitor, which may be connected in parallel to the first capacitor, and

outputting the sampling signal, if the comparison yields that the voltages at both capacitors are equal.

20. Method according to claim 19, wherein the second capacitor is discharged after outputting the sampling signal.

21. Method according to claim 19, wherein the first capacitor is charged from a constant current source, until the voltage pulse ends.

22. Method according to claim 19, wherein the first and the second capacitor are connected in parallel, when the voltage pulse ends, so that the second capacitor is charged by means of the first capacitor and that the same voltage is lying at both capacitors.

23. Method according to claim 19, wherein the capacitors are separated from each other and that the first capacitor is discharged, when the primary-side switch is closed.

24. Method according to claim 16, wherein the starting of the voltage pulse is defined by a falling of the control signal for the primary-side switch to low potential, and that the end of the voltage pulse is defined by a rising of the demagnetization signal, wherein the demagnetization signal is at high potential, when the voltage at the auxiliary winding is below predetermined threshold value and at low potential, when the voltage at the auxiliary winding is above a predetermined threshold value.

25. Method according to claim 16, wherein the step of adjusting the output power comprises adapting the time period during which the primary-side switch is opened.

26. Method according to claim 16, wherein the primary-side switch is a power bipolar transistor, a field effect transistor or an insulated gate bipolar transistor (IGBT).

27. Method according to claim 16, wherein the sampling and storing of the height of the voltage pulse is performed by means of a sample and hold element (S &

H).

28. Method for controlling the output power of a primary-controlled switched mode power supply unit with a transformer and a primary-side switch, wherein the transformer comprises an auxiliary winding, wherein after the opening of the primary-side switch a first voltage pulse and after the closing of the primary-side switch a second voltage pulse is induced, and wherein the method comprises the following steps:

during one switching cycle, sampling and storing a height of the first voltage pulse for generating a controlled variable,

comparing the controlled variable with a reference value and adjusting the output power dependent on the result of said comparison,

sampling a height of the second voltage pulse and comparing same with a threshold value and setting a secure operating modus of the switched mode power supply unit, in case that the comparison indicates a transgression of said threshold value.

29. Method according to claim 28, wherein the setting of a secure operational modus comprises switching off a driver, which controls the primary-side switch.

30. Primary-controlled switched mode power supply unit with a transformer and a primary-side switch, wherein the transformer comprises an auxiliary winding, wherein after the opening of the primary-side switch a first voltage pulse is induced and after the closing of the primary-side switch a second voltage pulse is induced, wherein same is operable to perform the method according to claims 28.

31. Method for controlling the output power of a primary-controlled switched mode power supply unit with a transformer and a primary-side switch, wherein said primary-side switch is controllable by means of a driver and wherein the method comprises the following steps:

detecting the current which is flowing through a primary winding of the transformer,

comparing the detected current with a first threshold value and with a second threshold value,

switching the driver into a high impedance state, when the detected current transgresses the first threshold value,

opening the primary-side switch, when the detected current transgresses the second threshold value.

32. Method according to claim 31, wherein the first threshold value amounts to 80% of the second threshold value.

33. Method according to claim 31, wherein the driver is a push-pull driver.