Use of a rectified image voltage for controlling the switch on the primary side of a switched-mode power supply

• US 6,608,768 B2
• Filed: 07/10/2002
• Issued: 08/19/2003
• Est. Priority Date: 08/07/2001
• Status: Active Grant

• Current Assignee: Salcomp OY
• Original Assignee: Salcomp OY
• Inventors: Sula, Matti
• Primary Examiner(s): Vu, Bao Q.
• Application Number:
  US10/193,772
• Publication Number:
  US 20030030422A1
• Time in Patent Office: 405 Days
• Field of Search: 363/18, 363/19, 363/97
• US Class Current: 363/18
• CPC Class Codes: H02M 3/3385 with automatic control of output voltage or current H02M3/33561 takes precedence

Abstract

A switched-mode power supply has primary and secondary sides, and a transformer (M1) with a primary winding, a secondary winding and an auxiliary winding. The primary winding and the auxiliary winding are connected to the primary side, and the secondary winding is connected to the secondary side. A switch element (T3) is connected to the primary winding in order to interrupt a current flowing in the primary winding. The switched-mode power supply contains a freely oscillating circuit (R5, C3, R7, T1, T2, R8, R9, R10, R11) for generating switching pulses to the switch element (T3). A rectifying and charge storing coupling (D4, C5) is connected between the terminals of the auxiliary winding in order to generate such an image voltage which on the primary side represents a certain voltage of the secondary side. The switched-mode power supply further contains a coupling (D3, R9) from said rectifying and charge storing coupling (D4, C5) to said freely oscillating circuit (R5, C3, R7, T1, T2, R8, R9, R10, R11) in order to control how switching pulses are generated on the basis of the value of said image voltage.

Claims

1. A switched-mode power supply, comprising:

a primary side and a secondary side,

a transformer with a primary winding, a secondary winding and an auxiliary winding, of which the primary winding and the auxiliary winding are connected to the primary side and the secondary winding is connected to the secondary side,

a switch element connected to the primary winding for interrupting a current flowing in the primary winding,

a freely oscillating circuit for generating switching pulses to the switch element,

a rectifying and charge storing coupling between the terminals of the auxiliary winding for generating an image voltage, which on the primary side represents a certain voltage of the secondary side, and

a coupling from said rectifying and charge storing coupling to said freely oscillating circuit for controlling, on the basis of the value of said image voltage, how switching pulses are generated.

2. A switched-mode power supply according to claim 1, wherein:

the freely oscillating circuit contains a semiconductor switch having a certain electrode whose potential determines when the semiconductor switch is in conducting state, which conducting state is arranged to correspond to the end of a switching pulse, and

the coupling from said rectifying and charge storing coupling to said freely oscillating circuit is arranged to change the potential of said certain electrode as a response to a change in the image voltage.

3. A switched-mode power supply according claim 2, wherein:

said semiconductor switch is a thyristor coupling which is triggered as a response to a sufficiently positive potential at its certain electrode, and

the coupling from said rectifying and charge storing coupling to said freely oscillating circuit is arranged to change the potential of said certain electrode of said thyristor coupling to be more positive in a situation where the image voltage exceeds a certain limiting value.

4. A switched-mode power supply according to claim 3, which switched-mode power supply is a primary-controlled switched-mode power supply where the primary control equals the coupling from said rectifying and charge storing coupling to said freely oscillating circuit.

5. A switched-mode power supply according to claim 3, which switched-mode power supply is a secondary-controlled switched-mode power supply.

6. A switched-mode power supply according to claim 5, wherein:

on the secondary side there is a circuit for measuring a certain electrical quantity, which circuit is arranged to generate an activating signal to the primary side in a situation where the measurement indicates that said electrical quantity has exceeded a certain limiting value,

the switched-mode power supply includes a switch component which is arranged to react on said activating signal by changing the potential of said electrode of the thyristor coupling to be more positive, and

the operation of said measurement circuit and said switch component is arranged to be activated as a response to an increase in the value of the measured quantity, which increase is smaller than what would be needed in order to increase the image voltage to a value where the coupling from said rectifying and charge storing coupling to said freely oscillating circuit would change the potential of said certain electrode of the thyristor coupling to be more positive.

7. A switched-mode power supply according to claim 5, wherein:

the secondary side contains a circuit for measuring a certain electrical quantity, which circuit is arranged to provide an activating signal to the primary side in a situation where the measurement indicates that said electrical quantity is lower than a certain limiting value,

the switched-mode power supply contains a switch component which is arranged to react on said activating signal by changing the potential of said electrode of the thyristor coupling to be more negative.

8. A switched-mode power supply according to claim 1, wherein the rectifying and charge storing coupling connected between the terminals of the auxiliary winding contains a diode and a capacitor connected in series, of which the diode is arranged to enable a current flow in a loop formed by the auxiliary winding, the diode and the capacitor in that direction, in which the current tends to flow when the switch element connected to the primary winding is in the non-conducting state.