Short-circuiting rectifier for a switched-mode power supply
View Patent ↗A control circuit for controlling the output voltage of a primary-controlled switched mode power supply. The switched mode power supply includes a primary-side switch and a transformer with a primary winding, a secondary winding and a secondary-side auxiliary winding. The control circuit is connected to the auxiliary winding and a voltage tapping point between the secondary winding and the auxiliary winding. The control circuit includes a secondary-side switch and an RC element. The secondary-side switch short-circuits the connection between the output terminal and the voltage tapping point and the RC element provides positive and negative control pulses having a predefined time constant.
1. A control circuit for controlling an output voltage of a switched mode power supply, wherein said switched mode power supply comprises a primary-side switch and a transformer with a primary winding, a secondary winding and a secondary-side auxiliary winding, wherein a voltage tapping point is provided between the secondary winding and the auxiliary winding, wherein the control circuit is adapted to be connected to a connection of the auxiliary winding and to said voltage tapping point, wherein the control circuit is adapted to be connected to an output terminal of the switched mode power supply and the output voltage is adapted to be tapped parallel to the secondary winding, and wherein said control circuit comprises:
a secondary-side switch for short-circuiting a rectifier diode between said output terminal and said voltage tapping point, and
an RC element for providing positive and negative control pulses having a predefined time constant, said RC element being connected in parallel to the auxiliary winding.
2. A control circuit according to claim 1 , comprising a resistor for defining a switching voltage of the secondary-side switch, wherein said resistor is connected to the voltage tapping point and a control input of said secondary-side switch.
3. A control circuit according to claim 2 , wherein the resistor has a resistance value which is adapted to a predefined discharge of the capacitance of the RC element.
4. A control circuit according to claim 1 , wherein the secondary-side switch is a field effect transistor which includes an integrated rectifier diode.
5. A control circuit according to claim 1 , wherein the RC element is implemented such that the time constant corresponds to a period for which current flows through the rectifier diode.
6. A switched mode power supply for generating a controlled output voltage, said switched mode power supply comprising:
a primary-side switch for sampling an input voltage,
a transformer having a primary winding, a secondary winding, and a secondary auxiliary winding adapted to produce an auxiliary voltage, and
a control circuit for minimizing the power loss, said control circuit comprising:
a secondary-side switch for short-circuiting a rectifier diode between said output terminal and said voltage tapping point, and
an RC element for providing positive and negative control pulses having a predefined time constant, said RC element being connected in parallel to the auxiliary winding.
7. A switched mode power supply according to claim 6 , comprising a resistor for defining a switching voltage of the secondary-side switch, wherein said resistor is connected to the voltage tapping point and a control input of said secondary-side switch.
8. A switched mode power supply according to claim 7 , wherein the resistor has a resistance value which is adapted to a predefined discharge of the capacitance of the RC element.
9. A switched mode power supply according to claim 6 , wherein the secondary-side switch is a field effect transistor which includes an integrated rectifier diode.
10. A switched mode power supply according to claim 6 , wherein the RC element is implemented such that the time constant corresponds to a period for which current flows through the rectifier diode.
11. A method for controlling the output voltage of a switched mode power supply comprising a primary-side switch, a primary winding, a secondary winding, a secondary auxiliary winding adapted to produce an auxiliary voltage, and
a control circuit for controlling the output voltage, said secondary winding and said auxiliary winding having provided between them a voltage tapping point, and said method comprising the following steps:
primary-side sampling of an input voltage by the primary-side switch,
generating the output voltage of the switched mode power supply,
generating positive and negative control pulses by an RC element, said control pulses corresponding to a predefined time constant,
outputting said positive and negative control pulses to a secondary-side switch which includes a parallel diode,
temporarily short-circuiting the parallel diode by the secondary-side switch dependent upon the positive and negative control pulses for minimizing the power loss of the switched mode power supply.
12. A method according to claim 11 , wherein the predefined time constant corresponds to the period for which current flows through the parallel diode.