Primary-controlled SMPS with adjustable switching frequency for output voltage and current control
View Patent ↗The present invention relates to a method for controlling the output voltage of a primary-controlled switched-mode power supply unit having a primary-side switch and a transformer with an auxiliary winding in which an auxiliary voltage which images the output voltage is induced after the primary-side switch is opened. The voltage induced in the auxiliary winding is fed to a control circuit as the control variable. The present invention also relates to a control circuit for performing such a control method and to an associated switched-mode power supply unit. To adjust the output voltage and the output current in the simplest and most economical way while minimizing the cost of the components needed, the switching frequency of the primary-side switch is so adjusted in dependence on the auxiliary voltage in the present invention that the output voltage and the output current of the switched-mode power supply unit adopt values in accordance with a predetermined output characteristic. This may be a linear output characteristic.
1. Method for controlling the output voltage of a primary-controlled switched-mode power supply unit having a primary-side switch and a transformer with an auxiliary winding in which an auxiliary voltage which images the output voltage is induced after the primary-side switch is opened, wherein the switching frequency of the primary-side switch is so adjusted in dependence on the auxiliary voltage that the output voltage and the output current of the switched-mode power supply unit take values which comply with a predetermined output characteristic, said output characteristic specifying a correlation between the output voltage and the output current,
wherein the charging time of a charging capacitor, which is chargeable by means of a current source having a charging current that depends on the auxiliary voltage, determines the switching frequency of the primary-side switch by determining the instant when the primary switch is turned on.
2. Method according to claim 1 , wherein the switching frequency of the primary-side switch is adjusted in linear dependence on the auxiliary voltage for at least some auxiliary voltage values, a constant factor of this linear dependence being so chosen that the output current adopts a predetermined constant value.
3. Method according to claim 1 , wherein the charging current is reduced when the voltage across a second capacitor reaches a first limit value.
4. Method according to claim 3 , wherein the primary-side switch is switched off when the voltage across the second capacitor reaches a second limit value.
5. Method according to claim 1 , wherein the primary-side switch is switched on when the voltage across the charging capacitor reaches a first predetermined threshold value.
6. Switched-mode power supply unit with a control circuit for controlling the output voltage of a primary-controlled switched-mode power supply unit, said switched-mode power supply unit having a primary-side switch and a transformer with an auxiliary winding in which an auxiliary voltage is induced after the opening of the primary-side switch, said control circuit being designed for adjusting the switching frequency of the primary-side switch in linear dependence on the auxiliary voltage, a constant factor of this linear dependence being chosen in a way that the output current of the switched-mode power supply unit adopts a predetermined value, said output characteristic specifying a correlation between the output voltage and the output current,
wherein the control circuit comprises a charging capacitor, which is chargeable by means of a current source having a charging current that depends on the auxiliary voltage, and wherein said charging capacitor is connected to the primary-side switch in a way that the voltage across the charging capacitor controls the switching frequency of the primary-side switch by determining the instant when the primary switch is turned on.
7. Switched-mode power supply unit according to claim 6 , further comprising a control comparator for comparing the voltage across the charging capacitor with a first threshold value, said control comparator being designed for generating a switch-on signal for switching on the primary-side switch when the voltage across the charging capacitor reaches the first threshold value.
8. Switched-mode power supply unit according to claim 7 , wherein the charging current source is designed in a way that the charging current is reduced when the voltage across the charging capacitor reaches the first threshold value.
9. Switched-mode power supply unit according to claim 6 , further comprising an overvoltage protection circuit for comparing the voltage across the charging capacitor with a second threshold value that is designed for generating a switch-off signal for switching off the primary-side switch when the voltage across the charging capacitor reaches the second threshold value.
10. Control circuit for controlling the output voltage of a primary-controlled switched-mode power supply unit, said switched-mode power supply unit having a primary-side switch and a transformer with an auxiliary winding in which an auxiliary voltage is induced after the opening of the primary-side switch, said control circuit being designed for adjusting the switching frequency of the primary-side switch in linear dependence on the auxiliary voltage, a constant factor of this linear dependence being chosen in a way that the output voltage and the output current of the switched-mode power supply unit output values according to a predetermined output characteristic, said output characteristic specifying a correlation between the output voltage and the output current,
wherein the control circuit comprises a charging capacitor, which is chargeable by means of a current source having a charging current that depends on the auxiliary voltage, and wherein said charging capacitor is connected to the primary-side switch in a way that the voltage across the charging capacitor controls the switching frequency of the primary-side switch by determining the instant when the primary switch is turned on.
11. Control circuit according to claim 10 , further comprising a control comparator for comparing the voltage across the charging capacitor with a first threshold value, said control comparator being designed for generating a switch-on signal for switching on the primary-side switch when the voltage across the charging capacitor reaches the first threshold value.
12. Control circuit according to claim 10 , wherein the charging current source is designed in a way that the charging current is reduced when the voltage across the charging capacitor reaches the first threshold value.
13. Control circuit according to claim 10 , further comprising an overvoltage protection circuit for comparing the voltage across the charging capacitor with a second threshold value that is designed for generating a switch-off signal for switching off the primary-side switch when the voltage across the charging capacitor reaches the second threshold value.