Capacitor charger with a modulated current varying with an input voltage and method thereof

1. A capacitor charger connected with an input voltage, comprising:

a transformer having a primary winding connected with the input voltage and a secondary winding for transforming a primary current flowing through the primary winding to a secondary current flowing through the secondary winding;

a current control circuit for maintaining a magnitude of the primary current wherein the current control circuit comprises: a switch connected to the primary winding for switching the primary current; a voltage generator for providing a first voltage varying with the monitoring voltage; a current source for providing a reference current according to the first voltage; a current mirror for mirroring the reference current to generate a mirror current; a charge/discharge circuit for generating a second voltage according to the mirror current; and

a comparator for comparing the second voltage with a threshold to generate a comparison signal to adjust an on-time period of the switch; and

a current set circuit connected to the current control circuit for changing the magnitude of the primary current from a first value to a second lower value responsive to a change in a monitoring voltage corresponding to a decrease in the input voltage below a certain value.

2. The capacitor charger of claim 1 , further comprising a voltage generator for providing the monitoring voltage.

3. The capacitor charger of claim 2 , wherein the voltage generator includes a voltage divider for dividing the input voltage to determine the monitoring voltage.

4. The capacitor charger of claim 2 , wherein the voltage generator includes a voltage selection circuit for switching the monitoring voltage between a plurality of discrete setting values.

5. The capacitor charger of claim 1 , wherein the current control circuit further comprises control logics for generating a control signal according to the comparison signal to chive the switch.

6. The capacitor charger of claim 1 , wherein the voltage generator includes a voltage divider for dividing the input voltage to generate the first voltage.

7. The capacitor charger of claim 1 , wherein the current source comprises:

a transistor having a drain for conducting the reference current;

an operational amplifier having a non-inverting input connected with the first voltage, an inverting input connected to a source of the transistor, and an output connected to a gate of the transistor; and

a resistor connected to the inverting input for adjusting the reference current.

8. The capacitor charger of claim 7 , wherein the current set circuit includes a second comparator for comparing the monitoring voltage with a second threshold to generate a second comparison signal to determine a resistance of the resistor.

9. The capacitor charger of claim 1 , the current control circuit further comprising a maximum switch on-time setting circuit for setting a maximum on-time period for the switch, the maximum switch on-time setting circuit increasing the on-time period for the switch responsive to a decrease in the input voltage, the on-time period being at a maximum value in correspondence with the input voltage decreasing to the certain value.

10. A control method for a capacitor charger including a transformer having a primary winding connected with an input voltage and a secondary winding for transforming a primary current flowing through the primary winding to a secondary current flowing through the secondary winding, the control method comprising the steps of:

generating a monitoring voltage varying in correspondence with changes in a magnitude of the input voltage;

modulating the primary current by a control signal determined by a difference between a voltage corresponding to the input voltage and a reference voltage to maintain the primary current at a first magnitude during a reduction in the magnitude of the input voltage; and

changing the reference voltage responsive to the monitoring voltage reaching a threshold value corresponding to the input voltage decreasing to a certain value, the changed reference voltage subsequent to said monitoring voltage reaching said threshold value changes the primary current to a second magnitude, the second magnitude being lower than the first magnitude.

11. The control method of claim 10 , wherein the step of generating a monitoring voltage comprises dividing the input voltage for determining the monitoring voltage.

12. The control method of claim 10 , wherein the step of generating a monitoring voltage comprises comparing the input voltage with a second threshold.

13. The control method of claim 10 , wherein the step of modulating the primary current comprises the steps of:

generating a first voltage varying with the voltage corresponding to the input voltage;

generating a reference current according to the first voltage;

mirroring the reference current for generating a mirror current;

generating a second voltage according to the mirror current;

comparing the second voltage with a second threshold for generating a comparison signal to adjust an on-time switching period and

switching the primary current in accordance with the on-time switching period.

14. The control method of claim 13 , wherein the step of generating a first voltage includes the step of forming the voltage corresponding to the input voltage by dividing the input voltage with a voltage divider.

15. The control method of claim 13 , wherein the step of generating a reference current comprises applying the first voltage across a resistance for determining the reference current.

16. The control method of claim 15 , wherein the step of changing the reference voltage includes the step of switching a value of the resistance.

17. The control method of claim 13 , further comprising setting a maximum for the on-time switching period of the primary current, the on-time switching period being increased responsive to a decrease in the input voltage, the on-time switching period being at a maximum value in correspondence with the input voltage decreasing to the certain value.