Capacitor charging circuit with a soft-start function
View Patent ↗When a primary winding current increases to an upper limit determined by a reference voltage having a soft-start characteristic, a power switch is turned OFF. When a secondary winding current decreases to a lower limit determined by another predetermined reference voltage, the power switch is turned ON. A minimum ON-time limiting unit prevents the power switch from being turned OFF before a minimum ON-time expires. The minimum ON-time may be provided with a soft-start modulation. A minimum OFF-time limiting unit prevents the power switch from being turned ON before a minimum OFF-time expires. A maximum ON-time limiting unit prevents the power switch from still remaining ON after a maximum ON-time expires.
1. A capacitor charging circuit for controlling a transformer such that a voltage source coupled to a primary winding of the transformer charges a capacitive load coupled to a secondary winding of the transformer, comprising:
a power switch coupled to the primary winding such that a primary winding current is allowed to flow during an ON-time of the power switch but is terminated during an OFF-time of the power switch;
a switch controller for controlling the ON-time and the OFF-time;
a soft-start circuit for modulating the ON-time to gradually increase during an initial period of a charging process;
a first current detector for detecting the primary winding current to generate a primary current detection signal;
a reference voltage generator controlled by the soft-start circuit to generate a soft-start reference voltage; and
a first voltage comparator for comparing the primary current detection signal with the soft-start reference voltage so as to output an ON-time ending signal to the switch controller.
2. The capacitor charging circuit according to claim 1 , further comprising:
a second current detector for detecting a secondary winding current to generate a secondary current detection signal; and
a second voltage comparator for comparing the secondary current detection signal with a predetermined reference voltage so as to output an OFF-time ending signal to the switch controller.
3. The capacitor charging circuit according to claim 1 , further comprising:
a minimum ON-time limiting unit for preventing the power switch from being turned off before a minimum ON-time expires.
4. The capacitor charging circuit according to claim 3 , wherein:
the minimum ON-time limiting unit outputs a minimum ON-time limiting signal to the switch controller for determining the minimum ON-time.
5. The capacitor charging circuit according to claim 3 , wherein:
the minimum ON-time limiting unit is controlled by the soft-start circuit for modulating the minimum ON-time to gradually increase during the initial period of the charging process.
6. The capacitor charging circuit according to claim 1 , wherein:
a minimum OFF-time limiting unit for preventing the power switch from being turned on before a minimum OFF-time expires.
7. The capacitor charging circuit according to claim 6 , wherein:
the minimum OFF-time limiting unit outputs a minimum OFF-time limiting signal to the switch controller for determining the minimum OFF-time.
8. The capacitor charging circuit according to claim 1 , further comprising:
a maximum ON-time limiting unit for preventing the power switch from still remaining ON after a maximum ON-time expires.
9. The capacitor charging circuit according to claim 8 , wherein:
the maximum ON-time limiting unit outputs a maximum ON-time limiting signal to the switch controller for determining the maximum ON-time.
10. A capacitor charging circuit for controlling a transformer such that a voltage source coupled to a primary winding of the transformer charges a capacitive load coupled to a secondary winding of the transformer, comprising:
a power switch coupled to the primary winding such that a primary winding current is allowed to flow during an ON-time of the power switch but is terminated during an OFF-time of the power switch;
a switch controller for controlling the ON-time and the OFF-time;
a minimum ON-time limiting unit for preventing the power switch from being turned off before a minimum ON-time expires;
a first current detector for detecting the primary winding current to generate a primary current detection signal;
a reference voltage generator for generating a soft-start reference voltage; and
a first voltage comparator for comparing the primary current detection signal with the soft-start reference voltage so as to output an ON-time ending signal to the switch controller.
11. The capacitor charging circuit according to claim 10 , wherein:
the minimum ON-time limiting unit outputs a minimum ON-time limiting signal to the first voltage comparator for determining the minimum ON-time.
12. The capacitor charging circuit according to claim 10 , further comprising:
a second current detector for detecting a secondary winding current to generate a secondary current detection signal; and
a second voltage comparator for comparing the secondary current detection signal with a predetermined reference voltage so as to output an OFF-time ending signal to the switch controller.
13. A capacitor charging circuit for controlling a transformer such that a voltage source coupled to a primary winding of the transformer charges a capacitive load coupled to a secondary winding of the transformer, comprising:
a power switch coupled to the primary winding such that a primary winding current is allowed to flow during an ON-time of the power switch but is terminated during an OFF-time of the power switch;
a switch controller for controlling the ON-time and the OFF-time;
a minimum OFF-time limiting unit for preventing the power switch from being turned on before a minimum OFF-time expires;
a first current detector for detecting the primary winding current to generate a primary current detection signal;
a reference voltage generator for generating a soft-start reference voltage; and
a first voltage comparator for comparing the primary current detection signal with the soft-start reference voltage so as to output an ON-time ending signal to the switch controller.
14. The capacitor charging circuit according to claim 13 , wherein:
a second current detector for detecting a secondary winding current to generate a secondary current detection signal; and
a second voltage comparator for comparing the secondary current detection signal with a predetermined reference voltage so as to output an OFF-time ending signal to the switch controller.
15. The capacitor charging circuit according to claim 14 , wherein:
the minimum OFF-time limiting unit outputs a minimum OFF-time limiting signal to the second voltage comparator for determining the minimum OFF-time.
16. A capacitor charging circuit for controlling a transformer such that a voltage source coupled to a primary winding of the transformer charges a capacitive load coupled to a secondary winding of the transformer, comprising:
a power switch coupled to the primary winding such that a primary winding current is allowed to flow during an ON-time of the power switch but is terminated during an OFF-time of the power switch;
a switch controller for controlling the ON-time and the OFF-time;
a maximum ON-time limiting unit for preventing the power switch from still remaining ON after a maximum ON-time expires;
a first current detector for detecting the primary winding current to generate a primary current detection signal;
a reference voltage generator for generating a soft-start reference voltage; and
a first voltage comparator for comparing the primary current detection signal with the soft-start reference voltage so as to output an ON-time ending signal to the switch controller.
17. The capacitor charging circuit according to claim 16 , further comprising:
a second current detector for detecting a secondary winding current to generate a secondary current detection signal; and
a second voltage comparator for comparing the secondary current detection signal with a predetermined reference voltage so as to output an OFF-time ending signal to the switch controller.