System and method for charging a capacitor using a variable frequency, variable duty cycle current waveform

• US 6,411,064 B1
• Filed: 07/20/2000
• Issued: 06/25/2002
• Est. Priority Date: 07/20/2000
• Status: Expired due to Fees

• Current Assignee: Koninklijke Philips Electronics N.V. (Koninklijke Philips N.V.)
• Original Assignee: Koninklijke Philips Electronics N.V. (Koninklijke Philips N.V.)
• Inventors: Brink, Gregory D.
• Primary Examiner(s): TOATLEY, GREGORY J
• Application Number:
  US09/619,954
• Time in Patent Office: 705 Days
• Field of Search: 320/166, 320/167
• US Class Current: 320/166
• CPC Class Codes: A61N 1/3975 Power supply A61N1/378 takes precedence
• CPC Class Codes: A61N 1/3981 High voltage charging circuitry
• CPC Class Codes: H02M 3/33507 with automatic control of the output voltage or current, e.g. flyback converters H02M3/33561, H02M3/33569 take precedence

Abstract

A system and method for charging a high-voltage capacitor through the application of a current, the magnitude of which has a variable frequency, variable duty cycle waveform. Generally, energy is transferred from a power source to the high voltage capacitor via a magnetic element such as an inductor or transformer. For example, a pulsed voltage supply provides voltage pulses having a variable frequency and an adjustable duty cycle to a primary winding of a fly-back transformer. During a charging sequence in which current charge cycles are applied to the capacitor, the duty cycle of the variable frequency current waveform is controlled dynamically based on the rate at which energy can be transferred to the capacitor. Specifically, during a charge sequence, current pulses through the primary winding are controlled such that the transformer operates in a continuous mode during an initial portion of the charge sequence, and in a discontinuous mode during a subsequent portion of the charge sequence. During the continuous mode, the duration of the off or non-conduction time of the primary winding current waveform is limited to be less than or equal to a maximum time period. Within this maximum time period energy can be transferred efficiently from the transformer to the capacitor. Subsequent, inefficient conduction time is inhibited. This increases the speed at which the high voltage capacitor is charged.

Claims

1. A capacitor charging system, connected electrically to a capacitor, constructed and arranged to charge the capacitor by generating a variable frequency, variable duty cycle current waveform.

2. The system of claim 1, wherein the system generating a charge sequence of successive current charge cycles, and wherein the system comprises:

a transformer having a primary winding connected in series to a voltage source and a secondary winding across which the capacitor is electrically coupled; and

a controller for controlling a primary current through said primary winding such that during each of a first plurality of charge cycles of the charge sequence said transformer does not transfer all stored energy to the capacitor, and during each of a subsequent plurality of charge cycles of the charge sequence said transformer transfers substantially all stored energy to the capacitor.

3. The system of claim 2, wherein said transformer is a fly-back transformer, wherein said controller comprises:

a first current detector configured to determine when said primary current achieves a maximum current; and

a second current detector configured to determine when current flowing through said secondary winding is approximately zero

wherein said controller controls said primary current based at least in part on whether said primary current has achieved said maximum value and whether said secondary current is approximately zero.

4. A high-voltage capacitor charging system constructed and arranged to generate current pulses having a variable frequency to charge a capacitor, wherein during a charging sequence in which said current pulses are repeatedly applied to the capacitor, the duty cycle of the variable frequency current waveform is controlled dynamically based on the rate at which energy can be transferred to the high voltage capacitor.

5. A system for charging a high-voltage capacitor comprising:

a fly-back transformer having a primary winding, a core and a secondary winding that is out of phase with said primary winding, with the capacitor electrically coupled across said secondary winding; and

a charge controller for applying a charge sequence of successive current charge cycles to said primary winding, each charge cycle having an on time and an off time, wherein for each said charge cycle on time said controller enables said primary current to flow through said primary winding until said primary current reaches a maximum current, and for each said charge cycle off time said controller inhibits said primary current until either current through said secondary winding is approximately zero or a predetermined maximum charge cycle off time has transpired.

6. The system of claim 5,

wherein, for a first plurality of charge cycles said maximum off time transpires prior to said secondary winding current becoming approximately zero, resulting in said transformer storing energy in said core during each of said first plurality of charge cycles, and

wherein, for a second plurality of charge cycles occurring subsequent to said first plurality of charge cycles, said secondary winding current becomes zero prior to said maximum off time transpiring, resulting in said transformer transferring substantially all stored energy during of said second plurality of charge cycles.

7. The system of claim 6, wherein said charge controller comprises:

a power supply for generating a voltage across said primary winding;

a first current detector configured to determine when said primary current achieves a maximum current;

a secondary current detector configured to determine when current flowing through said secondary winding falls below a minimum current; and

control logic that controls said primary current based on whether said primary current has achieved said maximum value and a relative timing between when said secondary current reaches approximately zero and said maximum time duration.

8. The system of claim 7, wherein said power supply comprises:

a power source for providing a voltage and a current;

a switching element connected in series with the power source and said primary winding, wherein said switching element controls said current flowing in said primary winding.

9. The system of claim 8, wherein said switching element comprises:

a switching transistor.

10. The system of claim 9, wherein said switching transistor is a MOSFET.

11. The system of claim 8, wherein said control logic comprises a counter having a programmable clock input wherein said counter generates a signal having a period set to said maximum time duration.

12. A method for charging a capacitor comprising the steps of:

providing a fly-back transformer having a primary winding, a core and a secondary winding that is out of phase with the primary winding;

applying a charge sequence of successive current charge cycles to the primary winding, such that for each charge cycle on time the primary current is applied until the primary current reaches a maximum current, and for each charge cycle off time the primary current is inhibited until either current through the secondary winding is approximately zero or a predetermined maximum charge cycle off time has transpired.

13. The method of claim 12, wherein said maximum charge cycle off time is of a duration such that for a first plurality of charge cycles the maximum off time transpires prior to the secondary winding current becoming approximately zero, resulting in the transformer storing energy during each of the first plurality of charge cycles, and for a second plurality of charge cycles occurring subsequent to the first plurality of charge cycles, the secondary winding current becomes zero prior to the maximum off time transpiring, resulting in the transformer transferring substantially all stored energy during the second plurality of charge cycles.