Low loss digitally controlled voltage converter

• US 5,892,672 A
• Filed: 12/29/1997
• Issued: 04/06/1999
• Est. Priority Date: 07/04/1995
• Status: Expired due to Term

• Current Assignee: Infineon Technologies AG
• Original Assignee: Siemens AG
• Inventors: Preller, Peter
• Primary Examiner(s): STERRETT, JEFFREY L
• Application Number:
  US08/983,608
• Time in Patent Office: 463 Days
• Field of Search: 363/15, 363/16, 363/20, 363/21, 363/95, 363/97
• US Class Current: 363/97
• CPC Class Codes: H02M 3/33515 with digital control

Abstract

In a voltage converter for producing a controlled output voltage from an input voltage, particularly in an isolating-transformer switch-mode power supply, a digitally realized control circuit is provided. This circuit contains a counter apparatus whose counter state is compared with a digital comparison value derived from the output voltage to be controlled. The pulse-width-modulated driving of a switching element coupled with an inductive element is carried out dependent on the result of the comparison and on a signal indicating the state of demagnetization of the inductive element. By means of a further comparison value for comparison with the counter apparatus, a turn-on blocking voltage is produced for the switching element, in order to raise the switching frequency for standby operation.

Claims

1. A voltage converter for producing a controlled output voltage from an input voltage, comprising:

an inductive element connected between an input circuit and an output circuit, the inductive element also being connected with a switching element for pulse-by-pulse application of the input voltage to the inductive element;

a control circuit for driving the switching element to control the output voltage;

said control circuit having a first comparator whose output is connected to control said switching element and which has a first input connected to an output signal of a first digital counter unit and a second input connected with a digital comparison value derived from the output voltage via a second digital counter unit so that dependent on a comparison result, the switching element is turned off;

a circuit for generating at an output connected to said first digital counter unit a demagnetization signal indicating a state of demagnetization of the inductive element;

a comparison counter for creating a further comparison value, and a multiplexer circuit for feeding both the further comparison value and said digital comparison value derived from the output voltage to said second input of said comparator so that during determination of a low load operation, turning on of the switching element is first blocked and then released, dependent on a comparison result of said further comparison value with the output signal of the first digital counter unit.

2. The voltage converter according to claim 1 wherein the inductive element comprises a transformer with a primary winding to which said switching element is connected in series, said transformer also having a secondary winding for connecting to a load via a rectifier unit, and said control circuit for driving said switching element being constructed such that the switching element is turned on when a characteristic signal portion of a signal derived from a further transformer winding of said transformer via said demagnetization signal circuit occurs, said signal portion indicating said state of demagnetization of said transformer, and said switching element being turned off when said output signal of said first digital counter unit reaches said digital comparison value provided by said second digital counter unit.

3. The voltage converter according to claim 1 wherein said first digital counter unit is counted by a predetermined increment in a period formed by a turn-on phase and a turn-off phase of said switching element, and wherein a second comparator has an output connected to an input of said second digital counter unit and wherein a first input of said comparator is connected to a reference value and a second input of said second comparator is connected to a control voltage derived from a further winding of a transformer which comprises said inductive element.

4. The voltage converter according to claim 3 wherein a size of said increment of said second digital counter unit is greater for a period following a change of counting direction and for at least one subsequent period.

5. The voltage converter according to claim 1 wherein said demagnetization signal indicates a zero crossing, a memory element connected to said output of said first comparator which controls said switching element is set in order to turn on said switching element, said first digital counter unit being started, said first digital counter unit being clock-controlled by a reference clock-producing unit, and said memory element being reset when a counter state of said first digital counter unit reaches the comparison value output by said second digital counter unit.

6. The voltage converter according to claim 1 wherein said further comparison value generated by said comparison counter is compared with said output signal of said first digital counter unit when the second digital counter unit digital comparison value derived from the output voltage falls below a comparison threshold value input to said comparison unit.

7. The voltage converter according to claim 1 wherein said comparison counter has a counter state which is changed during a period by an increment in a first counting direction when a counter state of said second digital counter unit falls below a threshold comparison value, and is changed by an increment in a second opposed counting direction when the counter state of said second digital counter unit exceeds said threshold comparisoin value.

8. A voltage converter for producing a controlled output voltage from an input voltage, comprising:

a transformer having a primary connected to said input voltage and a secondary connected to provide said output voltage, said transformer having a further winding;

a switching element connected in series with said primary winding;

a control unit connected to control said switching element;

an output of said further winding connecting through a detection circuit to a first input of said control unit to provide a demagnetization signal and also via a rectifying circuit to a second input of said control unit to provide a control voltage;

said control unit comprising first and second comparators, a comparison unit, a multiplexer, first, and second digital counters, and a comparison counter;

said first counter providing a clock signal to a first input of said first comparator;

an output of said multiplexer connecting to a second input of said first comparator, an output of said first comparator connecting to control said switching element;

a first input of the multiplexer connecting via said second counter to said control voltage and also to a reference value, and said first input of said multiplexer also connecting to a first input of said comparison unit; and

a second input of said comparator unit connecting to a comparison value and an output of said comparator unit connecting to a control circuit, the control circuit connecting to an input of said comparison counter, and an output of said counter unit connecting to a second input of said multiplexer.

9. The voltage converter according to claim 8 wherein an output of said first comparator connects through a memory element to said switching element.

10. The voltage converter according to claim 8 wherein the detection circuit comprises a zero crossing detection circuit connected at its input to an output of said further winding for providing a signal indicating state of demagnetization of the inductive element.