Apparatus and method for balancing output current of transistors

Claim 3

An externally-matched transistor pair for use in a circuit having a first drive input point and a second drive input point, requiring matched transistors and having a first drive means and a second drive means, each of said drive means being for providing a finite amount of driving power to a respective one of said transistors, comprising: a first transistor having a drive terminal coupled to said first drive means, an output terminal connected to said first drive input point of said circuit, a common terminal coupled to a reference point of said circuit, and having a first set of electrical characteristics; a second transistor having a drive terminal coupled to said second drive means, an output terminal connected to a second drive input point of said circuit, a common terminal coupled to said reference point of said circuit and having a second set of electrical characteristics, said second set differing from said first set in at least one of said electrical characteristics; switching means for selectably connecting a switched terminal to said drive terminal of said first transistor or said drive terminal of said second transistor; and power absorpotion means connected between said switched terminal and said reference point; whereby said switching means is positioned to cause said power absorption means to absorb a portion of said driving power from the one of said driving means which is coupled to the one of said transistors which is conducting more current than the other of said transistors.

Claim 4

An externally-matched transistor pair for use in a circuit having a first drive input point and a second drive input point, requiring matched transistors and having a first drive means and a second drive means, each of said drive means being for providing a finite amount of driving power to a respective one of said transistors, comprising: a first transistor having a drive terminal coupled to said first drive means, an output terminal connected to said first drive input point of said circuit, a common terminal coupled to a reference point of said circuit, and having a first set of electrical characteristics; a second transistor having a drive terminal coupled to said second drive means, an output terminal connected to a second drive input point of said circuit, a common terminal coupled to said reference point of said circuit and having a second set of electrical characteristics, said second set differing from said first set in at least one of said electrical characteristics; switching means for selectably connecting a switched terminal to said drive terminal of said first transistor or said drive terminal of said second transistor; and power absorpotion means connected between said switched terminal and said reference point; whereby said switching means is positioned to cause said power absorption means to absorb a portion of said driving power from the one of said driving means which is coupled to the one of said transistors which is conducting more current than the other of said transistors.

Claim 5

The externally-matched transistor pair of claim 2 wherein said power absorption means is selectably variable.

Claim 6

The externally-matched transistor pair of claim 2 wherein said power absorption means is a variable resistor.

Claim 7

The externally-matched transistor pair of claim 2 wherein said first transistor and said second transistor are bipolar transistors.

Claim 8

An improved power supply usable with unmatched transistors, comprising: a DC power source having a first terminal of a first polarity and a second terminal of a second polarity; a transformer having a center-tapped primary winding and at least one secondary winding for providing a desired output power, the center-tap of said primary winding being connected to said first terminal of said DC power source; a first transistor having an output termianl connected to a first end of said primary winding, an input terminal, and a reference terminal coupled to said second terminal of said DC power source; a second transistor having an output terminal connected to a second end of said primary winding, an input terminal, and a reference terminal coupled to said second terminal of said DC power source; driving means having a first drive output and a second drive output, said first drive ouptut being connected to said input terminal of said first transistor for providing drive power for said first transistor, said second drive output being connected to said input terminal of said second transistor for providing drive power for said second transistor, switching means for connecting a switched terminal to a selected one of said input terminal of said first transistor and said input terminal of said second transistor, said selected one of said input terminals corresponding to the one of said transistors which is conducting more current than the other of said transistors; and a resistor connected between said switched terminal and said second terminal; wherein said resistor absorbs a predetermined portion of said drive power available at said selected one of said input terminals.

Claim 9

An improved power supply usable with unmatched transistors, comprising: a DC power source having a first terminal of a first polarity and a second terminal of a second polarity; a transformer having a center-tapped primary winding and at least one secondary winding for providing a desired output power, the center-tap of said primary winding being connected to said first terminal of said DC power source; a first transistor having an output termianl connected to a first end of said primary winding, an input terminal, and a reference terminal coupled to said second terminal of said DC power source; a second transistor having an output terminal connected to a second end of said primary winding, an input terminal, and a reference terminal coupled to said second terminal of said DC power source; driving means having a first drive output and a second drive output, said first drive ouptut being connected to said input terminal of said first transistor for providing drive power for said first transistor, said second drive output being connected to said input terminal of said second transistor for providing drive power for said second transistor, switching means for connecting a switched terminal to a selected one of said input terminal of said first transistor and said input terminal of said second transistor, said selected one of said input terminals corresponding to the one of said transistors which is conducting more current than the other of said transistors; and a resistor connected between said switched terminal and said second terminal; wherein said resistor absorbs a predetermined portion of said drive power available at said selected one of said input terminals.

Claim 10

The improved power supply of claim 6 wherein said resistor comprises a variable resistor.

Claim 11

The improved power supply of claim 6 wherein said drive means comprises a pulsewidth modulating regulator.

Claim 12

The improved power supply of claim 8 wherein said resistor comprises a variable resistor.

Claim 13

The improved power supply of claim 6 and further comprising: current sensing means for providing a current feedback signal responsive to an output current conducted by said first transistor and said second transistor; wherein said driving means is responsive to said current feedback signal for varying at least a first parameter of said first drive power and at least a first parameter of said second drive power.

Claim 14

The improved power supply of claim 10 wherein said first parameter of said first drive power and said second drive power is the width of a pulse of said drive power.

Claim 15

The improved power supply of claim 8 and further comprising: current sensing means for providing a current feedback signal responsive to an output current conducted by said first transistor and said second transistor; wherein said driving means is responsive to said current feedback signal for varying at least a first parameter of said first drive power and at least a first parameter of said second drive power.

Claim 16

The improved power supply of claim 12 wherein said first parameter of said first drive power and said second drive power is the width of a pulse of said drive power.

Claim 17

For use in a circuit having a first drive input point and a second drive input point, requiring a transistor pair having predetermined matched characteristics over a predetermined temperature range, and having a first drive means and a second drive means, each of said drive means being capable of producing a finite amount of driving power, an externallymatched transistor pair comprising: a first transistor having a drive terminal coupled to said first drive means, an output terminal connected to said first drive input point of said circuit, a common terminal coupled to a reference point of said circuit, and having a first set of electrical characteristics; a second transistor having a drive terminal connected to said second drive means, an output terminal connected to a second drive input point of said circuit, a common terminal connected to said reference point of said circuit and having a second set of electrical characteristics, said second set differing from said first set in at least one of said electrical characteristics at at least one point within said predetermined temperature range; first power absorption means electrically connected between said drive terminal of said first transistor and said reference point, and thermally connected to said second transistor for absorbing a portion of said driving power from said first drive means, said first power absorption means absorbing less of said driving power from said first drive means as the temperature of said second transistor increases; and second power absorption means electrically connected between said drive terminal of said second transistor and said reference point, and thermally connected to said first transistor for absorbing a portion of said driving power from said second drive means, said second power absorption means absorbing less of said driving power from said second drive means as the temperature of said first transistor increases.

Claim 18

For use in a circuit having a first drive input point and a second drive input point, requiring a transistor pair having predetermined matched characteristics over a predetermined temperature range, and having a first drive means and a second drive means, each of said drive means being capable of producing a finite amount of driving power, an externallymatched transistor pair comprising: a first transistor having a drive terminal coupled to said first drive means, an output terminal connected to said first drive input point of said circuit, a common terminal coupled to a reference point of said circuit, and having a first set of electrical characteristics; a second transistor having a drive terminal connected to said second drive means, an output terminal connected to a second drive input point of said circuit, a common terminal connected to said reference point of said circuit and having a second set of electrical characteristics, said second set differing from said first set in at least one of said electrical characteristics at at least one point within said predetermined temperature range; first power absorption means electrically connected between said drive terminal of said first transistor and said reference point, and thermally connected to said second transistor for absorbing a portion of said driving power from said first drive means, said first power absorption means absorbing less of said driving power from said first drive means as the temperature of said second transistor increases; and second power absorption means electrically connected between said drive terminal of said second transistor and said reference point, and thermally connected to said first transistor for absorbing a portion of said driving power from said second drive means, said second power absorption means absorbing less of said driving power from said second drive means as the temperature of said first transistor increases.

Claim 19

The externally-matched transistor pair of claim 14 wherein said first and second power absorption means are positive temperature coefficient resistors.

Claim 20

An improved power supply usable with unmatched transistors, comprising: a DC power source having a first terminal of a first polarity and a second terminal of a second polarity; a transformer having a center-tapped primary winding and at least one secondary winding for providing a desired output power, the center-tap of said primary winding being connected to said first terminal of said DC power source; a first transistor having an output terminal connected to a first end of said primary winding, an input terminal, and a reference terminal coupled to said second terminal of said DC power source; a second transistor having an output terminal connected to a second end of said primary winding, an input terminal, and a reference terminal coupled to said second terminal of said DC power source; driving means having a first drive output and a second drive ouptut, said first drive output being coupled to said input terminal of said first transistor for providing drive power to said first transistor, said second drive output being coupled to said input terminal of said second transistor for providing drive power to said second transistor; a first temperature sensitive resistor being electrically connected between said drive terminal of said first transistor and said second terminal of said DC power source and being thermally connected to said second transistor for absorbing a portion of said drive power provided to said first transistor; and a second temperature sensitive resistor being electrically connected between said drive terminal of said second transistor and said second terminal of said DC power source and being thermally connected to said first transistor for absorbing a portion of said drive power provided to said second transistor.

Claim 21

An improved power supply usable with unmatched transistors, comprising: a DC power source having a first terminal of a first polarity and a second terminal of a second polarity; a transformer having a center-tapped primary winding and at least one secondary winding for providing a desired output power, the center-tap of said primary winding being connected to said first terminal of said DC power source; a first transistor having an output terminal connected to a first end of said primary winding, an input terminal, and a reference terminal coupled to said second terminal of said DC power source; a second transistor having an output terminal connected to a second end of said primary winding, an input terminal, and a reference terminal coupled to said second terminal of said DC power source; driving means having a first drive output and a second drive ouptut, said first drive output being coupled to said input terminal of said first transistor for providing drive power to said first transistor, said second drive output being coupled to said input terminal of said second transistor for providing drive power to said second transistor; a first temperature sensitive resistor being electrically connected between said drive terminal of said first transistor and said second terminal of said DC power source and being thermally connected to said second transistor for absorbing a portion of said drive power provided to said first transistor; and a second temperature sensitive resistor being electrically connected between said drive terminal of said second transistor and said second terminal of said DC power source and being thermally connected to said first transistor for absorbing a portion of said drive power provided to said second transistor.

Claim 22

The improved power supply of claim 16 and further comprising: current sensing means for providing a current feedback signal responsive to an output current conducted by said first transistor and said second transistor, wherein said driving means is responsive to said current feedback signal for varying at least a first parameter of said first drive power and at least a first parameter of said second drive power.

Claim 23

The improved power supply of claim 17 wherein said first parameter of said first drive power and said second drive power is the width of a pulse of said drive power.

Claim 24

The improved power supply of claim 16 and further comprising: switching means for connecting a switched terminal to a selected one of said input terminal of said first transistor and said input terminal of said second transistor; said selected one of said input terminals corresponding to the one of said transistors which is conducting more current than the other of said transistors; and a resistor connected between said switched terminal and said second terminal; wherein said resistor absorbs a predetermined portion of said drive power available at said selected one of said input terminals