Dynamic AC impedance stabilization in a flyback LED driver

Patent metadata

Field	Value
Publication number	US-10128740-B1
Application number	US-201715695589-A
Country	US
Kind code	B1
Filing date	Sep 5, 2017
Priority date	Sep 6, 2016
Publication date	Nov 13, 2018
Grant date	Nov 13, 2018

Abstract

An apparatus and a method reduce the ripple on a load current through an LED load. The load current is provided by a flyback converter, which is responsive to a feedback signal to maintain the output current at or near a selected magnitude. The LED load has an AC resistance which is at least one cause of the ripple on the load current. A stabilizing resistor in the current path to the LED load reduces the magnitude of the ripple caused by the AC resistance. The stabilizing resistor may have a constant stabilizing resistance value for different LED loads; or the stabilizing resistor have a dynamic stabilizing resistance value such that the stabilizing resistance value is greater when the LED load has fewer LEDs and is smaller when the LED load has more LEDs.

First claim

What is claimed is:

1. A drive circuit for providing a load current to a load having an AC resistance, the drive circuit comprising:
an isolation transformer, which includes a primary winding and a secondary winding;
a flyback converter controller configured to selectively apply a pulsing DC voltage to the primary winding of the isolation transformer to generate a current in the secondary winding of the isolation transformer;
a diode coupled to the secondary winding of the isolation transformer, the diode configured to provide a rectified secondary current;
a secondary filter capacitor coupled to receive the rectified secondary current, the secondary filter capacitor including a first capacitor terminal and a second capacitor terminal;
a first output terminal coupled to the first capacitor terminal, and a second output terminal coupled to the second capacitor terminal, the first and second output terminals connectable to a load to provide a current path for a load current between the secondary filter capacitor and the load;
a current sensing circuit coupled to the current path to sense a magnitude of the load current and to provide a feedback signal to the flyback converter, the flyback converter responsive to the feedback signal to control the pulsing DC voltage to the primary winding of the isolation transformer to thereby control the magnitude of the load current, the load current including an AC ripple superimposed on an average DC current, the AC ripple having a magnitude inversely responsive to the AC resistance of the load, wherein the current sensing circuit includes a current sensing resistor in the current path; and
a stabilizing resistance connected in the current path, the stabilizing resistance having a resistance value selected to be much greater than a minimum AC resistance of the load connected between the first and second output terminals, wherein the stabilizing resistance is a dynamic resistance connected in series with the sensing resistor in the current path, the dynamic resistance responsive to a voltage across the first and second output terminals, the dynamic resistance having a first resistance value when the voltage across the first and second output terminals is below a threshold voltage, the dynamic resistance having a second resistance value when the voltage across the first and second output terminals is at least as great as the threshold voltage, the first resistance value much greater than the second resistance value and wherein the dynamic resistance comprises:
a stabilizing resistor having the first resistance value, the stabilizing resistor connected in series with the sensing resistor in the current path;
a semiconductor switch including a controlled conduction path connected across the stabilizing resistor, the semiconductor switch including a control input terminal responsive to a control input voltage to selectively turn on the controlled conduction path and apply a low resistance across the stabilizing resistor; and
a control input circuit that provides a control voltage to the control input terminal of the semiconductor switch, the control input circuit responsive to a load voltage between the first output terminal and the second output terminal to generate a first control voltage to the control input terminal to turn on the semiconductor switch when the load voltage is above a threshold voltage and to generate a second control voltage to the control input terminal to turn off the semiconductor switch when the load voltage is below the threshold voltage.

2. The drive circuit for providing a load current to a load as defined in claim 1 , wherein the first resistance value of the stabilizing resistance is at least 10 times the minimum AC resistance of the load.

3. The drive circuit for providing a load current to a load as defined in claim 2 , wherein the first resistance value of the stabilizing resistance is at least 40 times the minimum AC resistance of the load.

4. The drive circuit for providing a load current to a load as defined in claim 1 , wherein:
the load comprises light-emitting diodes (LEDs) connected in series; and
the threshold voltage is greater than a total forward voltage across a first number of LEDs in series and is less than total forward voltage across a second number of LEDs in series, the second number of LEDs in series being one greater than the first number of LEDs in series.

5. A drive circuit for providing a load current to a load having an AC resistance, the drive circuit comprising:
a current source configured to generate an output current having a nominal DC magnitude between a first output terminal and a second output terminal, the output current varying with respect to the nominal DC magnitude by a time-varying ripple magnitude responsive at least in part to the AC resistance of a load connectable between the first output terminal and the second output terminal;
a current sensing circuit that senses a magnitude of the output current and that generates a feedback signal to the current source responsive to the magnitude of the output current, the current source responsive to the feedback signal to maintain the magnitude of the output current at or near a selected magnitude, wherein the current sensing circuit includes a current sensing resistor in the current path; and
a stabilizing resistance connected in a current path from the current source to at least one of the first and second output terminals, the stabilizing resistance having a resistance value much greater than a minimum AC resistance of a load couplable to the first and second output terminals, wherein the stabilizing resistance is a dynamic resistance connected in series with the sensing resistor in the current path, the dynamic resistance responsive to a voltage across the first and second output terminals, the dynamic resistance having a first resistance value when the voltage across the first and second output terminals is below a threshold voltage, the dynamic resistance having a second resistance value when the voltage across the first and second output terminals is at least as great as the threshold voltage, the first resistance value much greater than the second resistance value, wherein the dynamic resistance comprises:
a stabilizing resistor having the first resistance value, the stabilizing resistor connected in series with the sensing resistor in the current path;
a semiconductor switch including a controlled conduction path connected across the stabilizing resistor, the semiconductor switch including a control input terminal responsive to a control input voltage to selectively turn on the controlled conduction path and apply a low resistance across the stabilizing resistor; and
a control input circuit that provides a control voltage to the control input terminal of the semiconductor switch, the control input circuit responsive to a load voltage between the first output terminal and the second output terminal to generate a first control voltage to the control input terminal to turn on the semiconductor switch when the load voltage is above a threshold voltage and to generate a second control voltage to the control input terminal to turn off the semiconductor switch when the load voltage is below the threshold voltage.

6. The drive circuit for providing a load current to a load as defined in claim 5 , wherein the first resistance value of the stabilizing resistance is at least 10 times the minimum AC resistance of the load.

7. The drive circuit for providing a load current to a load as defined in claim 6 , wherein the first resistance value of the stabilizing resistance is at least 40 times the minimum AC resistance of the load.

8. The drive circuit for providing a load current to a load as defined in claim 5 ,

Assignees

• Universal Lighting Tech Inc
• Universal Lighting Tech

Inventors

• Xiong Wei

Classifications

• H05B45/40

  Details of LED load circuits · CPC title

• H02M1/15 Primary

  using active elements · CPC title

• H02M3/33523

  with galvanic isolation between input and output of both the power stage and the feedback loop · CPC title

• H02M2001/0009

  Electricity · mapped topic

• H05B33/0815

  Electricity · mapped topic