LED drive circuit including a plurality of LEDs connected in series

Patent metadata

Field	Value
Publication number	US-10044289-B2
Application number	US-201615564669-A
Country	US
Kind code	B2
Filing date	Apr 8, 2016
Priority date	Apr 8, 2015
Publication date	Aug 7, 2018
Grant date	Aug 7, 2018

Abstract

The LED drive circuit has a rectifying circuit that rectifies an alternating-current voltage, an LED string including a plurality of LEDs connected in series, a first switch whose one end is connected to a cathode of an LED in a last stage of the LED string, a capacitor whose one end is connected to the other end of the first switch, a second switch that is connected between one end of the capacitor and the anode of the LED in the first stage of the LED string, and a third switch that is connected between the cathode of the LED in the last stage of the LED string and a second terminal of the rectifying circuit, and which turns on when the full-wave rectified voltage is lower than a charge start voltage and turns off when the full-wave rectified voltage is higher than or equal to the charge start voltage.

First claim

What is claimed is:

1. An LED drive circuit having:
a rectifying circuit that rectifies an alternating-current voltage and outputs a full-wave rectified voltage;
an LED string including a plurality of LEDs connected in series, in which an anode of an LED in a first stage of the LEDs is connected with a first terminal of the rectifying circuit;
a first switch whose one end is connected to a cathode of an LED in a last stage of the LED string;
a capacitor whose one end is connected to the other end of the first switch;
a second switch that is connected between the one end of the capacitor and the anode of the LED in the first stage of the LED string, and which turns on when the full-wave rectified voltage is lower than a threshold voltage of the LED string and turns off when the full-wave rectified voltage is higher than or equal the threshold voltage;
a third switch that is connected between the cathode of the LED in the last stage of the LED string and a second terminal of the rectifying circuit, and which turns on when the full-wave rectified voltage is lower than a charge start voltage and turns off when the full-wave rectified voltage is higher than or equal to the charge start voltage; and
a current limiting circuit whose current input terminal is connected to an other end of the capacitor and whose current output terminal is connected to the second terminal of the rectifying circuit,

wherein
the current limiting circuit has a first FET whose drain is connected to the other end of the capacitor and a first resistor whose one end is connected to a source of the first FET, and
the third switch includes a third FET whose drain is connected to the cathode of the LED in the last stage of the LED string and a third resistor whose one end is connected to a source of the third FET, and an other end of the first resistor is connected to one end of the third resistor and an other end of the third resistor is connected to the second terminal of the rectifying circuit.

2. The LED drive circuit according to claim 1 , wherein
the first switch includes a diode whose anode is connected to the cathode of the LED in the last stage of the LED string and whose cathode is connected to the one end of the capacitor, and
the second switch includes a diode whose anode is connected to the one end of the capacitor and whose cathode is connected to the anode of the LED in the first stage of the LED string.

3. The LED drive circuit according to claim 1 , further having:
a charge diode that is inserted between the other end of the first resistor and the one end of the third resistor, and whose anode is connected to the other end of the first resistor and whose cathode is connected to the one end of the third resistor; and
a discharge diode whose anode is connected to the other end of the third resistor and whose cathode is connected to the other end of the capacitor.

4. The LED drive circuit according to claim 1 , wherein
the LED string includes a first partial LED string and a second partial LED string whose anode of an LED in a first stage is connected to a cathode of an LED in a last stage of the first partial LED string, and
the LED drive circuit further has a fourth switch whose one end is connected to the cathode of the LED in the last stage of the first partial LED string and to the anode of the LED in the first stage of the second partial LED string and whose an other end is connected to the second terminal of the rectifying circuit.

5. The LED drive circuit according to claim 1 , wherein
the LED string includes:
a third partial LED string whose anode of an LED in a first stage is connected with the first terminal of the rectifying circuit; and
a fourth partial LED string whose anode of an LED in a first stage is connected with both the second switch and a cathode of an LED in a last stage of the third partial LED string, and

the LED drive circuit further has a second capacitor connected in parallel to the third partial LED string.

Assignees

• Citizen Watch Co Ltd
• Citizen Electronics

Inventors

• Akiyama Takashi

Classifications

• H05B45/44 Primary

  with an active control inside an LED matrix · CPC title

• H05B45/48

  having LEDs organised in strings and incorporating parallel shunting devices · CPC title

• H05B47/155

  Coordinated control of two or more light sources · CPC title

• H02M7/06 Primary

  using discharge tubes without control electrode or semiconductor devices without control electrode · CPC title

• H02M2001/0003

  Electricity · mapped topic