CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims priority to Korean Patent Application No. 10-2020-0064351, filed on May 28, 2020, the disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND

Technical Field

Exemplary embodiments relate to an alternating current (AC) direct light emitting diode (LED) driving apparatus, and more particularly to an AC direct LED driving apparatus including an LED driving capacitor.

Related Art

A light emitting diode (LED) driving apparatus using a converter has disadvantages that a system becomes complicated and it is difficult to reduce the size and weight of the system. Further, a power factor correction circuit is separately needed to improve a power factor, and an additional circuit is required to suppress electromagnetic waves generated at switching, thereby causing a problem of high production costs.

On the other hand, a linear-type LED driving apparatus, to which alternating current (AC) power is directly connected using no separate converters, has a simpler circuit than the converter type and exhibits great characteristics without additional correction circuits prepared for the power factor or the generation of the electromagnetic waves because commercial power, i.e., the AC power is directly used to control electric current. Further, there are advantages of longer life and higher reliability as compared with those of the converter type, in other words, a switching mode power supply (SMPS) type. Further, an AC-direct linear driving method has advantages of good power factor characteristics because current switching occurs in such a manner that a driving current increases as a higher voltage is input.

However, the AC-direct linear driving method, in particular, a sequential driving method exhibits a large current deviation between channels because the LEDs of the channels are sequentially driven according to the levels of the input voltage, in other words, the channels are driven in different driving periods of time and with different driving current levels.

To prevent such a current deviation, there has been proposed a synchronous multi-channel driving method by which the current deviation in time driving is minimized, but the deviation in a driving current level proportional to the input voltage still exists.

Further, the AC-direct driving method has a problem that a flicker occurs as the current is not supplied when the input voltage is lower than the minimum LED voltage drop.

Here, the flicker may refer to that periodic variation in light intensity is visually felt, and there are a percentage flicker and a flicker index as a method of quantifying a degree of flicker. In this case, the percentage flicker is a percentage into which a value obtained by dividing a difference between the highest light quantity and the lowest light quantity by a sum of them is converted, in which the higher the percentage flicker, the worse the flicker characteristics. Meanwhile, the flicker index is a value obtained by dividing light quantity exceeding average light quantity by the average light quantity, and ranges from ‘0’ to ‘1’, in which the lower the flicker index, the better the flicker characteristics.

To improve such a flicker characteristic, there is a need of using a capacitor or inductor which serves as storage of energy for driving an LED while power is not supplied. By connecting the capacitor to an input terminal or connecting the capacitor in parallel to the LED, current based on voltage stored in the capacitor is supplied to the LED while an input voltage is lower than an LED voltage. This method has been used a lot. Meanwhile, the linear driving method, in which AC voltage is directly used without the converter or the capacitor, always exhibits a percentage flicker of 100%, i.e., the worst value.

FIG. 1illustrates a conventional linear-type LED driving circuit, andFIG. 2shows voltage VINand current IINinput over time tin the driving current ofFIG. 1.

InFIG. 2, specific figures and units of time t, voltage VINand current IINare omitted because rough change thereof is important rather than the specific figures.

As shown inFIGS. 1 and 2, the current does not flow in the LED and therefore the percentage flicker has the highest value of 100% in a section where the input voltage VINis lower than a turn-on voltage of the LED.

FIG. 3illustrates a conventional LED driving circuit in which the capacitor is connected to a rectifying terminal, which is an LED driving circuit in which the capacitor is connected to the input terminal in order to decrease the flicker.FIG. 4is a graph showing input voltage and current characteristics in the circuit ofFIG. 3.

As shown therein, a rectifier is supplied with AC voltage from AC power VAC, rectifies the supplied voltage, and supplies the rectified voltage to an LED. Such a rectified voltage is charged in the capacitor and used as voltage for driving the LED.

FIG. 4shows rectified input power VIN, current ILEDflowing in one LED, and input current IIN.

When the input power VINis charged in the capacitor, a charged voltage lower than the input power is used as a reference voltage and the flow of the input current IINis controlled based on the reference voltage. As shown therein, in the conventional case, a high charged current IINis generated for a short charging time.

In the driving apparatus as shown inFIG. 3, when the LED voltage is set to be lower than the rectified input voltage, it is possible to lower the percentage flicker even up to “0%” but there is a problem that an inrush current is high because the capacitor is needed to be charged with high current for a short period of time. Further, due to a very low power factor not higher than ‘0.6’ and a high total harmonic distortion (THD) not lower than 40%, applicability is very limited.

In short, the method shown inFIG. 3, in which voltage rectified through the rectifier by using the capacitor at the input terminal is used for driving the LED has problems that an overcurrent breaker may operate due to the inrush current when many lights are driven in parallel and a high inrush current is continuously generated when a triode AC switch (TRIAC) is used because a high current is generated when power is supplied, even though the flicker characteristics are very improved. Further, the power factor and THD characteristics are bad and thus the use of the method is restricted.

FIG. 5illustrates another conventional LED driving circuit in which the capacitor is connected to a rectifying terminal, which is an LED driving circuit in which the capacitor is connected in parallel to the LED in order to decrease the flicker.FIG. 6is a graph showing an input voltage, an input current and an LED current in the circuit ofFIG. 5.

The driving circuit ofFIG. 5may be used to solve the problems of the conventional driving method shown inFIG. 3, i.e., the problems of a charging overcurrent and a low power factor. However, as shown inFIG. 6, the voltage stored in the capacitor is equal to the voltage of the LED, and thus there is a disadvantage that the LED current steeply decreases at discharging. Therefore, a capacitor of a higher value is required to exhibit the same flicker characteristic as that ofFIG. 3.

Meanwhile, regarding a recent LED light, a move to further strengthen flicker regulations has been actively made. The strengthening of the regulation is based on a research report that a sensitive human may feel dizzy or have a seizure when exposed to light having a high percentage flicker for a long time. In other words, the linear method directly using the AC power is not usable for a general light any more if it does not satisfy the flicker characteristics required in new regulations expected to be enacted in the future, even though the linear method has advantages of high reliability based on simpleness of a system, a lightweight, thin, compact and small system, low production costs, etc. on the contrary to the switching method. Further, the use of the conventional LED driving based on the capacitor as shown inFIG. 3is very limited due to the problems of an overcharged current and a low power factor.

Accordingly, a LED driving method is required to satisfy new flicker characteristics expected to be further strengthened in the future, have a high efficiency, and have lightweight, thin, compact and small features without using an inductor or transformer.

SUMMARY

Aspects of one or more exemplary embodiments are to provide an alternating current (AC) direct light emitting diode (LED) driving apparatus including an LED driving capacitor improved in flicker characteristics.

Further, aspects of one or more exemplary embodiments are to provide an AC direct LED driving apparatus including an LED driving capacitor, which satisfies high efficiency and high power-factor characteristics and decreases an overcharged current.

Further, aspects of one or more exemplary embodiments are to provide an AC-direct linear LED driving apparatus which satisfies a low total harmonic distortion (THD) characteristic while satisfying a uniform optical characteristic and a high optical efficiency.

Additional aspects will be set forth in part in the description which follows and, in part, will become apparent from the description, or may be learned by practice of the exemplary embodiments.

According to an aspect of an exemplary embodiment, there is provided an LED driving apparatus including: a rectifier configured to receive and rectify an alternating current (AC) voltage; an LED configured to emit light based on a rectified voltage received from the rectifier; a capacitor connected to a first terminal of the LED, and configured to drive the LED while alternating between charging and discharging sections according to a preset cycle; a first current driver connected to a second terminal of the LED and configured to control a path of current flowing in the LED and the capacitor based on different input voltage levels; a second current driver configured to control charging and discharging of the capacitor; and a first diode connected onto a current path of the capacitor and the second current driver, and configured to form a discharging path for driving the LED based on a charged voltage of the capacitor.

In the LED driving apparatus, the first current driver includes: a first metal oxide semiconductor field effect transistor (MOSFET) of which a drain terminal is connected to the second terminal of the LED; and a first resister connected between a source terminal of the first MOSFET and a ground terminal, and the second current driver includes: a second MOSFET of which a drain terminal is connected to the capacitor; and a second resister connected between a source terminal of the second MOSFET and the ground terminal.

In the LED driving apparatus, the first current driver includes: a first MOSFET of which a drain terminal is connected to the second terminal of the LED; and a first resister connected between a source terminal of the first MOSFET and a ground terminal, and the second current driver includes: a second resister connected between the capacitor and the ground terminal.

The LED driving apparatus further includes a second diode configured to prevent current from flowing from the second current driver to the capacitor.

In the LED driving apparatus, the first diode is connected in parallel to the second resister.

In the LED driving apparatus, the first current driver further includes a first cascode device connected in series between the LED and the first MOSFET, and the second current driver further includes a second cascode device connected in series between the capacitor and the second MOSFET.

In the LED driving apparatus, the first current driver further includes a first operational amplifier (OP AMP) of which an output terminal is connected to a gate terminal of the first MOSFET, and the second current driver further includes a second OP AMP of which an output terminal is connected to a gate terminal of the second MOSFET.

In the LED driving apparatus, the first current driver includes: a first MOSFET of which a drain terminal is connected to the second terminal of the LED; and a first resistor connected between a source terminal of the first MOSFET and a ground terminal, and the second current driver includes: a third diode of which an anode is connected to the rectifier and a cathode is connected to the first terminal of the LED; a second MOSFET of which a drain terminal is connected to the capacitor; a third MOSFET of which a drain terminal is connected to the anode of the third diode and which is source-coupled to the second MOSFET; and a second resister connected between a source terminal of the second MOSFET and the ground terminal.

In the LED driving apparatus, the second current driver further includes a third resister connected between the anode of the third diode and the drain terminal of the third MOSFET, and making current flowing in the third MOSFET be proportional to a level of an input voltage.

The LED driving apparatus further includes a second diode configured to prevent current from flowing from the second current driver to the capacitor.

In the LED driving apparatus, the first diode is connected in parallel to the second resister.

In the LED driving apparatus, the first current driver further includes a first cascode device connected in series between the LED and the first MOSFET, and the second current driver further includes: a second cascode device connected in series between the capacitor and the second MOSFET; and a third cascode device connected in series between the anode of the third diode and the third MOSFET.

In the LED driving apparatus, the first current driver further includes a first OP AMP of which an output terminal is connected to the gate terminal of the first MOSFET, and the second current driver further includes: a second OP AMP of which an output terminal is connected to the gate terminal of the second MOSFET; and a third OP AMP of which an output terminal is connected to the gate terminal of the third MOSFET.

In the LED driving apparatus, the first current driver includes: a first MOSFET of which a drain terminal is connected to the second terminal of the LED; and a first resister connected between the source terminal of the first MOSFET and a ground terminal, and the second current driver includes: a third diode of which an anode is connected to the rectifier and a cathode is connected to the first terminal of the LED; a second resister connected between the capacitor and the ground terminal; a third MOSFET of which a drain terminal is connected to the anode of the third diode; a third resister connected between the source terminal of the third MOSFET and the ground terminal; and a comparator configured to compare a predetermined voltage with voltage at opposite terminals of the second resister and control the third MOSFET to be turned on or off.

In the LED driving apparatus, the first current driver includes: a first MOSFET of which a drain terminal is connected to the second terminal of the LED; and a first resister connected between the source terminal of the first MOSFET and a ground terminal, and the second current driver includes: a third diode of which an anode is connected to the rectifier and a cathode is connected to the first terminal of the LED; a second resister connected between the capacitor and the ground terminal; a third MOSFET of which a drain terminal is connected to the anode of the third diode; a third resister connected between the source terminal of the third MOSFET and the ground terminal; and an inverter configured to detect voltage at opposite terminals of the second resister and control the third MOSFET to be turned on or off.

