LEDs are a light source with much higher energy efficiency than incandescent lamps, but they require dedicated Electronic drive circuits to ensure that they are not subjected to excessive stress, so that they continue to provide the long lifespan claimed in the product specifications. By designing a new lighting scheme, the compact size of the LED enables it to be placed in a flexible light bar, which is easy to hide in a cabinet or staircase. If it works within the range of design parameters and does not suffer excessive stress, the service life of the LED can be 100 times longer than that of an incandescent bulb.
The main operating parameters of the LED are relatively simple: keep the LED current constant and lower than the maximum current it can withstand. If the driving circuit maintains operation in the specifications of the LED, their light output will be constant and the service life will exceed 50,000 hours.
Buildings and indoor lighting fixtures are designed to work in all regions of the world and need to comply with global regulations. The luminaire needs to operate within the complete universal voltage specification range of 85 VAC to 265 VAC at a frequency of 50 or 60 Hz. Traditional power supplies provide accurate voltage output and different current levels. Connect a resistor in series with the LED to limit the current. The prerequisite for this type of design is to clearly know the voltage across the LED (string), and this voltage will not change with the temperature of the LED. The disadvantage is that the LED forward voltage does change with temperature.
LED manufacturers usually encode their components based on the forward voltage, allowing the luminaire manufacturer to construct a luminaire design that can match the forward voltage of the LED at a fixed temperature. Circuits that do not require LED coding are attractive because it saves the time of LED manufacturers and the price of LEDs will be cheaper. The positive voltage of the LED also has a negative temperature coefficient (that is, the positive voltage drops when the temperature rises), which may make the circuit thermally out of control, so designers are required to construct a protective circuit in the design.
The best way to drive the LED is to monitor the current and keep the current constant. This type of circuit is not affected by the positive voltage of the LED, no LED coding is required, and the influence of the negative temperature coefficient of the positive voltage of the LED is eliminated. Such circuits can be complex switching regulators or linear regulators with feedback loops. Complex switching regulators are very suitable for high light output applications, such as energy-efficient street lighting.
Building and indoor lighting fixtures are very suitable for simple, economical and robust hybrid circuits. The energy efficiency of their design may not be as high as a complex switching regulator, but low cost and simplicity make it an attractive choice. These circuits are capable of operating at a frequency of 50 or 60 Hz under the complete universal voltage specification of 85 VAC to 265 VAC.
The circuit shown in Figure 1 consists of a rectifier bridge, a chopper and a simple current regulator. The full-wave bridge is composed of diodes D1, D2, D3, and D4 (1N4004) to inject signals into the chopper circuit. Switch Q2 (NDD03N50Z) is turned on immediately, and capacitor C1 (22 µ F) starts to charge.
Resistor R1 (330 kΩ) and R2 (390 kΩ) are voltage dividers. When the cathode voltage of diode D5 (MMSZ5260BT1G) reaches 43.5 V, the Zener diode conducts electricity and turns on Q1 (MPSA44). When Q1 is turned on, the gate of Q2 is pulled to a low level, causing it to turn off. The circuit also includes diode D6 (MMSZ15T1G) to protect the gate of Q2.
The voltage across the capacitor C1 is maintained between 80 V and 90 V. The charge stored in C1 powers the constant current regulator (CCR) (NSI45020AT1G) and the LED string (22 LEDs in this circuit example). CCR maintains the current of the LED string at 20 mA. The circuit includes a resistor R4 (10 Ω, 1.0%) in series with the LED to measure the LED string current (200 mV = 20 mA).
The trace in Figure 2 clearly shows that when the rectifier bridge voltage increases above 80 V, the chopper circuit switches and limits the voltage applied to the current regulator circuit. Trace 1 is the output waveform of the rectifier bridge circuit. Trace 2 is the voltage across capacitor C1 in the output part of the chopper circuit. Trace 3 is the voltage across the current sense resistor (10 & Omega;, 1.0% = 200 mV = 20 mA).
The oscilloscope trace of the voltage waveform under the input voltage of 85 VAC shows that there is still enough design margin (head room) to keep Q1 in the on state for a longer time and keep the capacitor C1 in a fully charged state. When the input voltage drops to 54 VAC, the LED current starts to drop.
Under the condition of high input voltage of 265 VAC, the on-time of Q1 is extremely short. However, trace 2 shows that there is still enough energy to charge Q1 and maintain the LED current during the off period.
This circuit can be adjusted to work under the conditions of different LED arrays. CCR provides rated current up to 160 mA. To support higher currents, CCRs can be connected in parallel. The selection of the values of C1, R1 and R2 needs to match the type and number of LEDs used.
Building and indoor lighting fixtures can use the advantages of LEDs to obtain the long service life claimed by LEDs by using simple, economical and robust hybrid circuits. The circuit design described in this article uses a simple chopper and CCR, and they are configured to work at a frequency of 50 or 60 Hz at a complete universal voltage specification from 85 VAC to 265 VAC.