Driving circuits for architectural LEDs and indoor LEDs

　　LEDs are more efficient than incandescent bulbs and last 100 times longer, but they require specialized electronic drive circuits to avoid overload conditions. The main operating parameter is relatively simple: keep the current through the LED constant and below the specified maximum value.

　　Traditional power supplies have a precise voltage output, but the current varies. Connecting a resistor in series with an LED can control the current. This design assumes that the known voltage across the LED does not change with the temperature of the LED. Unfortunately, the forward voltage of an LED actually changes with temperature. LED manufacturers typically screen and sort their devices by forward voltage, allowing lighting manufacturers to manufacture products that meet this forward voltage at a fixed temperature. LED manufacturers can save time and get cheaper LEDs by using unscreened LEDs for circuits. LEDs also have a negative forward voltage-temperature coefficient, which can cause the driver circuit to enter thermal breakdown, requiring designers to take protective measures in circuit design.

　　The ideal solution for driving LEDs is a circuit that monitors the current and keeps it constant. The forward voltage of the LED does not affect this type of circuit, so no LED screening is required, and it is not affected by the negative forward voltage-temperature coefficient of the LED. These circuits can be complex switching regulators or simple linear regulators with a feedback loop. For applications with higher light output, such as street lights, complex switching regulators are ideal.

　　Simple hybrid circuits that are economical and durable can be used in architectural and indoor lighting fixtures. These circuit designs may be less efficient than complex switching regulators, but their low cost and simplicity make them attractive. These circuits can be used for all AC voltages from 85V to 265V at 50Hz or 60Hz.

　　The circuit of Figure 1 includes a bridge, a chopper, and a current regulator. The full-wave bridge includes diodes D1, D2, D3, and D4, and its output is fed into the chopper circuit. MOSFET tube Q2 is immediately turned on, and capacitor C1 begins to charge.

　　Resistors R1 and R2 form a voltage divider. When the voltage at the cathode of D5 reaches 43.5V, the Zener diode conducts, turning Q1 on, thereby pulling the gate of Q2 down, turning it off. Diode D6 is used to protect the gate of Q2.

　　The voltage on C1 is stabilized at 80V~90V. The charge on C1 supplies the CCR (constant current regulator) and the LED string. There are 22 LEDs in this circuit example. The CCR maintains the current through the LED string at 20mA. The circuit includes a resistor R4 in series with the LEDs to measure the current through the LED string.

　　Figure 2 shows the voltages at different parts of the cycle when the input voltage is 150V ac. Trace 1 is the output of the bridge rectifier circuit. Trace 2 is the voltage across C1, the output of the chopper circuit. Trace 3 is the voltage across the current sense resistor. These traces clearly show that when the output voltage of the bridge increases above 80V, the chopper circuit switches, limiting the voltage applied to the regulation circuit. Figure 3 shows the voltage when the input voltage is 85V ac.

　　The oscilloscope traces show that there is still enough design margin, Q1 can stay on for a longer period, during which C1 is fully charged. When the input voltage drops to 54V AC, the current through the LED begins to decrease.
Figure 4 shows the operation of the circuit when the input voltage is 265V AC. Trace 1 shows that due to its high input voltage, Q1 is on for a short time. Trace 2 shows that there is still enough energy to charge Q1 and maintain the current through the LED during the off cycle.

　　This circuit can be scaled to suit different LED arrays. CCRs are available with nominal currents up to 160mA. If higher currents are required, CCRs can be connected in parallel. The values ​​of C1, R1 and R2 will be tailored to the type and number of LEDs.