The duty cycle or switching frequency is regulated by a control circuit which outputs pulse-frequency-modulated (PFM) and/or pulse-width-modulated (PWM) control signals. ![]() The regulator also includes an inductance as an energy storage element which ramps up and stores energy when the power transistor is switched on and releases the energy into the LED load when the power transistor is switched off. The pass element, or the switching device, can be one or more bipolar junction transistors (BJT), metal oxide semiconductor field effect transistors (MOSFETs) or other types of transistors. A switching regulator operates by switching a pass element between its cut-off and saturation regions at a high frequency. What Is a Switching Power Supply?Īn LED switching power supply is an LED driver that uses a switching regulator as its DC-DC converter. Depending on the types of technique used in a DC-DC converter, LED power supplies fall into two broad categories: switched mode power supply (SMPS) and linear power supply. Electrical over stresses can cause the LEDs to fail faster than its expected lifetime. Aside from applying a sufficient forward voltage and supplying a constant current to the LED load, the DC-DC converter must ensure its output does not exceed the maximum rated voltage and current of an LED so as to prevent the LED from suffering an electrical over stress (EOS). This ripple that appears as a pulsed waveform must be filtered out to ensure a steady, non-flickering light output. ![]() The second power stage of AC-DC power conversion, which is a DC-DC stage, thus becomes a critical one since the residual of the AC input may exhibit large ripple on the output. The rectified voltage that the diode bridge outputs is an unregulated DC voltage which has a pulsed waveform. Since LED lighting involves both voltage and current regulation, the power source for an LED luminaire can be either called an LED driver or an LED power supply. In the past, an AC-DC converter that provides a regulated constant voltage is referred to as a power supply, while the AC-DC converter that provides a regulated constant current is called a driver. Unlike other AC-DC converters that merely step up or step down the rectified sinusoidal input voltage to provide a constant voltage, an LED power supply should be designed to supply a minimum amount of voltage to keep the p-n junction of LEDs forward biased for constant light output while regulating the peak magnitude of the current through the LEDs to ensure that the LEDs emit with appropriate intensity and color. As with usual AC-DC converters, an LED power supply comes with a full-wave rectifier which converts an alternating current into a direct one with current flowing in only one direction. What Is an LED Power Supply?Īn LED power supply operating on the line voltage is far more than a simple AC-DC converter that generates a DC supply from the AC mains input voltage. While LEDs are very efficient and have an impressively long lifetime, regulating the load to LEDs is enormously more challenging than conventional light sources. As a result, the luminaire will produce light flickering at 100 Hz or 120 Hz, twice the frequency of the incoming line voltage. ![]() If an LED luminaire is run on an alternating current (AC) source which periodically reverses its direction, light is only produced when the amplitude of the AC waveform is positive and power line voltage exceeds the sum of the forward voltages of the LEDs. As is well known, an LED is a semiconductor diode that emits light only when a forward bias is applied across its p-n junction. The LED driver or LED power supply is a critical component to an LED lighting system since it regulates power under supply voltage or load variations to LEDs that are sensitive to current and voltage fluctuations.
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