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LED Lighting: Switching Power Supplies vs. Linear Power Supplies

Abstract: LED power supplies fall into two broad categories: switched mode power supply (SMPS) and linear power supply. Each circuit architecture comes with its own advantages and disadvantages.

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. 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. 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 a result, the luminaire will produce light flickering at 100 Hz or 120 Hz, twice the frequency of the incoming line 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.

What Is an LED Power Supply?

An 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. 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. 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. 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. 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.

Types of LED Power Supplies

The rectified voltage that the diode bridge outputs is an unregulated DC voltage which has a pulsed waveform. 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. This ripple that appears as a pulsed waveform must be filtered out to ensure a steady, non-flickering light output. 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). Electrical over stresses can cause the LEDs to fail faster than its expected lifetime. 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.

What Is a Switching Power Supply?

An LED switching power supply is an LED driver that uses a switching regulator as its DC-DC converter. A switching regulator operates by switching a pass element between its cut-off and saturation regions at a high frequency. 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. 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 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. A switching regulator has a negative feedback loop which monitors the output load variations and input voltage changes. Switching regulators can be constructed using a variety of converter topologies such as buck, boost, buck-boost, cuk, SEPIC and flyback to step down and/or up the supply voltages.

What Is a Linear Power Supply?

A linear LED power supply is an LED driver that convert unregulated DC voltage to a regulated DC voltage using a linear regulator. A linear regulator operates in the linear region of a pass element to regulate the output. The regulator typically comprises an internal reference voltage, an error amplifier (operational amplifier), a feedback voltage divider, a pass transistor and an output capacitor. The error amplifier monitors the reference/feedback difference and controls the power switch to keep the output at desired value. Linear regulators are step-down converters, which are similar to the switching type buck regulators. All linear regulators have a minimum dropout voltage, below which the regulators will cease to operate. A low dropout (LDO) linear circuit can regulate the output voltage even when there is a very small input-to-output differential voltage.

Switching vs. Linear

Efficiency

There are inherent efficiency limitations with the linear power supply design. A linear regulator must maintain a dropout voltage which is the power wasted by the regulator. The power efficiency of a linear power supply is equivalent to the ratio of output power to input power. In order to improve circuit efficiency, the dropout voltage of a linear regulator must be kept at minimum. In low power systems, the quiescent current also needs to be minimized. Switching power supplies can exhibit up to 95% efficiency thanks to their mode of operation, in which a transistor changes 'ON' and 'OFF' states at high frequencies. When the transistor in its 'ON' state, it stays in its high gain mode (saturation) and the resistance is at the minimum value. Minimal switching resistance allows the converter to operate with high efficiency.

Thermal management

The voltage difference between the input and output of a linear regulator is continually dissipated as heat. This amount of heat can create a high degree of thermal stress on the circuit and the LEDs on the same board because most linear power supplies are implemented as driver-on-board (DOB) architecture in modern LED lighting systems. When designing an LED lighting systems using linear driver technology, additional thermal capacity to accommodate the heat flux of driver circuits must be taken into account. On the other side, high efficiency switching power supplies generate minimal amount of heat.

Flicker

Both switching and linear power supplies can generate ripple that causes flicker. In general LED lights driven by switching power supplies have a much low flicker percentage than those driven by linear power supplies. A switching power supply typically uses an electrolytic capacitor across the diode bridge to smooth out the peaks of the input waveform and a storage capacitor at the output to filter small harmonics. Linear power supplies essentially produce an intermediary DC voltage which have a high ripple percentage. Oftentimes even the input filtering capacitor is not used because a large capacitance on the primary side may reduce the power factor (PF). When flicker control and PF compliance are in conflict, flicker control is more likely to be sacrificed because there're no regulatory requirements on this parameter. And because linear power supplies for general lighting applications were born to serve the entry level market, it's rare to invest in ripple filtering circuits in these bare-bones driver solutions.

Electromagnetic interference (EMI)

The high frequency switching operation of switching regulators can generate radio noise which affects other electrical circuits. In order to compensate for this adverse effect, a filter circuit is typically used to depress the electromagnetic interference and to ensure that the product meet electromagnetic compatibility (EMC) requirements. On the reverse side, the complete absence of EMI makes linear power supplies appeal to medical, aeronautical and automotive lighting applications where EMI requirements are very demanding.

Electrical safety

Linear power supplies provide no galvanic isolation from the input circuit (connected to the utility power grid). The high-voltage breakdown path through the control circuitry may result in electrical shock hazards if an LED light is not properly insulated. Switching power supplies can be galvanically isolated from the AC mains using a high frequency transformer with primary and secondary windings, which provides enhanced electrical safety.

Circuit reliability

The use of unreliable electrolytic capacitors makes switching power supplies less reliable than its linear cousin. Among all circuit components, electrolytic capacitors typically have the shortest lifetime expectancy because the degradation process of an electrolytic capacitor can be accelerated by high temperatures. The loss of liquid electrolyte inside capacitor will cause the rise of equivalent series resistance (ESR) and the fall of capacitance. Linear driver circuits are designed without electrolytic capacitors, which renders them competitive for high reliability applications.

Cost

Linear circuits overwhelmingly beat switching circuits in terms of cost because linear circuits can be designed using a very few components. Switching power supplies are more complex circuits that usually include bulky and expensive components, such as coils, capacitors, inductors (or transformers) and switching transistors.

Form factor

Linear regulators are available as integrated circuit packages which can be mounted on the same metal-core PCB (MCPCB) as the LEDs. This eliminates the need for a separate driver circuit board and thus allows design of compact, low profile LED luminaires.

Input voltage range

Linear power supplies currently are unable to accept a universal AC input voltage (such as 100 to 277 VAC) because the input voltage must be higher than the output voltage for the regulator to work. Switching power supplies can be designed for larger fluctuation in input voltage.

 

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