Power Management

A power supply converts one voltage to another voltage that is useful to the target system. One of the most common types of power supplies is the type found in a desktop or laptop computer. This power supply converts AC power of 120VAC or 240VAC to a DC voltage that is used to power the computer. Often a transformer is used to step down the AC voltage. This also provides isolation from the AC mains, which improves the safety of the power supply. Rectification is done by a set of high voltage diodes, which converts the AC voltage into a series of wave pulses. A filter capacitor reduces ripple before the voltage is sent to a voltage regulator or DC/DC converter.

When constructing a power supply for a target application, almost every component selection is critical. The selection of the transformer and capacitors determine the output power.

The design of the power supply determines the final efficiency. Efficiency is calculated by taking the amount of power used by the end system and dividing it by the amount of power supplied by the voltage source. So if the power supply is pulling 30 Watts from the AC mains while the end system is using 25 Watts, then the efficiency is 25/30 = 83%. Higher efficiency can result in less heat, longer life for the power supply, and a more green system.

Rule of thumb for commercial applications is to design a power supply with 1½ to 2 times the power required by the end system, so if the system requires 12 Watts, then the power supply should be able to supply 18 to 24 Watts.

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A voltage regulator takes as an input an unregulated source of voltage and current and outputs a fixed, stable voltage at the required power. The voltage regulator is designed to maintain a stable output voltage regardless of any changes to the input voltage or load conditions. The specifications of a voltage regulator state how much the input voltage can vary, and the conditions under which it can maintain the output voltage and power under load.

Linear voltage regulators are three-terminal ICs that smooth the rectified AC signal into a nice, stable DC output. In linear regulated power supplies almost all the excess power is dissipated as waste heat. This heat must be taken into account in the design of the power supply and can require air vents in the power supply housings, and sometimes a fan. With very low ripple and high reliability, linear regulated power supplies are optimal for sensitive low power electronic equipment.

A linear voltage regulator is selected by matching the available input voltage and power to the required output voltage and power. Usually the required output voltage is fixed and has a specified maximum output current. The regulator must be able to handle a variable or unstable input voltage at a specific power rating.

Low Dropout Voltage Regulators (LDO) are a type of linear voltage regulator with a noticeably low quiescent current and very low voltage loss. LDOs require less headroom, or dropout voltage, to maintain a regulated output voltage. Dropout voltage is the minimum voltage potential needed across the regulator to keep working well. For digital circuits, low dropout regulators must be efficient and be able to handle rapidly changing demands for output currents. This is especially true in power-sensitive microcontroller-based systems where the microcontroller may turn peripherals on and off, change clock speed, and enter or leave different low power modes. An LDO must handle these rapidly changing current requirements with no lag.

While all voltage regulators can have some noise on the output voltage on the order of millivolts, LDOs have higher noise than standard linear voltage regulators. In general, the lower the dropout voltage, the higher the output noise.

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A switched-mode power supply (SMPS) is a type of DC/DC power supply that uses a switching regulator IC. The SMPS is a DC to DC converter, which means it takes in a DC voltage and then, very efficiently, converts that voltage to one or more DC output voltages. These DC output voltages can be a lower voltage (buck), or a higher voltage (boost).

The most efficient power supplies use a switching regulator. These regulators operate by rapidly switching the input voltage on and off, generating a square wave which usually drives some high Power MOSFETs. Depending on the topology of the power supply, the square wave is then smoothed out by a combination of resistors, capacitors, and a transformer. However, in the case of modern switching power supplies many of these components have been replaced by a single IC.

Because a switching regulator operates by rapidly switching a voltage signal on and off, the output voltage contains some noise in the form of ripple, or even sharp voltage spikes. This can interfere with nearby electronic equipment and so often requires some shielding.

The objective of a switching regulator is to waste as little power as possible, with an efficiency approaching the ideal but not attainable 100%.

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Power MOSFETs are voltage-controlled switches used in switching power supplies. While modern switching regulators contain on-chip MOSFETs to save board space, high power voltage supplies require external MOSFETs that can handle the output load.

Power MOSFETs are voltage-controlled switches often used in switching power supplies. They must also handle the voltage and current for the application, and be able to handle the heat generated. MOSFETs can switch higher then 200kHz which improves efficiency but generates noise. They can quickly respond to load variations and are available in varieties that can handle up to 500W of output power.

The on resistance of a Power MOSFET, specified as R_DS(ON) is one of the most critical parameters in a power supply circuit. It determines how much current the Power MOSFET can handle, and how much heat it will generate. The lower the R_DS(ON), the more current the MOSFET can handle and the less heat it will generate. Another important parameter is R_th thermal resistance. The lower the thermal resistance the better the Power MOSFET can dissipate heat. This lowers the temperature of the circuit which improves reliability and efficiency.

The drive circuit for a Power MOSFET is application-specific and determines its switching behavior and power dissipation. It’s important to note that the drive circuit cannot compensate for power losses due to R_DS(ON) and so the drive circuit is designed around that parameter. The drive circuit determines the switching speed of the Power MOSFET while minimizing EMI and voltage ringing.

Rectifier diodes are high current diodes used to change AC current into a pulsed DC current, which is then sent to the regulator. Rectifiers are arranged in a bridge configuration to achieve full-wave rectification.

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All power supplies need additional passive components such as capacitors, transformers, and resistors, and sometimes inductors.

When dealing with power supplies that are converting single-phase AC systems, the most common type of transformer is appropriately enough called a single-phase transformer. These have two coils, one primary and the one secondary. Three-phase transformers are used when converting a three-phase AC system and contain six coils, one pair for each phase. It’s crucial that the correct transformer be selected for a given application or the system can be damaged. For example, if a transformer is selected that is rated for more power than the circuit requires, the circuit will not draw more power than it needs but without the proper load on the transformer, the transformer may overheat and fail.

Capacitors are always found on the output of the voltage regulator for DC filtering, which is smoothing out the voltage ripple. These capacitors are placed on the PC board very close to the voltage regulator output, which is best for minimizing noise. These have a high capacitance and must be rated to withstand the power of the power supply. Usually electrolytic capacitors are found in power supplies as they can withstand voltages of up to 1500VDC. Capacitors are also used for power supply decoupling in other than the immediate output of the voltage regulator.

Some switched mode power supplies still require an inductor for storing electrical energy. Selecting the correct inductor for a buck regulator is important for insuring that the correct output voltage is generated with the lowest ripple. An inductor must be able to handle the maximum input and output voltages. Although all inductors have a small capacitance and resistance, at high switching frequencies these can become significant and are important in inductor selection.

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