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Home » Applications & Technologies » Power Supply Technology - AC/DC Power Supply Electronics
Applications & Technologies

AC/DC Power Supply

Compared with linear supplies, switched-mode power supplies(SMPSs) not only provide a substantial boost to energy efficiency but are also smaller and lighter – advantages that have seen SMPSs completely replace their linear counterparts in many applications.

EMI
Filter
PTC Fuse
TVS
Diode
Rectifier
PFC
Driver
PWM
Controller
Optocoupler
Comparator
85-270V AC
+ VOUT
COM
- VOUT

This design is for reference only. The design, as well as the products suggested, has not been tested for compatibility or interoperability.

EMI Filters for AC/DC Power Supplies

EMI stands for “Electromagnetic Interference” and is also known as electrical noise. Thus, an EMI filter is used for noise suppression and there is at least one EMI filter in all modern electronics. EMI itself can be created within the wiring of a circuit or it can be radiated through the air (referred to as “RFI.”) EMI shielding and filtering is required by government standards. EMI noise interferes with a circuit’s ability to operate properly. This is especially true with portable devices, since they are designed to save power, which typically means lower signal voltages. With lower voltage, there is less head room for noise to ride without becoming an unintended part of the signal. Devices with poor EMI prevention can interfere with air plane flight frequencies, causing static in the same radio band that is used to by traffic control towers to direct pilots landing and taking off. This is why all electronic devices are turned off before take-off and landing.

One of the biggest sources of EMI is in the generation of parasitic noise, which is created by the circuit itself. Unintended capacitance facilitates the transfer of high frequency, random noise into a circuit or other parts of a system. All AC/DC switch mode power supplies have EMI filters that allow them to comply with standards. Design engineers from all industries report that one of the more difficult tasks is achieving EMI safety compliance and that EMI is a significant factor in new product development, even at the system level. At the system level, it can be very difficult to find the source of the EMI noise and eradicate or prevent it.

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PTC Fuses for AC/DC Power Supplies

A Positive Thermal Coefficient fuse, or PTC thermistor, is a thermally sensitive semiconductor resistor that increases in resistance as temperature increases to limit current to a load. When the voltage input for a power supply is too far above or below what is specified, the excess current can damage components. Besides shutting down the power supply altogether, one can effectively limit the current by using the predictable resistive response of a PTC.

PTCs are usually made of ceramic or polymer and wired in series with the load to be protected. As current flows through the PTC, it heats up. Power is never completely consumed; a portion of energy is always wasted as heat. The better the efficiency of a device, the less heat it produces. PTCs and their cousin the NTC (Negative Temperature Coefficient) can more effectively respond to high in-rush currents as opposed to regular resistors. The criteria for selection of these thermistors is driven by the maximum current expected during normal, continuous operation of the load; the load’s capacitance, and how much of a reduction of in-rush current is needed. It is important to note that after it is first energized, a PTC or NTC will not recover for 1 or 2 minutes. Therefore, the thermistor may need to be bypassed if a shorter period of start/stop operation is expected. Note that in-rush current is the result of the higher energy required to energize a cold or fully de-energized load. Once energized, it takes some time for load components to lose stored energy, an indicator of total capacitive load.

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TVS Diodes for AC/DC Power Supplies

A transient voltage suppression (TVS) diode is used to protect electronic components from electrical overstress such as that caused by electrostatic discharge and inductive load switching, which can cause sudden, short-lived high-amplitude voltage transients in either a positive or negative direction. Suppressing transients can be achieved by the reduction of transients to keep them from propagating into the protected circuit and also by conducting large amounts of current to ground, away from the protected circuit without allowing damage. TVS diodes can attenuate transient amplitudes in both positive and negative directions as special diodes placed back-to-back. A TVS should be placed as near as possible to the signal input, ahead of any other component. A TVS diode works to limit voltage spikes by taking advantage of the natural silicon-avalanche action of its very rugged silicon p-n junction, and is well-suited for ESD protection. A TVS diode p-n junction has a larger cross-sectional area than either a normal diode or a Zener diode. The surge power and current that the TVS can withstand are proportional to the TVS junction area.

Although TVS diodes are very similar to Zener diodes, the TVS is designed and tested for transient voltage protection, whereas Zener diodes are designed and tested more for voltage regulation. Zeners are limited by their ability to dissipate energy; a large surge can generate much heat, creating an unacceptable peak temperature in the p-n junction. TVS diodes have a more robust p-n junction and cover a broad range of operating voltages (about 3V to 500V.) TVS diodes and TVS diode arrays are used extensively because of their wide voltage range and their cost-effectiveness.

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Rectifiers for AC/DC Power Supplies

A rectifier is an electrical circuit that can convert AC current to an imperfect DC current. In electronics, this is typically accomplished with the use of switching devices such as diodes or MOSFETs that allow only one portion of the AC cycle to pass through the rectifier circuit. AC current alternates in a sinusoidal waveform, swinging between the positive and negative poles, and the rectifier allows only half of that wave through, resulting in a waveform that is a series of humps instead of a snake-like sine wave. The rectified waveform is then further refined to achieve the steady-state, flat-line output that we commonly perceive of as DC (direct current.) Since nearly all electronic circuits operate with DC signals, rectifiers are widely employed throughout many industries.

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PFC ICs for AC/DC Power Supplies

Power factor is a unitless quantity that measures the ratio of AC power dissipated by the load (true power) to the total amount of AC power sent to the load (apparent power). A purely resistive, i.e. non-reactive, load dissipates 100% of the apparent power, and the circuit therefore has a unity power factor. In this case, the voltage and current are completely in phase. However, AC voltage and current delivered to reactive loads cause a phase shift between the two, effectively reducing power efficiency. This is undesirable, so circuits are designed to add power factor correction (PFC) to power supplies.

The power factor is “corrected” by a PFC control circuit which forces the current waveform to follow the voltage waveform as accurately as possible, driving it closer to a power factor of 1 and increasing efficiency. PFC is important in many applications, and switched-mode power supplies (SMPS) may have a power factor of 0.6 or lower without it.

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Drivers for AC/DC Power Supplies

Designers of power electronic circuits must often drive power switches that feed DC, AC, or power signals to a variety of workloads. Logic-level electronic circuits provide the driving signals. In general, however, the power sources and their loads have reference levels different from that of the control circuitry (ground). MOSFET selection begins by choosing devices that can handle the required current, then giving careful consideration to thermal dissipation in high current applications.

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PWM Controllers for AC/DC Power Supplies

Pulse Width Modulation (PWM) is widely used in switch mode power supplies that use digital control to provide the switching action. PWM itself is a controlled digital output signal. The PWM controller controls the rapid switching in a power supply by sending a pulse to the gate driver that drives a power MOSFET (or other switching device like a bipolar transistor, IGBT, etc.) One advantage of PWM is that the signal is digital. Digital signals are more immune to noise, because a digital signal is either a binary “1” or “0.” Therefore noise can only change a digital signal if it is big enough to change a logical “0” to register at the receiving end as a logical “1”, or vice versa.

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Optocouplers for AC/DC Power Supplies

An optocoupler is a device used to provide electrical isolation. Isolation is critical for protecting both an electronics system and the user from potentially hazardous voltages. An optocoupler consists of at least two functional parts: an LED to translate the electrical input signal into light waves, and a photodetector, such as a phototransistor or photodiode, to convert the optical signal back to an electrical output. There is no conductive path through the device, so the output is considered opto-isolated from the input.

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Comparators for AC/DC Power Supplies

The comparator is so named because it is a device used to compare two voltages. An open-loop operational amplifier (op-amp) is a simple example of a comparator; the output assumes one of two values corresponding with the greater input. Comparators are used in a wide variety of applications and are made according to a large range of specifications, such as maximum switching speed, power consumption, and supply voltage range. Hysteresis is also an important consideration; adding hysteresis to the device can prevent small, noise-related changes in input voltage from causing a series of rapid changes in output. For switched-mode power supply (SMPS) applications, a comparator is often used in the output stage as part of the feedback loop leading to a PWM controller or MCU.

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Diodes for AC/DC Power Supplies

The first silicon-based electronic component, diodes are passive devices which are found in virtually every electronic product or device. The ideal diode allows current to flow freely in one direction and completely prevents current from flowing in the opposite direction. Although, at their core, semiconductor diodes consist of a single P-N junction, there is vast array of different diode types and designs. The zener diode, for example, is designed to also conduct current in the opposite direction when reverse-biased at or above a specific voltage threshold known as the "breakdown voltage." AC/DC power supplies often employ diodes in a bridge-type configuration to rectify the AC input.

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Power MOSFETs for AC/DC Power Supplies

Metal-oxide-semiconductor field-effect transistors (MOSFETs) are by far the most common of transistors today, being used for flash memory, processors, random-access memory (RAM), and application-specific integrated circuits (ASICs), and more. MOSFETs can be conceptualized as a voltage-controlled device for limiting current flow.

MOSFETs are also for power switching circuits. Unlike bipolar junction transistors (BJTs), the competing type of power transistor, MOSFETs do not require a continuous flow of drive current to remain in the ON state. Additionally, MOSFETs can offer higher switching speeds, lower switching power losses, lower on-resistances, and reduced susceptibility to thermal runaway. In switched-mode power supplies (SMPSs), MOSFETS are often used as the switching elements as well as for power factor correction (PFC).

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Diodes for AC/DC Power Supplies

The first silicon-based electronic component, diodes are passive devices which are found in virtually every electronic product or device. The ideal diode allows current to flow freely in one direction and completely prevents current from flowing in the opposite direction. Although, at their core, semiconductor diodes consist of a single P-N junction, there is vast array of different diode types and designs. The zener diode, for example, is designed to also conduct current in the opposite direction when reverse-biased at or above a specific voltage threshold known as the "breakdown voltage." AC/DC power supplies often employ diodes in a bridge-type configuration to rectify the AC input.

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Power MOSFETS for AC/DC Power Supplies

Metal-oxide-semiconductor field-effect transistors (MOSFETs) are by far the most common of transistors today, being used for flash memory, processors, random-access memory (RAM), and application-specific integrated circuits (ASICs), and more. MOSFETs can be conceptualized as a voltage-controlled device for limiting current flow.

MOSFETs are also for power switching circuits. Unlike bipolar junction transistors (BJTs), the competing type of power transistor, MOSFETs do not require a continuous flow of drive current to remain in the ON state. Additionally, MOSFETs can offer higher switching speeds, lower switching power losses, lower on-resistances, and reduced susceptibility to thermal runaway. In switched-mode power supplies (SMPSs), MOSFETS are often used as the switching elements as well as for power factor correction (PFC).

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Capacitors for AC/DC Power Supplies

A capacitor is a passive electronic component that stores energy in the form of an electric field. As part of an electrical circuit, capacitors "oppose" changes in voltage by supplying (or drawing) current. An ideal capacitor is characterized simply by its capacitance value, the device’s ability to store charge. However, a real-world capacitor has many additional characteristics, such as tolerance rating, working voltage, leakage current, temperature coeffecient, and equivalent series resistance (ESR) – each of which may carry a different level of importance for any given application.

Many types of capacitors exist to perform a variety of functions for a variety of different applications. Decoupling capacitors protect electrical circuits from destructive voltage spikes and transients. Similarly, coupling capacitors serve to block direct current, which can cause damage to certain electronics, while only allowing the AC signal to pass. AC-to-DC power supplies use a reservoir capacitor to smooth the output of a rectifier stage.

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Zener Diodes for AC/DC Power Supplies

A zener diode is a special type of diode that is designed to operate in the reverse breakdown region without causing damage to the device. Compared to a conventional diode, zener diodes have breakdown voltages low enough to ensure a negative temperature coefficient (NTC) during the breakdown process – thereby avoiding thermal runaway which would damage the device. Zener diodes are most commonly used as a form of overvoltage protection, safely shunting excess electrical energy to ground when voltages exceed the diode’s breakdown voltage.

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Inductors for AC/DC Power Supplies

Like a capacitor, inductors are also passive energy-storing devices. Inductors, however, store energy in a magnetic field, and have the effect of opposing changes in current flow. An ideal inductor is characterized by a single value called inductance, which is measured in units called henries. Physical, real-world inductors generally consist of a coil of wire wrapped around a core of ferromagnetic material. However, not all inductors use a magnetic core, and the material used directly affects the non-ideal properties of the device such as eddy current losses, magnetic saturation, peak current, and high-frequency losses. Mutual inductance, formed by one or more inductors with coupled magnetic flux, is the principle that underlies another electronic device, the transformer. In a switched-mode power supply, one or more inductors can be used for both output filtering and energy storage (often implemented as a transformer).

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