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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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).
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.
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).
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.
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.
