Capacitors for Half-Bridge Converters
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.
PWM Control for Half-Bridge Converters
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.
Power MOSFETs for Half-Bridge Converters
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 Half-Bridge Converters
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.
Inductors for Half-Bridge Converters
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).
