Virtually all of today's electronic systems require some form of power conversion.
The trend toward lower power, portable equipment has driven the technology and
the requirement for converting power efficiently. Switchmode power converters,
often referred to simply as "switchers", offer a versatile way of achieving this goal.
Switching regulators are small, flexible, and allow either step-up (boost)
or step-down (buck) operation.
When switcher functions are integrated and include a switch which is part of the
basic power converter topology, these ICs are called “switching regulators”. When no
switches are included in the IC, but the signal for driving an external switch is
provided, it is called a “switching regulator controller”. Sometimes - usually for
higher power levels - the control is not entirely integrated, but other functions to
enhance the flexibility of the IC are included instead. It is important to know what you aregetting in your controller, and to know if your switching regulator is really aregulator or is it just the controller function.
The primary limitations of switching regulators as compared to linear regulators are
their output noise, EMI/RFI emissions, and the proper selection of external support
components. Although switching regulators do not necessarily require transformers,
they do use inductors.
One unique advantage of switching regulators lies in their ability to convert a given
supply voltage with a known voltage range to virtually any given desired output
voltage, with no “first order” limitations on efficiency. This is true regardless of
whether the output voltage is higher or lower than the input voltage - the same or
the opposite polarity.
Switchers also offer the advantage that, since they inherently require a magnetic
element, it is often a simple matter to “tap” an extra winding onto that element and,
often with just a diode and capacitor, generate a reasonably well regulated
additional output. If more outputs are needed, more such taps can be used. Since the
tap winding requires no electrical connection, it can be isolated from other circuitry,
or made to “float” atop other voltages.
Though switchers can be designed to accommodate a range of input/output
conditions, it is generally more costly in non-isolated systems to accommodate a
requirement for both voltage step-up and step-down. So generally it is preferable to
limit the input/output ranges such that one or the other case can exist, but not both,
and then a simpler converter design can be chosen.
The concerns of minimizing power dissipation and noise as well as the design
complexity and power converter versatility set forth the limitations and challenges
for designing switchers, whether with regulators or controllers.
The ideal switching regulator performs a voltage conversion and input/output energy transfer without loss of power by the use of purely reactive components. Although an actual switching regulator does have internal losses, efficiencies can be quite high, generally greater than 80 to 90%. Conservation of energy applies, so the input power equals the output power. This says that in stepdown (buck) designs, the input current is lower than the output current. On the other hand, in step-up (boost) designs, the input current is greater than the output current. Input currents can therefore be quite high in boost applications, and this should be kept in mind, especially when generating high output voltages from batteries.
Engineering Services
Source: http://www.ArticlePros.com/author.php?Mark
|
