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Demystifying Switching Power Supplies by Raymond A. Mack, Jr. | PDF Free Download.
This book is intended for those who need to understand how a switching power supply works. I intend to provide enough information so you can intelligently specify a custom off-line supply from a power supply manufacturer.
You should also gain enough information to be able to design a DC-DC converter. I have included basic analog design information for those whose primary electronics background is not analog circuits.
Then I build on that basic information to show how to design and analyze practical switching power supplies. Those with a strong background in analog circuitry may want to skim over the preliminary data.
In numerous places, I skip over the details of derivations and transformations of equations. The details of those transformations are left as an exercise for the reader.
There are two broad classes of power supplies: linear and switching. Linear supplies use time-continuous control of the output.
Switching supplies are time-sampled systems that use rectangular samples to control the output. This book explores each of the variations of switching power supplies.
The principles of switching power supplies have been used for over 100 years (though people didn’t know that’s what they were).
The ignition system used in a gasoline engine was the earliest version of a flyback switching power supply.
The next general use of switching supplies was in the high voltage section of televisions. Again, this is an example of a rudimentary flyback supply.
The flyback name comes from the short time period where the spot on the television CRT is moved from the right side of the screen back to the left side of the screen (it would “flyback”). The rapid change in current in the deflection coil causes a very large voltage to be generated.
This was used to advantage in televisions to create the large acceleration potential necessary for the CRT. Widespread switching supply use was limited to television high voltage service until the late 1960s because of limited capabilities of the three major components in a switching supply: the magnetics, the switch, and the rectifier.
Components were available for switching supply use in the early 1960s with the advent of high voltage bipolar transistors, but they weren’t economically feasible for low wattage uses until the price of semiconductors became reasonable.
Since 1970, advances in all component categories have changed the power supply market to the point where linear power supplies are almost nonexistent above the level provided by three-terminal linear regulators.
Advances in semiconductors allow single package switching power supplies with multi-watt capability. These designs use the IC, an inductor, and a couple of capacitors to produce a complete voltage regulator in a volume smaller than a single TO-3 switching transistor from the 1960s.
The price per watt of AC line operated power supplies has dropped to the point that it is not cost-effective to design and build such a supply in-house unless extremely large quantities are involved. Many companies market lines of standard output voltage supplies.
Most of these companies can also supply nonstandard voltages based on standard designs for nominal design fees. Most of the major linear IC manufacturers (Linear Technology, Maxim, TI, National Semiconductor, Analog Devices, etc.) provide a line of switching regulator circuits suitable for local voltage regulation or voltage conversion.
Modern devices from these manufacturers are extremely small and efficient. This is true especially of devices intended for battery-operated equipment where maximum operation between charging is important.
Modern devices frequently integrate the control circuit, the switch, and the required rectifiers in the same package. The passive component manufacturers have been busy improving components as well.
The magnetic materials companies (Ferroxcube, Siemens, Micrometals, Magnetics division of Spang & Co., etc.) have extended the useful range of transformers and chokes from the low kHz range (10–50 kHz) in the 60s to well above 1 MHz today.
This improvement has allowed much smaller filter capacitors and magnetic cores in modern designs. Capacitor manufacturers have also improved filter capacitors for use in switchers.
Ordinary electrolytic capacitors have a very large equivalent series resistance that causes them to dissipate power when a rapidly varying DC voltage is applied.
If this equivalent AC current is too high, these electrolytes will heat to the point of explosion. All electrolytic capacitor manufacturers now make lines of capacitors that are designed to limit this equivalent series resistance.
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