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DC to DC Converter (Switched Mode Power Supply)
Design
Why use dc/dc converters?
In many designs there arises the need to convert one
voltage to another. Linear regulators offer a simple
low cost solution, but the heat they generate is
often inefficient and bulky heat sinking is needed
to dissipate the heat.
A more elegant solution is a dc/dc converter
(switched mode power supply).
Many application notes and datasheets assume that
the reader has a basic understanding of switched
mode power supplies which can leave the circuit
designer with a circuit that works, but he is unsure
of exactly how.
This article takes the reader from the very basics
to the complex equations involved in dc/dc converter
design and shows that switched mode power supply
design can be designed with the most basic of
mathematical skills. All examples are backed up with
LTspice®
simulations.
All of the architectures below rely on a controller
IC to switch a voltage across an inductor to
generate the appropriate output voltage. To
understand how they work, we must first understand
how an inductor works.
Inductors
When a voltage is switched across an inductor, the
current through the inductor behaves according to
the equation:
where V is the voltage across the inductor (in
Volts), L is the inductance value (in Henries),
di is the change in current (in Amps) and
dt is the change in time (in seconds).
Thus if a fixed voltage is applied to an inductor
(of fixed value) this generates a current through
the inductor that ramps up linearly with time – a
constant change in current with time.
When the voltage is removed from the inductor, the
inductor generates its own voltage to try to
maintain the current flow. Harnessing this voltage
is what enables dc/dc converters to generate higher
and lower voltages from the input supply.
Any of the following design guides will explain the
operation of the inductor in more detail.
There are 4 basic types of non isolated dc/dc
converter:
Buck Converter Design
These convert a high voltage to a lower voltage,
mostly converting a positive high voltage to a
positive lower voltage.
Boost Converter Design
These convert a low voltage to a higher voltage,
mostly converting a positive low voltage to a
positive higher voltage.
Buck Boost Converter
Design:
These produce a fixed output voltage when the input
voltage is either higher or lower than the output
voltage.
Inverting
Converter Design
These produce a negative voltage from a positive
voltage. Many text books call this type of converter
a buck boost converter, however this nomenclature is
normally exclusively reserved for text books. Most
practicing engineers refer to a buck boost converter
as defined above.
There are also several different types of isolated
converter, but the one we will be discussing is the
flyback converter:
Flyback Converter Design
These use a transformer, but driven in such a way
that the primary and secondary windings appear as 2
separate inductors to the surrounding electronics.
Therefore, in architecture flyback converters are
very similar to boost converters.
Current Mode dc/dc Converter Operation
Explains how a current
mode dc/dc converter operates with respect to the
error amplifier, the feedback network, the current
sense resistor and the soft start pin
Feedback
Resistor Calculator
The feedback resistor values can be calculated using
this spreadsheet:
Feedback Resistor Calculator
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