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171 lines
6.6 KiB
Markdown
171 lines
6.6 KiB
Markdown
Simple DC electronic circuits can be powered by directly connecting a
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battery. However, more complex circuits usually require a constant
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input voltage for proper operation.
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This page is a small sidetrack to explain the different regulated DC
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power supply topologies, before looking at the **FunKey S** power
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supply schematics in details.
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If you are already comfortable with this subject, you can skip this
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section entirely!
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## Linear Regulators
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The easiest method to achieve a constant voltage viewed from the load
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despite a varying source voltage is to linearly control the resistance
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of the regulator in accordance with the load, resulting in a constant
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output voltage.
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### Shunt Regulator
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The simplest voltage regulator is the [shunt regulator][1], built
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around a Zener diode which most interesting characteristic is to
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maintain a constant voltage across itself when the current through it
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is sufficient to take it into the Zener breakdown region. A simple
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shunt regulator looks like this:
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### Series Regulator
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By adding a emitter-follower transistor to the simple shunt regulator,
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the small base current of the transistor forms a very light load on
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the Zener, thereby minimizing variation in Zener voltage due to
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variation in the load, resulting in a better regulation. Here is a
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schematic for this [series regulator][2]:
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### Integrated Linear Regulator
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In integrated voltage regulators, the discrete Zener diode is replaced
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by a more sophisticated (but easier to integrate) circuit built around
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a resistor divider feeding an operational amplifier, a voltage
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reference, and a transistor driving the emitter-follower pass
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transistor:
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Usually, the pass transistor and its driving transistor are combined
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into a single Darlington transistor plus a controllable current source
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like this:
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### LDO (Low Drop-Out) Regulator
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The above circuit works well, but its drop-out voltage (the difference
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between the input and output voltage) is rather high because of this
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transistor cascade, around 1.5V to 2.5V.
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By replacing the emitter-follower Darlington transistor by a PNP
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transistor in an open collector or open drain topology, the drop-out
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voltage is reduced to 0.7V or lower:
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## SMPS (Switched-Mode Power Supply) or DC/DC Converters
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A linear regulator provides the desired output voltage by dissipating
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excess power as heat in the Zener diode or in the pass
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transistor. Hence its maximum power efficiency is VOUT/ VIN since the
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voltage difference is wasted to heat the birds.
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In contrast, a Switched-Mode Power Supply changes output voltage and
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current by switching non-linear storage elements, such as inductors,
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transformers and capacitors between different electrical
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configurations.
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These elements are said to be non-linear because the inductor and
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transformer respond to changes in current by inducing its own voltage
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to counter the change in current, whereas a capacitor responds to
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changes in voltage by inducing its own current to counter the change
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in voltage.
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Thus, depending on the way the components are arranged, it is possible
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to obtain SMPS circuits that either have an output voltage higher than
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the input voltage ("Boost Converters"), or lower than the input
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voltage ("Buck Converters", as is it subtracts or “Bucks” the supply
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voltage).
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Because of technology, power inductors are easier to manufacture, take
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less space and are more stable over time than their counterpart
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capacitors. This is why most power DC/DC converters are built using
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inductors. Capacitor-based SMPS are generally used for lower power
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applications, such as for generating the +12V and -12V voltages
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required by true RS232 from a +3.3V or +5V power supply in the
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ubiquitous MAX232 drivers.
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### Boost Converter
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The most basic circuit for the Boost converter is the following:
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If the switch is driven by a square wave, the peak-to-peak voltage of
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the waveform measured across the switch can exceed the input voltage
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from the DC source. This is because the non-linear characteristic of
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the inductor, and this voltage adds to the source voltage while the
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switch is open.
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!!! warning
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In this converter, the output voltage is not isolated from the
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input voltage.
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### Buck Converter
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The corresponding basic circuit for the Buck converter is the
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following:
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The way this converter works is described in details
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[here][3]. Basically, when the switch is closed, the inductor will
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produce an opposing voltage across its terminals in response to the
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changing current, reducing the output voltage, and meanwhile the
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inductor stores this energy in the form of a magnetic field. When the
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switch is opened, the current will decrease and will produce a voltage
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drop across the inductor, and now the inductor becomes a current
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source, where the stored energy in the inductor's magnetic field is
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restored and fed to the load.
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!!! warning
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In this converter too, the output voltage is not isolated from the
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input voltage.
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### Isolated SMPS
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Isolated Switched-Mode Power Supplies use a transformer to isolate the
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input voltage from the output voltage, and thus can produce an output
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of higher or lower voltage than the input by adjusting the turns
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ratio.
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## Pros and Cons
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Linear regulators are simpler than SMPS, and their linear behavior
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produce a very clean output voltage, but their efficiency is directly
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proportional to the difference between the input and output voltage,
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which is dissipated as heat.
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However, for light loads and/or when the voltage drop-out is low, LDOs
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are very useful.
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OTOH, SMPS are more complex and require more components, but their
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efficiency is much better (typically 80-90%), resulting in less heat,
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with the drawback of a switching electrical noise pollution of both
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the input voltage (that may couple electrical switching noise back
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onto the mains power line) and the output voltage (with
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electromagnetic interference (EMI) and a ripple voltage at the
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switching frequency and all its harmonic frequencies).
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SMPS are thus almost exclusively used when heavy loads are used and/or
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when the voltage drop-out is important.
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[1]: https://en.wikipedia.org/wiki/Linear_regulator#Simple_shunt_regulator
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[2]: https://en.wikipedia.org/wiki/Linear_regulator#Simple_series_regulator
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[3]: https://en.wikipedia.org/wiki/Buck_converter#Concept
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--8<--
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includes/glossary.md
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--8<--
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