Switch Mode Power Supplies (Part 1)

Part 12

This is the begining of a series of posts dedicated to an original subject that I had in mind for sometimes now. After few attemps, I decided to share my early design which might drive us up to building a versatile lead-acid battery charger.

The aim of these posts is not to rewrite or reword the numerous publications related to switching power supplies. It is an attempt to show and explain how to build a pretty simple and yet powerful Switch Mode Power Supply (SMPS).

Let’s start with some fundamentals and explain the principles of regulated power supplies. There are multiples ways of getting a source of DC voltages: batteries, solar panels, alternators and dynamos, AC-DC converters, etc. However, most of these sources fail to deliver a constant DC voltage: batteries are getting slowly discharged, alternators and dynamos may have unstable driving speeds and deliver fluctuating voltages, AC-DC converters vary according to AC drifts and power consumption. If one really need a constant DC voltage, he will have to insert a voltage regulator in between the source of DC voltage and the load.

There are mainly two different ways of controlling the power supply output: you may use an analog regulator or a switched regulator.

An analog regulator (e.g. 78xx series regulators) is comparable to a tap with a feed back. If the flow at the exit of the tap is higher than expected the  tap is slightly closed, down to the expected value. And vice versa (I am over-simplifying, but that’s the idea). This type of regulator is very simple, cheap and comes most of the time integrated and built in one component (true for medium to low power regulators). However, these regulators suffer from a major drawback: power dissipation ! The reason is that the energy which is not used by the load has to be dissipated. This is the reason why most regulators of this kind feature large thermal dissipators to which are attached the regulators or transistors (high power). Let’s check that through an example:

On the left hand side is a source of DC voltage (~15V). On the right is the load to be fed by regulated voltage. This is a resistive load of 500 Ohm across which flows a current of 1/100 A, as the resistor is biased by the 5 V from the 7805 regulator. This same current flows through the 7805 regulator. As the voltage drop across the regulator is 10 V, the dissipated power is easy to compute.

From this example, we can draw some recommendations: avoid large voltage differences between the input and the output of the regulator, constrain the current used by the load and use appropriate heat dissipators. The good news however is that most modern regulators feature over-current and over-temperature protections which will protect the component located on the load side.

Switched regulators solve the heat dissipation problem to the cost of few external components and “intelligent” controllers. Back to the hydraulic comparison, on one side we have a reservoir of energy fitted with an on/off valve, a damper and a flow sensor. The idea is to switch the valve on and off  in order to drain just the required energy from the reservoir. As the on/off cycles create fast pressure changes a damper is fitted between the valve and the exit of the device. Based on this design, it is easy to understand that the longer the on time, the more the power flowing through the regulator. The flow sensor measures the amount of energy leaving the device and applies the feed back to the valve controller.

This is how it translates in electronic terms. This design is known as a buck converter: other component arrangements can be used for generating an output voltage which is higher than the voltage at the input (This is the booster design). Some are capable of generating a constant voltage whatever the input voltage: e.g. many portable devices feature buck-boost converts so that two 1.5 V battery can feed electronics requiring 3.0 V. Buck-Boost converters will deliver a constant 3 V from the fully charged state of the batteries (~3.3 V) down to their fully discharged state (~1.6 V). Many publications cover the principles of SMPS, among which this one or this one. The main benefit from the switched power supplies lies in the very low power dissipation across the switching element .

Further readings:

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