SMPS Tutorial (1): Introduction - Switched Mode Power Supplies and Power Conversion

Oh you hello this is the introduction video to my tutorial series about switched-mode power supplies in the course of this series I will go through the most common circuit topologies for SMPS as they are used in electrical engineering today each of the videos will deal with one topology in detail starting with the simpler topologies and then proceeding to the more complex ones all videos though will be ordered in a common structure first the circuit topology in its principle of operation second examples of real-world electronic devices in which these topologies are used today but a step-by-step design example of how the topology can be used in a circuit but this introduction video will not yet concentrate on one particular topology but is meant to offer in overview of a power conversion techniques in general for it is important as a design engineer to know the different alternatives that can be used to reach a certain goal switch mode power supplies offer many advantages over all the design techniques but that doesn't mean that they are the right choice for every application therefore I will now start to give you a broad overview of the kinds of circuits that can be used as power converters of some kind because it is the most common way of understanding we will view these power converters as voltage converters from now on though it would be also possible to describe them as current converters there have been many attempts to properly categorize all kinds of converters oftentimes you hear for example the differentiation between step up and step down converters as you can also read in the data sheets of many integrated devices at this point we will not use these terms the reason is that many converter topologies can be used for stepping down as well as stepping up voltages instead we will take a practical approach that is more problem-oriented let's say you have a specific problem while designing a circuit normally you have some kind of supply voltage that is delivered by your local power a battery a photovoltaic cell a diesel generator or maybe a wind turbine on the other side you have some kind of load maybe a controller board an LED an electric motor and so on all the possible different sources on the left-hand side may deliver voltages with different amplitudes and waveforms also on the right hand side each of the low devices will require likewise one or more different voltages with again different waveforms so to categorize the various topologies of converter circuits will order them by the waveforms of the input and output voltages first we make the common differentiation between AC and DC voltages the difference is that AC voltages reverse the polarity over time while DC voltages don't that however doesn't mean that DC voltages are always constant therefore there is an unlimited number of waveforms for AC as well as DC in our complex world for the purpose of understanding the principle of operation of most converters it is however sufficient to differentiate between just a couple of idealized waveforms let's start with the AC waveforms first we have square wave AC a waveform often found in switch mode power supplies they are however actually not squares but trapezoids with very steep slopes next we have so called modified square waves that can have varying levels of modification the higher the level of modification the more the square wave resemble a sinusoid another common waveform is the approximation of the effective value of a sinusoidal AC voltage by a pwm that changes its duty cycle periodically a technique that is utilized in most advanced power inverter circuits and of course another very common waveform is an actual sinusoid as for the DC voltages we will find constant values of course but also pulse DC voltages in different shapes most commonly the square wave which again has actually the form of a trapezoid then pulse DC in the form of a sawtooth which is oftentimes actually just a simplification of an exponential function like the charging curve of a capacitor also often found our DC voltages in the shape of the positive or negative half of a sinusoid in order to categorize all the different converter circuits we will use the following model we can view the actual circuit as a black box with an input and an output voltage that both have a certain amplitude waveform and frequency this model can be applied to all converters even if they are actually comprised of a system of several converters that reshape the voltages parameters internally for many times but if you firstly if the various waveforms aside and concentrate on our basic distinction between AC and DC voltages we can use these common four categories to order all the circuits AC to AC AC to DC DC to DC and DC to AC now that we have the four main categories we can try to make further practical distinctions to get a better overview of the variety of circuit topologies in existence AC to AC converters can be divided into such that do not offer electrical isolation and such that do the same is true for AC to DC converters DC to DC converters are always either voltage dividers linear regulators or switching regulators while linear regulators are actually also voltage dividers it makes sense to differentiate between the two for practical reasons in this tutorial I will use the terms switched mode power supply switching regulator and switching converter as synonymous i will also often use the abbreviation SMPS DC to AC converters are all so switching converters and you can for example differentiate between such inverters that have an output transformer and such that don't ok so now we can take a look at the actual circuit topologies used in power conversion AC to AC converters without electrical isolation the simplest way to step-down an AC voltage is to use a voltage divider of some kind this is an unloaded voltage divider the output voltage can be calculated by the formula V out equals V n times z2 divided by z1 plus z2 when a load is connected in parallel to one of the dividers resistors you call it a loaded voltage divider and the formula changes to V out equals V n times ZL paralleled with z2 divided by z1 plus ZL paralleled with z2 these formulas are called transformation functions and you can formulate such a function for every possible converter there is the impedances could represent many different combinations of resistors capacitors and coils most of the time however you will find the same simple configurations for example the resistive voltage divider in its simplest form the unloaded voltage divider is actually used in many applications a good example is this neon lamp tester for 230 volt mains the neon lamp inside the tester would be destroyed if directly connected from phase to neutral but because the neutral is connected to earth ground in the local transformer station it is possible to use the human body as a resistor that creates a voltage divider with the neon lamp in another small resistor inside the tester it is also possible to connect numerous loads of equal resistance in series to divide a smaller portion of the input voltage over each of the loads this is for example done in these terribly catchy Christmas lights all the little light bulbs are actually rated for 24 volts but can be powered from 230 volts because they divided the input voltage roughly by 10 another common but less known circuit is the loaded capacitive voltage divider it is comprised of two capacitors connected in series with this circuit it is possible to divide the mains voltage with only low dissipative losses the voltage across one of the capacitors can be rectified and regulated to supply small electronic circuits this technique is often used in these infrared motion detectors that you can see everywhere these days here you can see the actual circuit the resistor in series with the capacitor limits the current but almost all of the voltage drop happens over the two capacitors the voltage across the smaller capacitor is then rectified with a bridge rectifier in stabilized with a Zener diode and an integrated linear regulator in this way the circuit provides constant plus 24 volts and plus 8 volt DC from a 230 volts AC input another topology that is around for some decades but is also little known our phase fired controllers using thyristors or as shown here a bi-directional diode thyristor better known as tri egg Triax are actually rectifiers that can be actively activated by a voltage impulse at their gate and stopped conducting once the current through them drops under a minimum value that happens automatically with each cycle when Triax are used in AC circuits so by firing the triac with a controlled pulse at a certain time in each period the effective value of the output voltage can be varied the AC sine wave is literally cut within each cycle phase fired controllers are used in light dimmers and in vacuum cleaners these are phased fired controller circuits that I have salvaged from broken vacuum cleaners with the help of just one triac some discrete components and a potentiometer it is possible to control the 1500 watt motor inside the vacuum cleaner actual thyristors are used in high-power applications like for example electric locomotives but are rarely found in consumer products phase fired controllers do use a kind of pulse width modulation but are normally not considered to be switched-mode power supplies another possibility to convert AC voltages are Auto transformers autotransformers consists of a winding on a magnetic core like normal transformers but are special in that they do only have one winding and therefore do not supply electrical isolation autotransformers can be used to step-down voltages but can also be used the other way round to step them up a common application of Auto transformers or older lab power supplies like the one you see here this device is a special kind of auto transformer called variac it is simply a transformer with just one winding that has a moveable tab with which the output voltage can be changed AC to AC converters with electrical isolation the one component that offers electrical isolation in power converters is the isolation transformer oftentimes just called transformer transformers have at least one primary and one secondary winding that are wound on a common core that conducts the magnetic flux caused by the primary through the secondary inducing a voltage in it as you most probably know the output voltage is determined by the turns ratio between primary and secondary most transformers in commercial equipment are however more complicated transformers often have a center tab on the secondary and or on the primary winding or even various tabs on both windings or even several separate windings this is oftentimes done to get several different output voltages from just one transformer or to make it possible to power the transformer from different input voltages a good example for that is this transformer inside an audio amplifier the transformer has three separate primary and three separate secondary windings on the primary side a voltage selector switch configures the three primary windings in such a way that the transformer can be operated from 110 130 to 120 and 240 volt grids the upper secondary winding supplies the preamplifier while the two other secondary windings are connected together to supply two symmetrical voltages for the power amplifier stage as I will explain later the Transformers used in modern switch mode power supplies do also in most cases have several windings AC to DC converters with or without electrical isolation the simplest kind of AC to DC converter is the half wave rectifier it is simply one single diode that only lets the positive half wave of an AC input voltage through that pulse DC voltage is then usually you to charge a capacitor to create a constant DC voltage the voltage however is only constant in a theory in reality the capacitor will immediately start to discharge once no new charges come from the input as can be seen in this graph the difference between the value of the highest voltage and the lowest voltage across the capacitor is called ripple voltage the most common type of rectifier circuit is the full wave which rectifier it utilizes four diodes to allow a constant flow of energy from the input to the output using the positive as well as a negative half wave which also reduces the ripple voltage these two AC to DC converters can be used to directly rectify even high AC voltages for example in most offline switch mode power supplies the mains voltage is directly rectified by a bridge rectifier that charges an electrolytic capacitor that then acts as the voltage source for a DC to DC converter in conventional non switching offline power supplies however you will almost always find an isolation transformer that first steps down the mains voltage and is only zem rectified another AC to DC converter is the full wave rectifier with center tab unlike the other two rectifier topologies this circuit only works in conjunction with a transformer it provides full wave rectification and has low ripple but does only deliver half the output voltage of a bridge rectifier another advantage of this topology is that the voltage drop of the diodes is only half as big as with a bridge rectifier if you change the topology in this way you can create two symmetrical voltages another variance is to use a bridge rectifier with a center tapped transformer and define the center tab as chassis ground in this way you can also get two symmetrical DC voltages that are equal in size but reversed in polarity these two topologies can can be found in the power supply's of amplifier circuits another class of AC to DC converters are the voltage multipliers there is a symmetrical voltage doubler that rectifiers the AC input voltage and then charges two capacitors with it that are connected in series in this way the voltages add up to twice the value of the output voltage of a normal rectifier the voltage doubler can also be expanded into a voltage quadruple circuit in theory it would be possible to obtain 1.3 kilo volts DC from 230 volt mains AC this circuit is a bridge rectifier that can be turned into a voltage doubler by closing a switch it is a circuit that can sometimes be used to build a device that is normally rated for 240 volts but is supposed to run on 120 volts as well if it is necessary furthermore there is the asymmetrical voltage doubler that can be cascaded into a voltage multiplier cascade this converter circuit is sometimes called the kaurav Whorton generator because it was used by physicists Cockroft and wharton to build a particle accelerator the circuit can be used to generate DC voltages of up to several mega volts it was also used in CRT TV sets those were the most important circuit topologies for AC to AC and AC to DC conversion in part 2 of this video tutorial I will continue in this fashion to describe the basic topologies for DC to DC and DC to AC conversion which includes all types of switched-mode power supplies so if you like this video please watch the other parts too and please subscribe to my channel

Channel: The Post Apocalyptic Inventor

Views: 423,056

Keywords: Switched Mode Power Supplies, ATX, ATX Power Supplies, Switching Regulators, DC to DC Converters, Electrical Engineering (Industry), Electrical Engineering Technology (Field Of Study), Amateur Radio (Hobby), Hobby Electronics, Physics (Idea), Circuits, PSU, Buck Converter, Boost Converter, Flyback Convertert, Push Pull Converter, Forward Converter, Voltage Dividers, Linear Regulators, Bench Power Supply, Transformer (Invention), Rectifier, AC, DC, AC to DC, AC to AC, Inverters

Id: nbKwUSH9NiY

Length: 18min 37sec (1117 seconds)

Published: Sun Mar 02 2014