How SMPS works | What Components We Need? Switched Mode Power Supply

these are so called smps which means switch mode power supply this is one of the most commonly used supply with a relatively high efficiency an smps transfers power usually from ac to dc and converts voltage and current characteristics as needed for a specific project the opposite type of supplies are those who are using linear regulators and transformers these are not efficient at all and also could be very big and way too much as you can see here this is a 10 watt switch power supply and it only weighs 50 grams now this here is a 30 watt switch power supply and weight around 100 grams but now this one here is just 18 watts lineal power supply and it already weighs around 4 times more than the other supplies and also another compare this is the pcb of a 360 watt supply of 24 volts and weights just below 400 grams but now we have just the transformer of a 15 watts lineal power supply of 12 volts and it already weights double the weight but is for 7 times less power the cuts are also higher due to more material needed to fabricate for the metal and the copper wire and also for higher loads we need a lot more copper wire and also a lot thicker but as you can see the circuit is a lot more complex for the switch supply so how can this be cheaper have more efficiency better range for the voltage weight less and be smaller and smaller than the common transformer and linear regulator supplies well in this video i'll tell you step by step how a switch mode power supply works we will go through all the components that you can see on these pcbs and explain you why we need each one and by that you'll learn the process of a switch supply so guys make sure that you subscribe and activate the notification bell and also consider supporting me on patreon so let's get started [Music] this episode is sponsored by the pcb manufacturer company glc pcb their main services are the two layer pcbs for only two dollars also four and six layer pcbs the smt assembly process where you will get the pcbs with all the components already soldered in place and also the smt stencil for soldering smd components with solar paste the quality of the pcbs is amazing i use their services all the time and always get good results for only 2 dollars you have 5 pcbs of any color that you want so go to jlcpcb.com upload the gerber files of your design and order the pcbs in just a couple of minutes what's up my friends welcome back this is a so called lineal power supply in order to understand why the switch supplies are better we need to know what is wrong with the linear supplies i guess that you already know but a linear power supply uses a transformer to load the voltage from let's say 230 volts ac to something like 20 volts then we use a full bridge rectifier to get dc voltage and finally we apply a simple filter made with a capacitor it's also quite common to add a linear voltage regulator at this output so we could get very stable values for let's say 5 volts 12 volts 24 and so on we do that because the transformer ratio is not always perfect and the high voltage input is not always the same value and 100 stable so what is wrong with this setup well it has a lot of power losses weights too much and is expensive and for high loads is not a good option a part of the power is lost on the transformer and even more power is locked on the regulator because as you all know linear regulators are not that efficient and usually they lose power in the form of heat so first of all these transformers work at low frequency such as 50 or 60 hertz because that's the frequency we have on our home outlets and remember that the coil impedance is proportional to the frequency and the self-induction coefficient so 50 or 60 hertz is a very low frequency so if the frequency is low the self-induction coefficient must be high to achieve high values of self-induction coefficients we need to use metal cores which are very heavy and at the same time we must create the winding with the high number of loops so more weight together with more loops so more copper will result into a heavy big and more expensive transformer and even more if you need to supply high loads the copper winding of the transformer must be a lot thicker obviously in order to be able to handle high current so that will make the transformer even bigger heavier and cost a lot more but on the other side switch power supplies will work at very high frequencies higher than 20 kilohertz even more this can work with square signals higher frequency square signals will let us work with much smaller transformers which can wait a lot less we use these so called pulse transformers which usually will have a core made out of ferrite to reduce the losses due to eddy currents we achieve the high frequency square signals with the use of poor mosfes transistors these work in convotation mode so we reduce the dissipated power that's because a mosfet will dissipate small power when in saturation or cut-off mode it only dissipates a lot of power when we switch from saturation to cut-off or from cut-off to saturation so that's how we can get an efficiency a lot higher than linear power supplies less weight using small transformers and a lot more power because the output can now have thicker copper wires that don't have to be that long with a teacup or wire we can regulate the output and also have a decent current value but this will come at a cost because the circuit of these supplies is a lot more complex so let's study the schematic of a basic switch mode power supply these supplies would usually have five parts as you can see here from a to e a is the main input protection and filter b is the full bridge rectifier together with the primary filter of the supply the c part are the switching transistors and usually we use two but sometimes is just one part d is the transformer which this time is a lot smaller than a lineal power supply together with this transformer we have the second rectifier's diodes and finally part e is the output filter coil and capacitors and also the feedback now let's talk about each part separately starting with block a so here as you can see usually we have a safety fuse all these supplies have some sort of safety fuse at the input so if there is a short circuit over current and so on the fuse will blow up and protect the circuit okay so then we have the input filter which is called emc filter which stands for electromagnetic compatibility this filter is made up of some high voltage capacitors and the one to one ratio choke this component this choke is important for removing the high frequency noise the high voltage input signal from our homes usually simulates a perfect sinusoidal wave but the signal is not perfect nor clean it comes with a lot of high frequency interferences so once we pass through this emc filter design wave should be a lot cleaner without the high frequency noise so that's how this first block works okay so block b was the rectifier and the primary filter sometimes the supply could have four diodes in a bridge configuration as you can see here on this supply but other supplies might have just one integrate component that already has all four diodes inside one like this one here so this component as any other full bridge rectifier will rectify the signal so only the positive side of the wave will pass so then we store this positive voltage inside of these high voltage capacitors so now this will be dc voltage here in europe this input is usually 230 volts ac and the output from this filter is usually 320 volts dc so just to get this clear the input signal is something like this it has both the positive and the negative sides so this is ac of 230 volts the rectifier will grade this signal with only positive values but these are still ac so if we add the filter capacitor this capacitor will charge up and smooth the signal it won't be perfectly flat but it will be close enough to a dc value so that's how the second block works okay so the third block are the switching transistors and all the digital parts around it this is a little bit more complicated to switch the voltage and create a high frequency signal we use transistors sometimes we have just one as in this case for the supply but sometimes we have two or more transistors as we have on this bigger supply these are usually placed on the exterior of the pcb so we can easily place this aluminum plate and connect them to the metal case and use that as a heat dissipator the gate of this component is controlled by the pwm controller which is this ic here on the smaller power supply we have a different controller and any other supply might have a different one ok so for now the primary filter will give us some sort of dc voltage of high value around 320 volts for europe so now we connect this to the mosfets and using high voltage frequency signal at the gate we create the high frequency signal at the output of the transistors so these squared signals will be connected to the transformer coil and as we will see later due to this fast switching the coil will charge and discharge and as we have seen in the boost and back converters tutorials that will also create negative pulses so this will actually result into an alternating signal once again but this time of a little bit lower voltage a squared shape and way higher frequency so that's what the third block does the fourth block is the transformer and the rectifier diodes this here might look at some transistors but these are actually poreshot key rectifiers some supplies of low power might do this job with only one diode we don't need more than that as you can see this one here has only one component and this is once again a double shot key diode so now with these components the fast square pulses from the transformer are also rectified but this time we are using a half bridge rectifier okay so remember that the signals from the transformers were something like this alternating between positive and negative values because the coil of the transformer will charge and discharge at high speeds but now with the schottky half bridge rectifier we get rid of the negative pulses so now we only have positive but this time this positive voltage is variable and control with the pwm signal applied to the mosfets so that's what we do on the fourth block so finally we have the secondary filter which is made up of a coil and some low value capacitors this can be low value because now we are working at higher frequencies so by this formula here the higher is the frequency the lower could be the capacitor value so the coil together with the capacitors will create an lc filter and smooth the output to the required dc value okay so remember that from the last part we were left only with the positive square signals that were still alternating so now if you want dc we connect this filter at the output so once again these capacitors will charge up and discharge but since the signal is fast enough there is no time to fully discharge so we get a more or less steady dc voltage there is still a ripple at the output of a few millivolts but for some appliances that is good enough okay so now finally if you want a steady output there must be a feedback from the output to the pwm driver for that feedback we usually use an optocoupler as this component here on the pcb because you see for safety we must separate the primary side of high voltage from the secondary side of low voltage so the transformer is already doing that but now if you want to connect the feedback from the low voltage output back to the controller driver which is controlling the high voltage mosfets we need to still keep the insulation so these components are using light to send the signals instead of electricity so now we are insulated the feedback job is to always keep the steady voltage at the output if you want 12 volts at the output for example and there are some unwanted ripples at the input signal the output might increase or decrease and you don't want that so the feedback will tell the controller if the voltage is too high or too low and the driver will adapt the pwm signal by lowering or increasing the duty cycle of the signal as we have seen in the fightback converter tutorial we can change the output value in order to have always the voltage that we want some supplies could also have a current feedback for safety using a shunt resistor so if the supply gives more current that it could handle the feedback will inform that to the driver and once again this driver will lower its signal in order to keep the limit of the current value so let's see the entire process once again but all together ok so the first block will receive the alternating high voltage and protect the circuit with a fuse and then using the emc filter we get rid of the high frequency noise and get a clean sine wave then we pass this signal to the full bridge rectifier and get the signal rectified with only positive waves to create more or less a dc voltage we add a filter with some big capacitors so now this will charge up and create the high voltage dc then the controller together with the mosfets will chop up this high voltage dc into fast pulses of dc these pulses when applied to the transformer coil will start creating an alternating signal once again because the coil will charge and discharge then we apply the signal to the second half bridge rectifier and once again we get only the positive voltage we can change the value of this voltage with the pwm signal from the controller and finally we filter this with the coil and the capacitor and we get our lower and steady dc voltage output the feedback will always inform the value to the controller in order to keep always the same voltage that we want now different supplies might have some different components as you can see here this is also a switch supply but it has less components the big one has a second smaller transformer that is probably used to supply the digital part with under 40 volts since the digital part is already insulated using the small transformer there is no need for optocouplers for the feedback because as you can see this bigger pcb doesn't have any optocouplers on it or at least i can't see any so guys that's how basic switch mode power supply works and these were all the basic blocks it could have some more performance supplies might have even more blocks for the poor factor correction and so on pc power supplies like this one here usually have multiple value outputs and more feedback options for safety features so guys i hope that you have learned something new and that you like this video if so give it a like and also consider subscribing and activating the notification bell consider supporting me on patreon thanks again and see you later guys you

Channel: Electronoobs

Views: 152,703

Rating: 4.9439545 out of 5

Keywords: SMPS, homemade, DIY, Arduino, switched mode power supply, supply, how it works, theory, principle, parts, circuit, Schematic, transformer, power, compare, lineal

Id: cK5WLMFSGnw

Length: 16min 38sec (998 seconds)

Published: Sun Oct 11 2020