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Howto repair switch mode power supplies #1: basics, and block diagram of a PSU

The repair of switch mode power supplies (SMPS) is economically a good investment for electronics repair shops and for hobbyists. However, in contrast to the old low frequency transformers and linear power supply units (PSUs), SMPS are rather complex. Still, due to their low weight, high efficiency, and being able to provide stable voltage output, they have became an ubiquitous component in almost every electronic equipment.

I assume that the viewer has only a basic knowledge of electronics. In this video series we will cover SMPSs and ATX computer supplies starting from the basics and going into the details in the repair of such units. Here we look into the most important components and building blocks of an SMPS, and we cover the block diagram. Understanding the block diagram is highly important, since it provides an easy way for a technician to narrow down the potential components which are defective in an SMPS unit.

Due to high amounts of heat dissipated and high switching voltages, and frequencies, power supplies are the most frequent source of failure. Thus, although it takes more time and effort to understand the basics of SMPS, in the long turn it pays off to learn about their inner workings and also about their most frequent failure modes.

In this video we cover the power input, filtering stage and rectification portion of an SMPS. I explain the need for inrush current limiting. Next, the power distribution is explained on the high voltage DC side, where through the two different paths the stand-by and the high power chopping circuits will be fed, driving the transformers.

The main building blocks and how they are tied together in a modern switch mode power supply:
-power input plug or cable for mains voltage
-input protection (fuse, metal oxide varistor MOV)
-EMI/RFI filter (Xcaps, Ycaps, common mode chokes)
-rectifier diode bridge
-inrush current rimiting NTC Thermistor
-active or passive power factor correction (PFC) circuit
-bulk mains filter capacitor
-controller circuit for the FET of the stand-by power and main power
-switching FETs
-stand-by and main power transformers (high frequency ferrite)
-Schottky diodes for the secondary side rectification
-electrolytic filter caps
-voltage reference circuit
-optocoupler and feedback loop
-over-voltage and over-current protection circuit

43 thoughts on “Howto repair switch mode power supplies #1: basics, and block diagram of a PSU”

  1. eodguy83 says:

    Very helpful. Thank you for the video.

  2. weerobot says:

    Awesome better than all Universities in the World….!!

  3. 1973jdmc says:

    If you were a super hero you'd be MR FANTASTIC – a massive THANK YOU

  4. I have managed to blow the varistor on my MS305D power supply. I had it switched to 110V and plugged it into my 240V I heard a loud bang inside and the varistor literally exploded there is a mark on my GBU808 bridge rectifier where it exploded, would they be the only components that I should look at changing or could others have been taken out at the same time??

  5. Donald Henke says:

    HELP…why a atx psu will not take 6amp load when its allready on

  6. joe liang says:

    So great of the video and explanation. thank you so much!

  7. Thank you so much. That was amazing! God bless you always!

  8. John Dro says:

    Becouse of this vid, I subscribed to your channel and unsubscribed from my other electronics channels.

  9. LIGO says:

    Unfortunately, the poster interchanged current w/ voltage and voltage w/ current. Make no mistake! Voltage is stored via a capacitor and current is the flow of carriers or electrons. This concept must be understood for correct troubleshooting. Think of it this way, Voltage is the pressure behind the current flow. Current flow is the amount of electrons past a singular point in a given time! Just look at the relationship of current, voltage and resistance according to Ohm's law.

    Again, given a fixed resistance, the current (electron flow) will increase with more voltage (pressure) applied. Conversely, as the resistance is decreased w/ a fixed amount of voltage the current will decrease proportionately. I hope this clarifies the curious.

  10. Specifically referring to two things, the fact I modded my ytube handle successfully, and failed to sign out on first try… G@@g1e true is 3√|L

  11. hakeem ajede says:

    Subscribed before watching to the end!

  12. No shit.. It worked. Cool

  13. have you got a book that I could purchase

  14. Brij b says:

    Nothing can be better explaining SMPS than this video. Core level and clear explanation.

  15. WV591 says:

    TU for film and your time

  16. Thanks very much for the great videos. Love your Tux mouse pointer too

  17. Great video. Just amazing. You made it so simple. Can’t wait to watch the part 2 of the video.

  18. OGmolton1 says:

    Great video, really helpful, thank you

  19. Jason corr says:

    Hi I have a (ZHAOXIN PS-3005D DC Power Supply) but it is stuck on 50 V it does not adjust to a lower voltage do you know where I should look ?

  20. Great Geekax says:

    Very good explanation. I subscribe to your channel as your way of explaining things is easy.

  21. scotty k says:

    Kwit saying cirkwit. I like the clunking power switch.

  22. Bige4u says:

    Experience and hands on are the greatest teachers…. books, not so much other than giving you a basic idea on how things work for knowledge sake, luckily i never took the tech/trade schoold route, nothing but rip offs.

  23. Excellent tutorial. Very well explained. The school teachers should learn from this tutorial on how to teach. Once you listen to this tutorial you will remember electro magnetic theory all your life.

  24. You should skip the plug, switch and fuse

  25. smig smög says:

    only the cheapest psu's and weller soldering irons want to skip the input protection ;P

  26. Theodoros says:

    You are doing a great job. I am an EE and I would say you are explaining things very well and accurately, bloody well done!

  27. D G says:

    Damn.. this accent is amazing.. can understand every single word without thinking "w8.. what did he say"? 😀

  28. Relan Andrew says:

    really u have done a great job bro, we are waiting for new electronic tropic from u in future.

  29. Saiful Islam says:

    It's really nice video ! Thanks a lot…

  30. Kris C says:

    Great video thank you. I have a question for you. I have this old small psu from vintage apple computer. It is supposed to put out +5 , +12 and -12. I checked them with my multimeter, very small voltages were detected. Then I hear ticking sound from the PSU. Very consisting ticking sound as turn the power on. Slowly disappearing as turn the power off. What component would be gone bad? Any idea?

  31. Ross Wrega says:

    Great video. finally i understand switch mode

  32. Nun Ya says:

    Something about a "non-american" accent always makes the person talking seem smarter…

  33. After all these years on YouTube, I have only subscribed to 2 channels… until now. I am subscribing to your channel for the simple reason that you are not trying to impress everyone with how brilliant you are, and thus avoiding simple explanation of basics. Thank you. Please keep doing what you are doing. Thorough in depth discussion with basic terminology and component description.

  34. You teached me so much! Your diagramm is really easy to undertand aswell!
    Greetings from Germany.

  35. Bright 59 says:

    What do you have to consider when replacing bulk filter capacitors

  36. exellent video, Very helpful. thank you

  37. Anton Malhin says:

    please stop saying "sirkuit" and start saying "sorket".

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