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
-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