ATX Power Supply Teardown

Posted under Repair, Teardown, Technology on December 26th, 2021 by
ATX Power Supply Teardown
ATX power supply teardown
ATX power supply teardown
Power Supply Teardown

These beefy mountable power supply units power up custom-built desktop PCs. It’s one of those mass-manufactured switched-mode power supplies which are optimised for the cost, in any fashion possible, even from the branded companies. Images show a teardown of 500W Cooler Master PSU. Usual building blocks of a PSU like this contain the input AC filtering section(To remove noise from input and to avoid coupling noise from the power supply back to the main AC supply) made up of fuses, MOVs(Over voltage protection), common mode chokes and huge power line filters. These filtered AC gets fed to bridge rectifiers to convert it into DC and is followed by massive DC bulk capacitors(Be very careful while handling these as it can give you a good shock if you touch their leads before discharging).

The output DC is fed to switching MOSFETs which chop it up to high-frequency pulses to drive the large transformers. The switched-mode section would contain a feedback back loop to regulate the voltage to desired levels. The output side, based on the ATX standards, provides voltages to different portions of a motherboard. Standard voltage levels would be +3.3V, +5V, 12V, -12V(For RS232 signalling) and is broken out as a 24pin(20+4) ATX connector with a standard pinout. The output side would contain a protection IC for overvoltage, undervoltage and overcurrent to safeguard the motherboard. A fan is provided to cool down the heat produced in switching MOSFETs(to ensure long life).

The funny part about the PCB layout of an ATX power supply is that it’s usually one-sided(Ignoring the rule of thumb to the fact of having ground layers and close return paths) purely for cost reasons. The PCBs are usually of a lighter colour and more importantly, made of FR2 Phenolic Resin(Not the usual FR4) because they are cheaper. Hence these units are an engineering marvel in themselves to pass standards and keep the pricing down.

PS: I have been away for a month from my regular posts since I lost someone very close to me. Was not in a great mental space the last few weeks to share posts. Things are getting back to normal and posts would be mostly regular from now on.

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Back to Basics: Capacitor DC Bias Effect

Posted under Back to Basics, Educational, Product Design, Technology on November 21st, 2021 by

MLCC(Multi-Layer Ceramic Capacitor) which most of you are using in your circuits(The usual SMD type) are divided into Class 1(Dielectric: Titanium Oxide) and Class 2(Dielectric: barium titanate). These Class 2 capacitors have problems that most of you never would consider. The capacitance value drops very drastically with the applied DC bias voltage to it.

DC Bias Capacitors

Take a look at the graph of a standard Murata capacitor(Nominal Value: 4.7μF rated for 35V). Now as the DC voltage applied to the capacitor is increased, you can clearly see that capacitance drops close to 0.5μF when it’s near its rated voltage. This will be a major issue if you are using this capacitor in circuits where capacitance value is critical like for timing or filtering. The effective capacitance will totally screw up your results when you move from the paper design to the physical board. So please consider this when you design next time. For critical applications, try to use Class 1 capacitance which is much more stable. Class 2 caps show this behaviour because of the Barium titanate dielectric used to make them.

One way to counter this problem is to use capacitors that have a rated voltage much more than what is needed for critical applications or by having the same capacitances in parallel to increase the effective capacitance to the nominal ones you design for. For MLCCs used in critical parts of your circuit always, ALWAYS, buy from reputed capacitor brands where these graphs are provided and not from knock-off unbranded ones. Else it will come back to haunt you later on and you wouldn’t even know this part is the culprit. Capacitor Package Size and its values also have an effect on the amount of value degradation. So please refer to datasheets thoroughly before selecting.
PS: Capacitors and things associated with it is a big rabbit hole in themselves. It would itself need an entire series of posts. I can definitely consider this for the future if there is enough interest in these basic topics.

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