Switched Mode Power Supplies – a massive influence over modern day electronics

The Switching Power Supply Unit – Introduction  

In 1977, during a course at Cape Technikon, students were introduced to switching power supplies. These devices were relatively new at that time. The Switched Mode Power Supply Unit (SMPSU or SMPS) was being designed by various companies nearly 100 years ago.

https://en.wikipedia.org/wiki/Capacitor_plague

Switched Mode Power Supply Units in Different Fields

The lower weight of the SMPSU compared to linear power supplies, which used heavy iron cores, was a significant motivation for their development. In SMPSUs, ferrite cores are utilized instead. The size of the core in traditional transformers is controlled by lower frequencies and current handling capacity. The SMPSU focuses on frequency and current handling capability, making switching devices and rectification diode types crucial components. Modern circuits have advanced significantly in comparison to earlier designs from the 80s and 90s.

The Switched Mode Power Supply Unit – Mass Production of Computer Power Supplies

In 1977, Apple introduced the Apple II with an SMPSU, while IBM released personal computers with SMPSUs in the mid to late 80s with the XT range. IBM played a notable role in the development of SMPSUs.

The SMPSU Reliability – The 2000s and Beyond

From 1998 until the end of 2024, it was observed that while significant investments were made in processors, graphics cards, and cases, less focus was placed on the SMPSU. For reliable computer systems and home appliances, it is important to invest in both quality power supply units and their protection.

What Affects the Reliability of Power Supplies?

Here are some factors affecting reliability based on observations:

The Mains Transformer and Linear Regulation Circuit:

Iron-cored transformers do not perform well outside their frequency limits. Transformers designed for 50Hz can operate on 60Hz but will overheat at 40Hz. This poses issues when connected to generators with unreliable frequency control. Linear regulators, despite being bulky and inefficient, are reliable when designed properly. Filter capacitors may fail after several years but are typically replaceable.

The SMPSU:

Many SMPSUs lack sufficient input protection; devices must limit current draw on activation. Anti-surge devices may fail simultaneously with switching transistors. Dried-out capacitors can lead to catastrophic failures in switching due to changes in gate drive voltages and waveforms. Repairs can be costly and recurrent if all problematic components are not replaced. During the late 90s and early 2000s, SMPSUs often failed due to capacitor electrolyte issues known as “the capacitor plague“.

Working in the computer and peripheral distribution industry over this period was overwhelming, firstly because of the quantity of RMA (Returns) and secondly, possibly more importantly, clients, presuming that all their purchases had this problem, would return it for an exchange.  I picked this up chiefly in power supplies, as in this article, and motherboards. Strangely enough, in the power supply, there would be +5V but no +12V across the reservoir capacitors.

SMPSU Versus Linear PSU Power Factor Correction

SMPSUs can negatively impact the grid by causing harmonics through rectifiers and reservoir capacitors on the supply’s input side. In certain regions, such as the UK, active power factor correction is required for all SMPSUs to prevent inefficiencies.

Power Factor Considerations

With capacitive loads, the current leads the voltage, while with inductive loads, the voltage leads the current. Mains transformers are rated at VA to account for apparent power rather than actual DC draw in watts. This means transformers need to be rated higher than the load’s specified draw. Purely resistive loads equate apparent power to the load in watts.

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Modifying the SMPS