Android / Machine learning
Why PSUs always use protection
Your PC’s power supply unit has one job to do – and that’s convert the 240VAC AC mains power generated by the power utilities into DC power your PC components can munch on. The vast majority of the time, it all works smoothly, but on the rare occasion something electrically goes wrong in your PC, you want to know that problem won’t cause you even more grief. That’s the job of the myriad of protection systems inside PC PSUs. But what do these protection systems do? That’s what we look into this month.
Stuff happens
Mistakes are made, components die, computers fail – not often, hopefully, but it does happen. That’s why electrical engineers design in safety measures into PSUs, so that when things do go wrong, you’ve got some technology barriers to minimise the damage. For example, if a motherboard component fails, you want the system to hopefully shut down before it fries your SSD.
Voltage Rails
Your PC requires different voltages for different jobs and for this, your PSU delivers different voltage supplies or ‘rails’. The four key rails are +12V1DC, +12V2DC, +5VDC and +3.3VDC. The +12V1DC and +12V2DC rails both supply 12-volts DC voltage, the +12V2DC rail usually feeding the four-pin ATX/EPS12V CPU plug.
Each rail supplies a level of electrical current measured in amps proportional to the overall power rating of the PSU. For example, the Seasonic S12III-650 will supply up to 20-amps of current on the +3.3VDC and +5VDC rails, up to 54-amps on the combined +12VDC rail.
There are two key points here – first, each voltage rail must supply the rated voltage and ideally, it should never deviate from that. So, the +3.3VDC rail must supply +3.3VDC. In practice, however, you never get zero-deviation and the ATX12V PSU specifications allow the voltage on each rail to vary over a small range. For these four rails we’ve mentioned, it’s +/-5%. That means the +5VDC rail, for example, can drop no lower than +4.75VDC and go no higher than +5.25VDC. For the +12VDC rails, the lower bound is +11.4VDC and the upper, +12.6VDC. PSUs also usually have two other rails – a +5VSB rail and a -12VDC rail. The -12VDC rail is optional and only delivers a small current output, while the +5VSB is a ‘standby’ rail delivering up to 2.5-amps on a 650-watt PSU.
The second point is that a PSU should not be made to deliver more current than it is rated for. Remember in Iron Man near the end when Tony Stark tells Pepper Potts to “throw open all the circuits” on the arc reactor and it blows itself to bits? Dial the idea back down to ‘0.01’ and it’s why overloading electronic components for any length of time is bad - depending on how great the overload, they tend to go ‘snap, crackle, pop’.
Protection systems
At the start of this series, we talked about voltage, current and power. Multiply the first two together and you get the third. These three key parameters and ensuring they stay within rated limits is what most PSU protection systems are all about. They’re baked into the current ATX12V v2.4 PSU specification, Revision 1.31, an easier-read version of which is available from Intel (tinyurl.com/intelpsupdf, PDF).
All up, there are six common systems that make up most PSU protection frameworks:
Over-Power Protection (OPP)
– this kicks in when the total power being drawn from the PSU exceeds its maximum rating. In practice, the trigger threshold will vary from one manufacturer to the next because this is not strictly part of the ATX12V standard. It can typically be
anywhere from 10% to 50% over maximum depending on the PSU.
Over-Voltage Protection (OVP)
– this one is part of the ATX12V standard and triggers when the +3.3VDC, +5VDC or +12VDC rails exceed their nominal levels by roughly 25%. Interestingly, this over-voltage trigger level itself has a range. For example, over-voltage on the +12VDC rail can be considered ‘triggered’ if the voltage reaches a minimum of +13.4VDC up to a maximum of +15.6VDC. PSU makers choose somewhere in this range.
Under-Voltage Protection (UVP) – this is the other side of the OVP coin but again, is not part of the ATX12V standard, so it varies depending on the PSU manufacturer. Under-voltage can cause computer systems to crash, so it’s not a good thing. However, under-voltage is typically less dangerous to electronics in general than over-voltage.
Short-Circuit Protection (SCP)
– this is an important one and is included in ATX12V. A ‘shortcircuit’ occurs when there is a low-resistance path from any supply rail to the zero-volt rail or ‘ground’ on a power supply. Short circuits can occur in any number of ways, so short circuit protection protects the PSU and your PC components by noting what’s called the ‘impedance’ or the resistance of the electrical load on the PSU rail. If it falls below typically 0.1-ohms, SCP should come to the rescue.
These are the four protection systems you’ll find on any decent PSU, but the better models will also include:
Over-Temperature Protection (OTP) – this is an optional extra according to ATX12V, but if you buy a fan-less PSU, it is a must. OVP is designed to trigger when the temperature within the PSU exceeds a preset trip level. Excess heat and electronics notoriously do not mix, so it’s important that should the temperature exceed electricallysafe levels, the PSU shuts down before it blows up.
Over-Current Protection (OCP)
– PSUs are rated to deliver a certain amount of current on each voltage rail and in some cases, the total rated current might be less than the sum of the two rails combined. For example, the Seasonic
S12III-650 can do up to 20-amps on each of the +3.3VDC and +5VDC rails, but combined, the output power of these two rails cannot exceed 100-watts. So if the +5VDC rail is supplying 15-amps, that’s 75-watts of the 100 gone (5x15), leaving only 25-watts for the +3.3VDC rail, or only 7.57-amps (25/3.3). The more expensive PSUs monitor output current levels and auto shut-down if levels exceed a trigger point, again, depending on the PSU and its manufacturer.
Safe computing
PCs are fun, entertaining and help many of us earn a crust, but given they spend so much time plugged into an AC mains socket, it’s important they have the protection framework to ensure that the rare event of a component fail doesn’t end up a catastrophic one. This is just one of the reasons why a good, reliable PSU should be the first PC component you think about, not the last.