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This is a sequel to our Anatomy of Switching Power Supplies tutorial and we are going to explore in depth all power supply protections like over voltage (OVP), under voltage (UVP), over current (OCP), over power (OPP), over load (OLP), over temperature (OTP), no-load operation (NLO) and also the power good signal.
Usually power supplies have a monitoring integrated circuit on their secondary (see Figure 1), which is in charge of the power supply protections. Protections can also be built using stand-alone components instead of using a ready-made integrated circuit – the most common integrated circuit for this option is the LM339, which is a voltage comparator. Frequently the monitoring circuit is built on a small printed circuit board that is attached to the main printed circuit board from the power supply.
On power supplies based on the outdated half-bridge topology, protections can be provided by the PWM controller, which is physically present on the secondary. Some half-bridge power supplies with better design will provide a monitoring integrated circuit in addition to the PWM controller.
[nextpage title=”Power Good”]
When we first turn on the power supply, voltages are not immediately available on the power supply outputs: they increase until reaching their correct values. This increase happens is a fraction of a second (maximum of 20 ms or 0.02 s to be more exact).
In order to prevent these lower-than-normal voltages to be provided to the computer, the power supply has a signal called “power good” (also called “PWR_OK” or simply “PG”), which tells to the computer that the +12 V, +5 V and +3.3 V outputs are in their correct value and thus can be used, and the power supply is ready to work in a continuous fashion. This signal is available through pin eight (gray wire) from the main power supply connector.
There is also another reason for this signal to exist: the under voltage protection (UVP). As we will see in the next page, the under voltage protection shuts down the power supply if the outputs have a voltage below a certain level. If the UVP is active when the power supply is first turned on, the power supply would never turn on, because voltages are below the UVP trigger point. In other words, because when you first turn on the power supply voltages are below their values for a fraction of second, the UVP would prevent the power supply from being turned on. Therefore the under voltage protection circuit has to wait until the power good signal is active to be turned on.
This signal is generated by the monitoring integrated circuit or by the PWM controller (in the case of power supplies based on the half-bridge topology).
Below you can see the time diagram for the power good signal as available on the ATX12V specification. “VAC” is the input alternating voltage, i.e., the voltage from the wall. PS_ON# is the “power on” signal (i.e., you pushed the “standby” button from the computer case). “O/P’s” stand for “operating points.” And PWR_OK is the power good signal.
T1 is less than 500 ms, T2 is between 0.1 ms and 20 ms, T3 is between 100 ms and 500 ms, T4 is less or equal 10 ms, T5 is greater or equal to 16 ms and T6 is greater or equal to 1 ms. Just remembering that ms stands for millisecond and equals to 0.001 second.
[nextpage title=”Under and Over Voltage Protections (UVP and OVP)”]
We are going to talk about under and over voltage protections together because they are built using the same circuit. These protections monitor the +12 V, +5 V and +3.3 V outputs and shut down the power supply in case any of these outputs are above (OVP) or under (UVP) a certain value, also called “trigger point.” These are the most basic protections available and almost all power supplies have them, including the ultra-low-end models. This happens because all monitoring integrated circuits (PWM controllers, on the case of low-end power supplies based on the half-bridge topology) implement these protections – and also because ATX12V specification requires OVP.
One interesting thing that most people don’t know is that the ATX12V specification requires all PC power supplies to have over voltage protection (OVP) but the under voltage protection (UVP) is optional.
The problem with these two protections is that they are usually configured at trigger points that are too far away from the output nominal voltage. To give you a better idea, consider the over voltage protection trigger points required by the ATX12V specification:
|+12 V||13.4 V||15.0 V||15.6 V|
|+5 V||5.74 V||6.3 V||7.0 V|
|+3.3 V||3.76 V||4.2 V||4.3 V|
One manufacturer could build a power supply with an OVP configured at 15.6 V at +12 V output or 7 V at +5 V and still be compliant with the ATX12V specification. So this power supply could be delivering say 15 V at its +12 V output and the over voltage protection would not kick in, and it would be probably damaging your components due to this very high voltage.
In other words, ATX12V specification says that voltages must be between 5% their nominal values, but when comes to over voltage protection, it allows manufacturers to configure this protection up to 30% on +12 V, 40% on +5 V and 30% on +3.3 V.
How manufacturers choose OVP and UVP trigger points? By choosing the monitoring integrated circuit (or PWM controller, on the case of low-end power supplies based on the half-bridge topology), because the values for these protections are hard-wired inside this circuit.
For a real example, consider the popular monitoring integrated circuit PS223, which is used by several power supplies available on the market. This circuit provides the following trigger points for over voltage protection (OVP):
|+12 V||13.1 V||13.8 V||14.5 V|
|+5 V||5.7 V||6.1 V||6.5 V|
|+3.3 V||3.7 V||3.9 V||4.1 V|
And the following values for under voltage protection (UVP):
|+12 V||8.5 V||9.0 V||9.5 V|
|+5 V||3.3 V||3.5 V||3.7 V|
|+3.3 V||2.0 V||2.2 V||2.4 V|
Other circuits will present different trigger points.
Once again we’d like to call your attention on how far from the nominal voltages these protections are usually configured. For them to enter in action the power supply must be facing a very serious condition. In fact in our experience low-end power supplies (which don’t have any other protection besides OVP/UVP) will burn before these protections have a chance to enter in action.
[nextpage title=”Over Current Protection (OCP)”]
There are a lot of misconceptions about the over current protection (OCP) and an explanation of why this protection exists is in order.
There is an international safety regulation called IEC 60950-1 that states that no single conductor can carry more than 240 VA in computer equipments. Since computer power supplies deliver continuous current, this means that no output wire from the power supply can carry more than 240 W.
This way the ATX12V specification includes a requirement for an over current protection circuit in order to shut down any rail that pulls more than 240 W.
When it comes to the +12 V output, we have that this equals to a current of 20 A (P = V x I; therefore I = P / V or 240 W / 12 V).
Of course this relatively low limit would prevent manufacturers from building higher wattage units. So they came with the idea of breaking down the +12 V output into two or more group of wires, each group with its own over current protection. For example, two groups of wires with an OCP configured at 20 A each would double the maximum allowed power for the +12 V output from 240 W to 480 W.
Each group of wires with its
own separated over current protection (OCP) is called “rail” (although we personally prefer the term “virtual rail”). So a power supply with “two rails” means that its +12 V wires are divided into two groups and each group has its own OCP circuit.
Power supplies that have only one OCP circuit (or even no OCP at all) are called “single rail.”
Currently there are several single-rail power supplies with a current limit way above 20 A on the +12 V rail. How is this possible? If you pay attention, the IEC 60950-1 requirement is per conductor. So if you get a high-current rail and spread it into several wires and make sure no wire will carry more than 20 A/240 W, then you are good.
In summary, the difference between single-rail design and multiple-rail design is the presence of more than one OCP circuit for the +12 V wires on the later.
Some manufacturers add a color stripe on the +12 V (yellow) wires from the power supply in order to identify to each rail each wire is connected to.
Low-end power supplies, however, normally lie about the presence of two +12 rails. On their labels you will see the description of two +12 V rails (and even having some +12 V wires – usually the ones connected to the ATX12V/EPS12V cable – with a stripe on a different color), but inside the power supply these units don’t even have an over current protection (OCP) circuit and all wires are connected together on the same place, thus these units are in fact single-rail products.
So how can you visually identify the presence of separated over current protection circuits? Just looking at the wires is not enough, as a manufacturer can simply add wires with different colors to deceive you.
There are two basic components necessary to build the over current protection circuit: the power supply must have a monitoring integrated circuit supporting OCP (and with the number of channels compatible with the number of rails advertised by the manufacturer) and current sensors, also known as “shunts,” which are high wattage resistors with a known very low resistance. On Figures 3 and 4 you can see the most common physical aspects of these “shunts.”
Each “shunt” represent a +12 V rail. The two power supplies portrayed above have four sensors and thus they probably have four +12 V rails. If you follow the wires you will easily find out which wires are connected to which rail.
There is one detail, though. Some manufacturers use the same printed board for products with single-rail design and multiple-rail design. So you may find power supplies with more than one “shunt” that are in fact single-rail products, because even though the manufacturer added the "shunts", they are all connected to the same circuit, instead of using separated circuits.
So if you open a power supply and can find only one (or no) “shunt,” it is a single-rail design; if you find more than one “shunt,” it is probably a multiple-rail design (the “shunt” count tells you the number of +12 V rails), but it can be also a single-rail design. You can take a look at the datasheet of the monitoring integrated circuit to see how many overcurrent protection circuits (“OCP channels”) it has. If it has only one OCP channel, obviously you are facing a power supply with only one rail.
Although theoretically required by the ATX12V specification, several power supplies simply don’t carry this protection or only install it on the +5 V and +3.3 V rails but not on the +12 V, which doesn’t make any sense.
In order for you to understand even more how this circuit works, consider the schematics from Figure 5, which is based on the popular PS223 monitoring integrated circuit, which features four OCP channels. The components marked as RS5, RS33, RS12(1) and RS12(2) are the “shunts.” Notice how in this example the power supply has only two +12 V rails, as the other two OCP channels are being used to monitor the +5 V and +3.3 V outputs.
The OCP trigger point – i.e., the value at which it will kick in – is manually configured by the power supply manufacturer, usually by choosing the value of external resistors that are installed at one of the pins of the integrated circuit (resistors ROC5, ROC33, ROC12(1), ROC12(2) and RI in Figure 4).
[nextpage title=”Over Temperature Protection (OTP)”]
Over temperature protection, as the name implies, will shut down the power supply if the temperature inside the power supply reaches a certain level. Although several monitoring integrated circuits have this capability, not all power supplies implement this protection. This is an optional protection.
Opening a power supply you will easily spot a thermistor attached to the secondary heatsink (some power supplies use a tiny sensor soldered on the solder side of the printed circuit board, though). This thermistor is connected to the fan controller circuit, making the power supply to adjust the fan speed according to the power supply internal temperature. This thermistor is not used for the over temperature protection: power supplies with OTP usually have two thermistors, one for the fan circuit and a separated one for the OTP.
The trigger temperature for the over temperature protection is configured by the power supply manufacturer by choosing the value of a resistor that is connected to the monitoring integrated circuit (RT in Figure 5; on this same figure NTC is the thermal sensor – NTC stands for Negative Temperature Coefficient, meaning that the resistance of this component decreases with the temperature).
[nextpage title=”Other Protections”]
Over Power/Load Protection (OPP/OLP)
Over Power Protection (OPP) and Over Load Protection (OLP) are two different names for the same thing. This is an optional protection that shuts down the power supply in the case the unit starts delivering more power than a configured trigger point.
On low-end power supplies based on the half-bridge topology this protection is performed by the PWM controller integrated circuit – as long as it supports it, of course. On power supplies with active PFC circuit, this protection is implemented on the PFC controller.
In both cases what the circuit is really monitoring the total current pulled by the power supply from the power grid. If it increases above a certain value, the protection kicks in, shutting down the power supply.
Short-Circuit Protection (SCP)
Short-circuit protection is probably the oldest form of protection available, being very easy to implement (it is usual
ly implemented outside the monitoring integrated circuit using a couple of transistors). This is a required protection that will shut down the power supply in the case of any output to “short-circuit,” i.e., to touch the ground line (black wire), either accidentally or in case a component from the computer somehow burns.
No-Load Operation (NLO)
No-load operation is a required protection that allows the power supply to turn on and work correctly even if there is no load on its outputs. This is not exactly a “protection” like the ones we’ve seen so far, but more like a design requirement.
[nextpage title=”Comparison Between Monitoring Integrated Circuits”]
In the table below we compare the main protections supported by the most popular monitoring integrated circuits. We are separating the circuits in two kinds, first the ones with integrated PWM controller (used by low-cost power supplies based on the half-bridge topology) and then the circuits used on power supplies with more modern topologies.
Unless noted, OVP and UVP circuits monitors the main positive voltages (+12 V, +5 V and +3.3 V) only.
Obs: ATX2005 is also known by other names like 2005AZ, SDC2005, etc.
* Also monitors -12 V and -5 V outputs.
** Doesn’t monitor +12 V for this protection.