As the world continues to trend toward increased energy savings and green energy sources, more and more heavy machinery and vehicles are becoming electrified. Mechanical combustion engines are being replaced by electric motors as part of this technology trend. As these demands accelerate, higher expectations for reliable and safe power are spurring engineers to put all their brain power into coming up with the most efficient product designs. The last thing that R&D engineers want is to mess with the power supply reliability and create a potential safety issue.
Over Voltage and Over Current Protection
Today’s system DC power supplies incorporate a variety of features to protect both the device under test (DUT) as well as the power supply itself from damage due to a fault condition or setting mishap. Over voltage protect (OVP) and over current protect (OCP) are two core protection features that are found on most system DC power supplies to help protect against power-related damage. But is that all that you need to know? In this blog, we will also discuss power protection on your devices that do not operate on fixed voltage and current levels.
Over voltage protect helps ensure the DUT is protected against power-related damage in the event the voltage rises above an acceptable range of operation. As over voltage damage is almost instantaneous, the OVP level is set at reasonable margin below this level to be effective; yet it is set suitably higher than the maximum expected DUT operating voltage so transient voltages do not cause false tripping. Causes of over voltage conditions are often external to the DUT.
Over current protect helps ensure the DUT is protected against power-related damage in the event it fails in some fashion, causing excess current, such as an internal short or some other type of failure. The DUT can also draw excess current by consuming excess power due to overloading or from an internal problem that causes inefficient operation and excessive internal power dissipation.
OVP and OCP are depicted in Figure 1 below in an example DUT that operates at a set voltage level of about 48V and uses about 450W of power. In this case the OVP and OCP levels are set at around 10% higher to safeguard the DUT.
Figure 1. OVP and OCP settings to safeguard an example DUT.
Over Power Protection
However, not all DUTs operate over a limited range, as depicted in Figure 1. Consider, for example, that many (if not most) DC-to-DC converters operate over a wide voltage range while using relatively constant power. Similarly, many devices incorporate DC-to-DC converters to give them an extended range of input voltage operation. To illustrate with an example (see Figure 2), consider a DC-to-DC converter that operates from 24 to 48 volts and runs at 225 W. DC-to-DC converters operate very efficiently, so they dissipate a small amount of power and the rest is transferred to the load. If there is a problem with the DC-to-DC converter that causes it to run inefficiently, it could be quickly damaged due to overheating. While the fixed OCP level depicted here will also adequately protect it for over power at 24 volts, you can see that it does not work well to protect the DUT for over power at higher voltage levels.
Figure 2. Example DC-to-DC converter input V and I operating range.
A preferable alternative would be to have an over power protection limit, as depicted in Figure 3. This would provide an adequate safeguard regardless of the input voltage setting.
Figure 3. Example DC-to-DC converter input V and I operating range with over power protect.
Since an over power level setting is not a feature that is commonly found in system DC power supplies, this would then mean having to change the OCP level for each voltage setting change, which may not be convenient, desirable, or in some cases, practical to do. However, in the Keysight N6900A and N7900A advance power system DC power supplies, it is possible to continually sense the output power level in the configurable smart triggering system. This can then be used to create a logical expression to use the output power level to trigger an output protect shutdown. The N7906A software utility was used to graphically configure this logical expression, and then it was downloaded it into the advance power system DC power supply, as shown in Figure 4. Since the smart triggering system operates at hardware speeds within the instrument, it is fast-responding, an important consideration for implementing protection mechanisms.
Figure 4. N7906A software utility graphically configuring an over power protect shutdown.
A glitch delay was also added to prevent false triggers due to temporary peaks of power being drawn by the DUT during transient events. While the output power level is being used here to trigger a fault shutdown, it could just as easily be used to trigger a variety of other actions.
We have discussed that advance system power supplies can provide over voltage and over current protection as well as protection for over power conditions. For more information on protection against power related damage, download the Protect Against Power-related DUT Damage During Test application note.