Critical Power Immunity Testing of DC Powered Equipment
Reliable operation of on-board electrical equipment under all circumstance is a critical requirement to ensure the safety of passengers and crew. This is true for both AC powered and DC powered equipment. With the continued growth of on-board electrical systems, the amount and types of equipment that needs certification and compliance testing for power anomalies increases every year. In this month’s blog we will discuss compliance testing of DC powered products but much of the same issues apply to AC powered products which we will cover in a future blog post.
The requirements that electrical equipment has to meet are controlled by standardization bodies or governments. For commercial aviation, the governing body is the US based Radio Technical Commission for Aeronautics or RTCA for short. (www.rtca.org).
The RTCA is a private association and a private public partnership between a large group of companies involved with aviation technologies. RTCA publications cover many aspects of safety and design but the one of interest for this month’s blog is the DO160 standard. This standard covers many aspects of equipment operation and verification, including Section 16 which deals with power immunity.
DO160 Rev G, section 16
Currently at revision G, the DO160 standard is constantly updated to cover new developments and technologies that find their way in new airplane designs. Section 16 of the standard covers environmental conditions and test procedures for power input.
Recent additions to Section 16 along these lines have been the inclusion of wild frequency for AC powered equipment and a new 270Vdc bus voltage to support the increasing electrification of modern and ever larger air frames.
For DC testing, requirements and test procedures are included to cover both 28Vdc powered and 270Vdc powered equipment.
Other Test Standards
Some aircraft manufacturers issue their own test requirement standards to their vendors and subcontractors. These proprietary test standards are often based on DO160 but expanded or modified in specific ways to meet the OEM’s requirements. Some examples of proprietary test standards are listed in the table below.
Test Equipment Requirements
When dealing with DO160 or proprietary test requirements, the user must review the test equipment requirements called out in the standard. This will be different for AC or DC testing but often the same power source can be used if it is capable of both AC and DC output modes.
For DC test requirements, careful consideration should be given to the following DC source requirements:
- Required maximum DC output voltage. This is often considerably higher than the nominal test voltage.
- Voltage slew rate. DC transient testing typically requires voltage slew rates that conventional DC power supplies cannot support. AC sources with DC mode however support high voltage slew rates as they are designed to support high frequency AC and have no output capacitance.
- Maximum DC current supported. At lower than nominal input voltage, some EUT may draw more current which the power source used must be able to deliver.
The considerations we are reviewing in this blog are similar for both DO160 power testing and testing to any of these OEM test standards. Let’s take a closer looks at what this means for the power source specifications.
All loads must be capable of riding through DC voltage transients without any disruption in operation. Voltage transients can be caused by power cycling of other loads on the same DC bus or by transfers between different DC buses.
For example, Section 126.96.36.199c addresses the need to test for these voltage surges that can occur during normal operation. To support these tests for 270Vdc powered equipment, the power source used must be capable of outputting 400Vdc for 30 msec. That means a 300V dc power supply would be ok to power the EUT but not to perform compliance testing to DO160. For abnormal operation test conditions, this requirement increases to as much as 425Vdc, beyond the range of even some AC and DC source models.
Voltage Slew Rate
Using the same Voltage Transient example, the required slew rate to run this test is much higher than most DC power supplies can handle. The voltage rise time from 270Vdc to 400Vdc must occur in 1 msec – requiring a voltage slew rate of (400 – 270) / 0.001 = 130kV/s. The voltage fall time must be less than 5 msec or 26kV/s. These slew rates are supported on an AC and DC capable programmable source like the Pacific Power AFX series, but not on a regular programmable DC only power supply.
Another consideration is the current demanded by the EUT, especially at lower test voltage such as 14Vdc or 28Vdc. Most AC sources with DC mode have higher voltage ranges as they need to support AC voltage output to at least about 150Vrms. That means voltage ranges are typically 212Vdc at their lowest and full power is not available at 14Vdc or 18Vdc. This means the power source must be oversized to support the maximum required DC current. Fortunately, this is issue applies to 14Vdc and 28Vdc load testing only, not 270Vdc. These lower dc input loads typically are fairly low power so DC current requirements are low. For example, a 6kVA 3600AFX in DC mode can deliver 62.5Adc at 28Vdc or 1750W. (29% of the source’s full power rating).
Available Test Software
Many of the tests called out in these power test standards require extensive programming of voltage levels and durations. There are also several EUT performance tests that will require additional measurement equipment such as scopes, meters and power analyzers. Developing these test routines in house can be a time consuming and costly endeavor. To support its aviation and test lab customers, Pacific Power has developed extensive libraries of pre-written test sequences that are available to run using our Test Manager Windows software. These test sequences covers all available public and most OEM test standards for AC and DC power testing. The table below lists available sequences and the latest standard revision level.
Why does it matter?
Understanding the requirements for the test equipment used to perform compliance testing to avionics DC power test standards is important for some of the reasons outlined here. Additional considerations apply for AC testing which we will discuss in an upcoming blog entry. Always discuss your test requirements with our application engineers so they can help you determine the best power source available.
Do your homework and consult with our product specialist and application engineers to make sure your test needs for AC and or DC power are covered before making a selection. Also consider the ability to expand the power level of your test equipment as the level of electrical power used on new airplane demands keeps rising.