DC-DC Converter Testing and the Benefits of the PXB
DC-DC converters play a crucial role in the power chain for many power supply applications from medical equipment to space-grade satellites. These devices often undergo stringent testing to ensure they meet performance requirements. The level of testing and regulation depends heavily upon the application: military, medical, and space use cases are highly regulated to ensure the safety and integrity of the equipment. Bidirectional DC-DC converters are also used in many new applications including automotive, server, and renewable-energy systems in order to seamlessly regulate power to and from different systems (e.g., to simplify battery charge and discharge management in an electric vehicle).
However, testing bidirectional converters requires rewiring if the test equipment itself is unidirectional, making the process problematic. The range of DC-DC converters and their requirements can be broad, often causing manufacturers to have to switch between different types of equipment to test different types of converters.
The Kikusui PXB series bidirectional DC power supply alleviates testing problems because it tests the bidirectionality of a converter without changing an established setup. The PXB has the added benefit of being regenerative, redirecting wasted power in aging tests back to the local power grid. This benefit not only saves time by shortening the time-to-test, but also reduces total energy costs in long-running DC-DC converter aging tests. This article dives into the various testing requirements of DC-DC converters and how the PXB reduces the barriers to testing this equipment.
Common DC-DC Converter Tests
The typical setup for a DC-DC converter can be seen in Figure 1 in which a DC-DC converter is powered/controlled by a power supply, and excess energy is sent to an electronic load to sink power using transistors to simulate an ohmic resistance. The function of the DC-DC converter itself is to take an input DC voltage and produce a regulated DC output that is either a higher voltage (boost) or a lower voltage (buck).
Figure 1. Typical test set up for a DC-DC converter.
Testing Electrical Parameters
Electrical characterization of DC-DC converters requires testing for input voltage (VIN), input current (IIN), output voltage (VOUT), and load current (IOUT or ILOAD ). These parameters will determine the converter’s efficiency, or the ratio of output power to input power (POUT/PIN). Efficiency is often expressed in terms of a percentage, and while no real converter reaches the theoretical limit of 100%, it is important to get as close to that number as possible in order to avoid energy wastage.
Load Regulation
Load regulation refers to the changes in output voltage (VOUT) with respect to changes in the load current (ILOAD) under a constant input voltage (VIN). In some iterations of this test, the output voltage is measured at 50% the rated load current (ILOAD50%) and at 100% the rated load current (ILOAD100%), or over the entire range of load currents. This is measured in terms of a percent change as shown in Equation 1. Ideally, the output voltage remains constant even when load current fluctuates; however, changes will occur in the presence of an output impedance.
$$Load~regulation~=~\frac{V_{OUT} (I_{LOAD100 \%})~-~V_{OUT}(I_{LOAD50 \%})} {V_{OUT}(ideal)}~\times~100$$
Equation 1.
Line Regulation
Line regulation refers to the changes in output voltage (VOUT) with respect to changes in the input voltage (VIN). In this test, the output voltage must stay relatively constant while the input voltage is varied over a specified range or, over three specified input voltages: minimum, nominal, and maximum. This is also given in terms of a percent as shown in Equation 2.
$$Line~regulation~=~\frac{V_{OUT}(V_{IN(MAX)})~-~V_{OUT}(V_{IN(MIN)})}{V_{OUT}(ideal)}~\times~100$$
Equation 2.
Tests That Require an Oscilloscope
Other tests will require the use of an oscilloscope to measure turn-on time, hold-up time, dynamic line regulation, and ripple. Turn-on time, or start up time, is the time it takes between the application of an input power to when the power supply’s output is stable and within specifications. Hold-up time refers to the time taken for the power supply’s output to remain within specified limits after the loss of the applied input power. Dynamic line regulation measures the ability of the power supply to maintain the output to a specified steady-state voltage during changes in input power. The specifications define the changes in amplitude of the input power and the rate at which these occur. Ripple (or output noise) measures the AC component of the DC output due to an incomplete suppression of the alternating wave after rectification. This is typically measured using an oscilloscope to determine the amplitude, frequency, and nature of the ripple voltage.
Burn-in
Environmentally controlled stress testing and aging tests vary drastically depending upon the market or intended application the converter will be used in including: space, military, commercial, medical, telecom, etc. These tests can also vary by test requirements in terms of power cycling, elevated temperatures, temperature cycling, thermal shock, cold soak, vibration, mechanical shock, humidity, radiation, altitude, powerline perturbations, etc [1]. All, or some of these test parameters will be applied to a DC-DC converter in order to be deemed suitable for its end-application.
Changing Application Requirements for DC-DC Converters
There are many application-specific testing requirements for DC-DC converters in order to ensure these devices can either operate nominally within their specified lifetime or, operate safely to protect any personnel (or patient) in contact with this equipment. Medical devices, for example, will strictly follow IEC 60601-1 guidelines for isolation with three distinct classifications: body (B), body floating (BF), and cardiac floating (CF) that define creepage distances, insulation, and leakage current.
The automotive industry has guidelines for powerline perturbations (e.g., LV123, LV124, LV148). The LV123 standard specifies the requirements for high voltage (HV) components in cars and will test the interaction between HV and low voltage (LV) systems as well as test power supply operations at different voltage ranges and voltage ripples. These standards require simulations of the electrical disturbances that might be found within an automobile while driving in order to test the response of the component.
Military and aerospace converters might be subject to the MIL-PRF-38534 standard to test the converter’s stability with respect to radiation and classify it in terms of radiation hardness, probability of failure, expected lifetime, etc. Many applications have their own electromagnetic compliance (EMC) testing for components such as DC-DC converters. Comité International Spécial des Perturbations Radioélectriques (CISPR) 16-1-1 is the umbrella standard for commercial EMI testing with a variety of EMI standards referencing this one including CISPR 11, 12, 14-1, 15, 25, 32, 36 as well as ANSI C63.2, FCC, and MIL-STD-461. The EMI standards listed are specific to household appliances, automotive systems, multimedia equipment, military applications, and more.
Challenges When Testing a Bidirectional DC-DC Converter
The setup shown in Figure 1 has its drawbacks. First, more converters are being designed for bidirectional power transfer. The bidirectionality has utility in many new applications including automotive, server, and renewable-energy systems to, for instance, switch between energy storage and power distribution within an EV to simplify battery charge and discharge management. Testing the bidirectional features of a DC-DC converter in a traditional setup will lead to time spent readjusting and rewiring the test bench. Second, the power dissipation from the electronic load. While this may be feasible for smaller test setups, when scaled up, the energy dissipated grows drastically. And when this is applied to longer term burn-in tests, the power/cooling costs can be exorbitant.
The Benefits of Using the PXB in DC-DC Converter Testing
Bidirectional Power Supply for Aging Tests
As shown in Figure 2, the PXB bidirectional programmable DC supply can replace both the DC power supply and the electronic load of Figure 1. All tests with electrical characterization from input to output and from output to input with “charging” and “discharging” VIN, VOUT, ILOAD, POUT, load and line regulation, as well as application-specific tests such as power fluctuations (e.g., LV123, LV124, LV148) and load variations can be performed with minimal effort and no rewiring. This saves time when testing the bidirectional behavior of a DC-DC converter.
Another pertinent factor is the power saved. The PXB series has a regenerative function that returns input power to the local power grid as opposed to dissipating tens to hundreds of kilowatts as heat through an electronic load. This allows for efficiency aging tests with over 90% regeneration efficiency.
Figure 2. The PXB series can be used in power fluctuation and/or load variation tests to replace both the traditional DC power supply and electronic load to test the bidirectional behavior of a DC-DC converter.
Standards and Compliance Testing With the Sequence Function
The PXB series is programmable, and this capability is built into the unit. From the PXB’s Sequence Function, a total of 30 programs, and up to 10,000 steps can be created for all programs. Programs stored in the unit’s memory can be exported as a CSV file to a USB memory stick from the front panel. Programs can be created, edited, and executed from the front panel. Also, programs can be uploaded and executed from the Built In Web, an interface that can connect to the PXB via LAN. This feature allows continuous and rapid execution of power fluctuation or immunity tests (e.g., IEC 61000-4, LV123, DO160, etc.), which would be impossible using conventional DC power supplies (Figure 3).
Figure 3. Sample sequence run on the PXB series with 10 ms/div.
As shown in Figure 4, the sequence function is further augmented by the PXB’s quick rise/fall time of 10 ms to enable high-speed power fluctuation tests that cannot be carried out with ordinary DC power supplies.
Figure 4. A sample waveform with a rise time of 3.2 ms typical in constant voltage (CV) operation and 2 ms in constant current (CC) operation.
Conclusion
DC-DC converter testing can be quite involved from more straightforward electrical characterization to more involved tests that require an oscilloscope to measure parameters such as ripple, turn-on time, and the dynamic behavior of the converter. This testing can easily grow in complexity when considering some of the application-specific testing that is required of the DC-DC converter for EMC, power fluctuation testing, radiation testing, environmental testing, mechanical testing, etc. Moreover, more and more converters are designed to be bidirectional to, for instance, use a single power train for the charging and discharging of a battery in an EV. This complicates testing as it must be performed in both directions, causing test engineers to rearrange and rewire their setup. The PXB series is a bidirectional DC power supply and can therefore supply power at the input or the output, eliminating the need for rewiring. This benefit of the PXB is further bolstered by the fact that the power supply is regenerative, supplying wasted energy to neighboring equipment, saving the test facility power by both recycling power that would otherwise be wasted and by reducing the cooling necessary for sinking large amounts of power.
References
- Crandall, Earl. Power Supply Testing Handbook: Strategic Approaches in Test Cost Reduction. Chapman & Hall, 1997.
