Implementing a cost-effective solution for automotive DC-DC converter testing

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Posted: 04/13/2017

A DC-DC converter is an electronic control unit (ECU) that converts a source voltage from one level to another. To ensure the DC-DC ECU operates seamlessly, manufacturers first have to put the ECU through rigorous functional tests while it is still in the manufacturing facility. 

Functional testing of a DC-DC converter generally requires power input and load output simulations to measure the stability and integrity of voltage outputs, power efficiency etc. Power levels may range from low to high. Automotive applications typically use higher power DC-DC converters, with ranges beyond 200 W.

DC-DC converters are used in start-stop systems of automobiles – these systems automatically shut off engine combustion during standstill, restarting the engine only when the driver engages the accelerator. These systems are seeing increased implementation, driven by the industry’s efforts to create “greener” environments with reduction of carbon emissions.

DC-DC Converter functional testing

A DC-DC converter is used to provide in-vehicle comfort – maintaining a 12 V supply to prevent any form of interruption to the vehicle’s infotainment and fan ventilation systems during engine startup. Figure 1 is a simplified block diagram that illustrates the function of a typical DC-DC converter ECU during activation. Whenever the battery voltage decreases to a level lower than 12 V due to impulse start-up, a trigger signal will be sent to the ECU, to boost the voltage and maintain it at a constant output level at around 12 V. 

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Figure 1: Block diagram of automotive DC-DC converter functionality

Condition simulations

Functional testing of the DC-DC ECU requires a battery input emulator that is able to generate arbitrary waveform types of high power input to the ECU. Manufacturers typically create their defined impulse input pattern; figure 2 illustrates an example voltage impulse input emulated for testing purposes. A high power dynamic DC power supply is required to create an arbitrary voltage pattern with high inrush current to simulate the battery’s transient condition in the test process. 

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Figure 2: Example of impulse battery voltage pattern

To achieve the right functional testing scenario, you will need dynamic DC power supplies capable of generating pulses from 12V to 6V at around 1-2 ms falling time, meeting the requirements of transient response emulations for most automotive batteries. It is also important to choose the appropriate power supply to minimize your initial set-up costs. There are instruments such as the Keysight N7951A/N7971A with 1 kW and 2 kW options rated at 20 V offering selections for lower (< 300 W) or higher power (> 300 W) types of ECUs in the market. This allows you to have greater flexibility to work with different power needs while leveraging the same equipment.

In addition to input emulation, electronic or passive loads are required to simulate the effects of on-board vehicular electronic networks. A load switching solution is needed to provide the flexibility of load disconnection and connection to establish open/close loop circuitries for functionality checking. The solution must also be able to tolerate handling of high current for automotive applications. Test engineers often need to develop custom switches for load connectivity, in consideration of safety and protective circuitry in the event that the ECU fails. Particularly in the high-mix automotive manufacturing industry, frequent re-design of custom switches is required to cater to different ECU applications – this incurs time and expenses. Therefore, a standard load switching solution usually provides better return of investment. Some solution providers such as Keysight offer standard load switching solutions – these off-the-shelf solutions are qualified for long hours of high current operations of up to 40 A per channel, which are typical requirements for automotive manufacturing test.

Measurement of Power Efficiency

Power efficiency is generally defined as “Power Efficiency = VIOutput/VIInput x 100%”, where VIOutput and VIInput are the ECU output and input power consumption correspondingly; higher efficiency means less the power loss during conversion. Power analyzers are very useful for engineers who want to quickly measure AC/DC power consumption, efficiency and quality. Multi-channel analyzers can simultaneously measure both input and output powers at very high accuracies. However, it may not be necessary to use a high precision instrument in the production line, since functionality checking does not need the accuracy and speed for analysis or characterization during the design phase. In addition, functional validations usually test operations at critical levels only. Figure 2 illustrates the typical levels tested in phases A, B and C of a battery input signal.

One can use a digital multimeter (DMM) to measure both input and output voltages and currents when they are static. Voltage measurement is relatively easy to capture by probing inputs/outputs reference to ground. For current measurement, instead of using the DMM as an “ammeter” that only works for low current measurements, a current shunt method is used. A current transducer or sensing resistor is placed in series at all inputs/outputs, and a DMM is used to measure its differential voltage that eventually will be converted to a current using Ohm’s Law V = I x R. Lastly, power efficiency can be calculated using obtained inputs/outputs voltage and current results. 

Keysight’s TS-5000 load switching solution offers the capability of current sensing. The load cards are incorporated with either a sense resistor or current transducer on every single channel for current measurement purposes. The architecture of load cards and matrix switches allows interconnection to an inexpensive basic DMM, offering a much lower cost manufacturing solution for DC-DC power efficiency measurements.

Measurement of Stability

Stability validation is required to guarantee the health of the DC-DC converter during activation. A dynamic DC power supply is programmed to generate battery impulse patterns. A digitizer is then used to capture input impulse patterns for verification of desired falling and rising speeds. Besides input validation, the digitizer is also used for output stability measurements. Voltage output waveform is acquired during ECU activation. The full waveform illustrates the overall output stability – ripple, average, peak to peak and settling speed throughout boost mode. A digitizer with a minimal sampling rate of  0.1 us is recommended - this high resolution setting helps to capture any sudden glitch or spike. 

Automotive batteries typically operate at around 12.6 V, so the digitizer must also be capable of detecting input signals at > 10 V. Higher-power typed of DC-DC converters typically come with multiple inputs/outputs, and you will need a digitizer with more than two channels in order to measure all inputs and outputs simultaneously. Input and output waveform acquisitions need to be synchronized within same time frame, displaying the correlation between all inputs/outputs, and also shortening the total test time. 

Keysight’s M9217A/L453xA digitizer comes with two or four isolated input channel options for simultaneous measurement. The high input voltage at ±256 V also eliminates the need for input signal attenuation for typical data acquisition instrument with ±10 V dynamic range. For DC-DC converters with multiple inputs/outputs, the number of channels on the digitizer can be multiplied by configuring additional digitizers within the system, while synchronization can be achieved via its triggering capabilities. This scalability enables the user to upgrade the solution with the existing setup without migration needs to other instrumentation.

Conclusion

Cost of test is one of the key factors in the total cost of manufacturing an ECU. Automotive manufacturers often spend time on developing their own rack-and-stack test system, and may end up spending more on sourcing higher cost instrumentation.  Overall cost can be greatly reduced by choosing the right instrument, or using the right test methodology. Choosing cost effective instrumentation and load switching solutions with commercial test-executive software as the foundation can help to increase manufacturers’ competitiveness in the automotive industry while keeping costs manageable.

keysightlogoo
Posted: 04/13/2017

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