Creating a combined test adapter for a DC/DC power product presents a design challenge, but if implemented properly can speed test throughput and lower cost.
Production test of a finished electronic product often involves two techniques: in-circuit test (ICT) and functional component test (FCT). The ICT technique examines a non-powered circuit board to measure attributes such as inductance, capacitance, impedance, and resistance of individual components and to check for opens, shorts, and incorrect or misoriented components. FCT applies power to a device under test (DUT) and measures its input and output characteristics under load, typically using a completely different test adapter. It is possible, though, to use a single adapter for both tests with careful adapter design.
ICT typically uses a bed-of-nails probing approach and techniques such as direct digital synthesis (DDS) and Discrete Fourier Transform (DFT) to generate stimulus signals and to perform analogue measurement analysis. This allows an in-circuit analyser (ICA) to determine a DUT is within tolerances without having to power up the device. A relay multiplexer controls the interconnection between the nail contacts and the relevant analogue channel or digital driver/sensor (D/S) on the probe board (Figure 1).
Figure 1 This typical bed-of-nails probe uses a 2×16 relay multiplexer (only one channel is shown in the schematic).
While an ICA module can also be used to carry out limited FCT by applying power to the device and measuring its input and output characteristics, FCT typically requires its own test adapter. There are several practical reasons for using a separate adapter.
Separate adapters are typical
First, ICT bed-of-nails probes are not rated to carry the supply voltage or load current necessary to carry out a full-function test on powered-up devices. A dedicated FCT test-bed will typically have heavy-duty contacts designed to carry higher currents or voltages without overheating, arcing, or suffering from excessive wear. Further, because these heavy-duty contacts take up more space than typical ICT probes, FCT test adapters can typically check only one DUT at a time.
Second, the internal programmable power supplies, relays, and electronic loads in an ICT analyser are typically not designed for high current testing. Simply swapping the power supply units for more powerful versions can cause serious interference problems with the sensitive ICT analogue measurements. The higher currents can introduce measurement inaccuracies due to ground-bounce, voltage drop along wiring, and through transients generated from switching inductive loads. A dedicated FCT adapter usually makes its measurements at lower resolution and with heavier filtering, so it is less sensitive to interference. Also, the power supplies and relay contacts of a dedicated FCT adapter are more robust and able to switch more than one amp.
The relay interface hardware and the software control used to change the relay configuration in the ICA modules are also typically different. In an ICT application, configuration often uses a parallel input output (PIO) controller and relay driver (Figure 2). In these applications relay switching speed is not usually an issue; the relays are mainly reconfigured at the end of each DUT test to multiplex connections from one pin assembly to the next.
In an FCT test adapter, however, the relays must change the functional test setup for each separate test on each DUT, so the control data throughput to the relays is higher. In a dedicated FCT set-up, the need for higher throughput is not an issue as only one DUT gets checked at a time. A combined ICT/FCT adapter, however, will need to test multiple devices at the same time, making the relay control’s speed limitation a major bottleneck in production test.
Finally, while ICT measurements can be made in milliseconds, FCT procedures are typically much slower. Measurements made while the DUT is powered up cannot be made instantaneously; the outputs have to settle before a reliable measurement can be taken. Typically, then, the FCT process will take five to 10 times as long to complete as the ICT process for the same product. If testing is combined in one ICT/FCT platform, then, the FCT part could be a bottleneck in production. Keeping the two processes separated allows one ICT machine to feed several FCT testbeds used in parallel, increasing the throughput and reducing the bottleneck.
Despite these considerations, however, Recom Power found that, for the newly-developed DC/DC product series, the additional cost and testing time of using two separate test adapters was not acceptable. Combining the high-speed advantage of ICT with the practical quality assurance of 100% FCT, all in one test adapter, was a technically complex challenge: the product series covered devices with up to 6A output current and input voltages up to 60V. Each PCB panel contained forty partly-finished modules, which required parallel testing using heavy-duty power supplies. The data throughput was therefore very high, and any timing errors could be problematic. Recom contracted Elmatest in the Czech Republic to build a combined ICT/FCT test adapter for the manufacturing service provider, Teledyne Teststation LH.
Creating a combined ICT/FCT test adapter
Working in close cooperation, Elmatest application engineer Zdenek Martinek and Markus Stöger from Recom’s R&D department, realised that this was no ordinary project. There were several significant problems that needed to be solved: how to combine ICT/FCT in one multi-panel, how to handle the high relay control throughput, how to accelerate the FCT process, and how to cope with the high power levels without harming the sensitive probes. Fortunately, solutions were found for all of these issues.
The first problem that needed to be solved was how to combine ICT/FCT given the product’s multi-panel design. Each PCB contained 40 independent circuit modules, not part-built but complete products that were already finished, cased, and screen-printed. Not all of their internal nodes were accessible to the ICT pin panel; this was deliberate. The DC/DC converter switches at high internal frequencies and it is integral to the product concept that the metal case and its multi-layer PCB form a complete six-sided Faraday cage to avoid EMI issues. Any external connections to an internal high frequency switching node would form a pathway for EMI to pass through the EMC seal and to radiate, possibly causing measurement errors.
The solution to testing these enclosed and inaccessible modules was to include a test module on each multi-panel. The test module allowed access to all of its necessary ICT nodes so that we could verify that each panel is built correctly. Once the conventional ICT procedure is carried out on the panel using the test module, then the remaining modules need be FCT-checked only.
Figure 3 The multi-panel PCB (top and bottom shown) has an ICT test module in one corner to support board testing.
The code required to carry out a single test and measurement process is called a test vector. The arrangement of the inputs, outputs, and analogue channel configurations required to carry out the measurement gets transmitted to the test controller as a data ‘burst.’ These configurations are loaded into local on-board memory, then a timing strobe signal activates them simultaneously. The configuration stays latched until the test has been completed and the measurement data transferred back to the CPU. In the meantime, the next data burst can be pre-loaded into the registers to await the next strobe signal. This methodology is what allows ICT to achieve its very fast throughput of around 4µs per vector.
The standard relay drivers used in the GenRad Teststation, however, are driven from the PIO controller, which in turn receives its commands from the controlling PC via a MXIbus (Fig 2). This arrangement proved to be too slow for the project. The goal was to process different FCT measurements within a single test vector using the high-speed system controller to control the relay configuration. In order to accelerate the relay switching rate, a novel relay driver topology was implemented in the Recom test adapter, based on a technique called ‘active burst.’
In active burst, some of the relays are driven not from the PIO controller card but directly from the D/S outputs, which are kept active until the ICA measurements have been completed. Each D/S can be configured with nine separate functions: idle, drive low or high, sense low or high, hold, drive with deep serial memory, sense with deep serial memory, and collect CRC data. In this case, the drive function was used to directly power the relays. The D/S drive output is limited to TTL voltage and current levels, though, which are normally not sufficient to operate a relay without a separate driver. But by building the test adapter using Darlington transistor current amplifier relay coils, the D/S modules were able to operate the relays directly, bypassing the PIO controller. This direct operation made the relay control practically instantaneous and made the coding much simpler.
Accelerating the FCT
The second problem that needed to be solved was how to accelerate the test’s FCT process. Waiting for the analogue levels to settle would have made the overall testing unacceptably slow. The technique we applied was to utilize the processing power already inherent in the ICA system for tests such as component impedance measurements (Figure 4). We invoked waveform generation and analysis techniques such DDS and DFT, which are inherently faster than any analogue bridge balancing measurement technique.
The breakthrough was to realise that these same advanced techniques could also be used to determine the powered-up functional testing results. Instead of applying a fixed load, waiting for the output to stabilise, and then measuring the input and output currents and voltages, we could pulse the output load for a few milliseconds and then derive the final output characteristics from the processed results. This approach reduced the measurement time by up to 80%.
Figure 4 Measuring terminal impedance in ICT uses a digitally synthesized source to drive the component and DFT to analyse the result.
One significant development issue we faced was matching such dynamic load and supply switching with the ancient “spaghetti” software the GenRad test station used. This legacy software was a mix of Pascal, Assembler, and Basic. Further, GenRad ceased to exist as a separate company back in 2003. It is a tribute to the robustness of their design, however, that even today it is possible to piggy-back state-of-the-art operating systems on top of the original hardware.
Avoiding probe damage
The solution to the FCT acceleration problem, using pulsed load signals, also solved the third problem: how to avoid damaging the sensitive probes. Because we pulsed the load current for only a very short time, there was no noticeable local heating at the very fine contact area, even with 6A peak current through a probe rated at only 2A. We were able to program the on-time/off-time ratio so that, even with sequential measurements, the probe tip had time to cool down between pulses and would not burn or scorch. This pulsed load technique also meant that the power supplies were not overloaded.
One of the ICT tests is to measure the internal voltage divider resistances used to pre-set the product’s output voltage. We leveraged the results of this ICT test in the FCT test. The test system could automatically derive the output voltage, output current, and input voltage range from ICT and then pass these values on to the FCT test program to carry out the appropriate functional testing. This automation eliminated the possibility of operator error setting the FCT variables out-of-range and damaging either the product or the expensive pin boards or programmable power supplies.
The combination of allowing ICT access through a test module, direct drive of configuration relays by the test adapter’s driver/sensor lines, and pulsed load signals made the combined ICT/FCT test adapter possible. The net result of applying all of these techniques is a combined ICT/FCT test time of between 1.8 and 1.9 seconds per DC/DC module, meaning that a complete PCB multi-panel can be 100% tested in less than 80 seconds. This includes the time needed to remove the tested PCB and place the next PCB to be tested into the test adapter. The cumulative time- and cost-saving for a minimum production run of 5000 has been instrumental in the resulting success of the entire product series. As a result of this achievement, the Recom Power Module’s initial design has now been extended from a single series with eight variants to three different series with a total of 22 variants, all sharing the same footprint and test adapter.
Steve Roberts is the innovation manager of Recom Power.