Benchtop testing a battery for emitted resistance

You wouldn’t believe that batteries might fall short radiated resistance screening, but today’s more advanced Li-ion battery packs consist of battery monitoring system (BMS) circuitry that monitors the charge/discharge state of each cell, as well as guaranteeing the battery is automatically separated from the system its powering should it find a fault (numbers 1 and also 2).

photo of a failing battery

Figure 1 This battery was failing radiated immunity by”disconnecting”itself from the system.

photo of a BMS circuit board under test

Lately, I was asked by a clinical devices producer to define and also fix a battery that was”detaching”automatically during the formal compliance testing of their item. The IEC 60601-1-2 standard(4th edition)for clinical products was upgraded in 2014 and also one result was that the test levels for emitted immunity were raised to as high as 10-20V/ m, with an optimum of 28V/m in some wireless and two-way radio bands. This battery pack was failing at 5-10V/m at 100 as well as 127 MHz. Figure 2 The BMS circuitry keeps track of the charge/discharge state of each cell. I’ve formerly defined the approach I developed for benchtop testing of radiated immunity in past books as well as articles(see recommendations). The strategy just makes use of a USB-controlled RF synthesizer with connected H-field probe. The RF resource I normally utilize is the Windfreak Technologies SynthNV($ 599), which generates about +19 dBm(80 mW) which normally has adequate power to develop an extremely extreme RF field near the tip of the probe and is sufficient for most items. Brushing up the probe around your motherboard and linked electrical wiring commonly exposes delicate circuit nodes. I’ve successfully used this technique in lots of customer jobs that have similar RF immunity concerns. More than as soon as, a project supervisor said that they had actually desired I would certainly been asked in weeks previously after assisting their group isolate and also deal with the concern within a few hrs. This has actually proven to be an extremely effective fixing method! When it comes to this certain battery pack, brushing up the H-field probe around the board exposed numerous spots of level of sensitivity.

graph of battery RF voltage drop

Every sensitive circuit node created the major MOSFET switch to disconnect the battery and also the output voltage

would certainly go down in the direction of zero(Figure 3). Figure 3 When RF was related to the battery, the voltage would certainly go down to zero. After evaluating the results, it came to be evident the most evident”antenna-like”structure that was getting the RF and coupling it right into the circuit board was the main battery cable television itself. I determined that just combining the H-field probe straight to the booster cable really did not have sufficient power to create the failure, so I

needed to consider choices. One prompt thought was to make use of a typical existing probe to couple the RF energy directly right into the booster cable. That is, we’ll mimic an emitted RF immunity examination by utilizing a conducted RF immunity examination configuration. I confirmed the battery wire was certainly the problem by briefly clamping a # 43 product ferrite choke around the cable in between the current probe and battery, which dealt with the problem. Regrettably, there was no room in the item for ferrite

chokes. While there was no very easy means to compare this test with a traditional RF immunity test using transmitting antennas at the item, the vital thing when fixing any kind of problem is asking,”Am I able to simulate the failing?”As soon as the failure is copied, after that numerous mitigations can be attempted. The typical RF synthesizer I was made use of to didn’t have actually the power needed to reliably cause the failing when using the existing probe coupling, so I resorted to a new item from Tekbox Digital Solutions. They make a collection of high-powered RF amplifiers with built-in inflection, ranging from 22 dBm (160 mW)to 37 dBm (5W )result. These are “modulated”amplifiers and also when the inflection is switched on, it can supply 1 kHz 80%AM modulation and also pulse modulation at either 1 kHz or 217 Hz(for screening TDMA mobile phones).

I picked to make use of the Tekbox TBMDA3 (5W) amplifier (10 to 1000 MHz) and also to drive it with my Signal Hound VSG25A vector signal generator. The VSG25A can likewise generate a selection of inflections, so it is extremely helpful on its own, although the highest output is +10 dBm (10 mW) and I usually need even more than that for normal troubleshooting.

The optimum recommended RF input power to the TBMDA3 amplifier was +3 dBm for 37 dBm (5W) result, so the mix was an excellent match. I simply guaranteed the outcome of the VSG25A was set listed below +3 dBm. Most of the screening just required a drive level of -15 to absolutely no dBm.

Later on, when I was consulting Tekbox on this test configuration, they suggested placing a 10W 3 dB attenuator between the amplifier result as well as either H-field probe or current probe in order to maintain the RF output better matched to 50 Ohms. While the amplifier is rated to run securely right into a brief, ideal technique determines a much safer, much more reliable termination.

I connected a range probe to channel 1 of my Agilent MSO-X 3102A oscilloscope as well as connected a medium-sized (1-cm diameter) H-field probe stuck partway right into the existing probe to keep track of the RF on/off state to channel 2. By causing on network 2 and also choosing a sluggish move, I could make use of the RF to begin the move while observing the battery voltage on channel 1. Every time I applied RF, I might enjoy the battery voltage decline during the failure. See the examination setup in Figure 4.

diagram of the battery test setup

Figure 4 In the test setup, the DMM as well as oscilloscope monitored the battery voltage and also the scope likewise kept an eye on when RF was used. The laptop computer controlled the RF synthesizer, which was intensified as well as connected to

photo of the test setup

the RF current probe that injected the RF right into the booster cable. Acloseup photo of the RF current probe and H-field probe

photo of the examination configuration might be seen in Figure 5. Note, I’ve made use of a number of turns of the battery cable wound around a ferrite toroid. This aids route the RF in the direction of the battery, rather than having it divided in two directions.

Figure 6 reveals a closeup of exactly how the present probe, additional inductance, scope probe, as well as keeping an eye on probe are arranged. Figure 5 Here you can see the total test arrangement. Figure 6 This closeup reveals the RF present probe as well as H-field probe utilized to check the RF. This test setup made it especially easy to carry out the

troubleshooting. Evidently, the RF power was combining right into the dual MOSFET power button, which when prejudiced off, detached the battery. By including a 0.01 μF filter capacitor to that location of the circuit, I had the ability to decouple the RF from impacting the switch(Figure 7). Figure 7 Applying a stable RF injection into the battery wire, I could either connect or disconnect the filter capacitor as well as see the outcomes in actual time.

By keeping track of the oscilloscope, I had the ability to execute troubleshooting and also mitigation experiments in real time. In Figure 7, we see that upon application of RF, the failing response differs between 0.8 and also 3.2 seconds in this instance. I have discovered that benchtop troubleshooting of radiated resistance issues is quick and also simple when using extreme and also local RF fields(either unmodulated or regulated )to the circuit card or system cables via making use of H-field probes or present probes. Once the failure mode can be simulated and the location of level of sensitivity is recognized, it ends up being much simpler to attempt numerous mitigations to solve the issue. In some cases, as in this case, the RF level needs a boost through the use of a broadband power amplifier, such as the Tekbox TBMDA3. What can conveniently take weeks of trial and error, and repeatedly cycling to and fro between your center as well as the compliance test laboratory, can currently be decreased to a few minutes or hours utilizing this effective strategy.

Referrals

  1. André and also Wyatt, Troubleshooting Cookbook for Product Designers, SciTech Publishing, 2014.
  2. Wyatt, Troubleshooting Radiated Immunity for Medical Products– Case Study, Interference Technology, 2017.
  3. Wyatt, High-powered radiated resistance pre-compliance screening on your workbench, EDN, 2015.
  4. Wyatt, Review: Windfreak Technologies SynthNV RF generator (Part 1), EDN, 2013.
  5. Wyatt, Inexpensive Radiated Immunity Pre-Compliance Testing, Interference Technology, 2014.
  6. SynthNV: 34MHz– 4.4 GHz RF Signal Generator w/ Power Detector, Windfreak Technologies.
  7. VSG25A Vector Signal Generator, Signal Hound.
  8. Wyatt, Review: Signal Hound VSG25A vector signal generator, EDN, 2015.

Relevant posts:

Leave a Reply

Your email address will not be published. Required fields are marked *