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CS101 Conducted Susceptibility

Overview

Purpose

The MIL-STD-461 CS101 conducted susceptibility test injects voltage noise into the power lead of electronic units. This test is intended to ensure the unit is not susceptible to steady state electronic noise injected into the shared power bus. It also gives a basic check of sensitivity to power quality effects including harmonic distortion of AC power. Note that the conducted emissions test CE102 covers 10 kHz to 10 MHz while CS101 covers 30 Hz to 150 kHz while the bulk current injection test CS114 covers 10 kHz to 200 MHz. In this sense CS101 is trying to simulate direct voltage noise on the bus while CS114 is simulating inductive noise coupling from adjacent power cables.

CS101 Limit Line

The standard CS101 limit line is expressed as a voltage from 30 Hz to 150 kHz as shown below with a relaxation at 5 kHz to the limit line. A power curve is also given in the standard that limits to peak power injecting into a power line as 80 W. Note that these levels are in RMS so 126 dBµV = 2 Vrms = 2.828 Vpk.
MIL-STD-461G Figure CS101-1. CS101 voltage limit for all applications.

CS101 Test Setup

The general test setup, the CS101 signal injection setup, and the 50 µH LISN schematic are shown below. This setup is similar to CE102 with the addition of the signal injection transformer, a 10 µF line to return capacitor, and an oscilloscope to monitor the injected voltage. I don't recommend using an isolation transformer on the oscilloscope because it usually violates the NEC, UL and manufacturer grounding requirements. Instead, use differential probes to monitor the line voltage.
MIL-STD-461G Figure 2. General test setup.
MIL-STD-461G Figure CS101-4. Signal injection, DC or single phase AC.
MIL-STD-461G Figure 6. LISN schematic.

Design and Analysis

General Approach

CS101 is one of the easiest of the MIL-STD-461 tests to pass if a little design effort is made. This test is easy to simulate because the frequencies are low and CS101 only applies to the power supply line input. Three key guidelines are:

  • 1. The power supply input voltage range must be wide enough.
    This test is temporarily shifting the voltage seen by the input power supply up and down several volts. During this test the input voltage is set to the minimum and maximum voltage across the rated voltage range of the EUT. The operational voltage range must be large enough to accept this additional positive and negative voltage injected on top of the supply. Otherwise, the supply can shut down due to under-voltage conditions or be damaged due to over-voltage conditions.
  • 2. EMI filter resonances should be damped.
    If resonances occur in the EMI filter, then the CS101 injected voltage can be multiples several times due to resonances. This resonance may cause under-voltage or over-voltage conditions and cause shutdown or damage to the power supply. Adding damping resistors to the filter may be required. Damping resistances usually help with system power quality requirements because resonances in the CS101 frequency range may cause instability to converters.
  • 3. Isolate the power supply input or use chassis power return.
    Isolating the input power lines from chassis ground and all signal references is best practices. However, sometimes a non-isolated power supply, such as a 28 V to 5 V buck converter, is attempted due to the significant size, weight and power (SWaP) performance benefits. This can work if you are using a DC chassis power return and implement appropriate grounding and bonding rules at the system level. The grounding architecture is rarely
    Attempting to use a non-isolated DC power supply input will introduce susceptibility and emissions concerns for every non-isolated interface on the unit.

CS101 Simulations

I recommend getting the CE102 simulation running first in PSpice because emissions requirements are usually more difficult to meet. The CS101 simulation will build upon the CE102 simulation with the following steps:

  • 1. Add the injection transformer and 10 µF capacitor.
    The injection transformer must be added on the power input line but can be modeled as an ideal 1 VAC voltage source. The capacitor should be added at the LISN and an ideal capacitor model can be used.
  • 2. Remove the switch mode supply model.
    Most of the switch mode power supply model may be removed. It is important to leave the input capacitor bank to the switch converter because that will significantly affect the CS101 response including resonances.
  • 3. Create an AC simulation.
    The CS101 simulation should be an AC sweep simulation from 30 MHz to 10 kHz with a minimum number of points per decade of 462 to give a frequency resolution of 0.5% or better which is double the accuracy required by testing. The voltage should be probed at the input capacitor bank of the converter.
  • 4. Perform post processing and iterations.
    Once the simulation is complete, the output should be exported to a CSV file. Copy and paste the probe voltage data into a CS101 post processing Excel template to scale to the specified limit line. The pass-fail criteria is typically based on the operational voltage range. I recommend showing a minimum design margin of 6 dB in a CS101 analysis. If exceedances are observed due to resonances, then damping resistance may need to be added to EMI filter. These damping resistors are usually added in series to capacitors. If changes are made, then the CE102 simulation should be updated.
    This may take several iterations, but the design needs to close for both CS101 and CE102.

CS101 Pre-Compliance Testing

Bench testing for CS101 should follow the same CE102 setup but with the injection transformer, 10 µF capacitor, and monitor oscilloscope added to the setup. In testing the unit should be monitored for correct behavior while the injection level is increased in 1% frequency steps (232 points minimum per decade). Automating this testing with scripting is recommended.

  • 1. Transformer and driver.
    Most linear power isolation transformers designed for 60 Hz power applications can work as an injection transformer. A standard function generator can be used to generate the waveform, but it should be buffered with a power amplifier. Off the shelf amplifiers are readily available but this can be built as a power BJT push-pull circuit if needed.
  • 2. Return line.
    CS101 only requires testing of the input power lines and power returns are not required to be tested. I recommend testing the power return lines as well in pre-compliance testing, especially if a non-isolated power supply is used but not chassis power return.
  • 3. Design margin.
    The only way to quantify design margin with bench testing of CS101 is by over testing. This may not be appropriate for all designs and damage may occur if the any parts are over stressed. Also, CS101 can take a long time to completely step through all the frequencies so gradually increasing the injection level is generally not feasible.
    I recommend bench testing at the specified CS101 injection level to verify compliance, but a simulation design margin of 6 dB or more is sufficient to ensure a robust design.

  • Need Help?

    If the steps above seem daunting or you run into problems along the way, then please reach out to EMI Sleuth for help in closing the analysis and getting the unit passing.