Power Conversion Techniques for Automotive Emissions Requirements: EMI sources and CISPR 25 limits
This video series explores power conversion techniques for complying with automotive emissions requirements. In this video we discuss EMI sources in power converters, types of emissions (differential mode and common mode), and CISPR 25 limits on vehicle emissions.
Resources
Welcome to Part Two of the video series on Power-Conversion Techniques for Complying with Automotive Emissions Requirements. In this video, we discuss EMI sources in power converters, types of emissions, and CISPR 25 limits on vehicle emissions.
Switch mode power supplies are often the source of EMI in electronic systems. In the diagram below, you can see the switch node voltage alternates between the input voltage, around 13 volts, and ground, or 0 volts. The rise and fall times of the switch node are very short, which results in high dv by dt transitions. Similarly, the current through the converter switches have high di by dt edge rates. The current conducted by the switch goes from 0 amps to several amps very quickly, and vise versa at the end of the switch on time.
The input capacitor of this example buck converter filters the pulses of current drawn by the converter's inductor. This results in measurable voltage ripple-- in this case, around 50 millivolts. That 50 millivolts corresponds to about 94 dv microvolts of ripple at this switching frequency. This voltage ripple is one of the primary sources of conducted EMI.
Let's take a closer look at the input voltage ripple of an example buck converter. The time domain wave form is shown on the left. This time domain wave form can be converted to the frequency domain using the Fourier transform. Remember that any periodic signal can be expressed as the sum of sinusoidal wave forms. The Fourier transform breaks down the time domain signal into its various frequency components. These components can be visually represented with a magnitude versus frequency plot, as shown in the top right, or mathematically represented using a Fourier series which sums all the frequency components together as shown in the bottom right.
Next, let's examine automotive emissions requirements. CISPR 25 is a widely used emissions standard in the automotive industry. Car manufacturers may have slightly different requirements, but most pre-compliance testing uses CISPR 25 as a basis.
The graph on the right shows the emissions limits for radiative emissions in red, and conducted emissions in blue across the frequency range of interest. You can see that conducted limits range from 150 kilohertz to 108 megahertz, and radiated limits range from 150 kilohertz to 2.5 gigahertz. Due to the admissions limits in the AM band, most power converters have a switching frequency that is less than 450 kilohertz or greater than 2 megahertz.
If you're not familiar with the automotive EMI limits, it may be helpful to compare them to the EMI limits for IT equipment. CISPR 32, formerly known as CISPR 22, is a common EMI standard for IT equipment. The CISPR 32 Class B limit line is shown in red in the graph. This limit line starts at 150 kilohertz and is continuous up to 30 megahertz.
Now compare it to the CISPR 25 Class 5 limit line shown in blue. The CISPR 25 limits used in automotive applications go higher in frequency up to 108 megahertz, and have significantly lower limit levels. Over 20 dB microvolts lower in some cases. One other noticeable difference is that there are gaps in between frequency bands in CISPR 25, whereas CISPR 32 is a continuous line. And for some of these reasons, people sometimes say that the automotive CISPR 25 limits are more challenging.
Lastly, let's discuss the difference between differential mode noise and common mode noise. These terms are used to describe different types of noise emissions. Differential mode noise results from AC currents that flow out on one input wire and return on the other input wire. This noise type is what you would expect to see from the voltage in current ripple, caused from normal switching converter operation.
Differential mode noise tends to increase with load current and is attenuated with a pi filter. On the other hand, common mode noise is caused by currents that flow out in the same direction on both input wires. These currents return to the source via parasitic capacitance to the chassis ground. Common mode noise is mostly independent of load currents and can be filtered by a common mode choke. Better yet, careful design and layout of power converters can reduce common mode noise. This concludes part two of the video series. The next video takes a look at various techniques to reduce EMI caused by power converters.
This video is part of a series
-
Power conversion techniques for automotive emissions requirements
video-playlist (5 videos)