Of course, it is not possible to get around the laws of physics. According to the Maxwell equations, each alternating current generates an alternating electromagnetic field. This applies to any electrical conductor, which naturally also has a certain electrical capacitance and inductance and thus forms a resonant circuit. This resonant circuit emits electromagnetic energy into the environment at a specific frequency of f = one / (2π ∙ √ (LC)). Thus, the circuit acts as a transmitter, but can conversely also absorb electromagnetic energy and act as a receiver. Antennas are designed so that they radiate or receive as much power as possible.
EMI sources in switching power supplies
However, this antenna effect is by no means desirable in every application, and it can lead to adverse side effects. Down-converting switched mode power supplies, for example, have the sole purpose of transforming an electrical voltage into a lower voltage. However, they incidentally also emit electromagnetic waves and can thus interfere with other applications and affect, for example, the medium wave reception. This effect is referred to as electromagnetic interference or interference (EMI).
To maintain functionality, it is important to minimize EMI sources. The Comité International Spécial des Perturbations Radioélectriques (CISPR) has defined standards such as CISPR 25 for automotive applications and CISPR 22 for IT equipment.
But how can you reduce the emissions of a power supply design? One possibility is to provide the complete switching power supply with a metal shield, but this is in most applications, for space and cost reasons out of the question. On the other hand, it is better to start with reducing and optimizing measures right away at the EMI sources. On this topic, about which much has already been written, you will find two bibliographic references at the end of this article.
Minimization of current loops in the layout
What are the most important EMI sources in switched-mode power supplies and how can EMI volumes be reduced by using power modules? Switching power supplies switch the input voltage on and off at a frequency that can range from a few hundred kilohertz to several megahertz, resulting in steep current and voltage edges (dI / DT or dV / dt). According to the Maxwell equations mentioned above, the alternating currents and voltages generate alternating electromagnetic fields which propagate radially from their point of origin and become weaker with increasing distance.
Magnetic and electric fields act on conductive parts of an application (for example on printed circuit boards that act like antennas) and create additional interference in the lines, which in turn generate EMI. Since several watts of power are converted here, the noise emissions spread over a larger radius. The amount of radiated energy is directly proportional to the current (I) and the area (A) of the loop in which the current flows. Consequently, it contributes to EMI reduction when the area of the current and voltage loops is reduced.
A look at the pin layout gives an idea of the possibilities for creating a functional layout by minimizing the area of loops with steep voltage edges. At the switching node, for example, strong current and voltage changes (dI or do) occur. With a deliberately selected pin assignment, susceptible pins are kept away from those with a high noise incidence. For example, the switch node and boot pin should be placed as far as possible from the noise sensitive feedback pin. Also, it makes sense to arrange input and ground pins next to each other, as this makes routing the PCB and placing the input capacitors easier.
The Modified Evaluation Module (EVM) for the LMR23630 Simple Switcher Converter. The two input capacitors are located about 2.5 cm from the input pin to simulate a poor layout. The area of the current loop is larger than necessary and recommended in the datasheet. The switching node between the converter and inductor is marked in with a red oval. The loop area between IC and inductance has been reduced to a minimum.
The radiated electromagnetic emissions of the LMR23630 transducer, with only the loop area deviating between V IN, GND and input capacitor. In a good layout, the capacitors are placed as close as possible to the input and ground pins (that is, the loop area is as small as possible). In contrast, in the poor layout shown, the input capacitors are located 2.5 cm away from the input pin, resulting in a large loop area.
The red curve in the radiated EMI for the poor layout, while the blue curve is for a good layout with the same EVM. Even changing the area of just one loop has a huge impact. In the present case, the radiated electromagnetic emissions of the LMR23630 could be reduced by more than 20 dbμV / m.
Therefore, when designing with a buck converter or power module down, the placement of the input capacitors should be the first consideration. Power modules also have the advantage that the area of the critical loops between IC and inductance is already optimized, as both are interconnected within the module, resulting in a tiny loop area. Most of the inductors used in power modules are additionally screened so that no electromagnetic radiation comes out of the coil. The steep current and voltage edges occur very close to the inductance, and a portion of the electromagnetic field emitted by the switching node is shielded when the inductance is above the lead frame.
Steep flanks can lead to oscillations at the switching node, which in turn generates spurious emissions. In some cases, the boot pin is led out at the converter. By connecting a resistor in series with the boot capacitor, one increases the rise time (DT), which reduces the EMI, albeit at the cost of lower efficiency.
A picture of the interference emissions of the LMR23630 EVM. By placing the input capacitors 2.5 cm away from the pins, a poor layout was simulated to show how the placement of a boot capacitor affects EMI behavior. It may be easier to supplement a design with an additional boot capacitor than to completely change the layout. It is recommended always to design a boot capacitor in case of case. If it is not needed, a 0-ohm resistor can be installed in its place in its place.
Connecting a boot resistor in series with the boot capacitor will result in a lower EMI spectrum. In some frequency ranges, emissions are reduced by up to 6 dB. also highlights the impact on efficiency: extending the rise time with a 30.1-ohm resistor increases efficiency by more than 1 percent. This becomes even clearer by looking at the losses that increase from 1.9 to 2.1 W at full load (3 A). This increase of more than 10 percent can be critical and lead to thermal problems.
For synchronous converters, the current swing at the switching node can be reduced by placing a small Schottky diode between the switch node pin and a ground terminal, reducing the reverse recovery current – albeit at the cost of more component overhead. Similarly, a snubber network can be placed between switch node and ground, consisting of a capacitor and a resistor (each with a large housing). The snubber dissipates the energy of the oscillations at the switching node, but presupposes the knowledge of the exact oscillation frequency and requires the correct calculation of the additional components. He also comes at the expense of the efficiency of the switching power supply.
Parasitic inductances and capacities in the current paths
In synchronous buck converters, the various IC architectures contribute in different ways to the disturbances manifesting as radiated emissions. From a data, the sheet can be found in this regard but little. In fact, most datasheets are looking in vain for an EMI diagram, because the PCB layout, the components used, and other factors also affect EMI behavior. If you are lucky, the EVM User’s Guide will provide a diagram of the EMI behavior of the design in question. However, as soon as the layout and bill of material design deviate from the EVM, its EMI characteristics can be drastically different from those of the EVM. Power modules simplify the layout and allow a quick and simple design since there are only a few rules of thumb to consider. Thus, for example, it is desirable to minimize traces or cuts in the ground plane to a minimum. If this can not be achieved, they should be arranged parallel to the current flow.
Interference-sensitive nodes must be protected from high-fault nodes
Interference-sensitive nodes should be as short as possible and kept away from high-impact nodes. Table 1 shows an overview. For example, along trace between the resistive divider network and the feedback pin (FB) may act as an antenna and receive electromagnetic emissions emitted elsewhere. This causes the faults in the FB pin, which can lead to additional disturbances at the output and make the entire block unstable. Considering all these aspects when designing the layout for a switched buck regulator can be a real challenge.
Power Modules have the advantage of minimizing both the noise sensitive nodes and the high noise nodes, minimizing the chance of choosing a wrong layout. It is only necessary to keep the leading to the FB-pin traces short.
Many adjustment screws are available to fine-tune the EMI footprint of downshifting controllers. However, it may not be enough to stick to best practices. In any case, finding the best configuration in design takes a lot of valuable time. Because power modules include both the FETs and the inductor, creating and finalizing a power supply design with good EMI characteristics is quick and easy. The most critical aspect of a down-converter module design is the placement of a few external devices, which can significantly improve EMI performance.
EMI sources in switching power supplies and how they can be contained was the topic of the previous sections. Next, it’s about how modules can help reduce radiated emissions. For this purpose, measurements are compared, which were carried out with a converter and a power module based on the same IC. Both come from the Simple Switcher series of Texas Instruments (TI). The converter is of the type LMR23630 and is also available in the power module LMZM33603 used. The EVMs of both components have been partially modified to get the same amount of parts and ensure that the results depend solely on the component (converter or power module) used and the layout. Both EVMs have a good, optimized layout. In the aftermath, the layout was intentionally degraded by placing the capacitors farther from the input pins.
The efficiency of the LMR23630 converter
Four different EMI spectra of different design layouts. The design is progressively worse. The first measurement (good layout: blue curve) refers to the unmodified, good layout of the EVM, where all the input capacitors are located close to the input pin. In the second measurement (red trace), the two 4.7 μF capacitors are located 2.5 cm from the input pin, while the small 0.22 μF capacitor is still placed close to the input pin. For the third measurement (green curve), the small capacitor was placed at 2.5 cm distance, and at the fourth measurement (purple curve), this capacitor was eliminated.
It is clear how important the placement of the input capacitors is. If the small capacitor is placed at a greater distance from the input pin or omitted altogether, the conditions of the CISPR 22 Class A3M standard are no longer met. With a small distance between the small input capacitor and the input pin, the loop area for high frequencies is minimized. While the small capacitor filters high frequencies, the larger capacity capacitors act as filters for lower frequency noise.
Performance of the power module LMZM33603
Power modules usually have a built-in small input capacitor. The EVM layout is also progressively degraded. The blue curve also shows the EMI behavior of the unchanged layout. The red and green curves reflect the behavior of the worse layout, one with two 4.7-μF capacitors on the bottom of the board. In the green curve, the capacitors are located approximately 3.5 cm from the input pins. Also, the thick red line in the modified EVM and the critical loop area between V IN, the input capacitors and the ground. Although the EMI properties also deteriorate here, they remain within the CISPR 22 Class A3M standard.
Power modules forgive layout errors
Compares the characteristics of the LMR23630 converter (red curve) and the LMZM33603 power module (blue curve) in a common diagram. Both solutions have a poor but comparable layout in which all input capacitors are placed a long distance from the input pins.
It can be seen that the EMI performance of the power module LMZM33603 is better than that of the LMR23630. None of the layouts is perfect, but the power module would pass the CISPR test as opposed to the converter.
For further reading on a good layout for EMI containment, I recommend the application notes AN-2155 “Layout Tips for EMI Reduction in DC / DC Converters” and AN-643 ” EMI / RFI Board Design.”