How parasitics create an surprising EMI filter resonance


Electromagnetic interference (EMI) has a fame for being one probably the most troublesome points in power-supply designs. I feel that this fame comes, largely, from the truth that most EMI-related challenges should not issues that may be solved by trying on the schematic. This may be irritating, for the reason that schematic serves because the central place the place the engineer goes to grasp what the circuit does. Positive, you understand that there are related features within the design that aren’t within the schematic—issues like code.

You additionally know that the schematic doesn’t symbolize issues like printed wiring board parasitics. Nonetheless, in EMI, parasitics like these can have dominating results in your capability to fulfill the necessities, forcing you to have the mandatory expertise to acknowledge what varieties of parasitics can positively or negatively impression the EMI spectrum. This Energy Tip article will discover how some of these parasitics can create an surprising EMI filter resonance in a gallium nitride (GaN)-based onboard charger (OBC) for an electrical automobile (EV).

Determine 1 exhibits a high-level system illustration of the OBC. Its main operate is grid-to-vehicle voltage and present battery charging. A secondary operate is vehicle-to-grid energy movement in order that the EV can complement renewable vitality sources that may have a fluctuating peak capability.

Determine 1 The schematic exhibits a high-level system illustration of the onboard charger (OBC). Supply: IEEE

Now, let’s flip our consideration to the EMI concerns contained in the OBC.

Onboard charger’s EMI evaluation

EMI contains differential-mode (DM) and common-mode (CM) noise. For an OBC system, DM noise is especially generated by the enter present of the ability issue correction (PFC), whereas CM noise may end up from each the PFC and the conductor-inductor-inductor-inductor-capacitor (CLLLC). Determine 2 exhibits the cooling resolution (chilly plate) for the OBC within the decrease proper nook of the schematic. The chilly plate is vital in stopping the elements from overheating; nevertheless, its presence introduces parasitic capacitance that impacts the EMI.

Determine 2 Parasitic results inflicting EMI are proven within the decrease proper nook of the schematic. Supply: Texas Devices

As proven in Determine 2, there are parasitic capacitances between the switching nodes to the chilly plate, between the first and secondary grounds to the chilly plate, and between the first and secondary winding of the CLLLC transformer. These parasitic capacitances can generate or have an effect on CM noise-current ranges within the system.

With estimated parasitic capacitances, simulations present that within the worst case, the naked DM noise with solely the two.2-µF enter capacitor (CX1) is about 110 dBµV. Likewise, the naked CM noise with none CM filter is about 115 dBµV at about 350 kHz. Designing a two-stage EMI filter helps attenuate EMI noise under the Comité Worldwide Spécial des Perturbations Radioélectriques (CISPR) 32 normal.

The common-mode impedance of LCM1 and LCM2 at 350 kHz is about 3 kΩ. Their leakage inductances are about 6.4 µH, that are used for DM noise attenuation. CX1 and CX2 are 2.2-µF movie capacitors for DM noise attenuation, and CY1, CY2, CY3 and CY4 are 4.7-nF ceramic capacitors for CM noise attenuation.

Ideally, with the designed filter, each the naked CM noise and naked DM noise ought to be attenuated by greater than 65 dBµV, and the EMI noise ought to meet the CISPR 32 normal. There are nonetheless some sensible challenges to deal with, nevertheless.

EMI filter resonance

EMI filters are stuffed with resonances by design. In reality, it’s these resonances that allow the filter to attenuate noise and allow a system to move the EMI normal. Determine 3 exhibits a typical attenuation curve for an EMI filter. Discover that at frequencies above 100 kHz, the filter does a pleasant job of decreasing amplitude. There are some resonances under 100 kHz, nevertheless, that may very well be fairly problematic in the event that they exist on high of a switching frequency.

Determine 3 That is how typical EMI filter attenuation appears to be like like for an onboard charger. Supply: Texas Devices

Clearly, nobody would deliberately place a resonance on the switching frequency, however interconnect impedances, element parasitics or each can typically push the system to function in an unintentional manner.

Determine 4 exhibits a barely modified EMI filter in comparison with Determine 2. The variations are within the elements in purple. LP1 and LP2 symbolize the parasitic inductance of the interconnect between the EMI filter and the PFC enter. The presence of LP1 and LP2 required some native capacitance for the PFC present to movement by. Subsequently, transferring CX1 to the enter of the PFC and including CX0 elevated the attenuation of the filter. The 4 parts in purple mix collectively to create a resonance at 240 kHz. On this design, 240 kHz is the conversion-combined switching frequency of the two-phase PFC. This resonance goes to amplify the switching present and subsequently make the EMI worse at this frequency.

Determine 4 The EMI filter is proven with a resonance on the switching frequency. Supply: Texas Devices

Determine 5 exhibits the time-domain waveforms of the AC line present flowing by LP1 in magenta, together with the AC enter voltage in blue. Discover that the present has a big 240-kHz sine wave with a 28-A peak-to-peak amplitude. This sine wave is the direct results of the triangular PFC present flowing by the unintended amplifier created by the purple elements in Determine 4.

Determine 5 The time-domain waveforms of the AC line present are proven flowing by LP1 in magenta. Supply: Texas Devices

Damping a resonance like this may be difficult, for the reason that vital damper will usually require an inductor or capacitor bigger than these used within the circuit. One other attainable resolution is perhaps to decrease the inductance of the interconnect in order that the resonance is now not on high of the switching frequency. In principle this sounds good, however virtually talking, that interconnect is there for a cause; so, making it smaller isn’t actually viable.

Another choice is to think about the need of retaining each CX0 and CX1. You may’t take away CX1 for the reason that PFC requires some native enter capacitance for the high-frequency present. Nonetheless, CX0 is there to extend the capacitance, with the intent of accelerating the attenuation. Eradicating CX0 improved the EMI by roughly 6 dBµV. The amplitude is diminished by 50% and a big share of the require attenuation (65 dBµV) wanted to move the usual. That’s a fairly whole lot.

Two design takeaways

The takeaway right here is twofold. The primary is the unique premise: the schematic doesn’t inform the entire story for EMI. On this case, there was an unintended resonance attributable to interconnect inductance that amplified the switching frequency noise. Recognizing the foundation reason for the issue is at all times probably the most vital step in debugging.

The second takeaway is that typically much less of a usually good factor (filter capacitors) is best. You may normally tackle EMI points by including elements, however on this case, the presence of a element makes the problem worse. Subsequently, by eradicating CX1, we had been in a position to scale back the dimensions of the filter, decrease the system value, and enhance the EMI.

Brent McDonald is a system engineer within the Energy Provide Design Companies group at Texas Devices.

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