How you can characterize a transformer for EMI


Energy transformers are sometimes the principle supply of common-mode noise in remoted switching energy converters. Why? As a result of contained in the transformer, the windings on the first and secondary sides of the isolation barrier are in shut proximity—often separated by lower than 1 mm—leading to important parasitic capacitance between the adjoining windings.

The voltages that seem on these windings sometimes have giant AC content material. For instance, within the flyback converter proven in Determine 1, the first winding connects to the drain of the first swap, which has a voltage waveform with giant AC content material throughout many frequencies. This AC voltage injects common-mode currents from main to secondary via the parasitic capacitance, which is commonly the supply of many electromagnetic interference (EMI) issues.

Determine 1 Widespread mode noise created by a flyback energy transformer. Supply: Texas Devices

Fortunately, transformer design methods corresponding to shielding and common-mode balancing can reduce the transformer influence on EMI, as mentioned within the Texas Devices Energy Provide Design Seminar paper, “Flyback Transformer Design Concerns for Effectivity and EMI”. It may be fairly tough and time consuming, nonetheless, to verify how a lot your transformer contributes to EMI and the way to optimize the transformer development. For every transformer design that you just need to check, it’s worthwhile to solder the transformer into the PCB, take your energy converter to an EMI check fixture, and run the scans. If the EMI efficiency of your transformer is just not acceptable, it’s worthwhile to unsolder it out of your PCB and take a look at once more.

On this Energy Tip, I’ll present you a very simple solution to verify the EMI efficiency of your transformer earlier than ever soldering it into your board.

Utilizing solely a operate generator and an oscilloscope, you possibly can mimic the circumstances seen by the transformer within the circuit and measure the transformer’s common-mode EMI signature. The diagram in Determine 2 exhibits the way to configure this measurement for the transformer utilized in Determine 1. Discover that this transformer has two windings on the first (WP and WAUX) and one winding on the secondary (WS).

First, use a brief piece of wire to tie the AC quiet nodes collectively on the first. An AC quiet node is any pin on the transformer that ties to main floor within the circuit, both on to or via a capacitor. On this instance, each Pin 2 and Pin 3 are AC quiet nodes on the first aspect of the isolation barrier. If in case you have a transformer with a number of windings on the secondary, additionally, you will have to tie all the secondary quiet nodes collectively, however don’t join them to the first quiet node.

Determine 2 Transformer CMRR check setup that makes use of a brief piece of wire to tie the AC quiet nodes collectively on the first and secondary, and applies a small sinewave throughout the first winding to measure the ratio between the voltage induced between the first and secondary AC quiet nodes and the voltage injected by the operate generator, or CMRR. Supply: Texas Devices

Subsequent, use the operate generator to use a small sinewave throughout the first winding of the transformer. This mimics the first winding voltage, however now you might be testing at a single frequency with a protected and low voltage. The amplitude of the sign is just not important, because the parasitic capacitances of the transformer are largely unbiased of the voltage amplitude.

Lastly, utilizing one channel of the scope, measure the voltage injected by the operate generator. With one other channel, measure the voltage induced between the first and secondary AC quiet nodes. The ratio of those two alerts is actually the common-mode rejection ratio (CMRR) and is a sign of how a lot your energy transformer will contribute to common-mode noise at that frequency.

Determine 3 exhibits the outcomes of this check at 100 kHz for 2 totally different transformers. The development used for transformer #1 leads to a CMRR of –39.6 dB, whereas the CMRR for transformer #2 is larger, measuring –31.4 dB. This means that transformer #1 will produce much less common-mode noise than transformer #2. With the operate generator, you possibly can examine the transformers’ traits at totally different frequencies. 

Determine 3 The time area transformer CMRR check outcomes that signifies that transformer #1 produces much less common-mode noise than transformer #2 on the check frequency of 100 kHz. Supply: Texas Devices

Alternatively, you possibly can carry out this identical check utilizing a frequency response analyzer (FRA) to comb the frequency of the injected sign throughout the whole frequency vary of curiosity. Determine 4 exhibits the FRA measurements of the identical two transformers throughout a large frequency vary of 100 kHz to 30 MHz. Discover that the acquire may be very flat over a variety from 100 kHz to round 4 MHz. The acquire at 100 kHz correlates very properly with the operate generator check, indicating that the operate generator check at 100 kHz is adequate to characterize these transformers throughout this band of frequencies. At frequencies above a couple of megahertz, it is best to measure the CMRR of those transformers on the frequency of curiosity.

Determine 4 Frequency area transformer CMRR check outcomes for transformer #1 and #2 throughout a large frequency vary of 100 kHz to 4 MHz utilizing an FRA. Supply: Texas Devices

Determine 5 exhibits the outcomes of soldering each of those transformers into the PCB of a switching energy converter measuring the performed EMI in opposition to Comité Worldwide Spécial des Perturbations Radioélectriques (CISPR) 32 Class B limits. The highest restrict line corresponds to the quasi-peak measurement, and the decrease restrict line corresponds to the common measurement. As anticipated, the EMI outcomes for transformer #2 are worse than transformer #1. The truth is, transformer #1 passes with first rate margin, whereas transformer #2 barely fails.

Determine 5 Carried out EMI check outcomes for transformers the place transformer #1 passes with margin and transformer #2 barely fails. Supply: Texas Devices

Curiously, each transformers on this instance have the identical winding construction and development. The variations in CMRR are fully attributable to variations within the manufacturing course of, demonstrating how delicate EMI may be to transformer development. Small variations corresponding to the precise placement of particular person strands of wire inside the transformer or the thickness of insulating layers can have profound results.

For the instance in transformer development, it’s clear you can’t be assured that each one items in manufacturing will move CISPR 32 performed EMI limits. One resolution is to extend the EMI filtering within the circuit to supply extra margin. An alternative choice is to make use of the operate generator check to display screen each transformer pattern throughout manufacturing. This check is similar to the kinds of assessments generally used to check and display screen transformer flip ratios between windings, so no particular tools is required. Within the instance, solely passing transformers with a CMRR lower than –38 dB supply a excessive likelihood that each one items will move EMI when assembled into an influence converter system.

The transformer’s influence on EMI

Debugging EMI points is fraught with many obstacles and difficulties. The easy measurement method described on this Energy Tip can prevent important time and frustration on the solder bench and within the lab. In your subsequent remoted power-supply design, take a couple of minutes to measure the CMRR of your energy transformers earlier than soldering them into the circuit boards, after which examine the CMRR to the ensuing EMI. You’ll acquire a greater understanding of the transformer’s influence on EMI, and what degree of transformer CMRR will move EMI in your system.

Brian King is a Programs Supervisor and Senior Member Technical Workers at Texas Devices. With over 26 years of expertise in energy provide design, he has supported over 1300 enterprise alternatives and has designed over 750 distinctive energy provides utilizing a broad vary of TI energy provide controllers. Brian has revealed over 45 articles associated to energy provide design, and since 2016 has been the lead organizer and content material curator for the Texas Devices Energy Provide Design Seminar (PSDS) collection.

 

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