Easily understand EMI and how to suppress it

EMI is translated into Chinese as electromagnetic interference. In fact, all electrical equipment will have electromagnetic interference. It's just that the severity varies. Electromagnetic interference will affect the normal operation of various electrical equipment and interfere with the normal transmission of communication data. Although the harm to the human body has not yet been determined, it is generally believed to be harmful to the human body. Therefore, many countries and regions have strict regulations on the degree of electromagnetic interference of electrical appliances. Of course, power supplies are no exception, so we have reason to understand EMI and its suppression methods.

 The following describes EMI in conjunction with some expert literature.

First of all, EMI has three basic aspects

that is

Noise source: The source of interference. Like the source of infectious diseases.

Coupling pathway: carrier of transmission interference. Just like the carrier of infectious diseases, food, water, air...

Receiver: The object that is disturbed. The person who is infected.

If one of them is missing, electromagnetic interference will not work. Therefore, there are three ways to reduce the harm of electromagnetic interference:

1. Suppress interference at the source.

2. Cut off the transmission route

3. Enhanced resistance, this is the so-called EMC (electromagnetic compatibility)

Let me explain a few terms first:

Conducted interference: This is the way noise is transmitted through wires.

Radiated interference: that is, the noise is transmitted through space radiation.

Differential mode interference: interference caused by current due to the potential difference in the circuit itself, such as live wire and neutral wire, positive pole and negative pole.

Common mode interference: Interference caused by current due to the potential difference between the circuit and the ground.

The items we usually test in the laboratory are:

Conducted emission: Test whether the interference emitted by your power supply through conduction is qualified.

Radiated emission: Test whether the interference emitted by your power supply through radiation is qualified.

Conducted interference immunity: Can your power supply work normally in an environment with conducted interference?

Radiated interference immunity: In an environment with radiated interference, can your power supply work normally? First, let’s look at the source of the noise:

Any periodic voltage and current can be decomposed into sinusoidal waves of various frequencies through Fourier decomposition.

Therefore, when testing interference, it is necessary to test the noise intensity at various frequencies.

So what is the source of this noise in a switching power supply?

In a switching power supply, since the switch device is periodically opened and closed, the current and voltage in the circuit also change periodically. Then those changing currents and voltages are the real source of noise.

Then someone may ask, my switching frequency is 100KHz, but why is the noise measured from a few hundred K to

How many hundreds of MB are there?

We perform fast Fourier analysis on various waveforms with the same effective value and frequency:

Blue: Sine wave

Green: Triangle wave

Red: Square wave

It can be seen that the sine wave has only the fundamental component, but the triangle wave and square wave contain higher harmonics, and the square wave has the largest harmonics.

That is to say, if the current or voltage waveform is a non-sinusoidal signal, high-order harmonics can be decomposed.

So what happens if the same square wave has different rise and fall times ?

Red: Rise and fall times are both 100ns

Green: Rise and fall times are both 500ns

It can be seen that the red higher harmonics are significantly larger than the green ones.

We continue to analyze the following two waveforms,

A: There are square waves with severe high-frequency oscillations, such as the voltage waveform on MOS and diodes.

B: Use an absorption circuit to absorb the high-frequency oscillation of the square wave.

Perform fast Fourier analysis separately:

It can be seen that after the oscillation frequency (about 30M), the harmonics of waveform A are greater than those of waveform B.

Let’s look at the following waveforms. One is a current waveform with a turn-on spike, and the other is without a turn-on spike.

Perform Fourier analysis on the two waveforms:

It can be seen that the higher harmonics of the red waveform are larger than those of the green waveform.

Continue to analyze the two waveforms

Red: Fixed frequency signal

Green: Signal with slight frequency jitter

It can be seen that frequency jitter can reduce the energy in the low frequency band. Further, the spectrum energy in the low frequency band can be amplified:

It can be seen that frequency jitter disperses the spectrum energy, while the spectrum energy of a fixed frequency is concentrated at the harmonic frequency point of the fundamental wave, so the peak value is relatively high and easily exceeded.

Finally, let’s summarize briefly how to suppress EMI from the source.

1. For the selection of switching frequency, such as the conduction test 150K-30M, if conditions permit, you can choose a switching frequency such as 130K, so that the fundamental frequency can avoid the test.

2. Use frequency jitter technology. Frequency jitter can disperse energy and is good for EMI in low frequency bands.

3. Appropriately reduce the switching speed. Reducing the switching speed can reduce di/dt and dv/dt at the switching moment, which is good for EMI in the high-frequency band.

4. Use soft switching technology, such as PSFB, AHB and other ZVS can reduce di/dt, dv/dt at the switching time. It is good for EMI in the high frequency band. Resonance technology such as LLC can make some waveforms become sine waves, further reducing EMI.

5. Absorb some oscillation peaks. The oscillations on these tubes are often of high frequency and will emit a lot of EMI.

6. Use a diode with good reverse recovery. The reverse recovery current of the diode will not only cause high di/dt, but also cause high dv/dt together with parasitic inductance such as leakage inductance. Let 's take a look at the propagation path, which is summarized by Professors Poon and Pong

The communication channels are relatively intuitive and comprehensive

Let's first look at the conduction pathway:

The transmission of conducted interference is all through wires. During the test, the size of the interference conducted through the wires is tested.

That is to say, for the power supply, all conducted interference will be transmitted to the test receiver through the input line.

So how do these interferences reach the receiver? And how can we prevent these interferences from reaching the receiver?

Let's first look at the concept of differential mode. Differential mode current is easy to understand, as shown in the following figure.

The differential mode current forms a loop between the input live wire and the neutral wire (or the positive wire to the negative wire). It can be easily understood using Kirchhoff's theorem that the currents in the two wires are exactly equal.

This differential mode current contains not only the low-frequency component of the grid frequency (or DC), but also the high-frequency current of the switching frequency. If the current of the switching frequency is not sinusoidal, then there must be harmonic currents.

Now take the simplest DCM flyback power supply with PFC function as an example (as shown above), the current on its input line is as follows:

If you enlarge it:

It can be seen that the current waveform is composed of many triangular waves, but its average value is the sine wave of the power frequency. Then, if we do Fourier analysis on the input current, we can get:

It can be seen that in addition to the fundamental wave of the 100Khz switching frequency, there are also abundant harmonics. Continuing to analyze to higher frequencies, we can see:

If left unchecked, optical differential mode current can cause conduction to exceed the limit.

So how to block these high frequency currents? The simplest and most effective way is to add an input filter. Example 1: Add an RC filter at the input:

When doing Fourier analysis on the input current:

It can be seen that the high-frequency harmonics are significantly reduced.

If you add an LC filter:

Analyze the input current:

It can be seen that the filter effect is better, but it is higher at low frequencies. This is mainly caused by the resonance of the LC filter.

In actual circuits, due to the existence of various impedances, LC is less likely to cause resonance, but it may occur occasionally.

If the conduction test finds that the low frequency band exceeds the standard at places other than the switching frequency, it may be considered whether the filter is resonant.