The most popular understanding of switching power supply electromagnetic interference analysis

Let me first briefly introduce EMI: 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

Noise source: the source of interference, like the source of infectious diseases;

Coupling pathways: carriers of transmission interference, such as carriers of infectious diseases, food, water, air, etc.;

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

Explain the following terms:

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: Can your power supply work normally in an environment with radiated interference?

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 switching device is periodically opened and closed, the current and voltage in the circuit are also changing periodically. Then those changing currents and voltages are the real source of noise. Then someone may ask, my switching frequency is 100KHz, but why the noise tested ranges from hundreds of K to hundreds of M?

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?

The same is a 100KHz square wave

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 greater than those of the green waveform. Let’s continue to analyze the two waveforms.

Red: signal with fixed frequency, 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, for example, the conduction test is 150K-30M, then if conditions permit, a switching frequency such as 130K can be selected, 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. It 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 diodes with good reverse recovery. The reverse recovery current of the diode will not only bring high di/dt, but also cause high dv/dt together with parasitic inductance such as leakage inductance.

But in fact, the switching power supply is the source of EMI emission and cannot be fundamentally solved. Moreover, some methods of suppressing EMI from the source will also reduce efficiency, so it is particularly important to suppress EMI from the propagation path.

Next, let’s look at the transmission channels, which are summarized by Professors Poon and Pong. They are relatively intuitive and comprehensive.