Sinrace Power Supply Technology (Shenzhen) Co., Ltd.

The interfering resource of switching power supply EMI concentrated on the power switch, rectifier diodes, high frequency transformer, the external environment on the interference of switching power supply jitter, mainly from the power grid, lightning and external radiation and so on.

1. Switching power supply EMI source
The interfering resource of switching power supply EMI concentrated on the power switch, rectifier diodes, high frequency transformer, the external environment on the interference of switching power supply SMPS  jitter, mainly from the power grid, lightning and external radiation and so on.

(1) power switch
The power switch works in On-Off state of rapid cycling transition, dv / dt and di / dt are rapidly transform. Therefore, the power switch is not only a major source of interference for electric coupling, but also for magnetic coupling.

(2) high-frequency transformer
The EMI source of high-frequency transformer embodied in the leakage inductance of corresponding di / dt rapid cycle change. Thus, high-frequency transformer is an important source of interference for magnetic coupling.

(3) Rectifier diodes
EMI source of rectifier diode embodied in reverse recovery characteristics, the reverse recovery current in the making-breaking point (lead inductance, stray inductance, etc.) to produce high dv / dt, resulting in strong electromagnetic interference.

(4) PCB
Accurately speaking, PCB is the coupling channels of the interference resource, PCB's strengths and weaknesses, corresponding to the source of the EMI suppression good or bad.

2. The transmission channel classifications of switching power supply EMI

(A). Transmission channel by transmitting interference
(1) capacitive coupling
(2) inductive coupling
(3) resistance coupling
a. resistance generated by the public power transmission coupling resistance
b. common ground impedance coupling produces conduction resistance
c. the resistance of the impedance of public transmission line coupling
(B).Transmission channel of radiation interference
(1) In the switching power supply (SMPS), the components of radiation sources of interference and line can be assumed as antennas. We analyses electric dipole and magnetic dipole theory. Diodes, capacitors and power switches can be assumed to be dipole while induction coil can be assumed to be magnetic dipole.

(2)No shield, for the electric dipole, magnetic dipole and the resulting electromagnetic wave, the transmission channel is air (it can be assumed as free space);

(3) Shield, take the gaps and holes of the shield into account, in accordance with the mathematical model of leakage field analysis and processing.

3. 9 measures for switching power supply EMI Suppression
In the switching power supply (SMPS), voltage and current mutations, that is, high dv / dt and di / dt, is the main cause of its EMI. The design of switching power supply to achieve EMC technical measures based on the following two points:

(1) Minimize interference generated by the power source itself, the use of interference suppression methods or interfere with smaller components and circuits, and then rational distribution;

(2) By grounding, filtering, shielding the power supply EMI suppression techniques and enhancing the power of the EMS.
Separately speaking, 9 major measures are:
(1) reduce the dv / dt and di / dt (lower its peak, slow down the slope)
(2) the reasonable application of aviators to reduce the surge voltage
(3) damping network to suppress overshoot
(4) diode with soft recovery characteristics to reduce the high frequency EMI
(5) active power factor correction, and other harmonic correction
(6) using rational design of power line filter
(7) reasonable ground handling
(8) effective shielding
(9) PCB design reasonably
 
4. The control of high-frequency transformer leakage inductance
High-frequency transformer leakage inductance is one of the important reasons of generating the power switch turn-off voltage spikes. Therefore, control the leakage part of the solution caused by high-frequency transformer is the primary EMI problem.

Reduce the high frequency transformer leakage inductance two points of penetration: electrical design, process design!
(1) Select the appropriate core and reduce leakage. Leakage inductance is proportional to the square with the primary number of turns, reducing the number of turns would significantly reduce the leakage.
 
(2)Reduce the insulation between windings. There is a known "gold film" insulating layer, the thickness of 20 ~ 100um, impulse breakdown voltage is up to several thousand volts.
 
(3)Increasing the coupling between the windings and reduce leakage.

5. High-frequency transformer shielding

In order to prevent leakage of high-frequency transformer around the circuit, the magnetic interference shielding can be used, which could bring high-frequency transformer of the leakage magnetic field shielding. Shielded with copper foil generally produced around the transformer outside a circle, and the ground, compared with the leakage magnetic field shielding is a short loop, thereby inhibit the leakage magnetic field a wider range of leakage.

High frequency transformer of SMPS, core and winding between the relative displacements occur and resulting in high-frequency transformer noise at work (whistle, vibration). To prevent the noise, the need for transformers to reinforce measures:

(1)The core with epoxy resin (such as EE, EI core) of the three contact surfaces for bonding, inhibiting the production of relative displacement;
(2) "glass beads" (Glass beads) adhesively bonded core, the better effect.