EMI countermeasures and output noise countermeasures in the circuit design of insulated flyback converters

“From the path point of view, EMI is divided into conducted noise and radiated noise. According to the conduction mode, conducted noise can be subdivided into differential mode noise and common mode noise. A rough overview to master the most basic knowledge of this kind.

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This section explains the noise countermeasures on the Circuit. When designing a switching power supply, noise evaluation and countermeasures must be carried out.

First, revisit the terms related to noise a bit.

・EMI (Electro Magnetic Interference): electromagnetic interference

Radio waves and high-frequency electromagnetic waves become noise, which affects Electronic equipment, etc., or electromagnetic waves that can cause influence.
－Conducted noise: noise conducted through cables and substrate wiring
> Differential mode (normal mode) noise: noise that occurs between power lines and transmits in the same direction as the current
> Common mode noise: the noise coming back to the source of the signal through the metal shell, etc., through the free capacitor, etc.
－Radiated noise: the noise released into the air

・EMS (Electro Magnetic Susceptibility): Electromagnetic immunity

It refers to the ability and tolerance to not be damaged even if it is hindered or interfered by electromagnetic waves (EMI: conducted noise and radiated noise).

・EMC (Electro Magnetic Compatibility): Electromagnetic compatibility

EMI + EMS. Compatible countermeasures for radiation (Emission: emission) and immunity (Immunity: tolerance).

From the path point of view, EMI is divided into conducted noise and radiated noise. According to the conduction mode, conducted noise can be subdivided into differential mode noise and common mode noise. A rough overview to master the most basic knowledge of this kind.

EMI countermeasures

The EMI of the switching power supply circuit will affect other circuits and, so EMI countermeasures must be implemented. Basically, in the high-current switching nodes and lines, add integrated impedance and capacitors with bypass/filter functions, and resistor/capacitor circuits.

1) C12, R17: add RC snubber circuit to the output rectifier diode

Same as the input buffer, it reduces the peak wave generated when ON/OFF. Please refer to here for input buffering. C12 chooses 500V 1000pF, R17 chooses about 10Ω 1W.

2) C10: Add Y-capacitor between primary side and secondary side

A capacitor called Y-Capacitor is added between the ground on the primary side and the secondary side. In this regard, the inter-winding capacitor of the insulating transformer is one of the representative methods to reduce the common mode noise generated on the secondary side caused by the switching noise of the primary side. The rated voltage of the Y-capacitor must be equal to the insulation withstand voltage of the transformer. The capacitance value is about 2200pF.

3) C11: Add a capacitor between the drain and source of MOSFET Q1

In order to reduce the surge caused by the high-speed switching off, a capacitor is added between the drain and the source of the MOSFET. This is also a kind of snubber circuit, but it will increase the loss, so you must pay attention to the temperature rise. Here, a 10-100pF capacitor with a withstand voltage of 1kV is used.

The component constants mentioned above are reference values ​​at the beginning. The influence of noise must be confirmed before adjustment.

Output noise countermeasures

Needless to say, the output voltage of the switching power supply has ripples that depend on the switching frequency, as well as noise caused by high-frequency harmonics, inductances and capacitors. When these noises cause trouble, an LC filter can be added to the output to effectively solve the trouble.

Take the inductance L to 10μH and C10 to 10μF～100μF as the standard values ​​at the beginning, carefully observe the noise before making adjustments.

The above are the main noise countermeasures. Regardless of the method, it is necessary to measure the noise and confirm what effect the noise has on the equipment. Planning the measurement environment and equipment is indispensable for measuring noise. When it is not possible to quantitatively measure the value, it may be possible to understand whether it will cause an impact from the equipment’s S/N, etc. and performance level.

The countermeasures mentioned here are noise countermeasures applicable to the structure of the power supply circuit. The generation of noise is also related to substrate layout, component configuration, component performance, and so on. Depending on the actual situation, upgrade the LC filter from a simple L type to a π type and a T type, and add necessary shielding on the circuit board.

In addition, depending on the specifications of the equipment, it must also comply with noise and equipment-related specifications such as the International Special Committee on Radio Interference (CISPR) specifications. It is important to keep in mind when designing in the first time that the specifications must be complied with.

The theme of this item is “Isolated Flyback converter Circuit Design” as the final description of the circuit design. Next, we will enter the “Substrate Layout Example”.