Transformer Electromagnetic Compatibility Design of Flyback Switching Power Supply

With the development of power semiconductor device technology, the high power-to-volume ratio and high efficiency of switching power supplies have made them widely used in modern military, industrial and commercial instruments and equipment at all levels. As the clock frequency continues to increase, the electromagnetic compatibility (EMC) of equipment has attracted widespread attention. EMC design has become an indispensable and important part of the development and design of switching power supplies.

The suppression of conducted electromagnetic interference (EMI) noise must be considered at the beginning of product development. Generally, the installation of power line filters is a necessary measure to suppress conducted EMI. However, relying solely on filters at the power input end to suppress interference often leads to an increase in the inductance and capacitance of the components in the filter. The increase in inductance increases the volume; the increase in capacitance is limited by the leakage current safety standard. If the other parts of the circuit are properly designed, they can also perform similar work as filters. This paper proposes a phase dry winding method for the noise active node of the transformer. This design method can not only reduce the volume of the power line filter, but also reduce the cost.

1 Common-mode conducted interference of flyback switching power supplies

Conducted noise interference of electronic equipment refers to the electromagnetic interference that is conducted to the public power grid environment through the power line in the form of noise current when the equipment is connected to the power grid. Conducted interference is divided into common-mode interference and differential-mode interference. The common-mode interference current has equal phases on the neutral line and the phase line; the differential-mode interference current has opposite phases on the neutral line and the phase line. Differential-mode interference contributes less to the overall conducted interference and is mainly concentrated at the low-frequency end of the noise spectrum, making it easier to suppress. Common-mode interference contributes more to conducted interference and is mainly in the mid-frequency and high-frequency bands of the noise spectrum. Suppression of common-mode conducted interference is a difficult point in the conducted EMC design of electronic equipment and is also the most important task.

There are some nodes with drastic voltage changes in the circuit of a flyback switching power supply. Unlike other nodes with relatively stable potential in the circuit, the voltages of these nodes contain high-intensity high-frequency components [2]. These nodes with very active voltage changes are called noise-active nodes. The noise active node is the common mode conduction interference source in the switching power supply circuit. It acts on the stray capacitance to ground in the circuit to generate common mode noise current M. The stray capacitance to ground in the circuit that has a greater impact on EMI includes: the parasitic capacitance C of the drain of the power switch tube to ground, the parasitic capacitance Cp of the primary winding of the transformer to the secondary winding, the parasitic capacitance C of the secondary loop of the transformer to ground, the parasitic capacitance C of the primary and secondary windings of the transformer to the magnetic core. The distribution of these parasitic capacitances in the circuit is shown in Figure 1.

The common mode current in Figure 1 has three main coupling paths in the circuit: from the noise source - the d pole of the power switch tube is coupled to the ground through C; from the noise source through C. Coupled to the secondary circuit of the transformer, and then coupled to the ground through C; from the front and secondary coils of the transformer through C?C coupled to the transformer core, and then coupled to the ground through C. These three currents are the main factors that constitute the common mode noise current (as shown by the black arrow in Figure 1). The common mode current flows back through the ground wire at the input end of the power line, and is sampled and measured by the LISN.

2 EMC design of isolation transformer

2.1 Traditional transformer EMC design

In addition to the coupling of common mode noise through the d pole of the field effect tube to the ground, the noise voltage of the d pole of the switch tube couples the noise current to the loop where the secondary winding of the transformer is located through the parasitic capacitance of the transformer, and then couples to the ground through the parasitic capacitance of the secondary loop to the ground. It is also a way for common mode current to be generated. Therefore, trying to reduce the common mode current transmitted from the primary winding of the transformer to the secondary winding is an effective EMC design method. The traditional transformer EMC design method is to add an isolation layer between the two windings[3], as shown in Figure 2.

The design of directly connecting the metal isolation layer to the ground wire will increase the common-mode noise current and deteriorate the EMC performance. The isolation layer should be a node with stable potential in the circuit. For example, connecting the isolation layer in Figure 2 to the negative pole of the front stage of the circuit is a good connection method. Such a connection can effectively shunt the common-mode current that originally flows to the ground, thereby greatly reducing the conducted noise emission level of the power line.

2.2 Node phase balance method

In the circuit, the noise voltage active node is not single. Take the circuit analyzed in this article as an example: in addition to the d pole of the power switch tube, the other end U of the front winding of the transformer is also a noise voltage active node, and the change direction of the node voltage is opposite to that of the d pole voltage of the field tube. Therefore, the two ends of the secondary winding of the transformer are noise voltage active nodes with opposite phases. Figure 3 shows the distribution of the coils on the transformer skeleton after the node phase balance method is adopted.

The innermost layer of the transformer skeleton is half of the coil of the front winding, which is connected to the d pole of the power switch tube; the coil in the middle layer is the secondary winding; the outermost layer is the other half of the front winding, which is connected to the node U. Since the noise current is mainly coupled through the parasitic capacitance between the front and rear coil layers, winding the noise active nodes of the front and rear coils in opposite directions in pairs at the relative positions of the inner and outer layers can make most of the noise current cancel each other, greatly reducing the intensity of the noise current finally coupled to the secondary.

The circuit discussed in this article also has auxiliary power supplies for the front and secondary circuits, which are also powered by independent coils wound on the transformer. The existence of these two auxiliary coils provides an additional path for the propagation of noise current. The auxiliary coils are designed for the power supply of the control circuit. Although the power of the control circuit itself is very small, their existence increases the parasitic capacitance of the circuit to the ground, thereby sharing part of the work of coupling the common mode noise from the active node to the ground. However, sandwiching these windings between the windings of the front and rear coils can increase the distance between the front and rear windings, thereby reducing their inter-layer parasitic capacitance and the noise current can be reduced accordingly. Therefore, the final method of transformer winding should be as shown in Figure 4. The coil windings from the inside to the outside are: half of the front winding, half of the auxiliary winding, the rear winding, the other half of the auxiliary winding, and the other half of the front winding.

3 Experimental part

The effectiveness of the improved transformer winding method in improving the conducted EMC performance of the switching power supply can be verified by experiments.

3.1 Experimental method

The experiment was carried out according to the voltage method in the literature [43. The frequency band range is 0.15~30 MHz; the detection method of the spectrum analyzer is quasi-peak detection; the measurement bandwidth is 9 kHz; the horizontal axis of the spectrum (frequency) is in logarithmic form; the unit of the noise signal is dB/"Vl5j

3.2 Experimental results

Figure 5 is a comparison of the conducted noise spectrum of the experimental samples before and after the transformer design improvement.

The two parallel lines in Figure 5 are the quasi-peak detection limit and average detection limit of Class B in the CISPR22 standard issued by the International Special Committee on Radio Interference (CISPR for short); and the curve is the conducted noise spectrum of the switching power supply. It can be seen from the experimental results that compared with the traditional method, the new method has a better ability to suppress common-mode noise current, especially in the medium frequency band of 1 to 5MHz. In the lower frequency band, the conducted interference on the power line is mainly caused by the differential mode current; while in the medium and high frequency bands, the common mode current plays a major role. The method proposed in this paper has a strong suppression of the common mode current, and the experiment is consistent with the theory. In the frequency band above 10 MHz, the EMC performance is mainly determined by other parasitic parameters in the circuit, and has little to do with the transformer.

4 Conclusion

The noise active node in the switching power supply circuit is the common mode noise source in the circuit. To reduce the conducted interference level of the switching power supply, it is actually to reduce the common mode current intensity and increase the ground impedance of the noise source. In traditional isolated EMC design, the isolation layer is connected to the node with stable potential in the circuit (such as the negative pole of the transformer front stage) to suppress EMI interference more effectively than directly connecting to the ground line.

The noise active nodes in the switching power supply circuit usually exist in pairs, and the phases between these paired nodes are opposite. Taking advantage of this feature, the active node phase balance winding method is more effective in suppressing EMI than the traditional isolated design. Since there is no need to add an isolation metal layer, the size and cost of the transformer can be effectively reduced or reduced.