Transformer Electromagnetic Compatibility Design of Flyback Switching Power Supply

This article takes a flyback switching power supply as an example to explain the generation and propagation mechanism of its conducted common-mode interference. Based on the idea of ​​balancing the active nodes of noise, a new transformer EMC design method is proposed. Experimental verification shows that compared with the traditional design method, this method has a stronger ability to suppress conducted electromagnetic interference (EMI) and can reduce the production cost and process complexity of the transformer. This method is also applicable to other forms of switching power supplies with transformer topology.

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 addition of a power line filter is a necessary measure to suppress conducted EMI [1]. However, relying solely on the filter at the power input 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. Other parts of the circuit can also perform similar work as the filter if they are designed properly. 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 supply

Conducted noise interference of electronic equipment refers to 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, which is easier to suppress; common mode interference contributes more to conducted interference, and is mainly in the medium and high frequency bands of the noise spectrum. The suppression of common mode conducted interference is the difficulty in the conducted EMC design of electronic equipment, and it is also the most important task.

There are some nodes with drastic voltage changes in the flyback switching power supply circuit. 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. Noise-active nodes are the common-mode conduction interference sources in the switching power supply circuit. They act on the stray capacitance to ground in the circuit to generate common-mode noise current ICM. The stray capacitance to ground in the circuit that has a greater impact on EMI includes: the parasitic capacitance Cde of the drain of the power switch tube to ground, the parasitic capacitance Cpa of the primary winding of the transformer to the secondary winding; the parasitic capacitance Cae of the secondary loop of the transformer to ground, the parasitic capacitance Cpc and Cac of the primary and secondary windings of the transformer to the magnetic core, and the parasitic capacitance Cce of the transformer magnetic core to ground. The distribution of these parasitic capacitances in the circuit is shown in Figure 1.

Figure 1. Coupling paths of common-mode noise current in a circuit

There are three main coupling paths for the common mode current ICM in Figure 1: from the noise source - the d pole of the power switch tube is coupled to the ground through Cde; from the noise source is coupled to the secondary circuit of the transformer through Cpa, and then coupled to the ground through Cae; from the front and secondary coils of the transformer through Cpc and Cac to the transformer core, and then coupled to the ground through Cce. These three currents are the main factors that constitute the common mode noise current (shown by the black arrows 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 coupling common-mode noise through the FET d-pole to ground, the noise voltage at the d-pole of the switch tube couples the noise current to the loop where the transformer secondary winding 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, which 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.

Figure 2: Effect of transformer isolation layer on noise current

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 electrode of the circuit front stage 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 active node of noise voltage 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 Uin of the transformer front winding is also an active node of noise voltage, and the change direction of the node voltage is opposite to the d-pole voltage of the field tube. Therefore, the two ends of the transformer secondary winding are active nodes of noise voltage with opposite phases. Figure 3 shows the distribution of coils on the transformer skeleton after the node phase balance method is adopted.

Figure 3. Coupling of noise current inside the transformer

The innermost layer of the transformer skeleton is half of the front winding coil, 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 Uin. 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 that is 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 to power 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 common-mode noise from the active node to the ground. However, sandwiching these windings between the front and secondary coils can increase the distance between the front and rear windings, thereby reducing their inter-layer parasitic capacitance and noise current. Therefore, the final method of winding the transformer 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 improved transformer winding method in improving the conducted EMC performance of switching power supplies can be verified through experiments.

3.1 Experimental Methods

The experiment was conducted according to the voltage method in the literature [4]. The frequency range was 0.15 to 30 MHz; the detection method of the spectrum analyzer was quasi-peak detection; the measurement bandwidth was 9 kHz; the horizontal axis of the spectrum (frequency) was in logarithmic form; and the unit of the noise signal was dBμV [5].

Figure 4: Details of transformer improved winding method

3.2 Experimental Results

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

Figure 5. Noise spectrum before and after transformer design improvement

The two parallel lines in Figure 5 are the quasi-peak detection limit and the average detection limit of the b-level requirements 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 nodes in the switching power supply circuit are the common mode noise sources in the circuit. To reduce the conducted interference level of the switching power supply, it is actually necessary to reduce the common mode current intensity and increase the impedance of the noise source to the ground. In the 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.

Noise-active nodes in switching power supply circuits usually exist in pairs, and the phases between these paired nodes are opposite. Taking advantage of this feature, the phase-balanced winding method of active nodes is more effective in suppressing EMI than the traditional isolation 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.