LTMR4644-based voltage regulator solution-EEWORLD

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The active noise 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 necessary to reduce the common mode current intensity and increase the impedance of the noise source to the ground. This paper 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 noise node, a new transformer EMC design method is proposed. 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. With the continuous increase in clock frequency, the electromagnetic compatibility (EMC) of equipment has attracted widespread attention. EMC design has become an indispensable and important link in the development and design of switching power supplies.
The suppression of conducted electromagnetic interference (EMI) noise must be considered in the early stage of product development. Generally, adding a power line filter is a necessary measure to suppress conducted EMI [1]. However, relying solely on the filter 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 the filter. This paper proposes a phase dry winding method for the noise active node of the transformer. This design method can not only reduce the size of the power line filter, but also reduce the cost. Common-mode conducted interference of flyback switching power supply The 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. The 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; the common-mode interference contributes more to the conducted interference and is mainly in the mid-frequency and high-frequency bands of the noise spectrum. The 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 the 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. 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. In addition to the coupling of common mode noise through the d-pole of the field effect tube to 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 ground, which is also a way to generate common mode current. Therefore, it is an effective EMC design method to try to reduce the common-mode current transmitted from the primary winding of the transformer to the secondary winding. The traditional transformer EMC design method is to add an isolation layer between the two windings [3], as shown in Figure 2. Experimental verification of the solution The effectiveness of the improved transformer winding method in improving the conducted EMC performance of the switching power supply can be verified by experiment. The experiment was carried out according to the voltage method in the literature [4]. 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 dBμV [5]. 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 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.

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