Principles of EMC design of transformers for flyback switching power supplies
With the development of power semiconductor device technology, switching power supply high power-to-volume ratio and high efficiency characteristics make it in the modern, industrial and commercial equipment at all levels are widely used, and with the clock frequency continues to improve, the electromagnetic compatibility of the equipment (EMC) problem has attracted widespread attention.EMC design has become an essential and important link in the development and design of switching power supply.
Conducted electromagnetic interference (EMI) noise suppression must be considered in the early stages of product development. Often, the addition of power line filters is necessary to suppress conducted EMI l1l. However, relying solely on the power input filter to suppress interference often leads to increased inductance and capacitance of the components in the filter. The increase in inductance increases the size; the increase in capacitance is limited by the leakage current standard. Other parts of the circuit, if properly designed, can do a similar job to the filter. This paper proposes a transformer noise active node phase dry winding method, this design method can not only reduce the size of the power line filter, but also reduce costs.
Flyback switching power supply common mode conducted interference
Conducted noise interference of electronic equipment refers to: equipment in connection with the power supply grid work in the form of noise current through the power line to the public power grid environment to conduct electromagnetic interference. Conducted interference is divided into common mode interference and differential mode interference. Common mode interference current in the zero line and the phase of the phase line is equal; differential mode interference current in the zero line and the phase of the phase line on the opposite. Differential mode interference on the overall contribution of the conducted interference is small, and mainly concentrated in the low-frequency end of the noise spectrum, easier to suppress; common mode interference on the contribution of the conducted interference is larger, and 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 design of electronic equipment conducted EMC, but also * the main task.
There are some nodes in the circuit of the flyback switching power supply where the voltage changes dramatically. Unlike other nodes in the circuit where the potential is relatively stable, the voltages at these nodes contain high-intensity, high-frequency components [2]. These nodes with very active voltage variations are called noise active nodes. Noise active nodes are a source of common-mode conducted interference in switching power supply circuits, which acts on the stray capacitance to ground in the circuit to generate a common-mode noise current M . And the circuit of EMI has a greater impact on the ground stray capacitance: power switching tube drain to ground parasitic capacitance C Transformer's main side of the winding on the secondary side of the winding parasitic capacitance Cp ; Transformer's secondary side of the circuit to the ground parasitic capacitance C Transformer main and secondary side of the winding on the core of the parasitic capacitance of the C, C, and the transformer core of the parasitic capacitance of the C to the ground? These parasitic capacitance in the circuit is distributed as follows
Coupling path in the circuit there are mainly 3: from the noise source - power switching tube d-pole coupled to ground through C; from the noise source through the c. Coupled to the secondary circuit of the transformer, and then through the C coupled to ground; from the transformer's front and secondary coils through the C?C coupled to the transformer core, and then through the C coupled to ground. These three currents are the main contributors to the common mode noise current (shown by the black arrows in Figure 1). The common mode current is measured by sampling the LISN by returning it through the ground at the input of the power supply line.
