Transformer EMC Design Principles 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 instruments and equipment at all levels are widely used, and with the continuous improvement of the clock frequency, the electromagnetic compatibility of the equipment (EMC) problem has attracted widespread attention.EMC design has become a switching power supply development and design of the essential and important links.
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 increased capacitance of the components in the filter. The increase in inductance increases the size; the increase in capacitance is limited by leakage current standards. Other parts of the circuit, if properly designed, can do a similar job to the filter. This paper proposes a noise-active node phase-dry winding method for transformers, a design method that not only reduces the size of the power line filter, but also reduces the cost.
Flyback switching power supply common mode conducted interference
Conducted noise interference of electronic equipment refers to: electromagnetic interference in the form of noise current conducted through the power line to the public power grid environment when the equipment is connected to the power supply grid operation. 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 conduction interference contribution is small, and mainly concentrated in the low-frequency end of the noise spectrum, it is easier to inhibit; common-mode interference on the contribution of the conduction 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 .
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 c. Coupled to the secondary circuit of the transformer, and then coupled to ground through C; from the transformer's front and secondary coils through C?C coupled to the core of the transformer, and then coupled to ground through C. 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.
