1 กระแสไฟ EMI
There are many circuit topologies of switching power supply, which can be divided into full-bridge, half-bridge, push-pull, single-ended forward, single-ended flyback and other modes according to the combination of power switch tube and high-frequency transformer. In the medium and small power switching power supply module, the circuit topologies used more are push-pull, single-ended forward, single-ended flyback and so on. The block diagram of a typical single-ended forward switching power supply circuit is shown in Figure 1. It consists of a power switch tube Q1, a high-frequency transformer T, a rectifier diode Dl, a freewheeling diode D2, an output filter inductor L, and an output filter capacitor C. When working, the PWM control unit can send a pulse signal with variable pulse width to drive the switch tube Q1. When the switch tube Q1 is turned on, the DC energy at the input end is transmitted to the secondary through the high-frequency transformer. When the switch tube Q1 is turned off, High-frequency transformer for magnetic reset. The high-frequency pulse transmitted through the high-frequency transformer is rectified into a single-direction pulsating DC by the rectifier diode. After this pulsating DC is filtered by the output filter inductor and filter capacitor, the required DC voltage can be sent.
During the high-frequency switching process of the power switch transistor Q1, the pulses flowing through the power switch transistor and the high-frequency transformer will generate complicated harmonic voltages and harmonic currents. The noise generated by these harmonic voltages and harmonic currents can be transmitted to the public power supply terminal through the power input line, or to the load through the output line of the switching power supply, thereby causing interference to other systems or sensitive components. The noise spectrum of these noises conducted on the power line is shown in Figure 2. It can be seen from the figure that in the frequency band of several hundred kHz to 50 MHz, that is, the frequency band of the fundamental and several harmonics of the switching frequency Within the range, the amplitude of the interference noise far exceeds the range specified by GJBl51A, which will cause the electromagnetic compatibility indicators such as system conduction noise to exceed the standard.
2. กระแสรบกวนโหมดทั่วไป
The entire circuit components of the surface mount switching power supply module with metal package structure are all assembled on the substrate. Active devices such as PWM control chips, power switch tubes, and rectifier diodes are all surface-mounted package components. The voltage and current of the input and output are sent out by the leads.
แผ่นด้านล่างของเปลือกหลอดเป็นพาหะของพื้นผิวอลูมินา ด้านหน้าของพื้นผิวอลูมินาคือพื้นที่เดินสายและพื้นที่ประกอบของส่วนประกอบ แผ่นฐานโลหะติดอยู่ ค่าคงที่ไดอิเล็กตริกของพื้นผิวอลูมินาคือ 8 และความหนามักจะอยู่ในช่วง 0.5 ถึง 1 0 มม. ในพื้นที่ประกอบที่ด้านหน้าของพื้นผิวอลูมินาส่วนประกอบยึดพื้นผิว (เช่นชิปควบคุม PWM, op amps, แหล่งอ้างอิง, สวิตช์ MOSFET, ไดโอดวงจรเรียงกระแส) จะเชื่อมต่อกับสายไฟผ่านบัดกรี (เช่นกาวนําไฟฟ้า, บัดกรี reflow ฯลฯ ) แผ่นในพื้นที่จะเชื่อมต่อ แม้ว่าวิธีการเชื่อมต่อนี้จะถือเป็นวงจรลูป แต่ก็นํา Cp ความจุของกาฝากใหม่มาสู่วงจร
In the primary loop, the power switch chip, PWM control chip, operational amplifier chip, the traces of the positive and negative input lines of the power supply, etc. will generate parasitic capacitance Cp between the shell bottom plate, and the capacity of the parasitic capacitance depends on the thickness of the substrate. and the area they occupy on the floor. In this way, in the circuit, distributed capacitances Cp1, Cp2, ..., Cp6, etc. are formed between these components and their traces and the bottom plate of the housing. These distributed capacitances will cause noise currents under the combined influence of dV/dt, dI/dt and the reverse recovery current of the rectifier diode. These noise currents are equal in magnitude and phase between the positive and negative of the input power line and between the positive and negative of the output load line, and are called common-mode noise currents. The size of the common mode noise current is related to the size of the distributed capacitance, dV/dt, dI/dt, etc.
3. Primary differential mode interference current
The primary differential mode interference current, in the primary loop, the power switching tube Q1, the high-frequency transformer primary winding Lp and the input filter capacitor Ci constitute the input DC conversion circuit of the switching power supply. The DC energy is transferred to the secondary through the high frequency transformer. However, when the power switch Q1 switches, the fundamental wave and harmonics caused by the rise and fall of the high-frequency pulse will be transmitted to the input power supply terminal along the input filter capacitor Ci, and this noise current propagates along the positive and negative terminals of the input power supply line. It is called the primary differential mode interference current IDIFF. This differential mode interference current IDIFF flows to the common power supply terminal through the input power supply line, especially when the input filter capacitor Ci is insufficiently filtered, the interference to the input power supply line is great, and it will also interfere with other parts of the system through the common power supply terminal. Thus, the performance indicators of other parts are reduced.
