Under normal circumstances, the DC power input anti-reverse protection circuit uses the unidirectional conductivity of the diode to achieve anti-reverse protection. As shown in Figure 1 below:
Figure 1. Diode-type anti-reverse protection circuit
This connection is simple and reliable, but the power consumption is very large when a large current is input. Taking the automobile HID ballast as an example, the input current rating of the ballast reaches 8A. If Onsemi's fast recovery diode MUR3020PT is used, the rated tube voltage drop is 1V, then the power consumption must be at least: Pd=8A×1V=8W, This will have a great impact on the efficiency and heat dissipation of the ballast, and has no practical significance.
In practical applications, the diode anti-reverse circuit generally adopts the connection method of FIG. 2. The advantage of the anti-reverse circuit is that the circuit is simple, the cost is low, and there is no loss, but after the power is reversed, the diode is turned on to burn the insurance, and after the reverse connection, Need to change the fuse, usually used in the car radio circuit, provided that the device must be equipped with a fuse.
Figure 2. Diode-type anti-reverse protection circuit application (2) MOS tube type anti-reverse protection circuit
Therefore, the ballast utilizes the switching characteristics of the MOS tube, and the control circuit is turned on and off to design an anti-reverse protection circuit. Since the internal resistance of the power MOS tube is small, the HOMSEMI MOSFET Rds(on) can now be implemented. The milliohm level solves the problem of excessive voltage drop and excessive power consumption of the existing diode power supply anti-reverse connection scheme. The specific circuit is shown in Figure 3.
Figure 3. MOS tube type anti-reverse protection circuit
The MOS tube is connected in series between the power supply and the load through the S pin and the D pin. The resistor Rg provides a voltage offset for the MOS tube, and the switching characteristics of the MOS tube are used to control the conduction and disconnection of the circuit, thereby preventing the power supply from being reversed. The load is damaged. The Rds(on) of the HS50N06 power MOS tube that actually uses HOMSEMI has only 20mΩ of actual loss, and there is no need to add a heat sink at all. It solves the problem of excessive voltage drop and excessive power consumption in the existing diode power supply anti-reverse connection scheme.
Figure 4. Practical application of MOS tube type anti-reverse protection circuit in HID xenon lamp power supply (usually called ballast)
The G pin of the MOS transistor is connected to the D pin of the MOS transistor through Rg2, and is connected to the power supply terminal through Rg1. The two resistors provide a voltage offset for the MOS transistor to divide the voltage, causing the reaching gate voltage to drop and reach a safe voltage. Increasing the input loop yoke current inductance L and EMC absorbing capacitor is to prevent the ripple current returning from the transformer from being excessive, and to interfere with the working state of the anti-reverse tube HS50N06 and the interference of the bus. We have proved through trial and error that the Rg2 pull-down resistor is more important. During normal operation, the ripple of the switching tube DC/DC reaches about 30-50V. If the Q1 gate has a pull-down resistor, it can be divided to make the gate voltage drop and reach a safe voltage.
This paper mainly aims at the anti-reverse protection and hot-swap circuit design of the DC power supply part of the HID ballast for the automobile bus. This design not only meets the requirements of the reliability and maintainability of the power system, but also has low cost. The circuit designed in this paper has been widely used in the actual system of the company, and its practicability and stability have been verified by the system.
In a car, there is often a wire harness wrapped with more than a dozen or even dozens of wires. It is difficult to distinguish the direction of the car, and the electricity is invisible, so the car circuit is a very complicated thing for many people. . In this way, when the ballast is powered on or off, a large starting current and voltage fluctuations are generally generated, which will affect the normal operation of the device and even cause damage to the entire system. Similarly, when the 12 (24) V DC input power supply is abnormal and positive and negative reverse connection occurs, in order to prevent the ballast from being damaged, the ballast is usually required to have an anti-reverse protection circuit.
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