Automotive Bus Protection against Reverse Connection of HVDC Ballasts and Design of Hot Swap Circuits for HID Ballasts

Normally, the DC power input anti-reverse protection circuit uses the unidirectional conductivity of the diode to achieve reverse-connection protection. As shown in Figure 1 below:

Figure 1. Diode type reverse connection protection circuit

This connection is simple and reliable, but the power consumption is very large when the input current is large. For automotive HID ballasts, for example, ballast input current rating of 8A, such as the selection of Onsemi's fast recovery diode MUR3020PT, rated voltage drop of 1V, then the power consumption must reach at least: Pd = 8A × 1V = 8W, This will have a great impact on the efficiency and heat dissipation of the ballast. There is no practical significance.

In practical application, the diode anti-reverse connection circuit generally adopts the connection method of Fig. 2. The advantage of this anti-reverse connection circuit is that the circuit is simple, the cost is low, and there is no loss, but after the power supply is reversed, the diode conduction will burn the insurance, after reverse connection Need to replace the fuse, usually used in the car radio circuit, provided that the device must be equipped with fuses.

Figure 2. Diode type reverse connection protection circuit application (2) MOS type reverse connection protection circuit

Therefore, the present ballast utilizes the switching characteristics of the MOS tube, and the control circuit is turned on and off to design the reverse connection protection circuit. Since the internal resistance of the power MOS tube is small, the HOMSEMI MOSFET Rds(on) can already be realized. The milliohm level solves the problems of the current voltage drop and power consumption that are caused by the diode power supply reverse-rejection scheme. The specific circuit is shown in Figure 3.

Figure 3. MOS transistor anti-reverse protection circuit

The MOS transistor is serially connected between the power supply and the load through the S pin and the D pin. The resistor Rg provides a bias voltage for the MOS transistor. The switch characteristic of the MOS transistor is used to control the turn-on and turn-off of the circuit, thereby preventing the power supply from being reversed. Damage caused by the load. The actual use of HOMSEMI HS50N06 power MOS tube Rds (on) only 20mΩ actual loss is very small, no additional heat sink. It solves the problems of the existing voltage drop and excessive power consumption of the diode-powered reverse-connection protection scheme.

Figure 4. Practical application of MOS-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 through Rg1. The two resistors provide the voltage bias for the MOS tube to divide the voltage, so that the voltage to the gate is reduced to reach the safe voltage. Increasing the input loop yoke inductance L and EMC absorption capacitance is to prevent the ripple current returned from the transformer from being too large, which interferes with the operation state of the anti-reverse connection HS50N06 and the interference of the car bus. We have shown through trial and error that the Rg2 pull-down resistor is more important. During normal operation, the DC/DC ripple of the switch reaches about 30-50V. If the gate of Q1 has a pull-down resistor, it can divide the voltage so that the voltage reaching the gate can be reduced to a safe voltage.

This article mainly aims at the anti-reverse protection and the hot-swappable circuit design of the DC power supply of the HID ballast for the automotive bus. This design not only meets the requirements of the reliability and maintainability of the power system, but also has a low cost. The circuit designed in this paper has been widely used in the company's actual system, and its practicality and stability have been verified by the system.

In a car, often there are more than a dozen or even dozens of wires wrapped in a wire harness. It is difficult to distinguish their direction with dense wires. With electricity being invisible, the car circuit is very complicated for many people. . In this way, when the circuit is powered on or powered off the ballast, a large starting current and voltage fluctuation will generally occur. These phenomena will affect the normal operation of the equipment and even cause damage to the entire system. Similarly, when the 12(24)V DC input power is abnormal and there is a positive and negative reverse connection, in order to prevent the ballast from being damaged, the ballast is generally required to have a reverse connection protection circuit.

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