Protection Technology of DC Switching Regulated Power Supply
The high-power switching devices used in the current switching regulator are more expensive, and the control circuit is more complicated. In addition, the load of the switching regulator is generally an electronic system installed with a large number of highly integrated devices. Transistors and integrated devices have poor resistance to electrical and thermal shocks.
Therefore, the protection of the switching regulator should take into account the safety of the regulator itself and the load. There are many types of protection circuits, here are polarity protection, program protection, over-current protection, over-voltage protection, under-voltage protection and overheating protectionand other circuits. Several protection methods are usually selected to be combined to form a complete protection system.
Switching power supply protection circuit
The input to a DC switching regulator is generally an unregulated DC power supply. Due to wrong operation or accidental connection, the polarity of the switch will be damaged, which will damage the switching power supply.
The purpose of polarity protection is to make the switching regulator work only when connected to an unregulated DC power source with the correct polarity. Power supply polarity protection can be achieved by using unidirectional conduction devices. The simplest polarity protection circuit is shown in the figure. Since the diode D must flow through the total input current of the switching regulator, this circuit is more suitable for low-power switching regulators. In the case of high power, the polarity protection circuit is used as a link in the program protection, which can save the high-power diode required for the polarity protection, and the power consumption will also be reduced.
Program protection
The circuit of the switching regulated power supply is more complex, and can basically be divided into a low-power control part and a high-power switching part. Switching transistors are high-power. In order to protect the safety of switching transistors when turning on or off the power supply, low-power control circuits such as modulators and amplifiers must first work. To this end, it is necessary to ensure the correct boot procedure.
The input end of the switching regulator is generally connected with an input filter with a small inductance and a large capacitance. At the moment of power-on, a large surge current will flow through the filter capacitor, and this surge current can be several times the normal input current. Such a large surge current can melt the contacts of ordinary power switches or relays and blow the input fuse. In addition, inrush current can damage capacitors, shortening their life and causing premature failure. For this reason, a current-limiting resistor should be connected at startup, and the capacitor should be charged through this current-limiting resistor. In order not to cause the current limiting resistor to consume too much power and affect the normal operation of the switching regulator, a relay is used to automatically short-circuit it after the startup transient process, so that the DC power supply directly supplies power to the switching regulator ,as the picture shows. This circuit is called the "soft-start" circuit of a switching regulator.
3. Over current protection
When there are unexpected situations such as load short circuit, overload or control circuit failure, the current flowing through the switching transistor in the voltage stabilizer will be too large, which will increase the power consumption of the tube and cause heat. If there is no overcurrent protection device, the high-power switching transistor may be damaged. Therefore, overcurrent protection is commonly used in switching regulators. The most economical and easy way is to use a fuse. Due to the small heat capacity of the transistor, ordinary fuses generally cannot play a protective role, and fast-blow fuses are commonly used. This method has the advantage of easy protection, however, the specification of the fuse needs to be selected according to the requirements of the safe working area of the specific switch transistor. The disadvantage of this overcurrent protection measure is the inconvenience of frequent fuse replacement.
Current limiting protection and current cut-off protection commonly used in linear regulators can be applied in switching regulators. However, according to the characteristics of the switching regulator, the output of this protection circuit cannot directly control the switching transistor, but the output of the overcurrent protection must be converted into a pulse command to control the modulator to protect the switching transistor. In order to realize overcurrent protection, it is generally necessary to use a sampling resistor in series in the circuit, which will affect the efficiency of the power supply, so it is mostly used in low-power switching regulators. In the high-power switching regulated power supply, considering the power consumption, the access of the sampling resistor should be avoided as much as possible. Therefore, over-current protection is usually converted into over- and under-voltage protection.
4. Overvoltage protection
The overvoltage protection of switching regulators includes input overvoltage protection and output overvoltage protection. If the voltage of the unregulated DC power supply such as battery and rectifier used by the switching regulator is too high, the switching regulator cannot work normally, and even damage the internal devices. Therefore, it is necessary to use an input overvoltage protection circuit.
5. Under-voltage protection
When the output voltage is lower than the specified value, it reflects an abnormality in the input DC power supply, inside the switching regulator, or the output load. When the input DC power supply voltage drops below the specified value, the output voltage of the switching regulator will drop and the input current will increase, which will endanger both the switching transistor and the input power supply. Therefore, it is necessary to set under-voltage protection
6. Overheating protection
The high integration, light weight and small volume of switching regulators greatly increase the power density per unit volume, and the requirements of the components inside the power supply device for their working environment temperature also increase accordingly. Otherwise, circuit performance will deteriorate and components will fail prematurely. Therefore, overheating protection should be set in high-power switching regulators.
