Protection Technology of DC Switching Stabilized Power Supply
The high-power switching devices used in DC switching regulators are more expensive, and their control circuits are 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 are less able to withstand electrical and thermal shocks. Therefore, the protection of the switching regulator should take into account the safety of the regulator itself and the load. protection circuit
There are many types, here we introduce circuits such as polarity protection, program protection, overcurrent protection, overvoltage protection, undervoltage protection, and overheating protection. Several protection methods are usually selected to be combined to form a complete protection system.
1 polarity protection
The input of a DC switching regulator is generally an unregulated DC power supply. The switching regulated power supply will be damaged if its polarity is wrongly connected due to misoperation or accident. The purpose of polarity protection is to make a switching regulator work only when connected to an unregulated DC power source with the correct polarity. The polarity protection of the power supply can be realized by using a unidirectional conduction device. The simplest polarity protection circuit enters the total current, so this circuit is more suitable for low-power switching regulators. In the case of higher power, the polarity protection circuit is used as a link in the program protection, which can save
High power diodes required for polarity protection will also reduce power dissipation. For the convenience of operation, it is easy to identify whether the polarity is correct or not.
2 Program protection
The circuit of the switching regulated power supply is relatively complicated, and can basically be divided into a low-power control part and a high-power switching part. Switching transistors are of high power. In order to protect the safety of switching transistors when the power is turned on or off, low-power control circuits such as modulators and amplifiers must first work. To this end, to ensure the correct boot procedure. The input terminal of the switching regulator is generally connected with an input filter with a small inductor and a large capacitor. At the moment of power-on, the filter capacitor will flow a large surge current, which can be several times the normal input current. Such a large inrush current can melt the contacts of a normal power switch or the contacts of a relay and blow the input fuse. In addition, inrush current can also damage the capacitor, shortening its life and premature failure. For this reason, a current-limiting resistor should be connected when starting up, and the capacitor will be charged through this current-limiting resistor. In order not to cause the current limiting resistor to consume too much power, so as to affect the normal operation of the switching regulator, after the start-up transient process is over, a relay is used to automatically short it, so that the DC power supply directly supplies power to the switching regulator. . This circuit is called the "soft start" circuit of the switching regulator.
3 Overcurrent protection
When accidents such as load short circuit, overload or control circuit failure occur, the current flowing through the switching transistor in the voltage regulator will be too large, which will increase the power consumption of the tube and generate heat. If there is no overcurrent
Protection devices, high-power switching transistors may be damaged. Therefore, overcurrent protection is commonly used in switching regulators. The cheapest and easiest way is to use a fuse. Due to the small heat capacity of transistors, ordinary fuses generally cannot play a protective role, and fast fuses are commonly used.
Fast-blow fuse. This method has the advantage of easy protection, but the specification of the fuse needs to be selected according to the safe working area requirements of the specific switching transistor. The disadvantage of this overcurrent protection measure is the inconvenience of frequently replacing the fuse. The current limiting protection and current cutoff 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 over-current protection, it is generally necessary to use sampling resistors in series in the circuit, which will affect the efficiency of the power supply, so it is mostly used in the occasion of low-power switching regulators. In high-power switching regulated power supplies, considering power consumption, the connection of sampling resistors should be avoided as much as possible. Therefore, the over-current protection is usually converted into over-voltage and under-voltage protection.
4 Overvoltage protection
The over-voltage protection of the switching regulator includes input over-voltage protection and output over-voltage protection. If the voltage of the unregulated DC power source used by the switching voltage regulator, such as the battery and the rectifier, is too high, the switching voltage regulator cannot work normally, and even damage the internal devices. Therefore, it is necessary to use an input overvoltage protection circuit. A protection circuit composed of transistors and relays.
5 Undervoltage protection
When the output voltage is lower than the specified value, it means that there is an abnormality in the input DC power supply, inside the switching regulator, or in the output load. When the input DC supply voltage drops below the specified value, it will cause
The output voltage of the switching regulator drops and the input current increases, which endangers both the switching transistor and the input power. Therefore, undervoltage protection should be set up. Simple undervoltage protection
