Switching power supply output protection strategy
In all switching power supply designs, the protection of the load and the protection of the power supply due to load failure are very important aspects that need to be considered in the design. In military product design, a task called failure mode and effects analysis is often required. In this work, each part of the circuit is assumed to be an open-circuit fault and then a short-circuit fault case. In this case, analyze separately how each fault affects the rest of the circuit. Predicting and estimating faults in this way will make the design of the power supply more reliable. This is also what the power supply designer needs to do to protect the load circuit when input abnormalities and power supply load circuit failures occur. In the protection strategy, multiple protection circuits are often cascaded to ensure that the protection circuit itself can provide redundant protection when an abnormality occurs. Fuses and circuit breakers are usually used to provide this backup function.
The protection methods used in power supply and energy transmission systems should carefully consider the end user of the product and the function of the product, and in turn, the method of repairing the product. If the product is regularly maintained by maintenance personnel on the circuit, the circuit in turn considers the maintenance method of the product. If the product is regularly maintained by maintenance personnel, the circuit must have a circuit breaker, a self-guided startup circuit, and an overcurrent feedback circuit. If the function requirements of the product are not strict, these functions are not needed, and only a fuse, an over-current shutdown type starting circuit, or an over-current protection or quick circuit breaker is sufficient. Some protection methods need to send the device to the maintenance station for inspection before it can be used again after the protection occurs. These protection methods can be divided into the following three types:
1. Repair after failure (fuse, fuse resistor, etc.).
2. It will recover after failure (circuit breaker, current limiting link, voltage limiting protection circuit, etc.).
3. Shut down after failure, but will recover after the failure disappears (over-current shutdown type startup circuit, etc.).
There are three basic types of overcurrent protection. Current mode control, or primary peak current controller, limits the output power in the event of a "slight" short circuit in the circuit, but can eventually lead to a severe short circuit condition. When an overcurrent fault occurs, the load impedance becomes smaller and smaller, the output voltage decreases, but the output current continues to increase, which may burn out the circuits and devices on the circuit board. The method of reducing the output voltage can be combined with a fault shutdown type circuit. The constant current limiting method is realized by amplifying the voltage on the current detection resistor and comparing it with the reference voltage. When the current protection value is exceeded, the load will increase again, and the output current will be limited. The over-current feedback limiting circuit uses a small part of the output voltage as the upper limit reference value of the over-current fault. When the current passing through the current detection resistor exceeds the current upper limit value, the output voltage is reduced, so the output current is also reduced, which can prevent the load circuit from burning out.
There are two methods of overvoltage protection at the output: mandatory method and overvoltage blocking method. The forcing method is achieved by overvoltage clamping and fast tripping. The mandatory method assumes that when the power supply fails, the load current cannot be limited
The over-voltage blocking method is used in the assumption that the power supply continues to work, but the voltage feedback loop is open, or one of the output terminals is lightly loaded, and the voltage exceeds the rated maximum value. These methods have a separate comparator or transistor per output and a resistor divider connected to the output. The choice of protection scheme is related to economic cost and layout space. Designers can use their creativity to design protection circuits, but carefully check the situation under all operating conditions.
