Power adapter overload protection

　　Type 1 Form B: Over-power delay shutdown protection

　　One of the most effective overload protection methods for low-power, low-cost power adapters is this over-power delay shutdown protection technology. If the load power exceeds the predetermined maximum value and its duration also exceeds the specified safe working time, the power adapter will be turned off to stop power supply, and the switching cycle of the input power adapter will also be reset to the normal working state.

　　This technology not only provides the greatest protection for the power adapter and load, but is also the most cost-effective for small power adapters. Although this technology is not so popular among most users, don't ignore it, because it can effectively shut off the power adapter when an overload occurs. Continuous overload usually indicates that there is a fault in the equipment, and a full range of protection is provided to the load and the power adapter by shutting down.

　　But many technical requirements rule out the possibility of simple trip type protection, because they require automatic recovery in overload conditions. The user may have previously used a foldback or trip system that did not have sufficient current range guarantee and delayed shutdown, and therefore encountered bad events such as "lock-up" or noise shutdown, and thus required the use of automatic recovery technology. The power adapter designer should question this technical requirement. Modern switching power supply adapters can still deliver current well when the continuous working rating is exceeded for a short period of time. Even if a shutdown system is adopted, this kind of switching power supply adapter with delayed shutdown will not lock up.

　　In the time-delay trip system, the requirement of short-term transient current is allowed. Only when the current stress exceeds a safe value for a long time, the power adapter is turned off. The short-term transient current supply will not harm the power adapter Reliability will not have a great impact on the cost of the power adapter. Only the requirement of long-term continuous current will affect the cost and volume of the circuit. When the power adapter outputs a large transient current, its performance will be reduced to a certain extent, which may exceed the specified voltage error and ripple value. Typical examples of such loads that are susceptible to large and short transient currents are floppy disk drives and solenoid drives.

　　Type 1 Form C: Pulse-by-pulse over-power or over-current limit

　　This is a very useful protection technology, which is often used in additional secondary side current limiting protection.

　　In the previous switchgear, the input current was monitored in real time. If this current exceeds the specified limit current value, the conduction pulse will be terminated. In a discontinuous flyback converter, its maximum current determines the power of the circuit, and this type of protection circuit becomes a real power limit protection circuit.

　　For the switching circuit of the forward converter, its input power is a function of input current and input voltage. The type of protection adopted by this circuit provides a primary-side current-limiting protection technology. Under the condition of constant input voltage, this technology also provides an effective detection method for power limit protection.

　　The main advantage of the one-by-one fast pulse current limiting technology is that it provides protection for the primary switching devices under the action of abnormal transient stress such as the step saturation effect of the transformer.

　　Current-type control stipulates the pulse-by-pulse current limit of the primary side as a standard function of the control technology, which is also one of its main advantages.

　　Type 1 Form D: Constant power limit

　　The constant input power limit protects the primary circuit by limiting the maximum transmission power. But in the flyback converter, this technology can hardly protect the secondary output components. For example, in a discontinuous flyback converter, the peak current of the primary side has been limited, which gives a limited transfer power.

　　When the load resistance decreases and the load exceeds its limit value, the output voltage begins to drop. It is precisely because the product of the voltage and current of the input and the corresponding output is specified that when the output voltage starts to drop, the output current will rise. In the event of a short circuit, the secondary side current will become very large, consuming all the power in the power adapter. This form of power limitation is generally only used as a supplementary form of certain limitations, such as secondary-side current limiting in circuits with supplementary limitations.

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