Thread Content
The development of switching power supplies has driven the growth of related industries. When developing power supply products, we have also identified some common protection circuits for LED switching power supplies. Generally, a good power supply product should not only be efficient, stable, and reliable, but various protection mechanisms for the circuitry also need to be carefully designed and implemented. This ensures that LED switching power supply devices can provide effective protection for both the switching circuitry and the load in complex environments. Therefore, in this article, the author will guide you through some of the common protection circuits used in LED power supplies. In DC LED switching power supply circuits, it is necessary to protect the regulating transistor from being damaged in the event of a circuit short circuit or an increase in current. Its general approach is that when the output current exceeds a certain value, the control transistor enters a reverse-biased state, thereby turning off and automatically cutting off the circuit current. The overcurrent protection circuit is composed of transistor BG2 and voltage-dividing resistors R4 and R5. When the circuit is operating normally, the voltage across R4 and R5 causes the base potential of BG2 to be higher than its emitter potential, resulting in a reverse voltage being applied to the emitter junction. As a result, BG2 is in a cut-off state (equivalent to an open circuit), having no effect on the voltage regulation circuit. When the circuit is short-circuited, the output voltage becomes zero; the emitter of BG2 is effectively grounded, causing BG2 to be in a saturated conducting state (equivalent to a short circuit). This in turn results in the base and emitter of the control transistor BG1 being nearly short-circuited, putting it in a cut-off state and thereby stopping the current flow in the circuit, achieving protection as a result. The overvoltage protection of the switching regulator in DC LED switching power supplies includes input overvoltage protection and output overvoltage protection. If the voltage of the unregulated DC power supply used by the switching regulator (such as batteries and rectifiers) is too high, it can prevent the switching regulator from functioning properly and may even damage its internal components; therefore, it is essential to use an input overvoltage protection circuit in LED switching power supplies. When the voltage of the DC power supply applied is higher than the breakdown voltage of the voltage regulator diode, the diode breaks down, allowing current to flow through resistor R, which in turn causes transistor T to conduct. This triggers the relay, causing its normally closed contact to open and thus cutting off the input. The polarity protection circuit for the input power supply can be combined with the input overvoltage protection to form a polarity protection and overvoltage protection circuit. The circuit of a switching power supply is relatively complex; the input side of a switching regulator usually has an input filter consisting of a small inductor and a large capacitor. At the moment of power-up, a large inrush current flows through the filter capacitor, and this inrush current can be several times the normal input current. Such a large inrush current can melt the contacts of ordinary power switches or relays, as well as blow the input fuse.