Power switch with line ripple protection and durable internal power
Electrical energy has penetrated into every aspect of human daily life. It is hard to imagine what life would be like without a refrigerator, air conditioner or television. However, if the power supply fluctuates or fails, even with advanced technology, electrical equipment will be useless. A more critical issue is damage to the equipment due to fluctuations in the AC input line. Several reasons for line voltage fluctuations are: Increased demand leads to grid overload, improper infrastructure construction, or lightning strikes. In either case, when the line voltage rises due to fluctuations, the electrical equipment is at risk of failure of the power system. In this case, the power board that converts the AC line input to a commensurate DC voltage is usually the first to be damaged. This paper deals with the protection scheme of the flyback AC-DC converter, which is essential in preventing overvoltage of the line of the household appliance system. Avalanche resistant power switch One of the viable solutions for input line fluctuations or overvoltages is the use of an avalanche-resistant power switch. Most integrated power switches use MOSFETs as power switches internally. Avalanche breakdown occurs when the power switch has a high voltage above its rated voltage, and typical modern power MOSFETs are somewhat resistant to this situation. However, this level of tolerance can vary greatly depending on device technology or chip design. Figure 1 illustrates this difference. Both devices have very similar on-resistance specifications. However, in the non-clamp-inductive switch (UIS) test, the fault current level of the FSL series is more than 10 times higher. This means that the FSL series can withstand much higher current or electrical stresses before the equipment is damaged. In this example, it is 424 mJ and 3.67 mJ. The FSL series has built-in power MOSFETs with a vertical DMOS structure, while competing devices use a lateral DMOS structure. In general, the vertical DMOS structure used in Fairchild Semiconductor's FPSTM power switch products is more suitable for high voltage, high power MOSFETs. The lateral DMOS structure enables the MOSFET to be better integrated into the logic circuit, but it is not as resistant to high levels of electrical stress as the vertical DMOS structure. In fact, unclamped switching conditions are not common because most flyback converters have integrated snubber circuits that clamp high voltage spikes when the power switch is turned off. However, the snubber circuit cannot always maintain the spike voltage below the maximum rating of the power switch. In the case of abnormalities such as output short circuit or input line overvoltage, the power switch may be broken down. In this case, better avalanche tolerance helps maintain the stability of the flyback converter. FPS power switch with input overvoltage protection (FSL MRIN series) If the line is overvoltage due to fluctuations, the internal power MOSFET will turn off under various voltage conditions within the drain-source range and may cause avalanche breakdown. Even if the internal MOSFET of the power switch can withstand a certain degree of electrical stress under transient or abnormal conditions, it should be used as the last line of defense against faults, because if the stress exceeds the tolerance of the device, the device will suffer To permanent damage. Therefore, when the line voltage level is too high, it is better to turn off the switching action of the internal power MOSFET. At this point, a power switch that provides integrated protection for input line overvoltage is a great advantage for flyback converter designers. The FSL117MRIN is a new green mode power switch with an internal MOSFET rated avalanche tolerance of 700V and integrated input line overvoltage protection (LOVP). This line overvoltage protection provides additional protection against sudden rises in line input voltage and ensures safe operation of the flyback converter. Figure 2 shows the schematic of the line overvoltage protection function. The voltage detection circuit inside the controller is responsible for real-time monitoring of the line input voltage. When the line input voltage exceeds the preset level - the voltage on the VIN pin exceeds 1.95V - the controller will signal a fault and turn off the PWM output. The flyback converter will stop operating when the VIN pin voltage is as low as 1.89V. To prevent false triggering of line overvoltage protection, this function only triggers when the line overvoltage continues for more than 20us. Another important feature of the line overvoltage protection feature is automatic recovery. Even under fault conditions, the controller continuously monitors the line input voltage and turns on the PWM output when the overvoltage condition disappears. The FSL117MRIN is designed to meet customer needs, as home appliance applications often require automatic recovery mode to be unlatched. With line overvoltage protection, internal power MOSFETs are protected from potential avalanche breakdown threats. It also guarantees the stability of the flyback converter with line input voltage fluctuations. in conclusion Fairchild Semiconductor has released a new family of FPS power switches that integrate input line overvoltage protection (LOVP). It provides additional protection against line fluctuations as well as a rugged internal power MOSFET for the FPS power switch (FSL MRIN series). When the grid is very unstable, it provides important protection for the device. Figure 1 UIS characteristics of the internal MOSFET Figure 2 Input overvoltage protection Figure 3 Working waveform of air conditioning input overvoltage protection and recovery function
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Figure 3 shows the operation of the input overvoltage protection function in commercial air conditioners. The air conditioning system operates at an AC line input voltage of 320 VAC and the line voltage suddenly increases to 336 VAC. At this point, the line overvoltage protection function will be activated and the FPS power switch (FSL MRIN series) will stop running. When the line input voltage returns to normal level, the protection function will be cleared and the FPS power switch (FSL MRIN series) will perform the switching action again. 
