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Short circuit protection with a single transistor

Published on Apr 18 2011 // Protection Circuits

In some DC / DC converter, the chip-by-cycle current limit on the measures in the short period may be insufficient to prevent the failure. A non-synchronous boost converter inductor and through the clamping diodes to provide a short circuit at the input end to the direct pathway. When the load short circuit, whether in the current limit protection circuits how great path flows through the load current may damage the clamp diodes, inductors, and integrated circuits. In a SEPIC (single ended primary inductor converter) circuit, the coupling capacitance will interrupt this path. Therefore, when the load short circuit, they do not exist the current output from the input stream to the direct pathway. However, if the required minimum on-time is shorter than the specific duty cycle, the inductor current and switch current will rapidly increase, resulting in integrated circuit failure, input overload, or a combination of both cases. Even in some of the buck regulator, the duty cycle limitations sometimes make the switch on for too long that they can not keep control of the output is shorted, especially in high frequency integrated circuits for very high input voltage time. Method using a single transistor, you can overload or short circuit caused the load inductor current starts out of control, the VC pin (the error amplifier output) voltage drop, so you can prevent SEPIC circuit short-circuit fault (Figure 1).

 Short circuit protection with a single transistor 1

Short circuit protection with a single transistor 2

 Short circuit protection with a single transistor 3

VC pin voltage can be forced down to stop switching circuits, skip the shortest on-time switching cycle, so that each inductor in the current decline. During the short-circuit, L1 of the peak current (limited by reducing the number of switching cycles) and the peak current of L2 is equal to the peak switch current of 1.5A is less than LT1961EMS8E limit. Figure 2 shows the different input voltage, input current and short circuit short-circuit output current. Figure 3 shows the maximum load current and input voltage curve. L2 in the average current equal to the load current, and under all load conditions up to 600 mA. If the sense resistor current of 800 mA is detected, then it will know what happened overload protection circuit and to transistor.