Using one pin for soft-start function and blanking_time_window function
Original Publication Date: 2003-Feb-25
Included in the Prior Art Database: 2003-Feb-25
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In SMPS (Switching Mode Power Supply) the transformer peak current is slowly increased by the soft start function during start up in order to prevent transformer rattle during start up. This function can be achieved by charging one external capacitor through one internal resistor (figure. 1). So, the start level and the time constant of the gradually increasing primary current level can be adjusted by external capacitor. Another problem in SMPS is the overload protection or light load condition. The overload protection circuit forces the power switch to stop its operation if the load draws a higher current than the predetermined maximum current value. The burst mode circuit will force the power switch to stop its operation for a while and lets the peak current be lower value during non-skipping operation if the load draws a less current than the predetermined minimum current value (light load condition). A problem associated with the overload protection and burst mode operation is that it can trigger erroneously on load transients. A blanking time window is used for the overload protection and burst mode operation detection. In this invention only one pin is used for multipurpose. This one pin is connected to an external capacitor and used for soft start, which can stabilize the whole system during startup period, and also for blanking window, which can help the system safely detect over voltage and burst mode. This method is very flexible, powerful and adjustable, which will not affect the whole system regulation. Only one capacitor is needed for the soft start and blanking window. Figure 2 shows a block diagram of a SMPS circuit where one fb pin is connected to the optocoupler and internally it is pulled up by one resistor to the internal power supply. There is also one softs pin which is connected to one capacitor and internally connected to power supply through one ressistor or one internal current source.