Multifunctional Circuitry for Off-Line Converter
Original Publication Date: 2005-Feb-25
Included in the Prior Art Database: 2005-Feb-25
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In an off-line converter, the bus voltage is normally sensed by a voltage divider consisting of resistors. For this kind of voltage sensing, one pin is needed to conduct the sensed voltage into the Integrated Circuit (IC). For starting-up a controller IC in an off-line converter, a start-up cell is used. Figure 1 shows a simplified example using chip-by-chip technology. The start-up cell is a depletion n-channel MOSFET (Metal-Oxide Semiconductor Field-Effect Transistor; T in Figure 1), which, for example, is controlled by the power management circuit that turns the gate control switch (S) on or off. During the start-up phase, the switch S is off and the gate-source voltage is zero, which is higher than its on-threshold voltage VT. Therefore, the MOSFET T is conducting (cf. Figure. 2) and the VCC capacitor C1 is charged by the current through the start-up cell. After the voltage VCC reaches a certain level, the so-called IC turn-on threshold, S is switched on. The gate-source voltage then becomes the voltage VCC, which is lower than the MOSFET threshold voltage VT so that the MOSFET blocks. In this way the MOSFET T is used only during the start-up phase and not during normal operation. After the start-up phase the IC is powered by the auxiliary supply, which is usually coupled to the main transformer via the fly-back method. For CRT TV applications, the output voltage of the converter is lowered to minimize the power consumption during standby mode operation. Hence, the voltage from this auxiliary winding during standby mode operation is much lower than in normal operation, which means that there is no power from the auxiliary winding to supply the IC. Therefore, additional measures are necessary or the IC has to go into restart mode, which in praxis is not desirable.