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Multifunctional Circuitry for Off-Line Converter

IP.com Disclosure Number: IPCOM000038221D
Original Publication Date: 2005-Feb-25
Included in the Prior Art Database: 2005-Feb-25
Document File: 4 page(s) / 83K

Publishing Venue

Siemens

Related People

Juergen Carstens: CONTACT

Abstract

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.

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Multifunctional Circuitry for Off-Line Converter

Idea: Dr. Jing Hu, SG-Singapore; Dr. Leo Lorenz, SG-Singapore

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.

The above-mentioned problems can be avoided by a new solution, which makes full use of the depletion MOSFET, thereby saving one IC pin for voltage sensing. The power supply problem of the IC during the standby mode operation is also solved. Compared to the conventional circuit (Fig. 1), some sub-circuits are added. These are shown in detail in Figure 3. The states of the switches in different operating modes are summarized in Figure 4. During the start-up phase all switches are off. The output of the hysteretic controller is an open circuit. The depletion MOSFET conducts since its gate-source voltage is zero, due to the resistor R0, and the current through the start-up cell charges the VCC capacitor C1 through the internal diode D1. After the voltage VCC reaches the IC turn-on threshold, the IC changes...