1. Field of the Invention
The present invention relates to a surge protection circuit, and more particularly, to a surge protection circuit which can perform surge protection effectively.
2. Description of the Prior Art
In general, a communication system performing signal transmission via a transmission line, such as an asymmetric digital subscriber line (ADSL) system, a very high bitrate digital subscriber line (VDSL) system or a power line system (PLC) etc., may be affected by a surge, e.g. the high power caused by a wavelet of an adjacent lightning, leading to damages. Because an output terminal of a line driver of the communication system is directly connected to the transmission line and affected by the surge first, the transmission line usually comprises a protection circuit to prevent damages from surge.
However, the surge frequency is lower than the frequency of a conventional electrostatic discharge (ESD), i.e. different order. Therefore, the conventional ESD protection circuit is not enough for surge protection. It is necessary to have a surge protection circuit for performing surge protection. Furthermore, the surge test inputs a differential mode surge signal on a positive output terminal and a negative output terminal of the surge protection circuit, whereas the ESD signal of the conventional ESD test is input between a power supply and the output terminal or between a ground terminal and output terminal.
It is therefore an objective of the present invention to provide a surge protection circuit to perform surge protection effectively and reduce cost.
The present invention discloses a surge protection circuit for a line driver of a communication system. The surge protection circuit comprises a transistor, an inverter and a first RC circuit. The transistor comprises a first terminal, a second terminal and a third terminal, the first terminal is coupled to a positive output pad of the line driver, and the second terminal is coupled to a negative output pad of the line driver. The inverter comprises an input terminal and an output terminal coupled to the third terminal of the transistor. The first RC circuit comprises a resistor having a terminal coupled to the input terminal of the inverter and another terminal coupled to a power supply, and a capacitor having a terminal coupled to the input terminal of the inverter and another terminal coupled to a ground. Wherein, a first RC time constant of the first RC circuit is substantially equal to or greater than a period of a differential mode surge signal.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.
For example, please refer to
However, for a differential mode surge signal, a frequency of the differential mode surge signal is lower than that of the ESD signal, and therefore the ESD protection circuit 10 designed for discharging the ESD signal can not effectively discharge the differential mode surge signal for surge protection, i.e. because the period of the differential mode surge signal is greater than the RC time constant T1, the transistor MN1 may have turned off the discharge path P1 before the discharge path P1 completely discharges the differential mode surge signal. Besides, the lengthy discharge path P1 may also cause damages to other components. As a result, besides the ESD protection circuit, a surge protection circuit is also needed to perform surge protection.
For example, please refer to
As can be seen from the above, the ESD protection circuit 10 can not effectively perform surge protection. Also, since the added surge protection circuit is realized by using additional Zener diodes set on the circuit board, it will increase the cost. Thus, there is a need for improvement.
Please refer to
In detail, before applying the differential mode surge signal on the positive output pad LD_OUTP′ and the negative output pad LD_OUTN′, the capacitor voltage Vc′ of the capacitor 208 is at a high voltage level and the inverter 202 can generate a voltage Vg′ of a low voltage level to the gate of the transistor MN2, such that the transistor MN2 is turned off. After the differential mode surge signal is applied on the positive output pad LD_OUTP′ and the negative output pad LD_OUTN′, the capacitor voltage Vc′ is switched to be at a low voltage level and the inverter 202 can generate the voltage Vg′ of a high voltage level to the gate of the transistor MN2, such that the transistor MN2 is turned on to form the discharge path P2 between the positive output pad LD_OUTP′ and the negative output pad LD_OUTN of the line driver as shown in
Noticeably, the main spirit of the present invention is to add the surge protection circuit 20, whose structure is similar to that of the ESD protection circuit 10, between the positive output pad LD_OUTP′ and the negative output pad LD_OUTN′, and the RC time constant T2 of the RC circuit 204 is designed to be substantially equal to or greater than the period of the differential mode surge signal, so as to effectively form the direct discharge path P2 without causing damages to other components to perform surge protection. Therefore, there is no need to use additional diodes on the circuit board to save cost. Those skilled in the art should make modifications or alterations accordingly. For example, applications of the present invention are not limited to the line driver of the communication system, such as an asymmetric digital subscriber line (ADSL) system, a very high bitrate digital subscriber line (VDSL) system or a power line system, as long as the line driver of the communication system performs signal transmission via a transmission line.
Besides, the transistor MN2 of the surge protection circuit 20 is realized by an N-type MOS transistor, and may also be realized by a P-type MOS transistor in practice, as long as the structures of the inverter 202 and the RC circuit 204 are modified correspondingly. The transistor MN2 can be any type of transistors and not limit to MOS transistors. Moreover, the present invention not only uses the surge protection circuit 20 to perform surge protection, but also sets up the ESD protection circuit 10 shown in
As above, the conventional ESD protection circuit can not perform surge protection effectively, and setting up additional diodes on the circuit board as one kind of surge protection circuits will increase cost. To solve these problems, the present invention provides the surge protection circuit 20, whose structure is similar to that of the conventional ESD protection circuit 10, between the positive output pad LD_OUTP′ and the negative output pad LD_OUTN′, and the RC time constant T2 of the RC circuit 204 is designed to be substantially equal to or greater than the period of the differential mode surge signal. Therefore, the direct discharge path P2 can be effectively formed without causing damages to other components for surge protection and there is no need to use additional diodes on the circuit board so as to save cost.
To sum up, the present invention can form the direct discharge path without causing damages to other components for surge protection. In addition, the cost can be reduced because there is no need to add diodes on the circuit board.
Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention.
Number | Date | Country | Kind |
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100112085 | Apr 2011 | TW | national |