Patent Application
20110292556
Embodiments of this invention relate to electronic device having input terminals and/or output terminals, and, more particularly, to a circuit and method for protecting the electronic device from being damaged by excessive voltages applied to the input terminals and/or output terminals.
Electronic devices often have input terminals that receive signals of various types or output terminals from which output signals of various types are provided. In fact, many electronic devices have both such input terminals and output terminals. Such electronic device include internal circuitry or components that may be damaged if a voltage having a large magnitude is applied to an input terminal or an output terminal. For example, electronic test equipment may have output terminals from which precision output voltages or currents are provided. The magnitude of these output voltages or currents may be relatively low, and the circuits or components that provide these output voltages or currents may be damaged if a relatively large voltage or current, such as an AC supply voltage, it applied to the output terminals.
It may seem to be a relatively simple matter to protect these circuits or components using, for example, a low current fuse. However, the circuits or components coupled to the terminals may be damaged before a fuse could reach melting temperature. Also, the impedance between the terminals may be too high to allow enough current to flow through the fuse responsive to a high voltage, so that the fuse would not open to protect the internal circuit or component. Fast acting current sensing components might also be placed in parallel with the terminals. However, it may be important for all of the current supplied by an internal circuit or component flow from the output terminal, thus precluding the use of a current sensing component in parallel with the output terminals which might draw current from the internal circuit or component that would otherwise flow from the output terminals. For example, the resistance of a circuit component connected between the terminals may be measured by coupling a specific current between the terminals and then measuring the voltage between the terminals. If current from a circuit supplying current to the terminals is diverted to a current sensing component coupled between the terminals, the resistance measurement may be in error.
It may therefore be important to be able to quickly decouple the external input or output terminals of an electrical device in a manner that does not draw current from or change the voltage between the input or output terminals.
A protection system for an electronic device includes a current limiting device and a solid state relay coupled in series between an input or output terminal and the electronic device. The solid state relay may include an opto-transistor coupled between the terminal and the electronic device in series with the current limiting device, and a light emitting diode optically coupled to the opto-transistor. The current limiting device may be implemented with one or more enhancement mode field effect transistors. A voltage detection circuit detects a voltage applied to the terminal having a magnitude that is greater than a specific value. The detection circuit then causes the solid state relay to close. For example, the voltage detection circuit may apply a voltage to a light emitting diode used in a solid state relay thereby causing current to flow through the first light emitting diode.
An embodiment of a prior art protection circuit 10 for an electronic device is shown in
In operation, the FET's 18, 24, being depletion mode FET's, are conductive until the source-to-gate voltage reaches a threshold voltage for the FET's. However, since the respective sources and gates of the FET's 18, 24 are coupled to each other, the FET's 18, 24 never become non-conductive. Instead, the FET's 18, 24 initially act as resistors when the applied voltage is increased so that the current through the FET's 18, 24 increases accordingly. However, when the FET reaches saturation, the current through the FET's 18, 24 remains constant. The FET's 18, 24 thus act as current limiters until the voltage detection circuit 30 resets the relay 20 to open the relay contact 20a. In theory, the current limiting effect of the FET's 18, 24 protects an internal circuit or component connected to the terminal 28 until the relay contact 20a is opened. However, in practice, since the power dissipated by the FET's 18, 24 continues to increase with the applied voltage, the FET's 18, 24 may become damaged or destroyed before the relay contact 20a can be opened. For example, although the voltage detector 30 may be able to very quickly detect a significant voltage applied to the terminals 14, 16, one relay 20 used in the prior art protection circuit 10 may require about 7 ms for the contact 20a to be opened. Yet the FET's 18, 24 may be damaged or destroyed if the applied voltage increases sufficiently during these 7 ms. Thus, the prior art protection circuit 10 may be inadequate in many instances.
A protection circuit 100 according to one example of the invention is shown in
In operation, if a relatively high voltage is applied between the terminals 14, 16, the current may initially flow through the FET's 18, 24 and the solid state relay 120. However, the FET's 18, 24 will then limit the current flow to a level that prevents the voltage drop across the solid state relay 120 from exceeding its maximum operating voltage. The voltage detector 30 will quickly sense the relatively high voltage, and will then apply a signal to the LED 124. The LED 124 in the solid state relay 120 will then couple light to the opto-FET 130, which causes it to turn OFF. Significantly, the solid state relay 120 may terminate the flow of current through the FET's 18, 24 before the FET's can be damaged because it may respond much more quickly than the relay 20. Additionally, the relay 20 will respond before an excessive voltage is placed across the solid state relay 120.
Although the detection voltage at which the voltage detector 30 applies a signal to the relay coil 20b may be fixed in some embodiments, in other embodiments the detection voltage may be dynamic. More specifically, the voltage detector may be programmed with a detection voltage that varies as a function of variations in the voltage that is applied between the terminals 14, 16 during normal operation so that the detection voltage is always greater than the normal operating voltage.
Another embodiment of a protection circuit 150 according is shown in
In operation, if a relatively high voltage is applied between the terminals 14, 16, the current may initially flow through the FET's 18, 24 and the solid state relay 120. However, the FET's 18, 24 will then limit the current flow to a level that prevents the voltage drop across the solid state relay 120 from exceeding its maximum operating voltage. Depending on the polarity of the applied voltage, one of the LED's 174, 178 in the solid state relays 176, 180, respectively, will be conductive to turn ON its respective transistor 184, 186 when the current through the LED reaches a specific level. The magnitude of the voltage applied to the terminals 14, 16 at which the transistors 184, 186 turn ON may be set by the selection of the value of the resistor 172. Thus, when the applied voltage reaches a specific level, the latch 194 will be set by the comparator 190 to drive the output of the inverter 196 low and cause current to flow through the LED 124 in the solid state relay 120. At the same time, a low at the output of the inverter 198 may reset the relay 20 to open the relay contact 20a after the delay of the relay 20. The current flowing through the LED 124 will then illuminate the LED, thereby turning OFF the LED 124 in the solid state relay 120. Significantly, the solid state relay 120 may terminate the flow of current through the FET's 18, 24 before the FET's can be damaged because it can respond much more quickly than the relay 20. Additionally, the relay 20 will respond before an excessive voltage is placed across the solid state relay 120. It should be noted that any current flowing through the resistor 172 and one of the LED's 174, 176 is returned to the current path between the terminal 14 and the electronic device 160 so that the detection circuit 170 does not divert current flowing from the electronic device 160 to the terminal 14 or vice versa.
Although the present invention has been described with reference to the disclosed embodiments, persons skilled in the art will recognize that changes may be made in form and detail without departing from the invention. For example, although the terminal 14 has been describe in various places as an output terminal, it may alternatively be in other embodiments an input terminal or an input/output terminal. Such modifications are well within the skill of those ordinarily skilled in the art. Accordingly, the invention is not limited except as by the appended claims.
