1. Field of the Invention
The present invention relates to a control circuit, particularly to a control circuit for current detection.
2. Description of the Related Art
Power over Ethernet (PoE) is a system, which obtains power through cables. In other words, when users' internet equipments are provided with PoE, the internet equipments are connected to the cables to work normally without coupling to a plug. The above-mentioned function of PoE is very convenient.
As shown in
Furthermore, when the transistor 18 is turned on, all power noise is transmitted to PD 14 through the cables 22. At the same time, the signals transmitted on the cables 22 will be distorted by interference from the power noise.
In view of the problems and shortcomings of the prior art, the present invention provides a method and a control circuit for current detection, so as to solve the afore-mentioned problems of the prior art.
An objective of the present invention is to provide a control circuit for current detection. The control circuit comprises an operational amplifier disposed between two transistors whereby the transistor copies the output current output from the control circuit. The output current is detected without consuming additional power and the operational amplifier is turned off in order to save power when detection of the output current isn't required.
Another objective of the present invention is to provide a control circuit for current detection, which comprises a capacitor and an operational amplifier disposed between a positive voltage terminal thereof and a negative voltage terminal thereof. When power noise is generated at the positive voltage terminal, the capacitor transmits the power noise from the positive voltage terminal to the negative voltage terminal by utilizing the virtual short characteristic for a position between a positive input terminal and a negative input terminal of the operational amplifier whereby a voltage drop of the power noise between the positive voltage terminal and the negative voltage terminal is eliminated. Besides, the capacitor can avoid generation of incorrect signals.
To achieve the abovementioned objectives, the present invention provides a control circuit for current detection, which is installed inside Power Sourcing Equipment (PSE) of Power over Ethernet (PoE), and which is coupled to Power Device (PD) to detect an output current output from PSE to PD. The control circuit comprises at least one first Field Effect Transistor (FET) coupled to the positive input terminal of a first operational amplifier and the output terminal of a second operational amplifier, wherein the first FET is coupled to PD through a positive voltage terminal and a negative voltage terminal. The negative voltage terminal is coupled to the negative input terminal of the second operational amplifier. The positive voltage terminal is coupled to the positive input terminal of the second operational amplifier through a capacitor. The output current is passed through the first FET. Also, the positive voltage terminal is coupled to a voltage source. When an input voltage provided by the voltage source generates a power noise at the positive voltage terminal, the capacitor transmits the power noise to the negative voltage terminal by utilizing a virtual short characteristic for a position between the positive input terminal and the negative input terminal of the second operational amplifier whereby a voltage drop of the power noise between the positive voltage terminal and the negative voltage terminal is eliminated. Furthermore, the first FET and the negative input terminal of the first operational amplifier are coupled to at least one second FET, wherein the second FET forms a current mirror with the first FET by utilizing the virtual short characteristic for a position between the positive input terminal and the negative input terminal of the first operational amplifier. The second FET copies output current to generate a copy current by a scale whereby the output current is detected by the copy current.
Below, the embodiments are described in detail in cooperation with the drawings to make easily understood the characteristics, technical contents and accomplishments of the present invention.
The present invention is applied to Power over Ethernet (PoE). Below is a description of a first embodiment. Refer to
The control circuit for current detection comprises a positive voltage terminal 24 and a negative voltage terminal 26. The positive voltage terminal 24 and the negative voltage terminal 26 are respectively coupled to PD. The positive voltage terminal 24 coupled to a voltage source 28 providing a direct-current (DC) input voltage of 48 V is used to output the output current. The voltage source 28 is coupled to a current mode digital-to-analog converter (DAC) 30. The current mode digital-to-analog converter (DAC) 30 is coupled to a side of a resistor 32. The resistor 32 with another side thereof is separately coupled to the source of at least one first N-channel Metal Oxide Semiconductor Field Effect Transistor (NMOSFET) 36 and a ground terminal. The drain of the first NMOSFET 36 is coupled to negative voltage terminal 26. The current mode DAC 30 receives an input voltage provided by the voltage source 28 to generate a constant voltage of 0.4 V at a node between the resistor and the current mode DAC. The embodiment is exemplified by the one first NMOSFET 36.
The control circuit of the present invention further comprises a first operational amplifier 34 with a negative input terminal thereof coupled to the drain of the first NMOSFET 36, output terminal thereof coupled to the gate of the first NMOSFET 36 and a positive input terminal thereof coupled to the resistor 32 and the current mode DAC 30. The first operational amplifier 34 receives the constant voltage to turn on the first NMOSFET 36. Thus, after the control circuit outputs the output current from the positive voltage terminal 24 to PD, the output current will flow back to the negative voltage terminal 26 and pass through the first NMOSFET 36.
The gate of the first NMOSFET 36 is coupled to the gate of at least one second NMOSFET 40. The embodiment is exemplified by the one second NMOSFET 40. The drain and the source of the second NMOSFET 40 are separately coupled to a negative input terminal of a second operational amplifier 38 and the ground terminal. A positive input terminal of the second operational amplifier 38 is coupled to the drain of the first NMOSFET 36. From
To save current consumption the device specification for the first NMOSFET 36 and the second NMOSFET 40 is adjusted. For example, adjusting the ratio of the length and width of the channel whereby the scale is greater than 0 and less than 1, or the scale is greater than 0 and equal to 1. Also, when the first NMOSFET 36 is a plurality of first NMOSFETs and the second NMOSFET 40 is a plurality of second NMOSFETs, the device specifications for the first NMOSFETs 36 and the second NMOSFETs 40 are equal and the number of the second NMOSFETs 40 is less than that of the first NMOSFETs 36. Thus, the scale is greater than 0 and less than 1, or the scale is greater than 0 and equal to 1.
The drain of the second NMOSFET 40 is coupled to the source of a third NMOSFET 42. The gate and the drain of the third NMOSFET 42 is separately coupled to the output terminal of the second operational amplifier 38 and a current mode analog-to-digital converter (ADC) 44. The second operational amplifier 38 receives the constant voltage to turn on the third NMOSFET 42 by utilizing the virtual-short of the first operational amplifier 34. After turning on the third NMOSFET 42, the third NMOSFET 42 transmits the copy current generated by the second NMOSFET 40 to the current mode ADC 44, and then the current mode ADC 44 detects the output current through the copy current.
The second operational amplifier 38 is further coupled to an electric switch 46 controlling the switching status of the second operational amplifier 38. When the control circuit doesn't detect the output current, the electric switch 46 turns the second operational amplifier 38 off to save power.
When the voltage source 28 starts to provide DC voltage, the current mode DAC 30 receives an input voltage provided by the voltage source 28 to generate a constant voltage at a node between the resistor and the current mode DAC 30. The positive voltage terminal 24 outputs an output current to PD since the voltage source 28 has begun operating. Next, the first operational amplifier 34 receives the constant voltage to turn on the first NMOSFET 36 whereby the output current transmitted from PD to PSE is passed through the first NMOSFET 36. Since the first NMOSFET 36 is turned on, the second NMOSFET 40 is also turned on. The second NMOSFET 40 can form a current mirror by utilizing the virtual-short theorem of the first NMOSFET 36 and the second NMOSFET 40. Therefore, the second NMOSFET 40 copies the output current to generate a copy current. Because the second operational amplifier 38 has already received the constant voltage by utilizing the virtual-short theorem of the first NMOSFET 36 to turn on the third NMOSFET 42 at this time, the third NMOSFET 42 transmits the copy current to the current mode ADC 44. Then, the current mode ADC 44 can detect the output current without consuming additional power.
When the control circuit doesn't detect the output current, the electric switch 46 turns the second operational amplifier 38 off. Thus, the third NMOSFET 42 and the second NMOSFET 40 are turned off in turn. In other words, the third NMOSFET 42, the second NMOSFET 40, and the current mode ADC 44 all suspend operation, so as to save power.
Refer to
In conclusion, the present invention comprises an operational amplifier disposed between two transistors whereby the transistor copies the output current output from the control circuit, and then the output current is detected without consuming additional power. In addition, the present invention can turn off the operational amplifier to save power when detection of the output current is not required.
The embodiments described above are only to exemplify the present invention but not to limit the scope of the present invention. Therefore, any equivalent modification or variation according to the shape, structures, characteristics and spirit disclosed in the present invention is to be also included within the scope of the present invention.
Number | Date | Country | Kind |
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98138934 | Nov 2009 | TW | national |