AC VOLTAGE CLIPPER IN MOS-TECHNOLOGY

Abstract: AC voltage clipper for an MOS-circuit having two input terminals (In1, In2) receiving an ac supply voltage, wherein one input terminal (In1) is connected to a point of common voltage through a first MOS-transistor (MCL41) and the other input terminal (In2) is connected to said point of common voltage through a second MOS-transistor (MCL42). The gates of MOS-transistors (MCL41, MCL42) are connected to each other and receive a gate voltage (Vg4) of a control circuit (MCL2, MCL3, DCL11, DCL12), in such a way that both transistors (MCL41, MCL42) will conduct when the absolute value of the ac supply voltage, being applied as an input signal to the control circuit (MCL2, MCL3, DCL11, DCL12) exceeds a predetermined threshold value.

Description: AC voltage clipper in MOS-technology. The invention applies to an ac voltage clipper to be used in MOS-circuits with an ac power supply. In that case the ac voltage has to be rectified by a rectifier. To protect the rectifier against damage caused by ac over-voltages at its input, it is necessary to limit the ac voltage by a clipper, if said voltage exceeds a predetermined threshold. The clipper should especially be applied in circuits where an ac voltage of highly varying amplitude is provided e.g. in the case of a LC-resonator influenced by an ac magnetic field. The object of the invention is to provide an ac voltage clip¬ per that has a short response time and a low current consumption when the clipper is not activated. The ac voltage clipper according to the invention therefore is characterized in that the MOS-circuit has two input terminals which receive an ac supply voltage, which input terminals are each connected to one of two input terminals of the ac voltage clipper, the first input terminal of which is connected to a point of common voltage through a first MOS-transistor and the other input terminal is connected to said point of common voltage through a second MOS- transistor, the gates of the first and second MOS-transistors being connected to each other and receiving a gate voltage of a control circuit which gate voltage depends on the absolute value of the ac supply voltage, in such a way that when this absolute value exceeds a predetermined threshold value the first and second MOS-transistor are forced to be conductive. In addition to a very short response time (nε range) and an extremely low current consumption (nA range) when the clipper is not activated, the ac voltage clipper according to the invention offers the advantage of a first order behaviour, a high current sinking capability (above 100 mA) and a small chip area. This is an unique combination of features for an ac voltage clipper in CMOS- technology. The invention will be further explained with reference to the drawings, in which Figure 1 shows an ac voltage clipper according to the inven¬ tion. Figure 2 shows the ac voltage clipper of figure 1 in CMOS- technology. Figure 3 shows the connection of a rectifier to a clipper according to the invention. Figure 4 shows a clipper to be connected to the output of the rectifier according to figure 3. Figure 3 shows a rectifier converting an ac voltage, provided between the input terminals In1 and In2, into a dc voltage VDDUNS. Since the rectifier itself is no part of the present inven- tion, a detailed description thereof will be omitted. To protect the rectifier against too high ac voltages at its input terminals, basically the output voltage of the rectifier can be simply limited with a single MOS-transistor, as is demonstrated in figure 4. However, MOS-bridge-rectifiers, as the one shown in figure 3, often have a rather large internal resistance, so that this kind of clipper still may result in high voltage differences and is therefore not efficient. The best way of voltage clipping is to limit the ac voltage directly at the input of the rectifier. For this reason the circuit of figure 1 has been designed. In the clipper circuit according to the invention input In1 ' (which is connected to In1 ) is connected to the point of common voltage VSS through a NMOS-transistor MC 41, while input In2' (which is connected to In2) is connected to the point of common voltage VSS through a NMOS-transistor MCL42 (see figure 3) . The gates of transistors MCL 1 and MCL42 are connected to each other and through a depletion MOS-transistor MCL3, which is in "diode" configuration, they are connected to the voltage VSS. MOS-transistor MCL3 has a lower threshold voltage than the MOS- transistors MC 41 and MCL42. Input Inl' is connected to the gate of transistors MCL41 and MCL42 through a diode DC 11 and a PMOS- transistor MCL2, while input In2' is connected to the point of connection of the diode DCL11 and PMOS-transistor MCL2 through a diode DCL12. The gate of PMOS-transistor MCL2 receives a reference voltage Vref. If no depletion MOS transistor is available MOS- transistor MCL3 can be replaced by a current source or a resistance. In fact, diodes DCL11 and DCL12, MOS-transistor MCL2 and MOS-transistor MCL3 (or a resistor or current source instead) constitute a control circuit for the gate voltages of MOS- transistors MC 41 and MCL42. The circuit of figure 1 functions as follows. If: |v(lnl) - V(In2)| < Vref + |vth MCL2| + Vdf where: |vth MCL2| = absolute value of the threshold voltage of MC 2 Vdf = forward biased voltage drop of DCL11 or DCL12 then the current through DCL11/DCL12, MCL2 and MC 3 approximately equals zero. Since MCL3 is a depletion MOS-transistor (and therefore may conduct current without a biasing gate voltage) and the current through MCL3 is very small, the voltage drop of MCL3 will be very small. Therefore, the gate voltages of MCL41 and MCL42 is too low for MCL41 and MCL42 to conduct. However, as soon as |v(In1) - V(In2)| > Vref + |vth MCL2| + Vdf DC 11/DCL12, MCL2 and MCL3 will conduct current. MCL3 is designed in such a way - very low ratio of channel width to channel length (W/ <<1) - that already a small current increase through MCL3 will be enough to generate a strong voltage increase across -MCL3. There¬ fore, the gate voltages of MCL41 and MCL42 will increase very fast, forcing them into conduction and limiting voltage |V(In1 )-V(In2) | . If MCL41 and MC 42 have a very high ratio of channel width to channel length (W/L>>1), then |v(In1) - V(In2)| is limited at the value of Vref + |vthMCL2| + Vdf plus a small voltage (of about 100 mV), which is required to drive a small current through DCL11/DCL12 and MCL2. The clipper has a high current sinking capacity: above 100 mA is easily obtained. The response time of the clipper is short, varying between some ns to some tens of ns, depending on the circuit dimensions. The clipper reaches its maximum current sinking capacity when the gate voltages of MCL41 and MCL42 reach the value of Vref + |vthMCL2|. By varying the value of Vref the maximum current sinking capacity of the clipper may be varied. An interesting application of the clipper may be obtained by using the gate voltage Vg4 of MOS-transistors MCL41 and MCL42 as a modulation signal. This is possible because the gate voltage Vg4 depends on the amplitude of the input signal |v(In1) - V(In2)|. Figure 2 shows an embodiment of the circuit of figure 1 in CMOS-technology, in which the diodes DCL11 and DCL12 are realized as diode connected MOS-transistors MC 11 and MC 12. The reference voltage Vref may for instance equal dc voltage VDD, VDD being a stabilized power supply voltage derived from VDDUNS through a voltage regulator. It is to be noted that the proposed circuit has very well defined starting characteristics. As long as Vref = VDD = 0 at least a double threshold voltage is necessary to activate the clipper. In case of a supply voltage rising too fast the clipper will therefore be directly activated, even if the power supply voltage is not yet present (VDD ~ 0). If the clipper is activated there always remains a well defined "overtension" , i.e. the thres¬ hold voltage of MCL2 to pass enough energy to the rectifier. This guarantees that the clipper will never sink all the energy from the rectifier, which would prevent the other circuits on the semicon- ductor chip from receiving supply voltage. If a protection of the transistors MCL41 and MCL42 is neces¬ sary, an additional MOS-transistor (M12, M34) may be arranged between points 1 and 2, and 3 and 4 respectively (compare figure 2), the gates of which are connected to a predetermined voltage, for instance, that of the point of connection between MCL11/MC 12 and MCL2. Application of the clipper is not limited to the protection of rectifiers. It may be applied everywhere where the level of an ac voltage can be limited by current sinking. For instance, this may be the case with an ac voltage source from a C-resonator, which is being influenced by an ac magnetic field. Also, the device according to the invention may be applied to a usual ac power supply, such as the mains. However, in that case in at least one of the cables between the mains and the input terminals In1 and In2 a current limiting resistance is to be installed. It will be evident that, where in the figures and the des¬ cription PMOS and NMOS-transistors are mentioned, also a polarity- reversed embodiment may be applied by commonly known modifications of the dc voltages, the current direction of the diodes, etc.

Claims: Claims 1. An ac voltage clipper for a MOS circuit characterized in that the MOS circuit comprises two input terminals (In1, In2) receiving an ac supply voltage (V(In1) - V(In2)), which input ter¬ minals are respectively connected to one of two input terminals (In 1 , In2') of the ac voltage clipper, the first input terminal (In1 ' ) is connected to a point of common voltage through a first MOS-transistor (MC 41 ) and the other input terminal (In2') is con- nected to said point of common voltage (VSS) through a second MOS- transistor (MC 42), the gates of the first and second MOS-transis- tors (MCL41, MCL42) being connected to each other and receiving a gate voltage (Vg4) of a control circuit (MCL2, MCL3, DCL11/MCL11 and DCL12/MCL12), which gate voltage (Vg4) depends on the absolute value of the ac supply voltage |v(ln1) - V(In2)|, in such a way that when this absolute value |v(ln1) - V(ln2)| exceeds a pre¬ determined threshold voltage the first and second MOS-transistors (MCL41, MCL42) are forced to be conductive. 2. An ac voltage clipper according to claim 1, characterized in that the control circuit comprises at least a third diode con¬ nected MOS-transistor (MC 3) connecting the gates of the first (MCL41) and second MOS-transistor (MCL42) to the point of common voltage (VSS). 3. An ac voltage clipper according to claim 2, characterized in that the third MOS-transistor (MCL3) is of a depletion MOS tran¬ sistor having a lower threshold voltage than MOS-transistors MCL41 and MCL42. 4. An ac voltage clipper according to claim 1, characterized in that the control circuit comprises at least a resistance or a current source connecting the gats-s of the first (MCL41 ) and second MOS-transistor (MCL42) to the poir,. of common voltage (VSS). 5. An ac voltage clipper according to one of the claims 2 to 4, characterized in that the control circuit also comprises a first diode (DCL11), the anode of which is connected to one input termi- nai (In1'), a second diode (DC 12) the anode of which is connected to the other input terminal (In2'), the cathodes of both diodes (DC 11, DCL12) being connected to each other and to the gates of the first (MC 41 ) and second MOS-transistor (MC 42) through a fourth MOS-transistor (MCL2), said fourth MOS-transistor (MCL2) receiving a predetermined voltage (Vref) at its gate. 6. An ac voltage clipper according to one of the preceding claims characterized in that a fifth MOS-transistor (M12) is ar- ranged between the first MOS-transistor (MCL41) and the one input terminal (Inl 1 ) and a sixth MOS-transistor (M34) is arranged between the second MOS-transistor (MC 42) and the other input ter¬ minal (In2'). 7. An ac voltage clipper according to one of the claims 2, 3, 5 or 6, characterized in that W/L << 1 for the third MOS-transistor (MC 3) and W/L >> 1 for the first (MCL41), the second (MC 42) and fourth MOS-transistor (MCL2), where is the channel width and L the channel length of the MOS-transistor concerned. 8. An ac voltage clipper according to any of the preceding claims characterized in that the gate voltages (Vg4) of the first (MCL41 ) and second MOS-transistor (MC 42) are modulation outputs. 9. An ac voltage clipper according to any of the preceding claims characterized in that the ac voltage is derived from the main supply voltage and a current limiting resistance is arranged between at least one of the two input terminals (In1, In2) and the main supply voltage. *****

Inventors: SEESINK PETRUS HENDRIKUS, SEESINK, PETRUS, HENDRIKUS
Applicants: SIERRA SEMICONDUCTOR BV, SIERRA SEMICONDUCTOR B.V.
Application year: 1992
Main classification: H02H9
IPC codes: H02H7, H02H9