JPH0415516B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an isolated process input device used as an input means of a process control device, and in particular to a DC-AC converter used for isolated transmission and a method for controlling drive signals in switch elements of synchronous rectifier circuits. The present invention aims to provide an insulated process input device that can significantly improve the effects of leakage.

<Background of the Invention> For example, when a weak analog signal from a detection end such as a thermocouple is input into a process control device, an insulated process input device is used for the purpose of removing common mode voltage.

The basic configuration of an isolated process input device is that a DC-AC converter is placed on the primary winding side of an isolation transformer, and this DC-AC converter interrupts the analog input signal from the detection end to convert it into an AC signal. This AC signal is then applied to the primary winding of the isolation transformer. The isolation transformer has a structure in which an alternating current signal is generated by magnetic coupling in the secondary winding, and this alternating current signal is synchronously rectified to reproduce an analog input signal.

FIG. 1 shows a conventional isolated analog input device with the simplest structure. In the figure, 101 is an isolation transformer, 102 is a DC-AC converter connected to the primary winding side of the isolation transformer 101, and 103 is a filter. That is, an analog input signal 106 from a detection end (not particularly shown here) is applied to input terminals 104 and 105. Filter 103
An ordinary low-pass filter consisting of an impedance element such as a resistor and a capacitor is used. This is inserted for the purpose of removing AC induced noise. The analog input voltage stored in the capacitor of the filter 103 is transferred to the DC-AC converter 10.
2 intermittently applied to the primary winding of the isolation transformer 101, and a pulse signal with a period matching the intermittent period of the DC-AC converter 102 is induced in the secondary winding of the insulation transformer 101. The pulse signal is synchronously rectified by a synchronous rectifier circuit 107, a voltage having a value and polarity corresponding to the analog input signal 106 is held in a hold circuit 108, and the held value is outputted from output terminals 109 and 110 and sent to a process control device. be taken in.

111 indicates a drive signal source. This drive signal source causes the DC-AC converter 102 and the synchronous rectifier circuit 10 to
7 are driven in synchronization with each other to achieve isolated transmission.

The insulated process control device shown in FIG. 1 has a simple structure and can be manufactured at low cost. However, on the other hand, there are the following drawbacks.

That is, the circuit structure shown in FIG. 1 is particularly simple in that the DC-AC converter 102 and the synchronous rectifier circuit 107 are constructed by one switch element. This structure is hereinafter abbreviated as a one-phase drive system. Insulation transformer 10 when using one-phase drive system
The primary winding of 1 is always provided with only a single pulse of single polarity, ie having the polarity and magnitude of the analog input signal. Immediately after this single pulse is transmitted to the hold circuit 108 through the synchronous rectifier circuit 107, the switch elements constituting the DC-AC converter 102 and the synchronous rectifier circuit 107 are both turned off, so that the electromagnetic energy accumulated in the isolation transformer 101 is loses its outlet.

Therefore, the electromagnetic energy accumulated in the isolation transformer 101 is spontaneously released and gradually disappears. Therefore, it takes a long time for the energy stored in the isolation transformer 101 to be completely released, so a long downtime must be taken before the next isolated transmission is performed. This makes high-speed transmission impossible and has the drawback of not having the ability to transmit changes at the detection end to the process control device with high density.

In order to overcome this drawback, the present applicant has already proposed an insulated process input device as shown in FIG. The insulated process input device shown in FIG. 2 is known from Japanese Utility Model Publication No. 52-38897.

The circuit shown in FIG. 2 is characterized by the structure of the DC-AC converter 102 and the synchronous rectifier circuit 107. In other words, the DC-AC converter 102 and the synchronous rectifier circuit 107 both have two switch elements 201 and 2.
02 are connected in parallel, and one of the switch elements 20
1 and a capacitor 302 connected in series.

In this structure, each switch element 201 and 20 of the DC-AC converter 102 and the synchronous rectifier circuit 107
2 are alternately turned on and off by a drive signal given from a drive signal source 111. Filter 1
If the voltage of the analog input signal 106 stored in the capacitor 03 is e _i as shown in FIG. As shown in FIG. 3B, the primary winding of the transformer 101 is given a rectangular wave voltage 301 that swings positive and negative, e _i and -e _i , centered around the common voltage e _c .
That is, when the switch element 202 is on, the voltage e _i of the analog input signal 106 stored in the capacitor of the filter 103 is applied to the primary winding of the isolation transformer 101. When the switch element 201 is turned on, a back electromotive force -e _i is generated in the primary winding of the isolation transformer 101, and this back electromotive force -e _i is combined with the analog input voltage e _i stored in the capacitor of the filter 103. are added and provided to the capacitor 302. Therefore, 2e _i is stored in the capacitor 302 as shown in FIG. 3C. Filter 1 like this
The voltage stored in the capacitor 302 is always stabilized to be twice the voltage stored in the capacitor 03, and as a result, the primary winding of the isolation transformer 101 has the voltage shown in FIG. 3B. A rectangular liquid voltage 301 as shown is provided.

In this way, according to the circuit structure shown in FIG. 2, currents with equal values in the positive and negative directions flow through the primary winding of the isolation transformer 101, so it is necessary to wait for the residual energy in the isolation transformer 101 to disappear due to spontaneous release. There is no.

Therefore, the circuit shown in FIG. 2 enables high-speed transmission, and allows fast changes at the detection end to be transmitted to the process control device with high density.

In addition, in the synchronous rectifier circuit 107, the isolation transformer 101
The rectangular wave generated in the secondary winding is synchronously rectified, and a voltage equal to the voltage e _i of the analog input signal 106 is reproduced in the capacitor 204 forming the hold circuit 108 . Also, the capacitor 3 of the synchronous rectifier circuit 107
2e _i is stored in 02.

Here, ``Kokoku No. 52-38897'' does not mention any measures for suppressing the drive signals that are coupled inside the switch elements 201 and 202.

Furthermore, ``Kokoku No. 52-38897'' describes that it is possible to detect disconnection at the detection end with the circuit structure. However, no consideration is given to the fact that an error occurs due to the current superimposed on the circuit for disconnection detection, and how to minimize this error.

That is, in the circuit structure shown in FIG. 2, the switch elements 201 and 202 are MOS type FETs in order to minimize the coupling between the drive signal system and the analog signal system. Even if a Moss-type FT is used, there is a drawback that drive signals leak into the analog signal system due to capacitive coupling between the gate, source, and drain. In this regard, according to the two-phase excitation circuit shown in FIG. 2, the leakage components of the drive signal have an antiphase relationship with each other as shown in FIG. 3D and E, so the leakage component of the drive signal due to the electrostatic coupling is This cancellation is explained in column 5, lines 9 to 18 of ``Publication of Utility Model Publication No. 52-38897''.

By the way, it is common knowledge that a MOS type FET generally has a substrate, and the substrate is connected to the source or drain electrode.

This idea is correct when using a MOS type FET as a normal amplification active element. However, the DC-AC converter 102 and the synchronous rectifier circuit 1 as shown in FIG.
07, it has been found that if the substrate is connected to the source or drain, the drive signal component leaks into the analog signal system. This leakage occurs when a charging/discharging current flows through the gate leakage capacitance formed between the gate and the substrate when a drive signal whose polarity changes from one to the other is applied to the gate electrodes of the switch elements 201 and 202. This charging/discharging current flows only when the drive signal changes from one polarity to the other. Therefore, since the two switch elements 201 and 202 operate complementary to each other in response to drive signals that alternately change from one polarity to the other, the leakage component due to the charge/discharge current flowing through the gate leakage capacitance is There are no components that cancel each other out, so they cannot be removed.

<Object of the Invention> An object of the invention is to provide an insulated process input device that can eliminate the influence of drive signal leakage due to charge/discharge current flowing through the gate leakage capacitance of a MOS FET.

<Embodiment of the Invention> FIG. 4 shows an embodiment of the invention. In this invention, the DC-AC converter 102 and the synchronous rectifier circuit 107 are connected to first and second capacitors 402 and 403, each of which has one end connected to the common side end 401 of the primary winding and the secondary winding of the isolation transformer 101.
and first and second switch elements 405 and 4 connected between the other end 404 of the isolation transformer 101 and the other ends of the first and second capacitors 402 and 403, respectively.
06.

Here, the structure that is particularly characteristic of this invention is the first one,
The structure is such that the substrates of the second switch elements 405 and 406 are commonly connected and connected to the common side ends of the primary and secondary windings of the isolation transformer 101.

Further, in this invention, two isolation transformers 411 and 412 are provided in the drive signal source 111, and Zener diodes Z _1a and Z _1b and Z _2b and Z _2a are connected to both ends of each secondary winding side of the isolation transformers 411 and 412. Zener diodes Z _1a and Z _1b are connected in series in opposite directions to each other.
and the isolation transformer 101 at the connection midpoint of Z _2a and Z _2b .
is connected to the common side terminal 401 of each of the primary and secondary windings, and both ends of each secondary winding of the isolation transformers 411 and 412 are connected to the gate electrodes of the first and second switch elements.

<Operation of the invention> According to the circuit structure of the invention, the first capacitor (also serving as the capacitor of the filter 103) 40
The analog input signal stored in the transformer 2 is applied to both ends of the primary winding of the isolation transformer 101 when the first switch element 405 is turned on.

Next, the first switch element 405 is turned off and at the same time the second switch element 406 is turned on. When the switch element 406 is on, the isolation transformer 1
A back electromotive force is generated in the primary winding 01, and this back electromotive force is applied to the second capacitor 403. Therefore, the first capacitor 40 is connected to the second capacitor 403.
A voltage is stored that is equal to the voltage stored at 2, that is, equal to the analog input voltage and of opposite polarity.

In this way, the first and second switch elements 405
and 406 alternately turn on and off, a rectangular wave voltage as shown in FIG. 3B is applied to the primary winding of the isolation transformer 101, and a similar rectangular wave is transmitted to the secondary winding. Ru.

Similarly, in the synchronous rectifier circuit 107, the first and second switch elements 405 and 406 alternately operate on and off to synchronously rectify the rectangular wave voltage generated in the secondary winding of the isolation transformer 101. Therefore, synchronous rectifier circuit 1
In the first and second capacitors 402 and 403 of No. 07, a voltage approximately equal to the analog input voltage and having a different polarity is reproduced.

<Effects of the Invention> Therefore, in the case of the circuit structure of the present invention, as in the case of FIG. 2, currents in opposite directions alternately flow through the primary winding of the isolation transformer 101, and no electromagnetic energy is accumulated. Therefore, the inconvenience that makes high-speed transmission impossible as explained in FIG. 1 does not occur.

Furthermore, according to the circuit of the present invention, since the substrates of the first and second switch elements 405 and 406 are connected to the common side, the influence of drive signal leakage due to charge/discharge current flowing through the gate leakage capacitance can be ignored. It can be made smaller.

5 and 6, 501 and 601 are the first switch element 405 and the second switch element 4.
06 is a gate leakage capacitance, 502 and 602 are gate drive signal sources, and 503 and 603 are equivalent diodes formed by the channel terminal and substrate on the side to which the first and second capacitors 402 and 403 are connected.

This equivalent circuit represents the effect of the bipolar mode operating state instantaneously occurring in switch elements 405 and 406 when the drive signal is inverted. When the drive signal is inverted, that is, when the drive signal rises to positive polarity, the equivalent diodes 503 and 603 formed by the channel terminal and the substrate become conductive, and the gate leakage capacitances 501 and 6 are reduced.
A charging current flows through 01. When adopting the conventional substrate connection structure, that is, when connecting the substrate to the channel connected to the first and second capacitors, this gate leakage capacitance 501, 60
The charging current flowing through the capacitor 1 flows into the first capacitor 402 and the second capacitor 403. Due to this inflow, the first and second capacitors 402 and 403 are charged with voltages in the same direction, causing an error in the analog input value.

On the other hand, in this invention, since the substrate is connected to the common side, the gate leakage capacitances 501 and 60
The charging current flowing through the capacitor 1 is shunted to the common through the equivalent diodes 503 and 603, and the amount flowing into the first and second capacitors can be reduced.

Therefore, according to this invention, the gate leakage capacitance 501
The amount of leakage of the drive signal component due to the charging current flowing through and 601 can be minimized. As a result, weak analog input signals can be isolated and transmitted with high precision.

In this invention, the first or second switch element 4
A structure can be added in which the drive signal of 05 or 406 is superimposed on the analog signal system through a resistor 413 or 414 as shown by the dotted line in FIG. By adding this structure, if a disconnection occurs on the detection end side, the accumulation of superimposed current will cause the first
The voltages of the second capacitors 402 and 403 swing out to the positive or negative side, and this phenomenon allows a disconnection on the detection end side to be detected.

When the connection structure of the resistor 414 is adopted here, the superimposed current flowing toward the detection end causes the filter 10 to
A voltage drop occurs in the impedance element constituting 3, and this voltage drop becomes an error. Therefore, in order to obtain the highest accuracy in reading analog input signals, the structure in which the superimposed current is injected for detecting disconnection is the resistor 41.
It is most desirable to adopt connection structure 3.

As explained above, according to the present invention, an insulated process input device capable of high-speed transmission can be obtained. Furthermore, it is possible to obtain an insulated process input device in which the amount of transmission errors caused by leakage components of drive signals is extremely small. Furthermore, it is possible to provide a disconnection detection function to an isolated PROMOS input device that is capable of high-speed transmission and has an extremely small amount of error caused by leakage components of drive signals. Therefore, by combining them, it is possible to obtain an insulated process input device with high precision and high reliability.

[Brief explanation of drawings]

1 and 2 are connection diagrams for explaining a conventionally known isolated process input device, and FIG. 3 is a connection diagram for explaining a conventionally known insulated process input device.
A waveform diagram for explaining the operation of the known isolated process input device shown in the figure, FIG. 4 is a connection diagram showing an embodiment of the present invention, and FIGS. 5 and 6 are effects of the present invention. FIG. 2 is an equivalent circuit diagram for explaining. 101: Isolation transformer, 102: DC-AC converter, 103: Filter, 106: Analog input signal, 107: Synchronous rectifier circuit, 111: Drive signal source, 401: Common side end of winding of isolation transformer,
402: first capacitor, 403: second capacitor, 405: first switch element, 406: second switch element.

Claims (1)

[Scope of Claims] 1 A: an isolation transformer; B: first and second capacitors, one end of which is connected to the common side end of the primary winding of the isolation transformer; and C: each of these first and second capacitors, and others. D first and second switch elements connected between one end of the primary winding and the other end of the primary winding, the substrate being connected to the common end of the primary winding; 1. 2nd
A synchronous rectifier circuit connected to the secondary winding of the isolation transformer symmetrically with a DC-AC converter constituted by a switch element, and E both ends of the first or second capacitor of the DC-AC converter. An insulated type comprising: an analog signal source that provides an analog signal through an impedance element to F; and a drive signal source that operates complementary to the first and second switch elements of the DC-AC converter and the synchronous rectifier circuit. Process input device.