CN111669032A - Self-driving circuit and method of capacitive isolation field effect tube - Google Patents

Abstract

The invention relates to a PLC output field effect tube self-driving circuit. On the basis of the prior art, the invention adds a simple carrier signal f (provided by an IO port of an MCU or generated by oscillation of a simple inverter and shared multiple paths), a logic gate U1, a rectifier diode D1 and an energy storage capacitor C3, can be applied to self-driven output of a PLC field effect transistor, has the advantages of simple scheme, low cost and small occupied space of components, and is suitable for the control driving requirements of high-density multi-output loops such as a PLC. In particular, an isolated power supply is not required, so that the complexity of the circuit is reduced and the cost is reduced. In addition, the field effect transistor is adopted for driving output, and compared with triode driving output, the driving power consumption can be reduced, the heat source in the PLC is reduced, and the reliability of the PLC is improved.

Description

Self-driving circuit and method of capacitive isolation field effect tube

Technical Field

The invention relates to a PLC output field effect transistor self-driving circuit adopting a capacitance isolation technology, and belongs to the output driving technology in the field of industrial control.

Background

At present, PLC output is conventionally provided with three types of relay output, silicon controlled output and transistor (including triode and field effect transistor) output. The output of the relay is generally commonly used for alternating current or direct current high voltage and large current driving, and has the defects of larger volume and longer switching time; the output of the controlled silicon can only be used for alternating current driving; the transistor output is typically used for relatively low voltage (< 48VDC) and relatively low current (< 2A).

The output of the transistor is divided into triode output drive and field effect transistor output drive. The field effect transistor output drive can accomplish to switch on the pressure drop very little, and the less advantage of power loss, nevertheless because field effect transistor gate voltage is higher, prior art if adopt self-drive can lead to switching on the pressure drop too big certainly, consequently need be equipped with extra drive power supply, and general PLC output stage is kept apart, additionally provides the complexity that isolation power supply can increase technical scheme and increase cost. The triode driving output can be self-driven without providing an additional driving power supply, the output conduction voltage drop is small (less than or equal to 1.5V), and the technical scheme is very simple.

For the PLC application, because there are many control output loops, the common products have 6, 8, 12, 18, 24, 32 output paths, especially the PLC with more than 12 output paths, the internal space thereof is very compact, the space that each output loop can occupy is limited, and the volume of the output driving transistor is very limited. In fact, for a PLC product with multiple outputs, the output drives the transistor chip package, and a device with a package power larger than that of SOT-89 cannot be selected, that is, the device package power is not larger than 0.5W. How to improve the output driving current and reduce the power consumption of the output driving triode under the limited condition is a difficult problem. The utility model of the technology is authorized, and the number of the notice is CN 210038526U.

Related patent technologies of field effect transistor self-driving applied to PLC are not discovered for a while. Other applications are similar to the techniques: the invention patent of patent publication No. CN 106787633B (referred to as "comparison document 1" hereinafter) and the utility model patent of CN 210157172U (referred to as "comparison document 2" hereinafter) relate to the field of power switch tube driving of switching power supplies.

In addition, there are companies using capacitive isolation technology, texas instruments (Ti) abroad, and fens (shanghai) limited nationwide, and related literature "magnetic field immunity of digital capacitive isolator" (hereinafter referred to as "reference 3", by the authors: Thomas Kugelstadt, texas instruments (Ti) advanced application engineers), and a data manual "2 Pai _ pi 11xM1x _20200416_ CN" (hereinafter referred to as "reference 4", a data manual of semiconductor related devices).

As shown in fig. 1, the conventional invention has the advantages of self-driving, simple circuit, small occupied space on the PCBA, low cost and the like. But can also only be applied to higher frequency continuous or discrete pulse drives, such as the switching tube drive of a switching power supply. The requirement of 0-200 kHz control frequency in the field of PLC application cannot be met.

As shown in fig. 2 and 3, the driving capability of the prior utility model is only enhanced, and the working frequency range is not enlarged.

In summary, the driving signals of the prior invention patent and the prior utility model patent are continuous or discrete pulses, and do not need to drive continuous conduction signals or low-frequency pulse signals (limited by the selectable range of the capacitance value of the isolation capacitor), and the requirement of the PLC application field for controlling the frequency of 0-hundreds of kHz can not be met.

The capacitive isolation techniques mentioned in prior publications, reference 3 and reference 4, do not have the above-mentioned limitations, and can transmit signals in a frequency range of 0 to several hundred MHz, for example, "products of surge semiconductors" can realize isolation withstand voltage levels of 1.5kV rms to 5.0kV vrms and DC to 600Mbps signal transmission ". However, no matter the Ti technology or surge technology, the advantage is that signal transmission is limited by the capacity of a silicon chip capacitor, and energy cannot be transmitted while capacitive isolation is achieved, so that, with these prior art technologies, an isolation power supply must be provided, single-side power supply signal transmission cannot be achieved, modulation and demodulation required for signal transmission are divided into two isolated sides, single-side power supply is only available, modulation cannot be demodulated, or modulation cannot be performed without a demodulator to work normally.

Disclosure of Invention

The purpose of the invention is: the technical scheme that a drive signal with lower frequency (such as 0-200 kHz) is adopted to modulate a carrier signal with 1MHz (or higher frequency) solves the requirement of controlling the frequency of 0-200 kHz in the field of PLC application.

In order to achieve the above object, a technical solution of the present invention is to provide a self-driving circuit of a PLC output fet, which is characterized by comprising a logic and gate U1, wherein a low-frequency control signal DO and a high-frequency carrier signal f are respectively input to two input terminals of a logic and gate U1; the output end of the logic AND gate U1 is connected with one end of an isolation capacitor C1, and the other end of the isolation capacitor C1 is connected with the anode of a rectifier diode D1 and the cathode of a clamping diode D2; the anode of the clamping diode D2 is connected with one end of the isolation capacitor C2, and the other end of the isolation capacitor C2 is grounded; the cathode of the rectifier diode D1 is connected with one end of the discharge resistor R1, one end of the energy storage capacitor C3 and the gate of the output field effect transistor Q1; the other end of the discharge resistor R1, the other end of the energy storage capacitor C3 and the source electrode of the output field effect transistor Q1 are connected with one end of the isolation capacitor C2; the drain and source of the output fet Q1 form the output.

Another technical solution of the present invention is to provide a self-driving method for a PLC output field effect transistor, wherein the self-driving method for a PLC output field effect transistor includes the following steps:

the control signal DO modulates a carrier signal f connected to the other input end of the logic AND gate U1 through one input end of the logic AND gate U1 and is output to the output end of the logic AND gate U1, and is applied to the anode of the rectifier diode D1 and two ends of the clamping diode D2 through the isolation capacitor C1 and the isolation capacitor C2, and the clamping diode D2 is used for ensuring that a positive pulse signal passes through the rectifier diode D1;

the positive pulse signal passing through the rectifier diode D1 maintains a high level on the gate of the output field effect transistor Q1 for a period of time under the action of the energy storage capacitor C3 and the discharge resistor R1, the maintenance time is related to the discharge resistor R1, the energy storage capacitor C3 and the input capacitor Ci of the output field effect transistor Q1, appropriate parameters are selected, the maintenance time is 1.5/f-2/f, so as to respond to the control signal well, and the high level maintenance period is the conduction period of the drain and the source of the output field effect transistor Q1.

Preferably, the control signal DO is a square wave signal of 0-200 kHz; the carrier signal f is a square wave signal with a frequency of at least 1 MHz.

The beneficial effects of the invention are: on the basis of the prior art scheme, a simple carrier signal f (provided by an IO port of an MCU or generated by oscillation of a simple inverter and shared by multiple paths) and a logic gate U1, as well as a rectifier diode D1 and an energy storage capacitor C3 are added, so that the self-driven output circuit can be applied to self-driven output of a PLC field effect transistor, and is simple in scheme, low in cost, small in occupied space of components and parts, and suitable for control driving requirements of high-density multi-output loops such as a PLC.

In particular, an isolated power supply is not required, so that the complexity of the circuit is reduced and the cost is reduced. In addition, the field effect transistor is adopted for driving output, and compared with triode driving output, the driving power consumption can be reduced, the heat source in the PLC is reduced, and the reliability of the PLC is improved.

Drawings

FIG. 1 is a schematic circuit diagram of the invention patent CN 106787633B (compare with FIG. 4 of document 1);

FIG. 2 is a schematic diagram of a reference of the utility model CN 210157172U (compare with FIG. 1 of document 2);

FIG. 3 is a second schematic diagram of the CN 210157172U reference (FIG. 2 of the comparison document 2);

FIG. 4 is a schematic circuit diagram of the present invention;

FIG. 5 is a waveform of the control terminal of the embodiment;

figure 6 is a drive-end waveform of an embodiment;

FIG. 7 is a graph 2 of the magnetic field immunity of the digital capacitive isolator in the comparison document 3;

FIG. 8 is a graph of FIG. 3 comparing the magnetic field immunity of the digital capacitive isolator of document 3;

FIG. 9 is the drawing of FIG. 1 comparing the document 4 data manual "2 Pai _ π 11xM1x _20200416_ CN".

Detailed Description

The invention will be further illustrated with reference to the following specific examples. It should be understood that these examples are for illustrative purposes only and are not intended to limit the scope of the present invention. Further, it should be understood that various changes or modifications of the present invention may be made by those skilled in the art after reading the teaching of the present invention, and such equivalents may fall within the scope of the present invention as defined in the appended claims.

As shown in fig. 4, the technical solution of the present invention is to add a logic and gate U1 and a carrier signal f to the driving control terminal based on the prior art solution shown in fig. 1, and add a rectifier diode D1 and an energy storage capacitor C3 to the signals passing through the isolation capacitors C1 and C2 in addition to the clamp diode D2 of the prior art solution.

As shown in fig. 4, the self-driving circuit of a PLC output fet according to the present invention includes a logic and gate U1, wherein a low-frequency control signal DO and a high-frequency carrier signal f are respectively input to an input terminal B and an input terminal a of the logic and gate U1; the output end Y of the logic AND gate U1 is connected with one end of an isolation capacitor C1, and the other end of the isolation capacitor C1 is connected with the anode of a rectifier diode D1 and the cathode of a clamping diode D2; the anode of the clamping diode D2 is connected with one end of the isolation capacitor C2, and the other end of the isolation capacitor C2 is grounded; the cathode of the rectifier diode D1 is connected with one end of the discharge resistor R1, one end of the energy storage capacitor C3 and the gate of the output field effect transistor Q1; the other end of the discharge resistor R1, the other end of the energy storage capacitor C3 and the source electrode of the output field effect transistor Q1 are connected with one end of the isolation capacitor C2; the drain and source of the output fet Q1 form the outputs OD and COM.

The working principle of the prior art scheme is as follows: the control system in fig. 1 outputs a control pulse which is applied across the clamp diode D1 and across the resistor R1 through the isolation capacitors C1 and C2, the D1 clamp diode ensuring that across R1 is a positive pulse which is applied through R1 to the gate of the driver fet Q1 so that the drain D is conductive with the source S during the positive pulse. When the capacities of C1 and C2 are sufficiently large, the control pulse waveform at the drive control terminal can be reproduced well.

The working principle of the invention is as follows: the control signal DO (e.g., 0 to 200kHz) in fig. 4 modulates the carrier signal f (e.g., 1MHz or higher) connected to the input terminal a of the and gate U1 through the input terminal B of the and gate U1, and outputs the modulated signal to the output terminal Y of the and gate U1, through the isolation capacitors C1 and C2, and across the anode of the rectifier diode D1 and the clamp diode D2, and the clamp diode D2 functions to ensure that a positive pulse signal passes through the rectifier diode D1. The positive pulse signal passing through the rectifier diode D1 can maintain a high level for a period of time on the grid G of the output field effect tube Q1 under the action of the energy storage capacitor C3 and the discharge resistor R1, the maintaining time is related to the input capacitor Ci of the discharge resistor R1, the energy storage capacitor C3 and the output field effect tube Q1, appropriate parameters are selected to enable the maintaining time to be 1.5/f-2/f, the control signal can be responded well, and large time delay is not generated. The high-holding period is the conduction period of the drain D and the source S of the output fet Q1. Increasing f appropriately is advantageous to reduce the delay in the on state.

In this embodiment, the logic and gate U1 is NC7S08 (two-input and gate), the carrier signal f is a 2MHz square wave, the control signal DO is a 100kHz square wave, and the output waveforms of the input terminal B and the output terminal Y are shown in fig. 5.

C1 and C2 are 4.7nF, C3 is 470pF, R1 is 1k Ω, BAT54 is used for D1 and D2, and N-channel fet SI2304 is used for Q1.

The waveform between the two terminals of the rectifier diode D1 and the gate-source GS of the output fet Q1 is shown in fig. 6.

The values of R1 and C3 are related to f, the input capacitance of the output field effect tube Q1 is Ci, and the time constant tau is R1 x (C3+ Ci) approximately equals to 1.5/f.

f is increased to 4MHz, R1 and C3 parameters are reduced, and the DO working frequency can reach more than 200kHz at most.

On the basis of the invention of the comparison document 1, the invention well solves the problem that the frequency range of signal transmission is expanded to direct current on the premise of synchronous transmission of signal transmission and energy by adding a proper modulation and demodulation technical means, so that the application field of the invention of the comparison document 1 is expanded from the power drive of a switching power supply to the output isolation drive of automatic control equipment (such as PLC).

Claims (3)

1. A PLC output field effect tube self-driving circuit is characterized by comprising a logic AND gate U1, wherein a low-frequency control signal DO and a high-frequency carrier signal f are respectively input into two input ends of a logic AND gate U1; the output end of the logic AND gate U1 is connected with one end of an isolation capacitor C1, and the other end of the isolation capacitor C1 is connected with the anode of a rectifier diode D1 and the cathode of a clamping diode D2; the anode of the clamping diode D2 is connected with one end of the isolation capacitor C2, and the other end of the isolation capacitor C2 is grounded; the cathode of the rectifier diode D1 is connected with one end of the discharge resistor R1, one end of the energy storage capacitor C3 and the gate of the output field effect transistor Q1; the other end of the discharge resistor R1, the other end of the energy storage capacitor C3 and the source electrode of the output field effect transistor Q1 are connected with one end of the isolation capacitor C2; the drain and source of the output fet Q1 form the output.

2. A self-driving method of a PLC output fet, which is characterized by using the self-driving circuit of a PLC output fet according to claim 1, comprising the steps of:

the control signal DO modulates a carrier signal f connected to the other input end of the logic AND gate U1 through one input end of the logic AND gate U1 and is output to the output end of the logic AND gate U1, and is applied to the anode of the rectifier diode D1 and two ends of the clamping diode D2 through the isolation capacitor C1 and the isolation capacitor C2, and the clamping diode D2 is used for ensuring that a positive pulse signal passes through the rectifier diode D1;

the positive pulse signal passing through the rectifier diode D1 maintains a high level on the gate of the output field effect transistor Q1 for a period of time under the action of the energy storage capacitor C3 and the discharge resistor R1, the maintenance time is related to the discharge resistor R1, the energy storage capacitor C3 and the input capacitor Ci of the output field effect transistor Q1, appropriate parameters are selected, the maintenance time is 1.5/f-2/f, so as to respond to the control signal well, and the high level maintenance period is the conduction period of the drain and the source of the output field effect transistor Q1.

3. The self-driving method of a PLC output FET as claimed in claim 2, wherein said control signal DO is a square wave signal of 0-200 kHz; the carrier signal f is a square wave signal with a frequency of at least 1 MHz.

Images