PWM (Pulse Width Modulation) isolation type 0-20mA output circuit

Abstract

The utility model discloses a PWM isolation type 0-20mA output circuit, which comprises an isolation circuit and a PWM input isolation circuit, the output end of the isolation circuit is connected with the input end of a shaping circuit, the output end of the shaping circuit is connected with the input end of a filter circuit, and the output end of the filter circuit is connected with the output end of the isolation circuit. Relative to a power supply loop high-end power supply circuit, accurate voltage values are provided for a shaping circuit and a filter circuit, so that the PWM amplitude of the shaping circuit and the filter circuit does not change along with the change of power supply voltage, the output end of the filter circuit is connected with the input end of a V-I circuit, and the output end of the V-I circuit is in direct proportion to the PWM pulse width. According to the utility model, the power supply circuit is added to carry out accurate amplitude processing on the isolated PWM pulse, errors caused by power supply errors or fluctuations are eliminated, the PWM pulse input end and the current output end are electrically isolated, the PWM pulse width ratio is accurately converted into a current output value, and the PWM pulse-width ratio conversion circuit has the remarkable advantages of simple circuit structure, easy implementation and the like.

Description

Technical Field

[0001] The utility model relates to the field of digital technology, in particular to a PWM isolated 0-20mA output circuit. Background Art

[0002] In intelligent measurement and control situations, intelligent controllers usually output control currents such as 0-20mA and 4-20mA to control actuators, such as valve opening, as inverter input control parameters, etc. Since the circuit loop control signal is minimally affected by the transmission line interference and the output impedance is very high, current signals are one of the most common signal transmission forms in measurement and control situations.

[0003] However, in complex electrical environments, such as centralized central air conditioning (DDC) systems in buildings, where inverter-based fan speed control and water pump flow control are crucial, the non-isolated current signal output by the intelligent controller cannot be directly connected to the inverter input due to the unequal potential between the (weak-current) intelligent controller and the high-voltage cabinet. Doing so would result in crosstalk, interference with the intelligent controller, or even damage to the inverter in the cabinet. To overcome these issues, the intelligent controller's output—an isolated current signal—is a promising technical solution, significantly expanding the scope and reliability of intelligent controllers. Furthermore, common PWM isolation solutions fail to address the amplitude of the isolated PWM pulses. This unstable amplitude can lead to poor overall circuit stability and accuracy. Designing a highly accurate and cost-effective isolated current output loop for mass-produced products presents a significant challenge. Summary of the Invention

[0004] In response to the problems existing in the prior art, the utility model provides a PWM isolated 0-20mA output circuit, which not only improves the application range of the intelligent controller, but also improves the reliability of the intelligent controller with high precision and extremely low cost.

[0005] The purpose of the utility model is achieved through the following technical solutions.

[0006] A PWM isolated 0-20mA output circuit includes an isolation circuit and a PWM input isolation circuit. The output end of the isolation circuit is connected to the input end of a shaping circuit, which is also connected to the input end of a filtering circuit. A high-end power supply circuit relative to a power supply loop provides precise voltage values for the shaping circuit and the filtering circuit, so that the PWM amplitude does not change with changes in the power supply voltage. The output end of the filtering circuit is connected to the input of a VI circuit, and the output of the VI circuit is a current proportional to the PWM pulse width.

[0007] The isolation circuit includes a photoelectric coupling device, a pulse transformer, a digital isolation chip based on the charge gate principle, and a chip and device that transmits signals from the input end to the output end and electrically isolates the two sides.

[0008] The power supply circuit includes a reference power supply chip, an analog power supply component or a switching power supply component to stabilize the high-end voltage.

[0009] The filtering circuit includes a passive or active single-stage or multi-stage low-frequency filtering circuit.

[0010] The VI circuit includes a VI conversion circuit composed of discrete components such as MOS tubes and triodes or constructed by an operational amplifier.

[0011] Compared with the existing technology, the advantages of the present invention are: the present invention eliminates the error between the output current and the PWM pulse width caused by the change of the PWM output pulse amplitude with the power supply voltage through extremely simple electronic circuit design or modification, thereby improving the applicability of the intelligent controller and improving the reliability of the intelligent controller. It has many advantages such as high precision, simple circuit, low cost, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 This is a PWM isolated 0-20mA output circuit diagram of the utility model.

[0013] Figure 2 A diagram showing an embodiment of a PWM isolated 0-20mA output circuit. DETAILED DESCRIPTION

[0014] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0015] like Figure 1 As shown, the input end of the isolation circuit is usually connected to the output of the CPU and other internal intelligent controllers. The PWM pulse signal output by the CPU is isolated by the isolation circuit and electrically isolated from the subsequent circuits. The shaping circuit shapes the PWM signal transmitted by the isolation circuit to keep the PWM duty cycle unchanged. The precise voltage value output by the high-end (+ end) power supply circuit is used to power the shaping circuit to keep the "amplitude" of the PWM pulse unchanged. The shaped PWM signal is input to the filter circuit and converted into a DC signal by the filter circuit. The DC signal is converted into a current output by the V-I conversion circuit.

[0016] like Figure 2As shown, the PWM signal is connected to the cathode of the input side of the optocoupler U1 (isolation circuit), the anode of U1 is connected to the current limiting resistor R1, R1 is connected to the 3.3V (5V) inside the (primary side) intelligent controller, the emitter on the output side of U1 is connected to the pull-down resistor R3 and the input end of U3, and the collector is connected to 24V (bus voltage VC). When PWM is high, no current flows through the input side of U1, the output side of U1 is in the cut-off state, and the voltage value on the resistor R3 (VP, relative to VD) is low. When PWM is low, the output side of U1 is in the on state, VP is high, and the isolated inverse transmission of the PWM signal is realized through U1.

[0017] The TL431 controllable precision voltage regulator and resistor R2 form a power supply circuit. The reference terminal VREF and cathode of TL431 are connected to 24V (bus voltage VC), the anode is connected to one end of R2 (VD), and the other end of R2 is connected to the 24V ground (GND). Since the reference terminal VREF and cathode of TL431 are directly connected, the voltage between VC and VD is 2.5V, and this voltage value will not change due to fluctuations in VC.

[0018] The shaping circuit is composed of a Schmitt inverter U3. The positive power terminal of U3 is connected to VC, and the negative power terminal of U3 is connected to VD. That is, the power supply voltage of U3 is a highly stable 2.5V. Therefore, the "amplitude" of the PWM after shaping by U3 will not fluctuate with the change of VC, realizing the inverted output of PWM.

[0019] After the PWM is inverted by U1 and then inverted by U3, the PWM at the output of U3 is a same-phase signal.

[0020] Resistor R4 and capacitor C1, resistor R5 and capacitor C5 form a two-stage passive filter to filter out the AC component in the PWM signal, so that the output (VA) DC voltage is strictly (only) proportional to the PWM duty cycle.

[0021] Resistors R6 and R7, transistors T1 and T2 and operational amplifier U4 form a V-I conversion circuit. The left side of R6 is connected to VC, and the right side is connected to the - input terminal (VB) of U4. The positive power supply terminal of U4 is connected to 24V, and the negative terminal is connected to the 24V ground wire (GND). The output terminal of U4 is connected to one end of resistor R7, and the other end of R7 is connected to the base of a PNP type composite tube composed of transistors T1 and T2. The emitter of the composite tube is connected to the right end (VB) of R6, and the collector outputs current IO.

[0022] Since the amplification factor of the composite tube composed of T1 and T2 is extremely high, the current flowing through R7 can be ignored. At the same time, the input impedance of the operational amplifier is extremely high, and the current flowing into the U4-terminal can be ignored. The voltage difference between the two terminals of R6 is VC-VB. From the virtual terminal characteristic of U4, we know that VA=VB, so the voltage difference between the two terminals of R6 is VC-VA. Therefore, the output current value IO is (VC-VA) / R6.

[0023] The output voltage range of the power supply circuit relative to VC is the possible variation range of the VA voltage.

[0024] For example, when the PWM duty cycle is 100%, VA is 0V relative to VD, (VC-VA) = 2.5V, and the required output current value IO is 20mA, R6 = 125Ω.

[0025] Assuming R6 is 125Ω, when the PWM duty cycle is 50%, IO is 10mA; when the PWM duty cycle is 20%, IO is 4mA. Clearly, the output current is proportional to the VA voltage, which is in turn proportional to the duty cycle of the PWM pulse signal at the input.

[0026] The output current is directly proportional to the voltage value determined by the power supply circuit to supply power to the shaping circuit, and inversely proportional to the value of the current sampling resistor R6. Keeping the R6 parameter unchanged and doubling the VA value (determined by the VD value), the output current value will also double. If the VA value remains unchanged and you want to output 1A current, the R6 value is 2.5Ω.

[0027] By changing the values of VA and R6 at the same time, any output current value can be combined.

[0028] The resolution of current change depends entirely on the resolution of PWM. Taking 10-bit PWM and 20mA current output as an example, the maximum current resolution is 20 / 1024=0.0195mA. If the PWM resolution is 12 bits, the current resolution is 0.00488mA.

[0029] To reduce the power consumption of the composite diodes T1 and T2, a voltage regulator diode Z1 is added. Assuming the maximum output current of this circuit is 20mA, the current loop sampling resistor of the subsequent device is 250Ω, and Z1 is set to 6V, the power consumption of T1 is estimated to be (24-6-2.5-10) × 20 = 110mW.

[0030] In this circuit, the 24V GND and the GND of the CPU inside the intelligent controller are independent of each other, thus greatly improving the CPU's anti-interference ability.

[0031] In this circuit embodiment, the use of the inexpensive and high-quality bandgap reference device TL431 can ensure that the "P-I" conversion of mass-produced products has extremely high consistency.

[0032] In order to overcome the device parameter error, correction technology is adopted to further improve the accuracy of the circuit. The output PWM duty cycle is set to 80%. The theoretical value of the output current should be 16.000mA. If the actual value is 16.010mA, the correction coefficient λ when the intelligent controller outputs PWM is (16.00 / 16.010).

[0033] If the PCB of this circuit is designed to be relatively independent, it becomes a universal PWM isolated current output module.

[0034] If a high-precision resistor is connected in series with the ground at the output end of this circuit, the voltage value across the resistor will be converted into an isolated universal voltage signal output circuit.

[0035] The utility model adds a power supply circuit to accurately process the "amplitude" of the isolated PWM pulse, thereby eliminating errors caused by power supply errors or fluctuations. The PWM pulse input end and the current output end are electrically isolated, and the PWM pulse width ratio is accurately converted into the current output value. It has significant advantages such as simple circuit structure and easy implementation.

[0036] The above embodiments are merely examples for the purpose of clearly illustrating the present invention and are not intended to limit the manner in which the present invention may be implemented. Those skilled in the art will appreciate that other variations or modifications based on the above description are possible. It is not necessary and impossible to enumerate all possible embodiments here. However, such obvious variations or modifications arising from the spirit of the present invention remain within the scope of protection of the present invention.

Claims

1. A PWM isolated 0-20mA output circuit, characterized by: It includes an isolation circuit and a PWM input isolation circuit. The output end of the isolation circuit is connected to the input end of the shaping circuit, and the output end of the shaping circuit is connected to the input end of the filtering circuit. The high-end power supply circuit relative to the power supply loop provides accurate voltage values for the shaping circuit and the filtering circuit, so that the PWM amplitude does not change with the change of the power supply voltage. The output end of the filtering circuit is connected to the input of the VI circuit, and the output of the VI circuit is a current proportional to the PWM pulse width.

2. A PWM isolated 0-20mA output circuit according to claim 1, characterized in that: The isolation circuit includes a photoelectric coupling device, a pulse transformer, a digital isolation chip based on the charge gate principle, and a chip and device that transmits signals from the input end to the output end and electrically isolates the two sides.

3. The PWM isolated 0-20mA output circuit according to claim 1, wherein: The power supply circuit includes a reference power supply chip, an analog power supply component or a switching power supply component to stabilize the high-end voltage.

4. The PWM isolated 0-20mA output circuit according to claim 1, wherein: The filtering circuit includes a passive or active single-stage or multi-stage low-frequency filtering circuit.

5. The PWM isolated 0-20mA output circuit according to claim 1, wherein: The VI circuit includes a VI conversion circuit composed of discrete components such as MOS tubes and triodes or constructed by an operational amplifier.

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