Isolation method

Galvanic isolation

Application range and characteristics of galvanic isolation

The isolation of analog signals in YUTONG's various series of two-wire isolators is galvanically isolated. In the two-wire model, there is no classification of safety barriers, and there is no classification of 1 in multiple outputs.

The galvanically isolated output is current, the load is low impedance, and the current in the output coil is the output current of the two-wire isolator. Therefore, when the input DC signal is modulated into an AC signal, the corresponding electric power needs to be output on the one hand, and also consumes the power generated by the lowest voltage (<2V) that maintains the modulation and demodulation circuit operation is removed.

The internal resistance of the current transformer is not an ideal value infinity. The specific resistance can be measured simply. For example, the load changes by 50Ω, the output current changes by 5μA, and the internal resistance of the current transformer is equal to (20mA╳50Ω/5μA=)200KΩ. It is connected in parallel with the load, so it splits the output signal. For example, when the load is 600Ω, the load voltage is 12V, the shunt in the internal resistance of 200KΩ is (12V/200KΩ=) 60μA, and the output current is reduced to 19.94mA. The accuracy is around 0.4% FS.

In order to overcome the above error, the load voltage can be detected and compensated according to the magnitude of the load voltage to compensate the shunt of the internal resistance of the current transformer, so that the impact of the load change on the output is reduced by more than 40 times, and the typical value of mass production is <±0.01%FS.

It is also possible to reduce the load. For example, the load in the above example is reduced to 10Ω, the output sampling voltage is 10ΩX20mA=0.2V, and the internal resistance 200K is 0.2V/200K=1 microamperes, which can be ignored. In order to solve the problem that the load is too small and not practical, it is necessary to convert the above 0.2V voltage into 20mA current by using a V/I conversion circuit.

The current transmission cannot be used in parallel mode. Because the shunt is generated, the current isolation cannot use the parallel connection of the output coils as the same. One transformer can only output one signal. In the one-input and multiple-out mode, multiple transformers and multiple sets of modulation and demodulation circuits need to be added to achieve, which is not conducive to reducing heat and reducing volume.

Galvanic isolator schematic

Passive galvanic isolation

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Voltage isolation

The scope and characteristics of voltage isolation 

The isolation of analog signals in each series of four-wire safety barriers (isolators) of YUTONG is voltage isolation.

The output of the voltage isolation is voltage, the load is high impedance, and the current in the output coil is nanoampere. Therefore, only micro power is required when modulating the input DC signal into an AC signal. A prerequisite for reducing the size of the transformer is provided.

Magnetoelectric isolation requires only one cycle. When the modulation frequency is 100KHZ, the response time is as long as 10 microseconds, which tends to zero. Because it is much smaller than the minimum delay time of other hardware and software, magnetoelectric isolation can be regarded as no cycle.

The amplitudes of the modulated and demodulated signals are continuous, with no conversion bits and steps, and the resolution is in principle higher than subsequent test circuits and devices.

Analog signals need to be used in ESD redundancy or other backup processes. The voltage isolation method can add one winding on the basis of one transformer, and can increase the isolated output voltage without adding a new transformer. Adding only 0.4mW of power consumption provides a prerequisite for reducing the number of transformers, circuits and power consumption.

The feedback signal (demodulation 0) is equal to the input signal, and the feedback coil is also equal to the number of turns and the magnetic circuit of the other output coils, so the output signal and the input signal are simply equal. The feedback voltage isolation circuit is an invention technology, and its practical feature is that only the coil number accuracy is required, the process is easy to implement, and the magnetic material and the magnetic circuit are not demanded because the latter is difficult in the process.

Low drift. . . . . . . . . . . . Typical value of 20nV/°C.

Low imbalance. . . . . . . . . . . . Typical value of 20μV.

High precision. . . . . . . . . . . . 0.01% FS maximum.

Micro power consumption. . . . . . . . . . . . 0.4mW/each way. 

Transformer feedback voltage isolation schematic

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Photoelectric isolation

The scope and characteristics of photoelectric isolation

The scope and characteristics of photoelectric isolation
YUTONG's series of four-wire safety barriers (isolators) are isolated by photoelectric isolation. YUTONG's earlier digital meters also used opto-isolated (PWM) to isolate analog signals.

The optocoupler is suitable for switching signal isolation. The amplitude of the analog signal is converted into a switching signal (PWM component) in the single-chip microcomputer, and the analog signal size is represented by the ratio of the switching time in one cycle, so that the optical isolation of the analog signal can be realized. The longer the set period, the higher the resolution. When the time interval is 20,000 in one cycle, the resolution of the output current is 1μA (the resolution of 14-bit DA). If the conversion is performed at a very low speed of 1 second period, Each time interval is 50/2=25μS, which exceeds the limit of the high speed optocoupler. The conversion speed and resolution of photoelectric isolation depend on the speed of the optocoupler. To accelerate the optocoupler, it is necessary to multiply the power consumption of the optocoupler, which is not conducive to the heat dissipation of the whole machine, and its effect is limited.

The conversion cycle and resolution are directly related to the system function safety. The shorter the cycle time, the stronger the real-time performance is to ensure that the control system does not use the wrong signal; the higher the resolution, the higher the realism of the signal to ensure the resolution of the control system itself is valid. The basic requirements are simple: 1. The sampling period of the conversion meter is lower than the period of the system control itself. 2. The resolution of the conversion meter is higher than the resolution of the control system.

YUTONG's series of universal safety barriers/isolators replace the PWM digital-to-analog converter with a 14-bit DA. The conversion cycle of the whole machine is shortened to 200mS and the resolution is 1.25μA, which can meet the requirements of most systems. Even so, the above-mentioned series of general-purpose instruments still have a problem that the sampling period is not short enough, and the possibility of mis-sampling with the sampling period of the system itself cannot be completely eliminated. Therefore, it is recommended to use the non-periodic, continuous analog instrument with the same function in the part with high system functional safety requirements, and use the above-mentioned general-purpose instrument reasonably in connection with display safety and other functions.

Analog photoelectric isolation schematic 

Switching photoelectric isolation schematic