Line-shaped optocoupler introduction
Optical isolation is a very common signal isolation form. Common optocoupler components and their peripheral circuits. Since the optocoupler circuit is simple, it is often used in a digital isolation circuit or a data transmission circuit, such as a 20 mA current loop of the UART protocol. For analog signals, optical coupling is poor because the input and output is poor, and the application is limited to analog signal isolation with a large change in temperature.
For high frequency AC analog signals, transformer isolation is the most common choice, but for DC signals are not applicable. Some manufacturers provide an isolation amplifier as an analog signal isolation solution, such as the AD202 of ADI, can provide a linearity of 0.025% from DC to a few k, but the internal voltage-frequency conversion is performed inside this isolation device. The AC signal performs transformer isolation, and then performs frequency-voltage conversion to obtain an isolation effect. The integrated quarantine amplifier internal circuit is complicated, large, high cost, and is not suitable for large-scale applications.
A better choice for analog signal isolation is to use a linear optocoupler. The linear optocoupler isolation principle is not different from ordinary optocoupler, but the single-handed single-handed single-handed mode of ordinary optocoupler is slightly changed, increasing a light receiving circuit for feedback for feedback. Thus, although the two light receiving circuits are non-linear, the nonlinear characteristics of the two light receiving circuits are the same, so that the non-linearity of the direct passage can be offset by the nonlinearity of the feedback path, thereby achieving linearity Isolation purpose.
Linear optocouplers on the market have several options, such as Agilent's HCNR200 / 201, TI subsidiary TOAS TIL300, CLARE LOC111, etc. Here is an example of HCNR200 / 201 as an example.
2. Chip introduction and principle description
The internal block diagram of HCNR200 / 201 is shown below
Be
Where 1, 2 is introduced as an isolation signal, and 3, 4 pins are used for feedback, and 5, 6 pins are used to output. The current between the 1, 2 pins is recorded as the current between the IF, 3, 4 pins and the 5, 6 pins, respectively, respectively, respectively, IPD1 and IPD2, respectively. The input signal is converted by the voltage-current conversion, and the change in the voltage is reflected on the current IF, IPD1 and IPD2 are basically connected to the IF, and the linear coefficients are recorded as K1 and K2, respectively.
Be
K1 and K2 are generally small (HCNR 200 is 0.50%), and the temperature varies greatly (HCNR200 varies between 0.25% to 0.75%), but the design of the chip makes K1 and K2 equal. It can be seen later that in a reasonable peripheral circuit design, the ratio K3 of the output / input ratio is truly affecting the output / input ratio, and the linear optocoupler is utilizing this characteristic to achieve satisfactory linearity.
The internal structure of HCNR200 and HCNR201 is identical, and the difference is on some indicators. The HCNR201 provides higher linearity relative to HCNR200.
Some indicators using HCNR200 / 201 are shown below:
Be
* Linearity: HCNR200: 0.25%, HCNR201: 0.05%;
* Linear coefficient K3: HCNR200: 15%, HCNR201: 5%;
* Temperature coefficient: -65ppm / oc;
* Isolation voltage: 1414V;
* Signal bandwidth: DC to greater than 1 MHz.
As can be seen from the above, the linear optocoupler is trifted, and the linear optocoupler is truly the current. To really isolate the voltage, it is necessary to increase the operational amplifier and other auxiliary circuits at the output and output. The Typical circuit of HCNR200 / 201 is analyzed below, and how to achieve feedback and current-voltage, voltage-current conversion in the circuit.
3. Typical circuit analysis
AGILENT has given a variety of practical circuits at the HCNR200 / 201 manual, which is a more typical as shown below:
If the input voltage is Vin, the output voltage is VOUT, the two current transfer coefficients of the optocoupler guarantee are K1, K2, obviously, and between the relationships depending on and between the relationships.
Be
The circuit of the preamplifies is shown, as shown below:
The voltage of the transportation negative is that the voltage of the op amp output is that both the following relationships are met in the case of incapacity in op amp.
VO = VOO-GVI (1)
Among them, the output voltage when the input difference model is 0, and G is the gain of op amp, which is generally relatively large.
Ignore the input current of the op amp, which can be considered that the current of R1 is IP1, according to the ohmic law of R1:
Be
The current at both ends of R3 is IF, according to Ohm's law:
Be
Among them, the voltage of the optically coupled 2 pin, in view of the voltage at which the LED is turned on is substantially unchanged, here is treated as a constant.
According to the characteristics of the optocoupler,
K1 = IP1 / IF (4)
The expression will be subjected to the above formula, available:
The above equation is deformable:
Considering that G is particularly large, you can do the following:
In this way, the relationship between the output and the input voltage is as follows:
It can be seen that in the above circuit, the output and input are proportional, and the proportional coefficient is determined only by K3 and R1, R2. Generally, R1 = R2 is selected, and only the purpose of only isolation is not enlarged.
4. Auxiliary circuit and parameter determination
The above derivation is assumed that all circuits are working within linearity. If you want to do this, you need to make a reasonable selection of op amp, and determine the resistance of the resistance.
4.1 op amp selection
The op amp can be a single power supply or a positive and negative power supply, which is given above is an example of a single power supply. In order to enable the input range from 0 to Vcc, it is necessary to make a full swing operation, in addition, the operating speed of the op amp, the presence rate does not affect the performance of the entire circuit. TI's LMV321 single-running circuit can satisfy the above requirements, which can be used as peripheral circuits in HCNR200 / 201.
4.2 Value Determination
The selection of the resistor needs to consider the linear range and the maximum working current IFMax of the linear optocoupler. K1 is known, IFMAX has also determined the maximum ipd1max of IPD1, so that since the range of VO is minimum, it can be 0, so that the maximum value is generally taken in consideration of IFMAX.
In addition, since the op amp is satisfied with the virtual short feature of the depth negative feedback state, it is considered that IPD1 is limited, and the determination of R2 can be determined according to the required magnification, for example, if it is not necessary, only R2 = R1 I.e.
In addition, since the optocoupler generates some high-frequency noise, it is typically in parallel capacitance at R2, which constitutes a low pass filter, and the value of the specific capacitance is determined by the input frequency and the noise frequency.
4.3 Parameter Determination Example
Assume that VCC = 5V is determined, between 0-4V, output equal to input, using an LMV321 op amp, and the above circuit, the following is given below.
* Determine if the IFMAX: HCNR200 / 201 manual is about 25 mA of the recommended device work;
* Determine R3: R3 = 5V / 25mA = 200;
* Determine R1 :;
* Determine R2: R2 = R1 = 32K.
Our other product:
