1. Linear light pair introduction
The light isolation is one kind of very commonly used signal isolation form. Commonly used light pair component and peripheral circuit composition. Because the light pair electric circuit is simple, uses frequently in the digit isolating circuit or in the data transmission electric circuit, like UART agreement 20mA electric current link. Regarding simulated signal, light pair because of input output linear bad, and is big along with the temperature change, has limited it in the simulated signal isolation application.
Regarding the high frequency exchange simulated signal, the transformer isolation is the most common choice, but is not actually suitable regarding the branch signal. Some factories provide isolate the amplifier to take the simulated signal isolation the solution, if ADI AD202, can provide from the cocurrent provides 0.025% linearity to several K frequencies, but this kind of isolation component interior carries on the voltage - frequency to transform first, the pair process exchange signal carries on the transformer isolation, then carries on the frequency - voltage transformation to obtain the isolation effect. The integrated isolation amplifier internal circuit is complex, the volume is big, the cost is high, does not suit the large-scale application.
A simulated signal isolation’s quite good choice is the use linear light pair. The linear light pair’s isolation principle and the ordinary light pair does not have the difference, is only receives the ordinary light pair single bill the pattern to change slightly, increases the light which uses in feeding back to accept the electric circuit to use in feeding back. Thus, although two light accept the electric circuit are non-linear, but two light accept electric circuit’s nonlinear response are the same, like this, may counter-balance through path’s misalignment through feedback path’s misalignment, thus achieves realizes the linear isolation goal.
In the market linear light pair has the chip which in several may choose, like Agilent Corporation’s HCNR200/201, TI subsidiary company TOAS TIL300, CLARE LOC111 and so on. Here take HCNR200/201 as the example introduction
2. Chip introduction and principle explanation
The HCNR200/201 internal diagram as follows shows
And 1, 2 direct to take the isolation signal the input, 3, 4 pins use in feeding back, 5, 6 pins use in outputting. 1st, between 2 pin’s electric currents record make between IF,3, 4 pins and between 5, 6 pin electric currents record separately make IPD1 and IPD2. The input signal transforms after the voltage - electric current, the voltage change manifests on electric current IF, IPD1 and IPD2 become the linear relationship basically with IF, the time scale factor respectively records is K1 and K2, namely
K1 and K2 are very small (HCNR200 are generally 0.50%), and is big along with the temperature change (the HCNR200 range of variation in 0.25% to 0.75% between), but the chip design causes K1 and K2 is equal. In behind may see that in reasonable peripheral circuit design, the one who affects the output/input ratio truly is the two’s ratio K3, the linear light pair is using this characteristic to be able to achieve satisfaction the linearity.
HCNR200 and the HCNR201 internal structure is completely same, the difference lies in some targets. Is opposite provides a higher linearity in HCNR200, HCNR201.
Uses HCNR200/201 to carry on the isolation some targets as follows to show:
* linearity: HCNR200:0.25%, HCNR201:0.05%;
* time scale factor K3:HCNR200:15%, HCNR201:5%;
* temperature coefficient: -65ppm/oC;
* isolation voltage: 1414V;
* signal band width: Directs current to is bigger than 1MHz.
May see from above, is the same with the ordinary light pair, what linear light pair true isolation is the electric current, if wants to isolate the voltage truly, needs to output and to lose the source to increase the operational amplifier and so on auxiliary circuit. Below carries on the analysis to the HCNR200/201 typical circuit, how to realize the feedback as well as the electric current - voltage, the voltage - electric current transformation to the electric circuit in carries on the inferential reasoning and the explanation.
3. Typical circuit analysis
In the Agilent Corporation’s HCNR200/201 handbook has given many kinds of practical electric circuits, a more typical one kind of following chart shows:
Figure 2
Supposes inputs the terminal voltage is Vin, outputs the terminal voltage is Vout, the light pair guarantee two electric current transmittance respectively is K1, K2, obviously, is decided with between relations by and between relations.
Before, the level will transport the electric circuit which puts to raise looked that the following chart shows:
Supposes transports puts negative end the voltage is, transports puts out-port’s voltage is, in transports puts in not the saturated situation the two to satisfy the following relations:
Vo=Voo-GVi (1)
And is when transports puts the input difference mold is 0 output voltages, G is the gain which transports puts, generally is quite big.
Neglects transports puts negative end the input current, may think that passes R1 the electric current is IP1, according to R1 ohm’s law:
Through the R3 both sides’ electric current is IF, according to ohm’s law:
And, for the light pair 2 foot’s voltages, considers time the LED breakover the voltage () basic invariable, here achievement constant treatment.
According to light pair characteristic, namely
K1=IP1/IF (4)
And the expression substitution above equation, may result in:
The above equation may obtain after the distortion:
The expression substitution (3) the formula may result in:
Considered G is specially big, then below may do approximate:
Thus, the output and input voltage’s relations are as follows:
Obviously, in the above electric circuit, the output and the input is proportional, and the scale-up factor only by K3 and R1, R2 determined. Generally chooses R1=R2, serves purpose which the isolation does not only enlarge.
4. Auxiliary circuit and parameter determination
The above inferential reasoning is hypothesis all electric circuits is the work in the linearity range, if wants to achieve this point to need to transport puts carries on the reasonable shaping, and determination resistance resistance number.
4.1 transport puts the shaping
Transports puts may be the single power source power supply or the positive and negative power source power supply, what above gives is the single power source power supply example. For can enable the input range from 0 to VCC, needs to transport puts can the full amplitude of vibration work, moreover, will transport the working speed which, pressure suspending rate will put not to affect the entire electric circuit’s performance. TI Corporation’s LMV321 Shan Yun puts the electric circuit to be able to satisfy the above request, may take HCNR200/201 the peripheral circuit.
4.2 resistance number determinations
The resistance shaping needs to consider that transports puts the linearity range and linear light pair biggest operating current IFmax. In the K1 known situation, IFmax has determined IPD1 maximum value IPD1max, like this, because the Vo scope is smallest may be 0, like this, because
Considered IFmax is greatly advantageous to the energy transmission, like this, takes generally
Moreover, because works in the depth negative feedback condition transports puts satisfies the empty short characteristic, therefore, considers IPD1 the limit,
Thus,
The R2 determination may according to the enlargement factor which needs determine, for example, if does not need the method, only need R2=R1 then.
Moreover because the light pair will have some high frequency noises, usually in the R2 place shunt capacitance, constitution low pass filter, concrete electric capacity’s value by input frequency as well as noise frequency determination.
4.3 parameter definite examples
The supposition determines Vcc=5V, inputs between 0-4V, the output is equal to the input, uses LMV321 to transport puts the chip as well as the above electric circuit, below gives the parameter definite process.
* determined that IFmax:HCNR200/201 in the handbook recommends the component work about 25mA;
* determines R3:R3=5V/25mA=200;
* determines R1: ;
* determines R2:R2=R1=32K.
5. Summary
This article has given linear light use and so on in pair simple introduction as well as circuit design, parameter choice matters needing attention and the reference design, and gives the corresponding inferential reasoning and the explanation to electric circuit’s design method, refers for general electronic engineer.
