Table of contents
PCB Modular Design 20——Optocoupler Module PCB Layout and Routing Design Specifications
1 Introduction
The full name of optocoupler is optocoupler. The technical parameters mainly include LED forward voltage drop VF, forward current IF, current transfer ratio CTR, insulation resistance between input stage and output stage, collector-emitter reverse breakdown voltage V(BR)CEO, collector Electrode-emitter saturation voltage drop VCE(SAT). In addition, parameters such as rise time, fall time, delay time and storage time.
An optocoupler is an isolated transmission device. When the original edge is given a signal, the auxiliary edge circuit will output an isolated signal. The isolation of optocouplers is easy to understand and will not be discussed here.
Use a simple diagram to illustrate the work of the optocoupler:
Input the signal Vin at the original edge, apply the LED on the LED and Ri to generate the input current If of the optocoupler, If drives the LED, and makes the phototransistor on the auxiliary edge Guide, the loop VCC and RL generate Ic, and Ic generates Vout through RL, so as to achieve the purpose of transmitting signals. The direct drive correlation between the original edge and the auxiliary edge is CTR (current transfer ratio), which needs to satisfy Ic≤If*CTR.
CTR: the minimum current ratio of the photosensitive triode of the light-emitting tube
2. Classification of optocouplers
1. According to the function, it is divided into linear optocoupler and nonlinear optocoupler
1. Linear optocoupler is an optical coupling device used for analog signal isolation. Like regular optocouplers, linear optocouplers truly isolate the current flow. Linear optocouplers can protect the object under test and the test circuit, and reduce the influence of environmental interference on the test circuit.
Principle: The isolation principle of the linear optocoupler is the same as that of the ordinary optocoupler, but the single receiving mode of the ordinary optocoupler is slightly changed, and an optical receiving circuit for feedback is added. Thus, although the two light receiving circuits are nonlinear, the nonlinear characteristics of the two light receiving circuits are the same. In this way, the nonlinearity of the feedback path can cancel the nonlinearity of the direct path, thereby achieving the purpose of linear isolation.
2. The current transmission characteristic curve of the nonlinear optocoupler is a nonlinear optocoupler. This optocoupler is suitable for the transmission of switching signals, not for the transmission of analog quantities. The commonly used 4N series optocouplers are nonlinear optocouplers. Whether this is linear or non-linear depends on whether CTR is linear or not. It is usually used for analog signals. Digital signals are generally rarely used to view the CTR-IF characteristic curve of the CTR-IF characteristic curve. Especially when transmitting small signals, its communication current transfer ratio (δCTR=δIC/δIF) is very close to the CTR value of the DC current transfer ratio.
2. According to the transmission signal, it can be divided into digital optocoupler (OC gate output type, totem pole output type, tri-state gate circuit output type, etc.) and linear optocoupler (can be divided into low drift type, high linear type, broadband type, single power supply type, dual power supply type, etc.). ).
3. According to the isolation characteristics, it can be divided into ordinary isolation optocouplers (generally optical glue seal is less than 5000V, and air seal is less than 2000V) and high-voltage isolation optocouplers (can be divided into 10kV, 20kV, 30kV, etc.).
4. According to the speed, it can be divided into low-speed photocoupler (output of phototransistor, photocell, etc.) and high-speed photocoupler (photodiode with signal processing circuit or photoelectric integrated circuit output).
Five, according to the output form
1, photosensitive device output type, including photodiode output type, photosensitive triode output type, photocell output type, photo thyristor output type and so on.
2. NPN transistor output type, including AC input type, DC input type, complementary output type, etc.
3. Darlington transistor output type, including AC input type and DC input type.
4. Logic gate circuit output type, including gate circuit output type, Schmidt trigger output type, tri-state gate circuit output type, etc.
5. Low conduction output type (output low order of magnitude).
6. Optical switch output type (lower on-resistance 10ω).
7. Power output type (II/MOE) output).
6. According to the package form,
it can be divided into coaxial type, dual in-line type, TO package type, flat package type, SMD package type, and optical fiber transmission type.
7. According to the channel, it can be divided into single-channel, dual-channel and multi-channel optocoupler.
8. According to the working voltage, it can be divided into low power supply voltage optocoupler (generally 5~15V) and high voltage optocoupler (generally greater than 30V).
9. According to the optical path, it can be divided into outer optical path photocoupler (also known as photoelectric continuous detector) and inner optical path optocoupler. External optical path optocouplers are divided into transmissive optocouplers and reflective optocouplers.
3. Key points of layout and routing analysis
What we need to pay attention to when designing the PCB is that we need to hollow out our optocoupler devices, that is, we need to hollow out the bottom of our optocoupler, and we also need to hollow out the bottom of our relays. Empty treatment is to hollow out our copper skin.
Since the function of the optocoupler device is to isolate signals, it is also necessary to consider that the signals at both ends of the optocoupler device should be isolated during design, and the middle needs to be hollowed out, and crosstalk between traces should be avoided.
