Optical coupling in the application of switching power supply
Optical coupling circuit in the main function is to realize the photoelectric conversion, isolation, avoid the mutual interference between the input and output situation.
In the process of different switching power supply design, optical coupling effect is also different, combined with TL431 is used, is the best way to reduce cost control switching power supply industry.
The basic parameters, optical coupling of the optical coupling in figure 1 internal structure made up of three basic parts: the leds, pervious to light insulation, photoelectric triode.
By light-emitting diodes, through insulating layer to the photoelectric triode, and realize the current transmission, isolation characteristic.
Can be seen from figure 1, the optical coupling of the main parameters are: 1, the current transfer ratio CTR:, luminous tube current and current ratio of the minimum light activated triode.
2, resistant to high voltage (
Pervious to light insulation)
: refers to the decoupling protection circuit and light themselves from physical damage caused by high pressure.
3, LED drive current IF: using high efficiency LED receiving amplifier with high gain, can reduce the drive current of the IF, at the same time, the smaller the IF current can reduce the power consumption of the system, and reduce the attenuation of LED, provide long-term reliability of the system.
4, common mode rejection ratio VCM refers to in every microsecond optical coupling can allow maximum common-mode voltage of the rate of rise and fall.
The main parameters is of vital importance to the application in industrial motor.
Such as motor start-up or braking process will lead to great common-mode noise.
Second, the optical coupling in the application of switch power supply, optical coupling features: has to signal transmissions, so as to realize the input and the output electrical isolation, i. e. , had no effect on the output signal of the input, with strong anti-interference ability, stable performance, high reliability, high transmission efficiency, etc. The is usually used with switching power supply control circuit.
Optical coupling in the typical application of switch power supply principle: sampling from the output end, gain error signal, and then transmit the signal through the transformation, isolation to the input PWM controller IC, by adjusting the size of the PWM duty cycle, high precision voltage regulator output.
By the combined use of optical coupling and TL431, constitute the most simple of switching power supply control circuit (
A feedback loop)
, to realize the voltage output, as shown in figure 2, v for the output voltage Vo after partial pressure for TL431 error amplifier inverting the sample model, the sampling signal Vs through light coupling diodes, TL431, IF resistor R1 is converted into a current signal, and then transferred to the optical coupling output, form error signal Vea, comparing with the triangular wave PWM controller Vt, rectangular pulse (
With a range of duty ratio of PWM signal Vb)
, and then adjust the power level device conduction and deadline, achieve the goal of stable output.
Three, optical coupling with the stability of the feedback loop feedback loop composed of TL431 for switching power supply is very important, if not enough phase margin and gain margin, dynamic characteristic of the power supply will become worse or led directly to the output oscillation, make the product damage or shorten service life.
When designing switching power supply control circuit, should give full consideration to the stability of the feedback loop, the negative feedback characteristics: in order to make the products that you can work in the worst situation, still remain stable, theory need at least 45 & deg;
The phase margin.
Figure 2 red box device elements form a feedback loop with type 2 compensation.
TL431 is the most commonly used benchmark and switch power secondary feedback error amplification device, the power supply in a different way has a great influence on its transfer function.
In research and development design phase, the engineer usually with the aid of the loop testing instrument, the stability of the loop debug shorten product development cycle, improve the stability and reliability of the product.
Four, decoupling the selection point of light coupling in the application of switching power supply is mainly to provide electrical isolation between the primary input and the output, and TL431 combination of feedback control loop, so the circuit design, must follow the following principles: 1, according to the product isolation between the input and output voltage, choose accord with domestic and international related optical coupling isolation breakdown voltage standard;
2, current transfer ratio (
The ideal range is 50% ~ 200%.
This is because when the CTR hours, LED to a larger working current of light coupling, this will increase the light coupling power consumption;
Start the circuit when the CTR is too large, or load mutation, may affect the normal output;
3, preferred linear optical coupling, because CTR value within a certain range, has good linear adjustment.
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