Application examples of linear optocouplers

　　The circuit of the multi output power converter is shown in Figure 3. Its input voltage is a quasi square wave voltage ranging from 36V to 90V, with three outputs: UO1=+5V (2A), UO2=+15V (0.17A), UO3=-15V (0.17A). UO1 is now designated as the main output, with a voltage regulation rate SV of ± 0.4%; UO2 and UO3 are used as auxiliary outputs, and the total power efficiency can reach 75% to 80%. A TOP104Y type three terminal single-chip switching power supply integrated circuit is used in the circuit. The main output winding voltage is rectified and filtered by VD2, C2, L1, and C3 to obtain a+5V voltage. VD2 adopts MBR735 type 35V/7.5A Schottky diode. The two auxiliary output windings and output circuit are completely symmetrical in structure. Due to the small output current of ± 15V, both rectifier tubes VD4 and VD5 use UF4002 type 100V/1A ultrafast recovery diodes. The optocoupler feedback precision switching power supply is composed of a linear optocoupler CNY17-2 and an adjustable precision parallel regulator TL431C, which can precisely adjust the+5V voltage. The feedback winding voltage is rectified and filtered by VD3 and C4 to obtain a 12V feedback voltage. The drain clamp protection circuit composed of P6KE120 transient voltage suppressor and UF4002 ultrafast recovery diode can absorb the peak voltage formed by high-frequency transformer leakage inductance and protect the power field-effect transistor MOSFET inside the chip from damage.

　　The external error amplifier consists of TL431C. When the+5V output voltage increases, the sampled voltage obtained after dividing by R3 and R4 is compared with the 2.5V bandgap reference voltage in TL431C, causing the cathode potential to decrease and the working current IF of the LED to increase. Then, the control terminal current IC is increased through the linear optocoupler IC2 (CNY17-2), reducing the output duty cycle of TOP104Y and maintaining UO1 unchanged, achieving the purpose of voltage stabilization+ The 5V voltage regulation value UO1 is set by TL431C and the forward voltage drop of the LED in the optocoupler. R1 is the current limiting resistor of the LED. The frequency response of the error amplifier is determined by C5, R2, and C6. The function of C5 is threefold: to filter out the peak voltage on the control terminal; Determine the automatic restart frequency; Compensate the control loop together with R2.

　　At the input end of the optocoupler, it can be seen that sometimes a resistor is connected in parallel, which is a current discharge resistor, mainly to provide a path for weak current signals and avoid misguiding the light emitting diode of the optocoupler

　　Another function is to divide the voltage with the current limiting resistor

　　At the same time, it is used in parallel with the capacitor at the input end of the optocoupler when the response speed is not high.

　　Assuming the conduction voltage of the optocoupler LED is 1.5V, the minimum current flowing when the optocoupler is turned off as shown in the datasheet is 300uA. So let's take this resistor as R=1.5/0.3k=5k, which means the maximum resistance is 5kohm. When the resistance is smaller, the anti-interference performance is stronger. The lower limit is calculated based on the input current, as long as the resistor shunt does not cause unstable operation of the optocoupler

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