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AN4129 Datasheet(PDF) 7 Page - Fairchild Semiconductor |
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AN4129 Datasheet(HTML) 7 Page - Fairchild Semiconductor |
7 / 12 page APPLICATION NOTE AN4129 7 ©2003 Fairchild Semiconductor Corporation 6. Output Voltage Sensing Resistor and Feedback Loop Design The output voltage sensing circuits cause power loss if the impedance is too low. The values of the output voltage sensing resistors are selected so that that power loss is under 10mW. The designed values are 27k Ω for the upper resistor and 7k Ω for the bottom resistor. To control output voltage, a KA431 and an optocoupler are used as shown in Fig. 15. Equation (15) is the compensator transfer function. In this equation, k is the current transfer ratio of the optocoupler and this value is nonlinear. Figure 15. Output Voltage Compensation Circuit The FAN7601 does not contain an LEB(Leading Edge Blanking) circuit, but the parasitic capacitance and resistance of the internal circuit work as an RC filter which filters switching noise. The parasitic capacitance is about 10pF and the parasitic resistance is about 20k Ω. These values are sufficient to filter switching noise; therefore a small capacitor can be used for Cf . The value of Rf should be 1000~2000 times higher than that of Rs. The filter resistor Rf causes some sensing delay so that the peak value of the filtered information is less than that of the real current information. The higher resistance causes a greater difference as shown in Fig. 16. The black line is the CS/FB voltage and the red and blue lines are the real current waveforms before filtering . The red line is the current waveform when the resistance is low and the blue line is the current waveform when the resistance is high. Because the current peak of the blue line is higher than that of the red line, more energy is transferred to the secondary side. Therefore standby power is lower with the higher resistance. But if the resistance is too high, the system can become unstable. The selected values are 10pF for Cf and 1kΩ for Rf . Figure 16. Current Sense Waveforms For the stability of the system, capacitor C1 must be high enough. C1 can be selected as in (16). The selected value is 1nF. R3 determines the control loop gain. If the value is too low, the system can become unstable. And if the value is too high, the output voltage regulation characteristics may be poor. The selected value is 1.5k Ω. If the value of Rfb is too high then the output voltage is not regulated at no load because the offset voltage of the CS/FB pin is lower than necessary. If the value is too low the audible noise increases. Because the current transfer ratio of the photocoupler is nonlinear, the value of Rfb has to be selected by experiment at no load. The selected value is 3.9k Ω. 7. Transformer Audible Noise Because the FAN7601 operates in burst mode at light load and no load, it has a switching period and a non-switching period. Figure 17 shows the gate and output voltage at no load. Burst operation frequency is about 114Hz. The burst operation frequency varies according to load condition and the frequency is in the range of the audible frequency. Therefore the transformer may generate audible noise. The audible noise level depends on the control loop characteristic. If the loop speed is fast, audible noise increases but power loss decreases. If the loop speed is slow, audible noise decreases but power loss increases. There has to be a compromise between power loss and audible noise. Varnishing the transformer helps in reducing audible noise. Vfb VO -------- k = Rf R3 -------- 1 Rf Cf s ⋅⋅ 1 + --------------------------------- 1 1 R1 C1 s ⋅⋅ ---------------------------- + ⋅⋅ ⋅ (15 CS/FB Vcc R fb R f R S C f Vo C1 R1 R2 R3 1V CS/FB Low resistance High resistance Filtered informations C1 10 fsw --------- > 1 2 π R1 ⋅ ------------------- ⋅ (16) |
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