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AN4138 Datasheet(PDF) 8 Page - Fairchild Semiconductor |
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AN4138 Datasheet(HTML) 8 Page - Fairchild Semiconductor |
8 / 20 page AN4138 APPLICATION NOTE 8 ©2003 Fairchild Semiconductor Corporation In the snubber design in this section, neither the lossy discharge of the inductor nor stray capacitance is considered. In the actual converter, the loss in the snubber network is less than the designed value due to this effects. Figure 12. MOSFET drain voltage and snubber capacitor voltage (12) STEP-12 : Design the Control circuit. In general, a battery charger employs constant current (CC) / constant voltage (CV) control circuit for an optimal charge of a battery. This design note presents two basic CC/CV control circuits for FPS flyback converters. A simple, low cost circuit using a transistor and shunt regulator (KA431) is presented first. The second circuit features highly accurate current control using an op amp together with a shunt regulator (KA431) and secondary bias winding. In the circuit analysis, it is assumed that the CTR of the opto-coupler is 100%. (a) Transistor and regulator (KA431) scheme Figure 13 shows the CC/CV control circuit using a transistor and KA431 for 5.2V/0.65A output application. This circuit is widely used when low cost and simplicity are major concerns. Since the transistor base-emitter voltage drop depends on the temperature, a temperature compensation circuit is required for temperature stability. To turn on the transistor (Q), about 0.7V voltage drop across the sensing resistor (Rsense) is required and this current control circuit should be used for output currents below 1A due to the power dissipated in current sense resistor. For output currents greater than 1A, or if output current accuracy and temperature stability are a key factor, the op amp current control circuits shown in Figure 15 should be used. Figure 13. Transistor and KA431 CC/CV control Constant voltage (CV) control : The voltage divider network of R1 and R2 should be designed to provide 2.5V to the reference pin of the KA431. The relationship between R1 and R2 is given by where Vo is the output voltage. By choosing R1 to be 2.2k Ω, R2 is obtained as The feedback capacitor (CF) introduces an integrator for CV control. To guarantee stable operation, CF of 470nF is chosen. The resistors Rbias and Rd should be designed to provide proper operating current for the KA431 and to guarantee the full swing of the feedback voltage for the FPS device chosen. In general, the minimum cathode voltage and current for the KA431 are 2.5V and 1mA, respectively. Therefore, Rbias and Rd should be designed to satisfy the following conditions. 0 V V DC max V RO V sn2 Effect of stray inductance (5-10V) BVdss Voltage Margin > 10% of BVdss N S D R C O KA431 817A R d R bias R 1 R 2 C F V O L P C P R sense R base Q R TH C B v FB 1:1 FPS R B GND I FB I o 250uA 510Ω Ω Ω Ω 56Ω Ω Ω Ω 470nF KSP2222 10kΩ Ω Ω Ω 510Ω Ω Ω Ω 1Ω Ω Ω Ω 5.2V / 0.65A 2.2kΩ Ω Ω Ω 2kΩ Ω Ω Ω R 2 2.5 R 1 ⋅ V o 2.5 – --------------------- = (28) R 2 2.5 2.2k Ω ⋅ 5.2V 2.5V – ------------------------------- 2k Ω == V o V OP – 2.5 – Rd -------------------------------------- I FB > (29) V OP R bias --------------1mA > (30) |
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