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AN4138 Datasheet(PDF) 3 Page - Fairchild Semiconductor |
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AN4138 Datasheet(HTML) 3 Page - Fairchild Semiconductor |
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3 / 20 page  APPLICATION NOTE AN4138 3 ©2002 Fairchild Semiconductor Corporation (3) STEP-3 : Determine the reflected output voltage (VRO). When the MOSFET in the FPS is turned off, the input voltage (VDC) together with the output voltage reflected to the primary (VRO) are imposed on the MOSFET as shown in Figure 4. After determining VRO, the maximum nominal MOSFET voltage (Vds nom) is obtained as where VDC max is specified in equation (3). The typical value for VRO is 65-85V. Figure 4. The output voltage reflected to the primary (4) STEP-4 : Determine the transformer primary side inductance (Lm) and the maximum duty ratio (Dmax). A Flyback converter has two kinds of operation modes; continuous conduction mode (CCM) and discontinuous conduction mode (DCM). The operation changes between CCM and DCM as the load condition and input voltage vary and each operation mode has their own advantages and disadvantages, respectively. The transformer size can be reduced using DCM because the average energy stored is lower compared to CCM. However, DCM inherently causes higher RMS current, which increases the conduction loss of the MOSFET and the current stress on the output capacitors. For low power applications under 10W where the MOSFET conduction loss is not so severe, it is typical to design the converter to operate in DCM for the entire operating range, or to operate in CCM only for low input voltage conditions in order to minimize the transformer size. Figure 5. Simplified flyback converter The design procedures for CCM and DCM are slightly different. Once the reflected output voltage (VRO) is determined in step-3, the flyback converter can be simplified as shown in Figure 5 by neglecting the voltage drops in MOSFET and diode. For CCM operation, the maximum duty ratio is given by where VDC min and V RO are specified in equations (2) and step-3, respectively. For DCM operation, the maximum duty ratio should be determined as smaller than the value obtained in equation (5). By reducing Dmax, the transformer size can be reduced. However, this increases the RMS value of the MOSFET drain current and Dmax should be determined by trade-off between the transformer size and MOSFET conduction loss. With the maximum duty ratio, the primary side inductance (Lm) of the transformer is obtained. The worst case in designing Lm is full load and minimum input voltage V ds nom V DC max V RO + = (4) - V RO + V DC + - Dra in GND FP S + V ds - 0 V V DC V RO L m V DC min V RO I ds I D I m I m I m I ds I ds D max D max I D I D min DC RO RO V V V + = min DC RO RO V V V + ≤ D max V RO V RO V DC min + ------------------------------------- = (5) |
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