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MIC2171WT Datasheet(PDF) 9 Page - Micrel Semiconductor |
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MIC2171WT Datasheet(HTML) 9 Page - Micrel Semiconductor |
9 / 12 page Micrel, Inc. MIC2171 May 2007 9 M9999-051107 For the practical example: VCE = 65V max. for the MIC2171 FCE = 0.8 VSEC = 5.6V then: 5.6 6.0 0.8 65 a − × ≤ a ≤ 8.2 (NPRI/NSEC) Next, calculate the maximum primary inductance required to store the needed output energy with a power switch duty cycle of 55%. (10) OUT 2 ON 2 IN(min) SW PRI P T V 0.5f L ≥ where: LPRI = maximum primary inductance fSW = device switching frequency (100kHz) VIN(min) = minimum input voltage TON = power switch on time then: 2.5 )2 10 (7.4 (3.22) 10 1 0.5 L 6 2 5 PRI − × × × × × ≥ LPRI ≥ 11.4µH Use a 12µH primary inductance to overcome circuit inefficiencies. To complete the design the inductance value of the secondary is found which will guarantee that the energy stored in the transformer during the power switch on time will be completed discharged into the output during the off-time. This is necessary when operating in discontinuous-mode. (11) OUT 2 OFF 2 SEC SW SEC P T V 0.5f L ≤ where: LSEC = maximum secondary inductance TOFF = power switch off time then: 2.5 ) 10 (2.6 (5.41) 10 1 0.5 L 2 6 2 5 SEC − × × × × × ≤ LSEC ≤ 7.9µH Finally, recalculate the transformer turns ratio to insure that it is less than the value earlier found in equation (9). (12) SEC PRI L L a ≤ then: 20 . 1 7.9 11.4 a = ≤ This ratio is less than the ratio calculated in equation (9). When specifying the transformer it is necessary to know the primary peak current which must be withstood without saturating the transformer core. (13) PRI ON IN(min) PEAK(pri) L T V I = so: PRI 6 PEAK(pri) L 10 7.6 3.22 I − × × = IPEAK(pri) = 2.1A Now find the minimum reverse voltage requirement for the output rectifier. This rectifier must have an average current rating greater than the maximum output current of 0.5A. (14) a F a) (V V V BR OUT IN(max) BR + ≥ where: VBR = output rectifier maximum peak reverse voltage rating a = transformer turns ratio (1.2) FBR = reverse voltage safety derating factor (0.8) then: 1.2 0.8 1.2) (5.0 6.0 VBR × × + ≥ VBR ≥ 12.5V A 1N5817 will safely handle voltage and current require- ments in this example. Forward Converters Micrel’s MIC2171 can be used in several circuit configurations to generate an output voltage which is less than the input voltage (buck or step-down topology). Figure 6 shows the MIC2171 in a voltage step-down application. Because of the internal architecture of these devices, more external components are required to implement a step-down regulator than with other devices offered by Micrel (refer to the LM257x or MIC457x family of buck switchers). However, for step-down conversion requiring a transformer (forward), the MIC2171 is a good choice. A 12V to 5V step-down converter using transformer isolation (forward) is shown in Figure 6. Unlike the isolated flyback converter which stores energy in the primary inductance during the controller’s on-time and releases it to the load during the off-time, the forward converter transfers energy to the output during the on- |
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