Electronic Components Datasheet Search |
|
MIC2171BT Datasheet(PDF) 8 Page - Micrel Semiconductor |
|
MIC2171BT Datasheet(HTML) 8 Page - Micrel Semiconductor |
8 / 10 page MIC2171 Micrel 4-10 1997 For our practical example: L1 4.178 .662 3.0 1 105 ≥ × () ×× × 0 2 2 L1 ≥ 12.4µH (use 15µH) Equation (3) solves for L1’s maximum current value. (3) I L1(peak) = V IN TON L1 Where: T ON = δ / fSW = 6.62×10 -6 sec I 4.178 6.62 10 15 10 L1(peak) -6 -6 = ×× × I L1(peak) = 1.84A Use a 15 µH inductor with a peak current rating of at least 2A. Flyback Conversion Flyback converter topology may be used in low power appli- cations where voltage isolation is required or whenever the input voltage can be less than or greater than the output voltage. As with the step-up converter the inductor (trans- former primary) current can be continuous or discontinuous. Discontinuous operation is recommended. Figure 2 shows a practical flyback converter design using the MIC2171. Switch Operation During Q1’s on time (Q1 is the internal NPN transistor—see block diagrams), energy is stored in T1’s primary inductance. During Q1’s off time, stored energy is partially discharged into C4 (output filter capacitor). Careful selection of a low ESR capacitor for C4 may provide satisfactory output ripple volt- age making additional filter stages unnecessary. C1 (input capacitor) may be reduced or eliminated if the MIC2171 is located near a low impedance voltage source. Output Diode The output diode allows T1 to store energy in its primary inductance (D2 nonconducting) and release energy into C4 (D2 conducting). The low forward voltage drop of a Schottky diode minimizes power loss in D2. Frequency Compensation A simple frequency compensation network consisting of R3 and C2 prevents output oscillations. High impedance output stages (transconductance type) in the MIC2171 often permit simplified loop-stability solutions to be connected to circuit ground, although a more conventional technique of connecting the components from the error amplifier output to its inverting input is also possible. Voltage Clipper Care must be taken to minimize T1’s leakage inductance, otherwise it may be necessary to incorporate the voltage clipper consisting of D1, R4, and C3 to avoid second break- down (failure) of the MIC2171’s internal power switch. Discontinuous Mode Design When designing a discontinuous flyback converter, first de- termine whether the device can safely handle the peak primary current demand placed on it by the output power. Equation (8) finds the maximum duty cycle required for a given input voltage and output power. If the duty cycle is greater than 0.8, discontinuous operation cannot be used. (8) δ 2 P I V – V OUT CL IN(min) SW ≥ () For a practical example let: (see Figure 2) P OUT = 5.0V × 0.5A = 2.5W V IN = 4.0V to 6.0V I CL = 2.5A when δ < 50% 1.67 (2 – δ) when δ ≥ 50% Then: V = V – I R IN min IN CL SW () × () V IN(min) = 4 – 0.78V V IN(min) = 3.22V δ ≥ 0.74 (74%), less than 0.8 so discontinous is permitted. A few iterations of equation (8) may be required if the duty cycle is found to be greater than 50%. Calculate the maximum transformer turns ratio a, or N PRI/NSEC, that will guarantee safe operation of the MIC2171 power switch. (9) a ≤ V CE FCE – VIN(max) V SEC Where: a = transformer maximum turns ratio V CE = power switch collector to emitter maximum voltage F CE = safety derating factor (0.8 for most commercial and industrial applications) V IN(max) = maximum input voltage V SEC = transformer secondary voltage (VOUT + VF) For the practical example: V CE = 65V max. for the MIC2171 F CE = 0.8 V SEC = 5.6V Then: a ≤ 65 × 0.8 – 6.0 5.6 a ≤ 8.2 (N PRI/NSEC) Next, calculate the maximum primary inductance required to store the needed output energy with a power switch duty cycle of 55%. |
Similar Part No. - MIC2171BT |
|
Similar Description - MIC2171BT |
|
|
Link URL |
Privacy Policy |
ALLDATASHEET.NET |
Does ALLDATASHEET help your business so far? [ DONATE ] |
About Alldatasheet | Advertisement | Contact us | Privacy Policy | Link Exchange | Manufacturer List All Rights Reserved©Alldatasheet.com |
Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
Family Site : ic2ic.com |
icmetro.com |