MIC2171 Datasheet, PDF (8/10 Pages) Micrel Semiconductor – 100kHz 2.5A Switching Regulator Preliminary Information

MIC2171
For our practical example:
L1 ≥
(4.178 × 0.662)²
2 × 3.0 × 1× 10⁵
L1 ≥ 12.4µH (use 15µH)
Equation (3) solves for L1’s maximum current value.
(3)
I_L1(peak) =
V_IN T_ON
L1
Where:
T_ON = δ / f_SW = 6.62×10^-6 sec
I_L1(peak) =
4.178 × 6.62 × 10^-6
15 × 10^-6
I_L1(peak) = 1.84A
Use a 15µH 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-
Micrel
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_OUT
I_CL V_IN(min) – V_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_IN(min) ^{= V}_IN ^{– I}_CL ^{× R}_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/N_SEC, that will guarantee safe operation of the MIC2171
power switch.
(9)
a ≤ ^{VCE 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 (V_OUT + V_F)
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/N_SEC)
Next, calculate the maximum primary inductance required to
store the needed output energy with a power switch duty
cycle of 55%.
4-10
1997