MIC2171_07 Datasheet, PDF (8/12 Pages) Micrel Semiconductor – 100kHz 2.5A Switching Regulator

Micrel, Inc.
where:
P_OUT = 12 × 0.25 = 3W
f_SW = 1×105Hz (100kHz)
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 / fSW = 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
applications 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 (transformer 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 voltage 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 non-conducting) 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
May 2007
MIC2171
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 breakdown (failure) of the
MIC2171’s internal power switch.
Discontinuous Mode Design
When designing a discontinuous flyback converter, first
determine 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 ≥}
2P_OUT
I_CL V_IN(min) − VSW
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 discontinuous 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 ≤ ^{VCEFCE − 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)
8
M9999-051107