MP172– NON-ISOLATED OFF-LINE REGULATOR
maximum power using Equation (7) and Equation
(8):
Topology Options
The MP172 can be used in common topologies
such as buck, boost, buck-boost, and flyback
(see Table 1).
Vominoff
, for CCM
)
(7)
(8)
P
Vo(Ipeak
omax
2L
1
1
Component selection below is based on the
typical application of MP173 (see it on page 1).
LIp2eak
, for DCM
P
omax
2
minoff
For mass production, tolerance on the
parameters (such as peak current limitation and
the minimum off time) should be taken into
consideration.
Component Selection
Input Capacitor
The input capacitor supplies the DC input voltage
for the converter. Figure 6 shows the typical DC
bus voltage waveform of a half-wave rectifier and
a full-wave rectifier.
Freewheeling Diode
Select a diode with a maximum reverse-voltage
rating greater than the maximum input voltage
and a current rating determined by the output
current.
VDC(max)
Vin
DC input voltage
AC input voltage
DC input voltage
The reverse recovery of the freewheeling diode
affects the efficiency and circuit operation during
a CCM condition, so use an ultra-fast diode such
as the EGC10JH.
VDC(min)
t
VDC(max)
Vin
Output Capacitor
The output capacitor is required to maintain the
DC output voltage. Estimate the output voltage
ripple using Equation (9) and Equation (10):
VDC( min)
AC input voltage
i
, for CCM
iRESR
(9)
VCCM_ripple
t
8fsCo
Figure 6—Input voltage waveform
2
Ipk I
Io
o
, for DCM (10)
Typically, the use of a half-wave rectifier requires
an input capacitor rated at 3 µF/W for the
universal input condition. When using a full-wave
rectifier, an input capacitor is chosen between
1.5~2 µF/W for the universal input condition. A
half-wave rectifier is recommended for a < 2 W
output application, otherwise use a full-wave
rectifier.
VDCM_ ripple
Ipk RESR
fsCo
Ipk
It is recommended to use ceramic, tantalum, or
low ESR electrolytic capacitors to reduce the
output voltage ripple.
Feedback Resistors
The resistor divider determines the output
voltage. Choose appropriate R1 and R2 values to
maintain VFB at 2.55 V. An excessively large
value for R2 should be avoided.
Under very low input voltage, the inductor current
ramps up slowly; it may not reach the current
limit during τmanon, so the MOSFET on time
should be less than the minimum value of τmanon
.
Sampling Capacitor
Inductor
The sampling capacitor (C3) samples and holds
the output voltage for feedback. With R1 and R2
fixed, a small sampling capacitor result in poor
regulation at light loads, and large sampling
capacitor affect the circuit operation. Roughly
estimate an optimal capacitor value using
Equation (11):
The MP172 has a minimum off-time limit that
determines the maximum power output. A power
inductor with a larger inductance increases the
maximum power. Using a very small inductor
may cause failure at full load. Estimate the
MP172 Rev. 1.0
9/30/2015
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13
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