MP171 –NON-ISOLATED OFFLINE REGULATOR

MP171 Rev. 1.0

www.MonolithicPower.com

13

9/30/2015

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Topology Options

MP171 can be used in common topologies such

as buck, boost, buck-boost, and flyback (see

Table 1).

Component selection below is based on the

typical application of MP173 (see it on page 1).

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.

Vin

DC input voltage

t

V

DC(min)

V

DC(max)

AC input voltage

Vin

DC input voltage

t

V

DC( min)

V

DC(max)

AC input voltage

Figure 6—Input voltage waveform

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.

Under very low input voltage, the inductor current

ramps up slowly; it may not reach the current

limit during

time should be less than the minimum value of

Inductor

The MP171 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

maximum power using Equation (7) and Equation

(8):



omax

o peak

V

PV (I

)

2L

, for CCM

(7)





2

o max

peak

minoff

11

PLI

2

, for DCM

(8)

For

mass

production,

tolerance

on

the

parameters (such as peak current limitation) and

the minimum off time should be taken into

consideration.

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.

The reverse recovery of the freewheeling diode

affects efficiency and circuit operation during a

CCM condition, so use an ultra-fast diode such

as the EGC10JH.

Output Capacitor

The output capacitor is required to maintain the

DC output voltage. Estimate the output voltage

ripple using Equation (9) and Equation (10):

CCM _ ripple

ESR

so

i

Vi R

8f C



  

, for

(9)

2

pk

o

o

DCM _ ripple

pk

ESR

so

pk

II

I

VI

R

fC

I

















, for DCM (10)

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

value for R2 should be avoided.

Sampling Capacitor

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):