MP174 – NON-ISOLATED OFF-LINE REGULATOR

MP174 Rev. 1.01

www.MonolithicPower.com

15

8/18/2016

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Topology Options

MP174 can be used in common topologies, such

as Buck, Buck-Boost, Boost and Flyback. As

illustrated in table 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 half-wave rectifier and

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 3uF/W for the

universal input condition. When using the full-

wave rectifier, input capacitor is chosen as as

1.5~2uF/W for universal input condiont. Avoid a

minimum DC voltage below 70V; a low DC input

voltage can cause thermal issue. Half-wave

rectifier is recommended for <2W output

application and full-wave rectifier is

recommended for >2W output application.

Inductor

The MP174 has a minimum off-time limit that

determines the maximum power output. The

maximum power increases as the inductor

increases. Using a very small inductor may

cause failure at full load, but a larger inductor

means a higher OLP load. It is recommended to

select an inductor with the minimum value that

can supply the rated power. Estimate the

maximum power with:



o max

o peak

V

PV (I

)

2L

, for CCM





2

o max

peak

minoff

11

PLI

2

, for DCM

For

mass

production,

tolerance

on

the

parameters, such as peak current limitation,

minimal

off

time,

should

be

taken

into

consideration.

2

2.5

3

3.5

4

4.5

5

5.5

6

0.25 0.35 0.45 0.55 0.65 0.75

INDUCTANCE(mH)

For a 3.6W output converter (12V, 0.3A), the

minimum inductor value is about 0.36mH. But the

switching frequency is too high using a 0.36mH

inductor, which causes poor efficiency. Usually, it

is recommended to use an inductor that make

the switching frequency is higher than 20 kHz but

not too high in large output current applications.

To reduce costs, use a standard off-the-shelf

inductor no less than the calculated value.

Freewheeling Diode

The diode should be selected based on

maximum input voltage and peak current.

The freewheeling diode’s reverse recovery can

affect efficiency and circuit operation for CCM

condiotn, so use an ultra fast diode such as the

EGC10JH.