MP174A – NON-ISOLATED OFF-LINE REGULATOR

MP174A Rev. 1.0

www.MonolithicPower.com

16

2/27/2019

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2019 MPS. All Rights Reserved.

Topology Options

The MP174A can be used in common topologies,

such as buck, buck-boost, boost, and flyback.

See 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 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 3μF/W for the

universal input condition. When using the full-

wave rectifier, choose an input capacitor between

1.5~2μF/W for a universal input condition. Avoid

a minimum DC voltage below 70V; a low DC

input voltage can cause thermal issues. A half-

wave rectifier is recommended for <2W output

application,

and

a

full-wave

rectifier

is

recommended for >2W output application.

Inductor

The MP174A 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 Equation (6) and Equation

(7):



 OMin_off

O _ max

O Peak

V

PV (I

)

2L

, for CCM

(6)





2

O_ max

Peak

Min_ off

11

PLI

2

, for DCM

(7)

For

mass

production,

tolerance

on

the

parameters, such as peak current limitation and

minimum off time, should be taken into

consideration.

Figure 7 shows an example of a PMin curve with a

12V output. IPeak=0.6A and TMin_off=15μs are used

as the worst case for PMin calculation.

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)

Figure 7: PMin vs. L at 12V

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

minimum inductor value is about 0.36mH. Using

a 0.36mH causes the switching frequency to be

too high, which causes poor efficiency. It is

recommended to use an inductor that allows a

switching frequency higher than 20kHz (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 the

maximum input voltage and peak current.

The freewheeling diode’s reverse recovery can

affect efficiency and circuit operation for a CCM

condition, so use an ultrafast diode, such as the

EGC10JH.