MP155 – ENERGY EFFICIENT OFF-LINE REGULATOR

MP155 Rev. 1.12

www.MonolithicPower.com

13

10/28/2013

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Topology Options

MP155 can be used in common topologies, such

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

find the Table.1 for more information.

Component Selection

Input Capacitor

The input capacitor supplies the converter’s DC

input voltage. Figure 7 shows the typical half-

wave rectifier’s DC bus voltage waveform.

Figure 7: Input Voltage Waveform

When using the half-wave rectifier, set the input

capacitor 3µF/W for the universal input condition.

When using the full-wave rectifier, choose a

smaller capacitor, but avoid a minimum DC

voltage below 70V to avoid thermal shutdown.

Inductor

MP155 has a minimum off time limit that

determines the maximum power output. The

maximum power increases with the inductor

value. Using a smaller inductor may cause the

output to fail at full load, but a larger inductor

results in a higher OLP load. The optimal

inductor value is the smallest that can supply the

rated power. The maximum power is:

τ

omax

o peak

V

PV (I

)

2L

, for CCM

=⋅

τ

2

o max

peak

minoff

11

PLI

2

, for DCM

To account for converter parameters—such as

peak current limit and minimum OFF time—

the maximum power, and selecting an inductor

Using output voltages 5V and 12V as examples,

INDUCTOR(mH)

0.6

0.7

0.8

0.9

1

1.1

1.2

0.6

1.1

1.6

2.1

2.6

INDUCTOR(mH)

0.6

1.1

1.6

2.1

2.6

0

0.5

1

1.5

2

2.5

3

When designing a 0.5W converter (5V, 0.1A),

estimate the minimum inductor value at 0.6mH

based on Figure 8. Similarly, for a 1.2W

converter (12V, 0.1A), estimate the minimum

inductor at 0.9mH based on Figure 9 .

Use a standard off-the-shelf inductor to reduce

costs. Use a standard inductance that exceeds

calculated inductance.

Freewheeling Diode

Choose a diode with a maximum reverse voltage

rating that exceeds the maximum input voltage,

and a current rating that exceeds the output

current.

The reverse recovery of the freewheeling diode

can affect the efficiency and circuit operation.

Select an ultra-fast diode, such as the EGC10JH.