MP024-10

– PRIMARY-SIDE CC/CV REGULATOR

MP024-10 Rev.1.0

www.MonolithicPower.com

19

4/17/2017

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© 2017 MPS. All Rights Reserved.

APPLICATION INFORMATION

Input Filter

The input filter helps to maintain the bus voltage

when the AC input voltage is low.

Figure 10 shows the typical input filter of an

application with a low power range. The bulk

capacitors (C1 and C2) filter the rectified AC

input. Usually, C1 and C2 are set at 2µF/W to

3µF/W for a universal input condition. For

single-range

applications,

cut

the

capacitor values in half. The inductor (L) forms

a pi (π) filter with C1 and C2 to restrain the

differential mode EMI noise. The resistor (R) in

parallel with L dampens the mid-frequency

band EMI noise, if necessary. Normally, R is

1kΩ to 10kΩ.

Figure 11 shows the typical DC bus voltage

waveform after the rectifier.

C1

+

L

R

C2

+

+

AC Input

DC Input

Figure 10: Input Filter

t

DC input voltage

AC input voltage

0

Figure 11: DC Input Voltage Waveform

A low DC input voltage causes insufficient

output current due to the maximum on-duty

limitation of the secondary side. To prevent this,

calculate the minimum input DC voltage with

Equation (11):

p

S Max

DC(min)

O

D

s

S Max

N

D

V

(V

V )

N

1 D









(11)

Output Capacitor

Low or very low ESR output capacitors are

recommended to meet the output voltage ripple

requirements. Low ESR capacitors improve the

output voltage regulation accuracy at high or

low

temperatures

and

can

provide

better

efficiency than high ESR output capacitors.

Output Diode Or Synchronous Rectifier

Schottky diodes are recommended for their fast

switching speed and low forward voltage drop

for better high or low temperature CV regulation

and efficiency.

If lower average efficiency (3% to 4%) is

acceptable, replace the output diode with a fast

or ultra-fast diode to reduce cost. Re-adjust the

resistor divider values for an accurate output

voltage since the forward voltage drop is higher

than the Schottky diode.

If the circuit has a high efficiency requirement, it

is recommended to use a synchronous rectifier

(SR) instead. The MP6906 is a suitable SR

controller where the output voltage is low and

there is no need for a secondary auxiliary

winding (see Figure 12). The MP6906 is

available in a SOT23-6 package, which only

requires

few

extra

PCB

dimensions

and

external components.

AGND

Np

Ns

Naux

T1

LL

5

VDD

3

PGND

6

VD

4

VG

1

VSS

2

MP6906

AGND

Vout

AGN D

V

CC

C

LL

R

LL

R

D

M

AGND

Figure 12: Synchronous Rectifier

Leakage Inductance

The

transformer’s

leakage

inductance

decreases system efficiency and affects either

the output current or voltage constant precision.

The transformer structure can be optimized to

minimize the leakage inductance. The leakage

inductance should be less than 5% of the

primary inductance.