MP023 – PRIMARY-SIDE CC/CV CONTROLLER

MP023 Rev. 1.1

www.MonolithicPower.com

17

7/18/2016

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APPLICATION INFORMATION

Input Filter

The input filter helps to convert the AC input

voltage to DC voltage. Figure 11 shows the

input filter, and Figure 12 shows the typical DC

bus voltage waveform after the rectifier.

Figure 11: Input Filter

Figure 12: DC Input Voltage Waveform

The bulk capacitors (C1 and C2) filter the

rectified AC input. 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 restrains the mid-frequency band EMI

noise. Normally, R is 1kΩ to 10kΩ.

Usually C1 and C2 are set at 2µF/W to 3µF/W

single-range applications, cut the capacitor

values in half.

A low DC input voltage causes insufficient

output current due to the maximum on-duty

limitation of the secondary side. To avoid this

situation, calculate the minimum input DC

voltage with Equation (11):

p

SMax

DC(min)

O

D

sS Max

N

D

V(V

V )

N1 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. Output capacitors with an

ESR of tens of mΩ provide better efficiency

than high ESR output capacitors.

Output Diode or Synchronous Rectifier

Schottky diodes are recommended for their fast

switching speeds and low forward-voltage

drops 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. Figure 13 shows a typical SR circuit.

The MP6906 is available in a SOT23-6 package,

which does not require extra PCB dimensions

or external components.

Figure 13: 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.