MP020-5 – OFFLINE, PRIMARY-SIDE REGULATOR WITH CC/CV CONTROL AND A 700V FET

MP020-5 Rev. 1.05

www.MonolithicPower.com

13

1/23/2014

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

© 2014 MPS. All Rights Reserved.

APPLICATION INFORMATION

COMPONENT SELECTION

Input Filter

The input filter helps convert the AC input to a

DC source through the rectifier. Figure 9 shows

the input filter, and Figure 10 shows the typical

DC bus voltage waveform.

C1

+

L

R

C2

+

+

AC Input

DC Input

Figure 9: Input Filter

t

DC input voltage

AC input voltage

0

Figure 10: DC Input Voltage Waveform

Bulk capacitors (C1 and C2) filter the rectified AC

input. The inductor (L) forms a 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, the

R is 1kΩ to 10kΩ.

C1 and C2 are usually set 2µF/W to 3µF/W for

the universal input condition. For 230VAC single-

range applications, halve the capacitor values.

Avoid very low minimum DC voltages to ensure

that the converter can supply the maximum

power load, which can be expressed as:

S

P

DC(min)

O

D

SS

D

N

V(V

V )

N1 D

≥⋅

+

⋅

−

the value of the input capacitors to increase the

Output Capacitor

Use low ESR or very low ESR output capacitors

to meet the output voltage ripple requirement

without using an LC post filter. In addition, using

low ESR capacitors improves output voltage

regulation and feedback voltage sampling at high

temperatures or low temperatures. Use an output

capacitor with an ESR lower than 100mΩ for

better efficiency over non-low ESR output

capacitors.

Output Diode

Use a Schottky diode because of its fast

switching speed and low forward-voltage drop for

better high or low temperature CV regulation and

efficiency.

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

acceptable, replace the output diode could with a

fast or ultra-fast diode to reduce costs. Be sure to

readjust the resistor divider values to for the

correct output voltage because of the forward

voltage drop is higher than the Schottky diode’s.

Leakage Inductance

The

transformer’s

leakage

inductance

will

decrease the system efficiency and affect the

output current or voltage constant precision.

Optimize the transformer structure to minimize

the leakage inductance. Aim for a leakage

inductance less than 5% of the primary

inductance.

RCD Snubber

The transfomer’s leakage inductance causes the

MOSFET drain voltage to spike and the

excessive ringing on the drain voltage waveform,

which affects the output voltage sampling 3.5µs

after the MOSFET turns off.

The RCD snubber circuit can limit the Drain

voltage spike. Figure 11 shows the RCD snubber

circuit.