MP4034 – OFFLINE LED DRIVER

MP4034 Rev. 1.02

www.MonolithicPower.com

13

12/4/2013

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

© 2013 MPS. All Rights Reserved.

APPLICATION INFORMATION

COMPONENT SELECTION

Input Filter

Input filter produces a DC source through the

rectifier from the AC input power Figure 9 shows

the input filter and Figure 10 shows the typical

DC bus voltage waveform.

Figure 9: Input Filter

Figure 10: DC Input-Voltage Waveform

Bulk capacitors C1 and C2 filter the rectified AC.

Inductor L forms a π filter with C1 and C2 to

restrain the differential-mode EMI noise. The

resistor (R) in parallel with the inductor (L)

restrains the mid-frequency-band EMI noise.

Normally, R is selected between 1kΩ and 10kΩ.

The DC input capacitors, C1 and C2, are usually

2µF/W to 3µF/W for the universal input condition.

For a 230VAC single-range application, the

capacitor can be half that value. Normally, the

minimum DC voltage should not be too low to

ensure the converter can supply the maximum

power to the load which can be express as

follows:

S

P

DC(min)

F

D

SS

D

N

V(N V

V )

N1 D

≥⋅

⋅

+

⋅

−

increase the value of the input capacitors to

Output Diode

Use a Schottky diode because of its fast

switching speed and low forward voltage drop for

better efficiency.

If a lower average efficiency (3%-4%) is

acceptable, replace the output diode with a PN-

junction diode or other non-Schottky diode to

lower costs.

Leakage Inductance

The

transformer

leakage

inductance

will

decrease the system efficiency and affect the

output

current

constant

precision.

The

transformer structure should be optimized to

improve the primary side and secondary side

coupling and minimize the transformer leakage

inductance of transformer. Aim for a leakage

inductance that is less than 5% inductance.

RCD Snubber

The transformer leakage inductance causes the

MOSFET drain voltage spike and the excessive

ringing on the drain voltage waveform.

The RCD snubber circuit can limit this Drain

voltage spike. Figure 11 shows the RCD snubber

circuit.

Figure 11: RCD Snubber