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MP4021A

Primary Side Control Offline LED Controller with Active PFC

Monolithic Power Systems 

MP4021A—PRIMARY SIDE CONTROL OFFLINE LED CONTROLLER WITH ACTIVE PFC

OPERATION

The MP4021A is a primary-side control offline

LED controller which incorporates all the features

for high-performance LED lighting. The LED

current can be accurately controlled with the real

current control method from the primary-side

information. Active Power Factor Correction

(PFC) eliminates unwanted harmonic noise to

pollute the AC line.

Start Up

Initially, VCC of the MP4021A is charged through

the start up resistor from the AC line. When VCC

reaches 13.6V, the control logic works and the

gate drive signal begins to switch. Then the

power supply is taken over by the auxiliary

winding.

The MP4021A will shut down when VCC drops

below 9V.

Boundary Conduction Mode Operation

During the external MOSFET on time (tON), the

rectified input voltage (VBUS) applies to the

primary-side inductor (Lm), and the primary

current (Ipri) increases linearly from zero to the

peak value (Ipk). When the external MOSFET

turns off, the energy stored in the inductor is

transferred to the secondary-side and turns on

the secondary-side diode to power the load. The

secondary current (Isec) begins to decrease

linearly from the peak value to zero. When the

secondary current decreases to zero, the

primary-side leakage inductance, magnetizing

inductance and all the parasitic capacitances

decrease the MOSFET drain-source voltage—

this decrease is also reflected on the auxiliary

winding (see Figure 2). The zero-current detector

in the ZCD pin generates the turn-on signal of the

external MOSFET when the ZCD voltage falls

below 0.31V (see Figure 3).

As a result, there are virtually no primary-switch

turn-on losses and no secondary-diode reverse-

recover losses. It ensures high efficiency and low

EMI noise.

VD S

VBUS +NVOUT

VBUS

turn-on

t off

ton

I pk Ipri

I sec / N

VZCD

0

Figure 2—Boundary Conduction Mode

Auxiliary Winding

+

Vcc

turn-on

signal

RZCD1

ZCD

0.31V

RZCD2

CZCD

Figure 3—Zero Current Detector

Real Current Control

The proprietary real-current control method

allows the MP4021A to control the secondary-

side LED current from primary-side information.

The mean output LED current can be calculated

approximately as:

Io

≈

N ⋅ VFB

2 ⋅Rs

N—Turn ratio of primary side to secondary side

VFB—The feedback reference voltage (typical

0.4V)

Rs—The sensing resistor connected between the

MOSFET source and GND.

MP4021A Rev.1.04

www.MonolithicPower.com

12

7/5/2013

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

© 2013 MPS. All Rights Reserved.

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