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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

PRIMARY-SIDE CONSTANT-VOLTAGE CONTROL

For LED lighting application, a single-stage PFC

converter needs large output capacitor to reduce

the 100Hz ripple. On the other hand, the two-

stage solution is also proposed to remove the

100Hz ripple from the LED current. Figure 11

shows a two-stage solution system based on

MP4021A and MP2489, offline isolated PFC

flyback constant-voltage regulator plus DC to DC

LED driver.

connected to FB with about 200kΩ impedance, if

there is no other external feedback signal on FB

pin, the sample signal from CS pin is regulated. If

there is external FB signal with low input

impedance (<20kΩ) applied in FB pin, the

external FB signal is regulated. This feature

makes the MP4021A suitable both for primary

side control application without other feedback

signals and direct control application with

external feedback signal applied.

Figure 11—Two-stage LED Driver Solution

The MP4021A can be designed as a primary-

side constant-voltage control, offline isolated

flyback controller with Active Power Factor

Correction (PFC). The output voltage VO can be

regulated without opto-coupler under a wide AC

main input voltage range in a single-stage flyback

converter.

Primary-side Voltage Control

The MP4021A can implement feedback control

function with FB pin.

RVS1

RVS2

DVS RVLIM

CVS

The FB pin is used to regulate the output voltage

by sampling the aux-winding. As a flyback

converter, during the flyback period, the primary

MOSFET is off and the secondary diode is

conducting current. Before the secondary current

drops to zero, the aux-winding voltage is

proportional to the output voltage.

VAUX

=

NAUX

NSEC

⋅ VO

A simple D-R-C circuitry sample-holds the aux-

winding voltage and a resistor divider reduces

the amplitude to match the internal 0.4V

reference. So the output voltage can be

calculated as:

VO

=

N SEC

N AUX

⋅ RVS1 + RVS2

R VS2

⋅ VREF

The current-limit resistor RVLIM helps to attenuate

the energy of the leakage inductance to improve

the voltage regulation precision.

FB

Aux-winding

COMP ICOMP

Real Current

CS

EA

0.4V

Control

CCOMP

REF

Figure 12—FB Pin Structure

Shown in Figure 12, the FB signal is fed to the

error amplifier and comparing with the 0.4V

reference. So, at steady state, the average value

of FB level will be regulated at 0.4V. The Real

Current Control block’s output is internally

Power Factor Correction

Same as CC mode, the MULT pin is connected

to the tap of the resistor divider from the rectified

instantaneous line voltage and fed as one input

of the multiplier. The COMP level, which is

regulated from the feedback signal, is fed as the

other input of the internal multiplier. The output of

the multiplier will be shaped as sinusoid wave too.

This signal provides the reference for the current

comparator and comparing with the primary side

inductor current which sets the primary peak

current shaped as sinusoid with the input line

voltage. High power factor can be achieved.

MP4021A Rev.1.04

www.MonolithicPower.com

16

7/5/2013

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© 2013 MPS. All Rights Reserved.

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