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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 Inductor Current Detection

The system is based on the peak current mode

control. A sensing resistor is tied from the Source

of the MOSFET and ground to sample the

primary inductor peak current. Different from

working in CC mode, since the MP4021A

regulates the FB signal, the Real Current Control

signal from CS pin is shielded. The CS pin is just

applied as primary peak current detector like

traditional CV converter.

Tuning the value of the CS sensing resistor can

change the COMP level. With larger sensing

resistor, the COMP level is higher and with

smaller sensing resistor, the COMP level is lower.

So, select a suitable sensing resistor to avoid the

COMP level to trigger Upper/Lower COMP

Voltage.

In order to avoid hitting the current limit, the

voltage across the sensing resistor should be

less than 80% of the worst case current limit

voltage. And the clamp voltage on CS pin is

typical 2.5V.

Loop Compensation

Unlike voltage mode has only a single pole roll

off due to the output filter, the MP4021A uses

peak-current-mode control to achieve more

stable control loop.

Since MP4021A is designed with PFC feature, so

the loop response is very slow with large COMP

cap. For Primary-side CC mode, the part

samples the primary-side current. With large

output cap, although the output voltage builds up

slowly, the primary current should be not small,

so the COMP cap can not be charged to

overshoot. For CV mode control, the feedback

voltage is from the output voltage of the aux-

winding, which reflects the output voltage of the

secondary winding at flyback period. The impact

of the slow loop response results in that the

COMP level is easy to be charged to overshoot

or drops to normal level slowly.

Figure 13—COMP Pin Structure

A series capacitor-resistor combination net sets a

pole-zero combination to control the

characteristics of the control system.

The CCOMP1 is a big-value capacitor (2.2µF

to10µF) to achieve high PF and stable loop. The

RCOMP1 in series with CCOMP adds a zero in the

loop to improve the phase margin, it is helpful to

limit the start-up voltage spike. As an option, a

small-value capacitor CCOMP2 can also be applied

to add a high-frequency pole to achieve a

beneficial effect of attenuating high-frequency

noise.

Feedforward Compensation

In order to limit the output voltage spike and

improve the voltage regulation, adding feed-

forward compensation from aux-winding

feedback voltage to CS pin, shown in Figure 14,

Aux-winding

RVS1

FB

RVS2

ZD

RFF1

DVS RVLIM

CVS

AC Line

Pri-winding

GATE

CS

RFF2

RSENSE

Figure 14—The Feedforward Compensation

on CS Pin

MP4021A Rev.1.04

www.MonolithicPower.com

17

7/5/2013

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