MP4027 – PRIMARY-SIDE CONTROL, OFFLINE LED CONTROLLER WITH ACTIVE PFC

MP4027 Rev. 1.02

www.MonolithicPower.com

10

12/9/2014

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© 2014 MPS. All Rights Reserved.

OPERATION

The MP4027 is a primary-side control, offline

LED controller. It incorporates all the features of

high-performance LED lighting. The LED

current is controlled accurately with the real-

current control method from primary-side

information. Active power factor correction

(PFC) eliminates unwanted harmonic noise on

the AC line. The rich protection features

achieve high safety and reliability in real

application.

Start Up

Initially, the VCC cap is charged by the start-up

resistor from the AC line. When VCC reaches

25.5V, the control logic activates and the gate

driver signal begins to switch; the power supply

is taken over by the auxiliary winding.

The chip shuts down when VCC drops below

9.5V.

The high hysteretic voltage allows for a small

VCC capacitor (typically 4.7μF) to shorten the

start-up time.

Valley Switching Mode

the energy stored in the inductor is transferred

to the secondary-side, which activates the

secondary-side diode to power the load. The

its peak value to zero. When the secondary

current decreases to zero, the MOSFET drain-

source voltage starts oscillating, which is

caused

by

the

primary-side

magnetizing

inductance and parasitic capacitances—the

voltage ring also is reflected on the auxiliary

winding (see Figure 2). To improve primary-

control precision, the chip monitors when ZCD

voltage falls to zero twice before the next

switching period. The zero-current detector

from CS/ZCD generates GATE turn-on signal

when the ZCD voltage falls below 0.295V the

second time (see Figure 3).

This virtually eliminates switch turn-on loss and

diode reverse-recovery losses, ensuring high

efficiency and low EMI noise.

Figure 2: Valley Switching Mode

Figure 3: Zero-Current Detector

Real-Current Control

The proprietary real-current-control method

allows the MP4027 to control the secondary-

side LED current using primary-side information.

The mean value of the output LED current is

calculated approximately as:

REF

o

S

NV

I

2R







 N—Turn ratio between primary side and

secondary side;