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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

Output Over-Voltage Protection (OVP)

Output over voltage protection can prevent the

components from damage in the over voltage

condition. The positive plateau of auxiliary

winding voltage is proportional to the output

voltage, the OVP uses the auxiliary winding

voltage instead of directly monitoring the output

voltage, the OVP sample is shown in Figure 8.

Once the ZCD pin voltage is higher than 5.4V,

the OVP signal will be triggered and latched, the

gate driver will be turned off and the IC work at

quiescent mode, the VCC voltage dropped below

the UVLO which will make the IC shut down and

the system restarts again. The output OVP

setting point can be calculated as:

VOUT _ OVP

⋅

NAUX

NSEC

⋅

R ZCD2

RZCD1 + RZCD2

= 5.4V

VOUT_OVP—Output over voltage protection point

NAUX—The auxiliary winding turns

NSEC—The secondary winding turns

Auxiliary Winding

+

Vcc

Latch

OVP

signal

RZ C D1

ZCD

5.4V

RZ C D2

1.5µs

Blanking

CZ C D

Figure 8—OVP Sample Unit

To avoid the mis-trigger OVP by the oscillation

spike after the switch turns off, the OVP sampling

has a tLEB_OVP blanking period, typical 1.5µs,

shown in Figure 9.

The current-limiting resistor between the output

of the aux-winding and the ZCD resistor divider

can also work as suppresser to avoid the OVP

mis-trigger.

VZCD

Sampling Here

0V

tLEB _OVP

Figure 9—ZCD Voltage and OVP Sample

Output Short Circuit Protection

The MP4021A clamps the CS pin voltage to less

than 2.5V to limit the available output power. If

the short circuit of the secondary-side occurs, the

voltage of the auxiliary winding will fall down

following the voltage of the secondary winding

and the VCC drops to less than UV threshold and

re-start the system.

As supplementary, tie a resistor divider form CS

sensing resistor to ZCD pin, shown in Figure 10.

When the power MOSFET in the primary-side is

turned on, the ZCD pin monitors the rising

primary-side current, once the ZCD pin reaches

OCP threshold, typical 0.6V, the gate driver will

be turned off to prevent the chip form damage

and the IC works at quiescent mode, the VCC

voltage dropped below the UVLO which will

make the IC shut down and the system restarts

again. Please note that the value of the resistors

to set the OCP threshold (ROCP1 & ROCP2) should

be much smaller than those of the ZCD zero-

current detector (RZCD1 & RZCD2)

VBUS

Primary Winding

Latch

GATE

PSR

control

CS

RCS

OCP

signal

ZCD

0.6V

D

280 ns

Blanking

ROCP1

ROCP2

Figure 10—OCP Sample Unit

MP4021A Rev.1.04

www.MonolithicPower.com

14

7/5/2013

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