MP4026 – PRIMARY SIDE CONTROL, OFFLINE LED CONTROLLER WITH ACTIVE PFC

MP4026 Rev. 1.1

www.MonolithicPower.com

12

9/9/2015

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© 2015 MPS. All Rights Reserved.

The output-OVP-set point is then:

⋅⋅

=

+

AUX

ZCD2

OUT _ OVP

SEC

ZCD1

ZCD2

NR

V5.1V

NR

R

Where:

is

the

output-over-voltage-

protection point,

turns, and

turns.

R1

Gate

CS/ZCD

5.1V

+

-

Latch

OVP

Blanking time

Gate_OFF

Figure 7: OVP Sampling Unit

To prevent a voltage spike from mis-triggering

OVP after the switch turns off, OVP sampling

0.3V) as shown in Figure 8.

0V

Sampling Here

t

LE B _OV P

Figure 8: ZCD Voltage and OVP Sampler

Output Short-Circuit Protection

If an output short occurs, the ZCD can not

detect the transformer’s zero-current-crossing

point, so the 190μs auto-restart timer triggers

the power MOSFET’s turn-on signal. Then the

switching frequency of the power circuit drops

to about 5kHz, and the output current is limited

to its nominal current. The auxiliary-winding

voltage drops to follow the secondary-winding

and the system restarts. This sequence limits

the output power and IC temperature rise if an

output short occurs.

Primary-Side Over-Current Protection

The

primary-side

over-current

protection

prevents device damage caused by extremely

excessive current, like primary winding short. If

the CS/ZCD pin voltage rising to 2.5V at gate

turn on interval, as shown in Figure 9, the

primary-side over-current protection signal will

be triggered and latched, the gate driver will be

turned off and the IC works at quiescent mode,

will make the IC shut down, and the system

restarts again.

To

avoid

mis-trigger

by

the

parasitic

capacitances discharging when the MOSFET

turns on, a LEB time is needed, this LEB time is

relatively

smaller

than

current

regulation

sensing LEB time, typical 200ns.

R1

Gate

CS/ZCD

2.5V

+

-

Latch

OCP

Blanking time

Gate_ON

Figure 9: Primary-side OCP Sampling Unit

Brown-Out Protection

The MP4026 has brown-out protection: the

internal peak detector detects the peak value of

the rectified sinusoid waveform in MULT pin. If

the peak value is less than the brown-out-

protection threshold 0.3V for 42ms, the IC

recognizes this condition as a brown-out,

quickly drops the COMP voltage to zero, and

disables the power circuit. If the peak value

exceeds 0.4V, the IC restarts and the COMP

voltage rises softly again. This feature prevents

both the transformer and LED currents from

saturating during fast ON/OFF switching. Figure

10 shows the brown-out waveforms.