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

MP4027 Rev. 1.02

www.MonolithicPower.com

12

12/9/2014

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2014 MPS. All Rights Reserved.

The output OVP setting point is calculated as:







AUX

ZCD2

OUT _ OVP

SEC

ZCD1

ZCD2

NR

V5.1V

NR

R

output

over-voltage

protection point;

R1

Gate

CS/ZCD

5.1V

+

-

Latch

OVP

Blanking time

Gate_OFF

Figure 7: OVP Sampling Unit

To prevent a voltage spike from an OVP mis-

0V

Sampling Here

t

LEB_OV P

Figure 8: ZCD Voltage and OVP Sampler

A current-limit resistor between the output of

the auxiliary winding and the ZCD resistor

divider also works as a suppresser to avoid an

OVP mis-trigger.

Output Short-Circuit Protection (SCP)

If an output short occurs, ZCD cannot detect

the transformer’s zero-current-crossing signal,

so the 190μs auto-restart timer triggers the

MOSFET’s turn-on signal. 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 voltage,

VCC drops to less than the UVLO threshold,

and then the system restarts. This sequence

limits both the output power and IC temperature

if an output short occurs.

Primary-Side Over-Current Protection (OCP)

The

primary-side

over-current

protection

prevents device damage from excessive current,

such as a primary winding short circuit. If the

CS/ZCD voltage rises to 2.46V during the gate

turn-on interval (see Figure 9), the primary-side

over-current protection signal is latched, turning

the gate driver off. When VCC drops below

UVLO, the IC restarts.

Figure 9: Primary-Side OCP Sampling Unit

Brown-Out Protection

The MP4027 has brown-out protection; the

internal peak detector detects the peak value of

the rectified sinusoid waveform on MULT. If the

peak value is less than the brown-out protection

threshold, 0.298V for typically 42ms, MP4027

identifies this as a brown-out, dropping COMP

to zero and disabling the power circuit. If the

peak value exceeds 0.398V, the IC restarts and

the COMP voltage rises again softly. This

feature prevents the transformer and LED

current from saturating during fast ON/OFF

switching (see Figure 10).