EM8671 datasheet(10/13 Pages) EXCELLIANCE

10 / 13 page

2013/01/30

REV.A.3

EM8671/A

Fig. 15

Soft Start

The EM8671/A has an internal soft-start circuit that

increases cycle-by-cycle current limit comparator

inverting input voltage slowly after it starts. The

typical soft-start time is 2mS. The pulse width to

the power MOSFET is progressively increased to

establish

the correct

working conditions for

transformers, rectifier diodes and capacitors. The

voltage on the output capacitors is progressively

increased

with

the

intention

smoothly

establishing the required output voltage. It also

helps prevent transformer saturation and reduces

the stress on the secondary diode during startup.

Slope compensation

In the conventional application, the problem of the

stability is a critical issue for current mode

controlling, when it operates in high than 50% of

the duty cycle. The EM8671/A built in saw-tooth

slope compensation. So it requires no extra

component.

Deep Burst Mode Operation

At no load or light load condition, majority of the

power dissipation in switching power supply is

form switching loss on the power MOSFET, the

core loss of the transformer and the loss on the

snubber. The magnitude of power loss is in

proportion to the number of switching events

within a fixed period of time. Reducing switching

events leads reduction on the power loss and

conserves the energy.

The

EM8671/A

adjusts

the

switching

mode

according to the load condition, the COMP pin

voltage drops below Deep Burst mode in-threshold

level (typical 1.3V). Device enters Deep Burst Mode

Control. The Gate drive output remains at off state

to minimize the switching loss and reduces the

standby power consumption. And when the COMP

pin voltage exceed the burst mode on threshold

level (typical 1.4V). The Gate drive output starts

active.

The

COMP

voltage

immediately

increases if there is a high load. When the COMP

voltage

exceed

the

Deep

Burst

mode

out-threshold level (typical 1.5V), the device goes

to normal mode. During the Deep Burst mode, the

CS level is controlled to 0.3V for EM8671. (0.1V for

EM8671A) Fig 16 shows the signals of Deep Burst

mode.

VCC

COMP

Vcs

Burst

Deep Burst

0.3 / 0.1V

1.3V

1.4V

1.5V

Fig. 16

Protection

The EM8671/A provides many protection functions

that intend to protect system from being damaged.

All the protection functions are listed as below:

Cycle-by-cycle current limit

The EM8671/A has over-current protection

thresholds. It is for cycle-by-cycle current limit,

which turns off MOSFET for the remainder of

the switching cycle when the sensing voltage of

MOSFET current reaches the threshold.

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