EM8636 datasheet(10/12 Pages) EXCELLIANCE

10 / 12 page

2014/3/21

A.1

EM8636

Soft Start

The EM8636 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 EM8636 built in saw-tooth

slope compensation. So it requires no extra

component.

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 EM8636 adjusts the switching mode according

to the load condition, the COMP pin voltage drops

below burst mode threshold level. Device enters

Burst Mode Control. The Gate drive output remains

at off state to minimize the switching loss and

reduces the standby power consumption.

Protection

The EM8636 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

EM8636

has

over-current

protection

thresholds (0.85V). 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.

Over-load / Open-loop Protection (OLP)

When feedback loop is open, as shown in Fig. 15,

no current flows through the photo-coupler

transistor, the EM8636 pulls up the COMP pin

voltage to 5.2V.

When the COMP pin voltage is above 4.2V

longer than 56mS, OLP is triggered. This

protection is also triggered when the SMPS

output drops below the normal value longer

than 56mS due to the overload condition.

Fig. 15

VCOMP

5.2V

4.2V

56mS

OLP triggers

Vout

PWM

comparator

COMP

VBias

4.2V

56mS

OLP

Feedback

open loop

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