ACT412

Rev 1, 30-Oct-13

- 8 -

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Copyright © 2013 Active-Semi, Inc.

Active-Semi Proprietary―For Authorized Recipients and Customers

ActivePSR

current limit compensation circuit to achieve

constant power over wide line and wide load range.

Frequency Fold-back

When the load drops to 75% of full load level,

ACT412 starts to decrease the switching frequency,

which is proportional to the load current ,to improve

the efficiency of the converter as show in Functional

Block Diagram.

This enables the application to meet all latest green

energy standards. The actual minimum switching

frequency is programmable with a small dummy

load (while still meeting standby power).

Valley Switching

ACT412 employed valley switching from medium

load to heavy load to reduce switching loss and

EMI. After the switch is turned off, the ringing

voltage from the auxiliary winding is applied to the

VFB pin through feedback network R5, R6.

Internally, the VFB pin is connected to an zero-

crossing detector to generate the switch turn on

signal when the conditions are met. In light load, the

frequency fold back scheme starts to take control to

determine the switch turn on signal, so thus the

switching frequency.

Figure 1:

Valley Switching at heavy load

Protection Features

The ACT412 provides full protection functions. The

following table summarizes all protection functions.

Auto-Restart Operation

ACT412 will enter auto-restart mode when a fault is

identified. There is a startup phase in the auto-

restart mode. After this startup phase the conditions

are checked whether the failure is still present.

Normal operation proceeds once the failure mode is

removed. Otherwise, new startup phase will be

initiated again.

To reduce the power loss during fault mode, the

startup delay control is implemented. The startup

delay time increases over lines.

Short Circuit Protection

When the secondary side output is short circuited,

the ACT412 enters hiccup mode operation. This

hiccup behavior continues until the short circuit is

removed.

FB Over Voltage Protection

The

ACT412

includes

output

over-voltage

protection circuitry, which shuts down the IC when

the output voltage is 40% above the normal

regulation voltage 4 consecutive switching cycles.

The ACT412 enters hiccup mode when an output

over voltage fault is detected.

VDD Over Voltage Protection

ACT412 can monitor the converter output voltage.

The voltage generated by the auxiliary winding

tracks converter’s output voltage through VDD,

design value for four consecutive cycles, IC will

FUNCTIONAL DESCRIPTION CONT’D

Vdrain_gndMosfet

t

DCvoltage

T

Ton

Possible Valley turn on

PROTECTION

FUNCTIONS

FAILURE

CONDITION

PROTECTION

MODE

(4 duty cycle)

Auto Restart

Over Temperature

T > 135˚C

Auto Restart

Short Winding/

Short Diode

Auto Restart

Over Load

Auto Restart

Output Short

Circuit

Auto Restart

Open Loop

No switching for

4 cycle

Auto Restart

Voltage

Auto Restart

VFB Over Voltage

(4 duty cycle)

Auto Restart