ACT412
Rev 1, 30-Oct-13
FUNCTIONAL DESCRIPTION CONT’D
current limit compensation circuit to achieve
constant power over wide line and wide load range.
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.
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.
PROTECTION
FUNCTIONS
FAILURE
CONDITION
PROTECTION
MODE
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).
VDD Over Voltage
VFB Over Voltage
Over Temperature
VDD > 20.5V
(4 duty cycle)
Auto Restart
VFB > 3V
(4 duty cycle)
Auto Restart
Valley Switching
T > 135˚C
Auto Restart
Auto Restart
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.
Short Winding/
Short Diode
VCS > 1.75V
Over Load
IPK = ILIMIT
Auto Restart
Auto Restart
Output Short
Circuit
VFB < 0.28V
Open Loop
No switching for
4 cycle
Auto Restart
Auto Restart
VCC Under
Voltage
VCC < 6.8V
Figure 1:
To reduce the power loss during fault mode, the
startup delay control is implemented. The startup
delay time increases over lines.
Valley Switching at heavy load
Short Circuit Protection
Vdrain_gndMosfet
DC voltage
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.
t
Possible Valley turn on
T
Ton
VDD Over Voltage Protection
Protection Features
The ACT412 provides full protection functions. The
following table summarizes all protection functions.
ACT412 can monitor the converter output voltage.
The voltage generated by the auxiliary winding
tracks converter’s output voltage through VDD,
which is in proportion to the turn ratio (VOUT+VDIODE
хNA/NS. When the VOUT is abnormally higher than
)
Auto-Restart Operation
ACT412 will enter auto-restart mode when a fault is
design value for four consecutive cycles, IC will
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