MP4021A 데이터 시트보기 (PDF) - Monolithic Power Systems
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MP4021A Datasheet PDF : 21 Pages
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MP4021A—PRIMARY SIDE CONTROL OFFLINE LED CONTROLLER WITH ACTIVE PFC
The higher output voltage is, the higher feed-
forward offset is. The Zener Diode is used to set
the compensation trigger level. The resistor
divider of RFF1 and RFF2 adjusts the action. Once
the output voltage rises up too high, the aux-
winding output also rises up to break the ZD to
add a feedforward bias on CS pin, then the
primary current is reduced quickly to prevent the
output voltage from rising to the higher level
further. With lower zener voltage range, the effect
of the feedfoward compensation is enhanced, but
the power loss is increased.
Dummy Load
For primary constant voltage control, sensing the
output voltage requires the secondary diode
conducts current, when the system is delivering
power to the output. When the load is very light,
the power consuming is very small. The
conduction of the secondary diode becomes very
short. The FB signal is hard to be coupled with
the real output voltage. So the output voltage
rises up to trigger OVP. The feedforward
compensation greatly reduces the power
delivered to output but a dummy load is still
required to dissipate the remaining power to limit
the voltage rising further. As a result, the dummy
load increases the power loss of whole system to
achieve the desired load regulation precision.
Protection
Different from CC mode control, CV mode makes
the system can work in no load condition. So the
OVP can not be triggered at normal operation.
But the MP4021A can still set a OVP threshold
thru ZCD R-divider to prevent the components
from damage in fault cases.
Other protection mechanisms, like SCP, OTP,
cycle by cycle current limit, are all same as
working in CC mode.
MP4021A Rev.1.04
www.MonolithicPower.com
18
7/5/2013
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© 2013 MPS. All Rights Reserved.
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