深圳由甲科技有限公司
YZ235X
OPERATION DESCRIPTION
The switch frequency reduces at light/no load condition
to improve the conversion efficiency. At light load or no
load condition, the FB input drops below burst mode
threshold level and device enters Burst Mode control.
The Gate drive output switches only when VDD voltage
drops below a preset level and FB input is active to
output an on state. Otherwise the gate drive remains at
off state to minimize the switching loss and reduces the
standby power consumption to the greatest extend.
The switching frequency control also eliminates the
audio noise at any loading conditions.
The YZ235X is a low power off-line SMPS Switcher
optimized for off-line flyback converter applications in sub
20W power range. The ‘Extended burst mode’ control
greatly reduces the standby power consumption and
helps the design easily to meet the international power
conservation requirements.
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Startup Current and Start up Control
Startup current of is designed to be very low so
YZ235X
that VDD could be charged up above UVLO threshold
level and device starts up quickly. A large value startup
resistor can therefore be used to minimize the power loss
yet achieve a reliable startup in application. For AC/DC
adaptor with universal input range design, a 2 MΩ, 1/8
W startup resistor could be used together with a VDD
capacitor to provide a fast startup and yet low power
dissipation design solution.
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Oscillator Operation
The switching frequency of
is internally fixed at
YZ235X
50KHZ. No external frequency setting components are
required for PCB design simplification.
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Current Sensing and Leading Edge Blanking
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Operating Current
The Operating current of
is low at 2mA.
YZ235X
Cycle-by-Cycle current limiting is offered in YZ235X
current mode PWM control. The switch current is
detected by a sense resistor into the sense pin. An
internal leading edge blanking circuit chops off the
sensed voltage spike at initial internal power MOSFET
on state due to snubber diode reverse recovery and
surge gate current of internal power MOSFET so that
the external RC filtering on sense input is no longer
needed. The current limiting comparator is disabled and
cannot turn off the internal power MOSFET during the
blanking period. The PWM duty cycle is determined by
the current sense input voltage and the FB input voltage.
Good efficiency is achieved with YZ235X low operating
current together with the ‘Extended burst mode’ control
features.
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Soft Start
YZ235X features an internal 4ms soft start to soften the
electrical stress occurring in the power supply during
startup. It is activated during the power on sequence. As
soon as VDD reaches UVLO(OFF), the peak current is
gradually increased from nearly zero to the maximum
level of 0.77V. Every restart up is followed by a soft start.
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Frequency shuffling for EMI improvement
frequency Shuffling (switching frequency
. The oscillation
The
modulation) is implemented in
YZ235X
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Internal Synchronized Slope Compensation
frequency is modulated so that the tone energy is spread
out. The spread spectrum minimizes the conduction
band EMI and therefore eases the system design.
Built-in slope compensation circuit adds voltage ramp
onto the current sense input voltage for PWM
generation. This greatly improves the close loop stability
at CCM and prevents the sub-harmonic oscillation and
thus reduces the output ripple voltage.
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Extended Burst Mode Operation
At light load or zero load condition, most of the power
dissipation in a switching mode power supply is from
switching loss on the mosfet, the core loss of the
ransformer and the loss on the snubber circuit. The
magnitude of power loss is in proportion to the switching
frequency. Lower switching frequency leads to the
reduction on the power loss and thus conserves the
energy. The switching frequency is internally adjusted at
no load or light load condition.
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Drive
The internal power MOSFET in
is driven by a
YZ235X
dedicated gate driver for power switch control. Too weak
the gate drive strength results in higher conduction and
switch loss of MOSFET while too strong gate drive
results the compromise of EMI.
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