MP4032-1—PRIMARY-SIDE-CONTROLLED, OFFLINE LED DRIVER WITH FULLY-INTEGRATED MOSFET
MP4032-1 Rev. 1.02 www.MonolithicPower.com 14
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APPLICATION INFORMATION
Components Selection
(Please refer to Application Note AN055 for detailed
design)
Input EMI Filter
Select EMI component values to pass EMI test
standards, as well as to account for the power
factor and inrush current when dimming turns on.
The input capacitance plays a primary role: a
small input capacitance can increase the power
factor and decrease the inrush current, so select
a relatively small X capacitor.
Input Bridge
The input bridge can use standard, slow-recovery,
low-cost diodes. When selecting diodes, take into
account these three items: the maximum input
RMS current; the maximum-input-line voltage;
and thermal performance.
Input Capacitor
The input capacitor mainly provides the
transformer’s switching frequency magnetizing
current. The maximum current occurs at the peak
of the input voltage. Limit the capacitor’s
maximum high-frequency voltage ripple to 10%,
or the voltage ripple may cause more primary-
peak-current spikes and degrade both the power
loss and the EMI performance.
−
>
⋅π⋅ ⋅ ⋅
pk _ max pri _ rms _ max
in
s _ min ac _ min
I2I
C
2f V 0.1
Input capacitor selection requires taking into
account the EMI filter, the power factor, and the
surge current at the dimming turn-on time. A
large capacitor improves EMI, but limits the
power factor and increases the inrush current.
Passive Bleeder and Active Damper
Since the LED lamp impedance is relatively large,
significant ringing occurs when the leading-edge
TRIAC dimmer turns on due to an inrush current
charging the input capacitance. The ringing may
cause the TRIAC current to fall below the holding
current and turn off the TRIAC dimmer, which
can cause flickering.
The typical application circuit incorporates both a
passive bleeder and an active damping circuit to
address this issue. The design details can be
found in the corresponding design tools.
Transformer
After accounting for the ratings of the primary
MOSFET and the secondary rectifier diode,
some applications allow for a range of turn ratios
N to be selected, which then requires the
following considerations: a small N leads to a
poor THD; a large N leads to a larger primary
inductance and a physically larger transformer.
Usually, the system will define a minimum
frequency f
s_min
at the peak of V
ac_min
. So setting
the value of f
s_min
can get the primary inductance
L
P
. The design details can also be found in the
design tools.
RCD Snubber
The peak voltage across the MOSFET at turn-off
includes the instantaneous input-line voltage, the
voltage reflected from the secondary side, and
the voltage spike due to the leakage inductance.
The RCD snubber protects the MOSFET from
over-voltage damage by absorbing the leakage
inductance energy and clamping the drain
voltage. The design details can be found in the
corresponding design tools.
Secondary Rectifier Diode
Choose a diode with an appropriate reverse-
voltage rating and current rating. The reverse
recovery of the freewheeling diode can affect the
efficiency and circuit operation, and a Schottky or
ultra-fast diode is recommended.
Output Capacitor
The output voltage ripple has two components:
the switching-frequency ripple associated with
the flyback converter, and the low-frequency
ripple associated with the input-line voltage
(120Hz). Selecting the output bulk capacitor
depends on the LED current ripple, the allowable
overvoltage and the desired voltage ripple.
Meanwhile, a pre-load resistor is necessary to
discharge the output voltage under open-load
conditions.
