MP150 – OFFLINE PRIMARY-SIDE REGULATOR
MP150 Rev. 1.15 www.MonolithicPower.com 14
9/25/2018 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
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Output Capacitor
The output capacitor maintains the DC output
voltage. Estimate the output voltage ripple as:
CCM _ ripple ESR
so
i
ViR
8f C
for CCM
2
pk o
o
DCM _ ripple pk ESR
so pk
II
I
VIR
fC I
for DCM
Use ceramic, tantalum, or low-ESR electrolytic
capacitors to lower the output voltage ripple.
Feedback Resistors
The resistor divider determines the output
voltage. Choose appropriate R1 and R2 values to
maintain the FB voltage at 2.5V. Avoid very large
values for R2 (typical values between 5k
to10k.
Feedback Capacitor
The feedback capacitor provides a sample-and-
hold function. Small capacitors result in poor
regulation at light load condition, and large
capacitors can impact circuit operation. Estimate
the capacitor range as per the following equation:
oo oo
FB
12o 12o
VC VC
1
C
2R R I R R I
Choose an appropriate value given practical
considerations.
Dummy Load
A dummy load maintains the load regulation. This
ensures sufficient inductor energy to charge the
sample-and-hold capacitor to detect the output
voltage. Start with a 3mA dummy load and adjust
as necessary.
Surge Performance
To obtain a good surge performance, select an
appropriate input capacitor that meets different
surge tests. Figure 10 shows the half-wave
rectifier. Table 2 shows the required capacitance
under normal conditions for different surge
voltages.
Figure 10: Half-Wave Rectifier
Table 2: Recommended Capacitor Values
Surge
voltage
500V 1000V 2000V
C1
1F 10F 22F
C2
1F 4.7F 10F
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