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Queation about Boost-up DC/DC Converter

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coolsw2003

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Hi, everybody,

I am designing a step-up DC-DC converter which can
converter 1.5V to 2.5V.

First, here is my simulation netlist.

L1 vdd n1 50u
Diode n1 vout power_diode
Cl vout gnd 220u
m1 n1 in gnd gnd ne w=350u l=0.25u
R1 vout x 20K
R2 x gnd 20K
rload vout gnd 33

Without the load resistor "rload", vout increase continuously more
then 5V when a pulse forsed to signal "in".
However, when the load resistor is connected the output vout stays
around 800mV and vout tends to decrease at very slow rate.

Can anybody help me what wrong with my simulation?


Thanks,
coolsw2003
 

goodboy_pl

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if u sim. only the netlist mentioned with a pulse voltage source and without feedback, this spike and even oscilation of output voltage will take place!

this is due to BAD start up; u must use soft start (small duty cycle st start up that increases slowly with time) if not u will see a large spike/osc. at output node voltage and inductor current.

in practice soft start is used is control ckt. is voltage mode OR current mode control which controls inductor current is used for preventing this conditions.

for simulation u can use a small soft ckt. in open loop or use a complete current mode ckt.

BEST!
 

goodboy_pl

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For boost Regulators also there is another problem, inrush current during power up; since there is no switch between input voltage and output node, during power up a sin current will pass through the input inductor and after that a voltage of two times of input voltage will be produced in output node.
for solving it usually a diode is connected parallel to the inductor to remove this resonance effect.
this method is used in low power regulators, for high power some methods is necessary to limit the diode inrush current like ptc @ input.

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