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Problem in simulating a step-up converter with LTspice IV (using LT1170)


Smuki
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Hello!

I am fairly new to DC-DC converters and LTspice and have, logically, stumbled on a problem when designing and simulating my first boost converter. The specifications for the converter are: input voltage ranging from 3-12V, output voltage 17V and output current ranging from 10mA to 1.01A. The chip used is LT1170 (fsw=100kHz).

I would first like to ask someone, if he or she could check the results for the component values I calculated:
L=1.25mH R1/R2=12.7 and for Co I chose 1000uF (the schematic is attached: Boost.asc). If the results are far off I would greatly appreciate if someone could write the right ones and I will try to figure out how to et them.

The second thing I would like to check is, weather it is OK to choose R1 and R2 so that they would also act as a shunt, to always guarantee the minimum 10mA current (for continuous mode).

Then there are the initial "bumps" in L current when running the transient analysis. The current rises to 9A in the start and is then somehow limited by something. What is the cause of this high current and how to limit that? Can anyone help me with that please?

The problem of the bumps gets even worse when output current is increased (attachment Boost2.asc) because now the output voltage and inductor current start oscillating. I have a feeling, that could be corrected with the proper values of C2 and R4. But my problem is that I do not know what this values are. I read something about how to choose them in LTC application note AN19, but the process for choosing C2 and R4 described there, doesn't work. At least not in the simulation.

Are there possibly any known issues with switching regulator IC spice models when simulating in LTspice? Is there something extra that needs to be taken into account when simulating.

My next step is to simply buy the elements and test it on protoboard, but the high currents shown in the simulation worry me because I wouldn't like to smoke my room.

I would greatly appreciate any help and would like to thank everyone in advance for their answers.
Cheers!

BoostConv.rar

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