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How to do AC simulation for constant on time buck converter in Simplis

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XKEPHY

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Hi guys,

I am trying to do the stability analysis for a V2 control constant on time buck converter. The attachment is the circuit I used. The only difference here is I don't have an Error Amp, instead, I connect Vref directly to the comparator.

I do the ordinary AC simulation, the result is kind of weird. There is double pole on LC resonance freq, which should not be the case for constant on time control. Also DC gain increases with inductor value, which doesn't make any sense.

I wonder ordinary AC simulation in Simplis is suitable for this kind of pseudo fixed freq circuit? Should I use Multi-Tone AC Analysis as they introduce on their website? Haven't try that since I don't have the DVM license.

BTW, is it convenient to do stability analysis in Spetre using PSS and PSTB? Looks like most people prefer Simplis.

Thanks
Screen Shot 2018-01-17 at 9.54.16 PM.png
 
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There are situations where current flows backwards through the coil, as son as forward current has dropoed to zero. It happens because the capacitor wants to discharge through the closed switch to ground. This is likely to occur when you have lengthy Off times.

As a test, try substituting a diode where you have the switch which is close to ground. (The diode should point upward.) This changes your converter to asynchronous operation. It should make your life easier even though it may not be as efficient as synchronous operation.
 

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