Combining synchronous buck and synchronous boost converters?

Thread Starter

CameronNielsen

Joined May 8, 2017
17
Trying to get a simple mock-up of a synchronous buck+boost converter in LTSpice - I have working models of a synchronous buck and synchronous boost converter but I am quite the novice with power electronics and the union of the two is giving me some trouble. (The capacitor/inductor values are placeholders for the time being until I get a working model - for the purposes of the model I am setting the Vin to 12.6 for the first ms and to 9.6 for the second ms)

Here is the synchronous buck converter (goal is to step down 12.6 V to 5 V primarily):




Here is the synchronous boost converter (goal is to step up 9.6 V to 20 V):




I figured connection the two in series would give me what I wanted as shown below:



But alas, it was not meant to be - instead of the combination of the above waveforms I get something quite different and the analysis hits a brick wall and starts chugging hard once the voltage switches to the 9.6 volts (boost mode). I realize the circuitry isn't identical but it seems frivolous to have inductor capacitor capacitor inductor between the two circuits.



Any idea why my two functional circuits have created this demon spawn?
 

Thread Starter

CameronNielsen

Joined May 8, 2017
17
That 'Cin1' that you left out is really important in a switcher: it makes sure that the supply is not pulled down by the converter
I removed the input/output capacitor filter nearest the inductor on each converter aince neither was truly at the input/output of the overall converter. Should I add the capacitor back in before or after the inductor (or does it even matter)?
 

Thread Starter

CameronNielsen

Joined May 8, 2017
17
Added the capacitor back in and the waveform is below - less noise but the circuit just gets pinned down at low voltages. Any other ideas? I am super stuck at the moment :/

 
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