Bi-directional Buck+Boost Converter MOSFET Driver Issues

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CameronNielsen

Joined May 8, 2017
17
Hello,

As part of a course I am taking I am tasked with designing a Bi-directional DC-DC converter which takes supplies energy from a battery at ~11 volts and delivers energy at 5 V (2 Amps), 12 V (3 A), and allow for charge of the battery at 20 V (3 A). The basic layout is follows but currently I am only tasked with developing the converter portion.



For the purposes of the project we are allowed two control signals and are designing within LTSpice - We are provided an example of a boost converter (which cannot satisfy the requirements of the project due to its inability to reduce voltage from input to output) that I have copied below for reference.



The only unusual components here are the blocks U5 and U3 - U5 essentially just an unregulated DC-DC converter which acts as a power supply for high side MOSFETs and U3 is a dead time circuit for generating complimentary signals from the PWM.

My approach to this problem was to create a synchronous buck+boost converter to address the project requirements - I started using ideal switches for the buck and boost stages individually - before moving to a combined circuit resembling a two-switch implementation of the buck+boost topology - before finally attempting to implement a fully synchronous buck+boost converter. I was able to achieve the buck and boost outputs (5 V and 20 V respectively) in the two switch implementation but transitioning to a synchronous converter everything has gone awry.

My current circuit is below along with the parameters in/out/control1/control2:





And the Vout waveform is shown below:



I think the "bones" of the converter are correct in terms of component location and control signal operation - but the gate drivers are not properly implemented and I am at a loss for how to correct them. Any help would be greatly appreciated!

(btw we are not allowed to use independent/dependent sources, ideal switches/transformers, switching controllers, or any other IC/chips and eventually the inductor I used will need to be replaced with a saturating inductor model similar to the boost example - I have attached the project description below for reference along with my current LTSpice schematic)
 

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Thread Starter

CameronNielsen

Joined May 8, 2017
17
Slight update - I got asynchronous buck and a synchronous boost converter to work with the specified inputs but the combination is still non-functional.

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?
 
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