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LTSpice Tutorial – Car USB power supply

This tutorial goes along with an upcoming Circuit Cellar article of mine on construction of a 5V / 2A USB power supply I built for my car. Why not simply purchase a cheap cigarette-lighter USB power supply? Well, two reasons. First, I needed an isolated power supply, something to keep the car’s power system hum and buzz from the audio when I used the line-in on the car’s radio. Second, it’s always more fun to build something yourself and learn something in the process. So go read the CC article!

For this tutorial, I wanted to outline some of the LTSpice usage in more detail than was possible to cover in the Circuit Cellar article. There are other LTSpice tutorials out there, like this one, but I wanted to do something a bit more specific for someone trying to follow along my 5V/2A isolated switching power supply design. The purpose here isn’t to walk you through the details of the specific switching power supply design I did (that’s more the Circuit Cellar article); here I more wanted to present an overview of how to use LTSpice for switching design so you can design your own circuit.

So here goes:
Step 1: running a working model and viewing the output
Step 2: modifying the model
Step 3: optimizing the model

Loading an LTSpice model for the first time pulls up the graphical view of the circuit design.

LTSpice Tutorial – running a model and viewing output

Let’s get LTSpice up and running with a working model, run a simulation and view the output. First, download the LTSpice application Second, from the LT3748 product page, download the LT3748 Demo Circuit – Automotive Isolated Flyback Controller. Third, Run LTSpice and open the LT3748_TA02.asc file. You should now have a window that looks like …

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LTSpice, changing a component's properties

LTSpice Tutorial – modifying the model

Now that we’ve got a working model, let’s begin making changes. First, let’s look at changing component values for resistors / capacitors / etc. Easy enough, just right mouse the component and you’ll get the properties window. Here you can manually type specific parameters such as the capacitor value and ESR or you can select …

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LTSpice resistor value variable

LTSpice Tutorial – Optimizing the model

So now the model generally works. But… for example I had problems with the ‘float’ that occurs when the circuit is unloaded. In the actual circuit I’d see numbers over 10VDC when the circuit was completely unloaded and there was no Zener diode was in place to limit the voltage. (Yes, read the application note …

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LTSpice Tutorial – Followup #1

Just spend a bunch of time trying to optimize the circuit around the ‘float’ when the circuit is unloaded, thought I would document some of those findings here…. The value of Rsense (R7 in the model) has a huge impact on the level of the unloaded float. Now I will say that the model is …

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