Modeling voltage regulator loss in Multisim

[mdknapp]

Hi.. I'm trying to model voltage regulator efficiency loss in Multisim.. I have a system with cascaded switching regulators and loads.

I have VR efficiency curves for all the regulators..  (ie spreadsheet of efficiency at different loads measured from lab data)

I'm trying to get overall system power using static loads.. I originally thought I could use ABM current sources inside of a sub circuit, using if statements (if output current >1 A, add 10% to input, if output current >2A, add 8% to input, etc.. but it looks like there is a problem with using ABM sources inside a sub circuit (according to some other threads)... I really need to be able to easily copy the regulators and make changes to the efficiency curves so just putting all of this in the main circuit seems prone to errors.

Does anyone have a good idea for how to do this? I've been using excel in the past but the spreadsheet has gotten really complicated.

Thanks

Mike

[yyao]

Hi Mike,

You are right, ABM sources and other SPICE mechanisms which rely on absolute node names don't work well in Subcircuits or Hierarchical Blocks. However, you can create a stepwise load response by using cascading a bunch of switches and current sources. Attached are two files. vreg.ms10 is a 12V source that has 10% efficiency at 1A and 18% efficiency at 2A. testbench.ms10 is a file which demonstrates the performance characteristics of the voltage regulator. Open the testbench and click  Simulate>>Analysis>>Transient Analysis. The Multisim grapher will open and plot the input current and the load current and you should see the desired efficiency degradation. If you want to model more complex behaviour, you can add other nonlinear sources and components to the Hierarchical Block model the behaviour of your regulator.

There's no limit to what you can model with a bit of creativity in Multisim. Hope that helps.

[mdknapp]

Yi,

Thanks for your help. Do you know if there is a way to import data from a text file.. ie

VR Load     VR Efficiency

1A               70%

2A               75%

3A               80%

etc? It seels like it would be extremely time consuming to dial that circuit in for each VR on the platform.

Thanks!

Mike

[yyao]

There isn't a way to do this that I can think of, but if you find a solution, please let us know!

[GarretF]

Hi Mike,

You are correct that ABM sources do not work well in subcircuits.  As Yi said, this is because the ABM source relies on absolute node names.

As an alternative, you can use the nonlinear dependent source (Sources > CONTROL_FUNCTION_BLOCKS > NONLINEAR_DEPENDENT). This source is essentially an ABM source wrapped up in a component (a .subckt) so it will work with hierarchical blocks/subcircuits.