High current Step-down converter efficiency problem

ronsimpson

Joined Oct 7, 2019
2,005
We are off on why simulation does not work. A simple model for an inductor does not include resistance of the wire, capacitance between the layers of wire and most importantly does not include saturation. The Cap in the circuit in post #1 needs to have internal series resistance and inductance. People often use "Perfect" inductors and capacitors and voltage sources and more, then do not know why real parts are not like that. In SPICE putting 200V on a 100V transistor work just fine. I am working with 300mhz transistors that handle 100A. I model the trace inductance in the PCB and in some cases also the trace to ground plane capacitance. SPICE has problems, but most of the time it is because we do not understand the entire circuit. There is a lot more to a circuit then the schematic.
I am using a 50 watt resistor that has 12nH of inductance and 35pF of capacitance that are not included in the schematic. At the speed I am running that makes a difference.
 

MisterBill2

Joined Jan 23, 2018
11,590
My assertion is that a user who does not know or understand the limitations of their simulation software will have problems if they trust the results without question. The fact is that components seldom burn up in simulation, nor do they fail from excess voltage application. And in simulations the "ground" or common bus wire, has zero resistance and never any current loop problems.
 

crutschow

Joined Mar 14, 2008
29,494
I agree with all the limitations of simulators (and there's likely a few more that haven't been mentioned).
But I don't think that's sufficient reason to discourage newbies (or anyone) from using simulators.
If they have problems with the simulations they can come here for help, just as they do when a real circuit doesn't work (or not work the way the simulator shows).
 

ronsimpson

Joined Oct 7, 2019
2,005
One reason for simulation is math or not doing the math. Some time I have a hard time choosing the right parts. Example; I need a complicated filter but the math hurts my head. I can choose C1 = 100pF to 1000pF in 10% steps. (and C1 = C2 = C4*2) Then run the simulation and see which value gets closest to what I want.
Example 2, all the resistors are 5% and the caps are 10%, what happens in all combinations of +5% and -5% of all the parts.
 

Papabravo

Joined Feb 24, 2006
18,097
When was the last time you did a Bode plot using a slide rule for the calculations? It does have everything you need for the calculations, but it is slower than molasses in January in Nome(64°30' N). For my Russian friends I could have said Murmansk(68°02' N), which is further North. I'm sure there is a colder place in Siberia, but I probably never heard of it.
 

MisterBill2

Joined Jan 23, 2018
11,590
Once again, the problems are for those who are not adequately aware of the limitations. and certainly over-simplification and leaving out variables will lead to results that can bring problems. An analogy is the brakes on a car. One must be aware of their limitations, and of the limitations of tire traction. Ignoring those limitations will eventually lead to problems.
 
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