# Is it appropriate to model a DC/DC converter as a variable resistance if I have input voltage and current datapoints?

So I am designing a circuit in MATLAB simulink which is basically a rectifier followed by a DC/DC converter.

Both simulations are working perfectly individually. However, when I attach the DC/DC converter to the rectifier the simulation becomes excruciatingly slow. It's impossible for me to analyze it.

Hence I am considering if it would be appropriate to model the DC/DC converter in a simpler way by using the current and voltage data points available from simulation of the individual DC/DC converter. Right now I am considering a variable resistor model, by dividing the input voltage by input current of the DC/DC converter at each time instant.

Will this be mathematically and electrically accurate or am I missing something?

• Have you fitted a smoothing capacitor on the output of the rectifier? Mar 1, 2022 at 16:30
• @Andy aka yes there is a capacitor at the output of rectifier. Mar 1, 2022 at 16:30
• I think you need to show your circuit. Mar 1, 2022 at 16:34
• You'll probably need to help the solver, it sounds like convergence problems. Without seeing any schematic, it's impossible to say what/who/where, but you could try to add some small capacitances across the rectifier diodes (series RC snubbers), or across the nodes that have the sharpest transitions. Mar 1, 2022 at 17:07
• The easiest way is to model the dc-dc converter with a constant power load which is a current source whose value is $i(t)=\frac{P_{out}}{v_{in}(t)}$. This mimics a converter featuring a negative incremental resistance and an infinite input voltage rejection. $v_{in}(t)$ represents the voltage across the current source. Mar 1, 2022 at 18:55

The easiest way to model a regulating dc-dc converter is to use a constant power source. It can be modeled as a voltage-controlled current source. The instantaneous current depends on the power absorbed by the dc-dc and the voltage across the current source terminals: $$\i(t)=\frac{P_{out}}{v_{in}(t)}\$$ in which $$\v_{in}(t)\$$ represents the instantaneous voltage controlling the current and $$\P_{out}\$$ the power delivered by the converter. This offers the advantage of modeling a negative incremental resistance and provides a good way to check the interaction with a front-end EMI filter.

The below drawing shows a typical application in a front-end rectifier and lets you assess the ripple amplitude when the bulk capacitor is loaded by the constant-power source:

You can also use a voltage-dependent resistor called a PWL resistance in SIMPLIS. Using an Excel sheet, you compute the resistance value based on the applied voltage and considering a constant power:

The divide-by-zero case when the bulk capacitor is discharged should often be considered by adding a .IC defining an initial charge.

• could you suggest a resource I could read about this from in detail? The modelling as a constant power source looks interesting. Mar 5, 2022 at 17:40
• I am not sure there are many details in the literature. I remember discussing it in my APEC 2017 seminar you can download from my site but I use it to test EMI stability which is also an important topic. Mar 5, 2022 at 17:47
• My DC/DC converter also needs to charge a battery with CC-CV control...I don't think it would be possible using this model. I made a detailed post about my exact issue today: electronics.stackexchange.com/questions/610916/… . Could you take a look? Mar 5, 2022 at 17:51

It depends on what you are trying to model. But a switch-mode DC/DC converter doesn't really look much like a variable resistor.

If it's stepping down the voltage, then it can deliver more current on the output than is drawn from the input. No resistor network can do that.