Mismatch Between Diode Current Flag and Branch Current Reading in PSIM

NikosDimitrakopoulos
NikosDimitrakopoulos
Altair Employee
edited October 21 in Altair HyperWorks

Mismatch Between Diode Current Flag and Branch Current Reading

 

Sometimes, when simple things don't work as expected, it can be unsettling. This was my experience when initially testing the following simple circuit:

  1. Step source

  2. Resistor

  3. Ideal diode

 

image

The step source output is:

image

 

We have a voltage transitioning from 0V to 50V at 5ms. Given this and the circuit configuration above, we should expect that the ideal diode model won't conduct current at any point in time.

Indeed, by enabling the diode's “current flag”, we observe a zero amp current waveform:

image

image

 

We’d expect the current meter in series with the diode to show the same reading, assuming no parallel paths. However, PSIM results beg to differ:

image

 

Is PSIM broken? Is the current meter or the diode model at fault?

Actually, everything is fine! We just discovered that an ideal switch is modeled with a very small resistance (R_switch_on, default 10µΩ) when on, and a very large resistance (R_switch_off, default 10MΩ) when off. These values can be changed in Simulation Control:

 

image

For example, an ideal diode is modeled as:

image

 

Please note that R_switch_off isn’t always added. If there’s already an RLC branch across it, R_switch_off won’t be added.

Historically, the displayed switch current only shows the current through the switch and R_switch_on, not including the current through R_switch_off. For example, the displayed current is onlyI(D1), not i(sw) from the image above.

 

Changing this behavior to include the parallel resistor current in the switch current could confuse things more, as the display would show a small negative current when the switch is off.

 

Thus, the above phenomenon is a feature, not a bug!

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