Saturday, December 19, 2015

Diode state determination

Diodes and switches are high value resistances when off and low value resistances when on. The way of detecting when the turn on is to measure the voltage across them at every simulation cycle and change their status when the forward voltage exceeds 1 Volt. For a device in off state this might mean a current of 0.5 microAmps.

When running the simulation of a three-phase inverter, I found that the diodes were trying to turn on at every simulation instant even though the switch across them was conduction. On the contrary, the diodes that were across switches that were on and conducting was significantly higher and therefore were turning on.

To describe this look at this switch/diode combination when the switch is on.

The switch is a part of a loop with a current i1. The diode is off and forms a loop with the switch. By mere observation, it is clear that because of its association with i1, i2 will be large enough for the diode to turn on. There is absolutely nothing wrong with the loop, but the result is blatantly wrong.

This leads me to think about why do I need to use loop analysis to determine the current though stiff elements? The loop i2 could be written in several different ways. If it is written with respect to the dc bus voltage, it would result in a reverse current that would keep the diode off.

The same goes for any stiff element. By making an element stiff, it ensures that it will not disrupt the currents in the circuit. So the effect of the diode can be removed by making it a high resistance. But if the current through the diode matters and in this case it does, loop analysis is not an accurate way to determine the current through the diode.

Since nodal analysis is available and is being used to determine freewheeling, why not use it to determine the current through all stiff elements after the loop analysis ODE is solved.

Here is the code:

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