Monday, December 10, 2012

More on power supply current source-to-sink crossover characteristics


On my earlier posting “Power supply current source-to-sink crossover characteristics” I showed what the effects on the output voltage of a unipolar two-quadrant-power supply were, resulting from the output current on the power supply transitioning between sourcing and sinking. In that example scenario, the power supply was maintaining a constant output voltage and the transitioning between sourcing and sinking current was dictated by the external device connected to and being powered by the power supply. This is perhaps the most common scenario one will encounter that will drive the power supply between sourcing current and sinking current.

Other scenarios do exist that will drive a unipolar two-quadrant power supply to transition between sourcing and sinking output current. One scenario is nearly identical to the earlier posting. However, instead of the device transitioning its voltage between being less and greater than the power supply powering it, the power supply instead transitions its voltage between being less and greater than the active device being normally powered.  A set up for evaluating this scenario on an Agilent N6781A two-quadrant DC source is depicted in Figure 1.



Figure 1: Evaluating current source-to-sink crossover on an N6781A operating in constant voltage

In this scenario having the DC source operating as a voltage source and transitioning between 1.5 and 4.5 volts causes the current to transition between -0.75 and +0.75A.  The voltage and current waveforms captured on an oscilloscope are shown in Figure 2.



Figure 2: Voltage and current waveforms for the set up in Figure 1

The waveforms in Figure 2 are as what should be expected. The actual transition points are where the current waveform passes through zero on the rising and falling edge. An expanded view to the current source-to-sink transition is shown in Figure 3.



Figure 3: Expanded voltage and current waveforms for the set up in Figure 1

As can be seen the voltage ramp transitions smoothly at the threshold point, or zero crossing point, of the current waveform. The reason being is that the DC is maintaining its operation as a voltage source. Its voltage feedback loop is always in control.


Yet one more scenario that will drive a unipolar two-quadrant source to transition between sourcing and sinking current is operate it as a current source and program is current setting between positive and negative values. In this case the device under test that was used is a voltage source.  One real-world example is cycling a rechargeable battery by alternately applying charging and discharging currents to it. The set up for evaluating this scenario, again using an N6781A two-quadrant DC source is depicted in Figure 4.



Figure 4: Evaluating current source-to-sink crossover on an N6781A operating in constant current

For Figure 4 the N6781A was set to operate in constant current and programmed to alternately transition between -0.75A and +0.75A current settings. The resulting voltage and current waveforms are shown in Figure 5.



Figure 5: Voltage and current waveforms for the set up in Figure 4

The waveforms in Figure 5 are as what should be expected. The actual transition points are where the current waveform passes through zero on the rising and falling edge. An expanded view to the current source-to-sink transition is shown in Figure 6.



Figure 6: Expanded voltage and current waveforms for the set up in Figure 4

As the N6781A is operating in current priority the interest is in how well it controls its current while transitioning through the zero-crossing point. As observed in Figure 6 it transitions smoothly through the zero-crossing point. The voltage performance is determined by the DUT, not the N6781A, as the N6781A is operating in constant current.

So what was found here is, for a unipolar two-quadrant DC source, transitioning between sourcing and sinking current should generally be virtually seamless as, under normal circumstances, should remain in either constant voltage or constant current during the entire transition.

2 comments:

  1. While testing this unit what is the length and gauge of the cable used between two units in the figure?

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  2. The cable used was approximately 3 feet of 12 AWG.

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