Power supply current source-to-sink crossover characteristics

A two-quadrant power supply is traditionally one that outputs unipolar voltage but is able to both source as well as sink current. For a positive polarity power source, when sourcing current it is operating in quadrant 1 as a conventional power source. When sinking current it is operating in quadrant 2 as an electronic load. Conversely, a negative polarity two-quadrant  power source operates in quadrants three and four. Further details on power supply operating quadrants are provided in a recent posting here in ‘Watt’s Up?”, “What is a bipolar (four-quadrant) power supply?” Often a number of questions come up when explaining two-quadrant power supply operation, including:

• What does it take to get the power supply operating as a voltage source to cross over from sourcing to sinking current?
• What effect does crossing over from sourcing to sinking current have on the power supply’s output?

For a two-quadrant voltage source to be able to operate in the second quadrant as an electronic load, the device it is normally powering must also be able to source current and power as well as normally draw current and power. Such an arrangement is depicted in Figure 1, where the device is normally a load, represented by a resistance, but also has a charging circuit, represented by a switch and a voltage source with current-limiting series resistance.

Figure 1: Voltage source and example load device arrangement for two-quadrant operation.

There is no particular control on a two-quadrant power supply that one has to change to get it to transition from sourcing current and power to sinking current and power from the device it is normally powering. It is simply when the source voltage is greater than the device’s voltage then the voltage source will be operating in quadrant one sourcing power and when the source voltage is less than the device’s voltage the voltage source will be operating in quadrant two as an electronic load. In figure 1, during charging the load device can source current back out of its input power terminals as long as the charger’s current-limited voltage is greater than the source voltage.

It is assumed that load device’s load and charge currents are lower than the positive and negative current limits of the voltage source so that the voltage source always remains in constant voltage (CV) operation. A step change in current is the most demanding from a transient standpoint, but as the voltage source is always in its constant voltage mode it handle the transition well as its voltage control amplifier is always in control. This is in stark contrast to a mode cross over between voltage and current where different control amplifiers need to exchange control of the power supply’s output. In this later case there can be a large transient while changing modes. See another posting, “Why Does My Power Supply Overshoot at Current Limit? Insights on Mode Crossover” for further information on this.  There is a specification given on voltage sources which quantifies the impact one should expect to see from a step change in current going from sourcing current to sinking current, which is its transient voltage response.  A transient voltage response measurement was taken on an N6781A two-quadrant DC source, stepping the load from 0.1 amps to 1.5 amps, roughly 50% of its rated output current.

Figure 2: Agilent N6781A transient voltage response measurement for 0.1A to 1.5A load step

However, the transient voltage response shown in Figure 2 was just for sourcing current. With a well-designed two-quadrant voltage source the transient voltage response should be virtually unchanged for any step change in current load, as long as it falls within the voltage source’s current range.  The transient voltage response for an N6781A was again capture in Figure 3, but now for stepping the load between -0.7A and +0.7A.

Figure 3: Agilent N6781A transient voltage response measurement for -0.7A to +0.7A load step

As can be seen in Figures 2 and 3 the voltage transient response for the N6781A remained unchanged regardless of whether the stepped load current was all positive or swung between positive and negative (sourcing and sinking).

While the transient voltage response addresses the dynamic current loading on the voltage source there is another specification that addresses the static current loading characteristic, which is the DC load regulation or load effect.  This is a very small effect on the order of 0.01% output change for many voltage sources. For example, for the N6781A the load effect in its 6 volt range is 400 microvolts for any load change. In the case of the N6781A being tested here the DC change was the same for both the 0.1 to 1.5 amp step and the -0.7 to +0.7 amp step change.

There are two more scenarios which will cause a two-quadrant power supply transition between current sourcing and sinking.  The first is very similar to above with the two-quadrant power supply operating in constant voltage (CV) mode, but instead of the DUT changing, the power supply changes its voltage level instead.  The final scenario is having the two-quadrant power supply operating in constant current with the DUT being a suitable voltage source that is able to source and sink power as well, like a battery for example. Here the two-quadrant power supply can be programmed to change from a positive current setting to a negative current setting, thus transitioning between sourcing and sinking current again, and its current regulating performance is now a consideration.  Both good topics for future postings!

Voltage Noise and You

One of the most important specifications for many of our customers on our power supplies is voltage noise.  We specify both peak to peak voltage noise and RMS voltage noise for frequencies ranging from 20 Hz to 20 MHz.  Agilent sells precision power supplies and you need to have low voltage noise in order to be a precision power supply and to make precision measurements. 

I sat down with some engineers today before sitting down to type up my blog post.  I wanted to find out some of the main causes for voltage noise in our supplies so that I can share it with you Watt's Up readers.  Please note that each of these reasons could be a blog post of its own but for brevity's sake I am going to come at this for a high level.  Since our newest supplies have the high power density, they are mostly switch mode power supplies.   The top cause of noise in switch mode power supplies is the frequency of the switching inverters.  This is inherent to the design of the supply.  The second most common cause of noise is common mode current noise that turns into normal mode voltage noise.  We have filtering and common mode chokes inside the supplies but some still gets through and comes out the output as normal mode voltage.  The third most likely cause is noise from operational amplifiers that makes its way to the outputs.  After talking to our designer I came out with a new respect for the challenges they face when having to deal with this!

 

I recently did a video on how to make this measurement:

I wanted to use the Watt’s Up blog to add some more information.

The general schematic for noise testing is:

One thing that I wanted to do in the video but was not able to for brevity’s sake was talk about the differential amplifier.  The differential amplifier is important for two key reasons.  The first reason is that the differential amplifier will greatly reduce the common mode noise on the output.  This is important because you want to measure the noise between the outputs.  The other benefit of the differential amplifier is that it multiplies the output by 10.  This multiplication enables us to use more accurate ranges in both the scope and the RMS voltmeter which is very important when you want to keep the measurement uncertainties of your measurements down.

When you do the noise test, typically you do it at the full power point.  To get the full current, you can either use an electronic load or a fixed resistor.  Typically on a lower noise supply (such as the N6761A) you want to use a fixed resistor since the electronic load will introduce its own noise to the measurement.  This is not really an issue for power supplies with higher noise because their output noise is higher than the noise from the load.  In all honesty, it also tends to be difficult to find 500 W resistors so the load is a great help in those instances too!

The last measurement item that I want to talk about is the scope setup.  The scope should be AC coupled and have the filter turned on (since we only specify noise to 20 MHz, we want to filter anything above this out). We recommend that you put the scope in peak detect mode and turn the statistics so that you can see the highest and the lowest voltages recorded.    This will give you the true variation in the power supply output.  Your peak to peak voltage noise will be this difference divided by the gain of the diff amp.  After taking the peak to peak measurement, I typically move the connector over to the RMS voltmeter to take that measurement.  There are no special settings for the RMS voltmeter, you just need to remember to divide by the gain of the differential amplifier.

That is a really quick overview of voltage noise.  Please feel free to leave any questions in the comments section of this blog.