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Showing posts with label low voltage load. Show all posts
Showing posts with label low voltage load. Show all posts

Friday, August 14, 2015

Not all two-quadrant power supplies are the same when operating near or at zero volts!

Occasionally when working with customers on power supply applications that require sourcing and sinking current which can be addressed with the proper choice of a two-quadrant power supply, I am told “we need a four-quadrant power supply to do this!” I ask why and it is explained to me that they want to sink current down near or at zero volts and it requires 4-quadrant operation to work. The reasoning why is the case is illustrated in Figure 1.

Figure 1: Power supply sinking current while regulating near or at zero volts at the DUT

As can be seen in the diagram, in practical applications when regulating a voltage at the DUT when sinking current, the voltage at the power supply’s output terminals will be lower than the voltage at the DUT, due to voltage drops in the wiring and connections. Often this means the power supply’s output voltage at its terminals will be negative in order to regulate the voltage at the DUT near or at zero volts.

Hence a four-quadrant power supply is required, right? Well, not necessarily. It all depends on the choice of the two-quadrant power supply as they’re not all the same! Some two-quadrant power supplies will regulate right down to zero volts even when sinking current, while others will not. This can be ascertained from reviewing their output characteristics.

Our N6781A, N6782A, N6785A and N6786A are examples of some of our two-quadrant power supplies that will regulate down to zero volts even when sinking current.  This is reflected in the graph of their output characteristics, shown in Figure 2.

Figure 2: Keysight N6781A, N6782A, N6785A and N6786A 2-quadrant output characteristics

What can be seen in Figure 2 is that these two-quadrant power supplies can source and sink their full output current rating, even along the horizontal zero volt axis of their V-I output characteristic plots. The reason why they are able to do this is because internally they do incorporate a negative voltage power rail that allows them to regulate at zero volts even when sinking current. While you cannot program a negative output voltage on them, making them two-quadrants instead of four, they are actually able to drive their output terminals negative by a small amount, if necessary. This will allow them to compensate for remote sense voltage drop in the wiring, in order to maintain zero volts at the DUT while sinking current. This also makes for a more complicated and more expensive design.

Our N6900A and N7900A series advanced power sources (APS) also have two-quadrant outputs. Their output characteristic is shown in Figure 3.

Figure 3: Keysight N6900A and N7900A series 2-quadrant output characteristics

Here, in comparison, a certain amount of minimum positive voltage is required when sinking current. It can be seen this minimum positive voltage is proportional to the amount of sink current as indicated by the sloping line that starts a small maximum voltage when at maximum sink current and tapers to zero volts at zero sink current.  Basically these series of 2-quadrant power supplies are not able to regulate down to zero volts when sinking current. The reason why is because they do not have an internal negative power voltage rail that is needed for regulating at zero volts when sinking current.

So when needing to source and sink current and power near or at zero volts do not immediately assume a 4-quadrant power supply is required. Depending on the design of a 2-quadrant power supply, it may meet the requirements, as not all 2-quadrant power supplies are the same! One way to tell is to look at its output characteristics.

Thursday, November 8, 2012

Configuring an Electronic Load for Zero Volt Operation

DC electronic loads are indispensable for testing a variety of DC power sources. There are a number of situations that call for testing DC power sources at low voltage, even right down to zero volts. Often this is also at relatively high current of many tens of amps or greater. Some examples include:
• Low output voltage power supplies and DC/DC converters (mostly for digital circuit power)
• Solar cell I-V testing, down to zero volts
• Fuel cell testing
• Single cell battery testing
• Power supply true output short-circuit testing, down to zero volts

It becomes challenging for the test engineer to find adequate DC electronic loads for low voltage operation, especially at high currents. Many DC electronic loads, including the ones Agilent Technologies provides, use multiple power FETs for their input loading element. While a power FET can actually operate down to zero volts, this is at zero current as well. At high current a few volts is typically needed for stable, dynamic operation at full current.  As one example Figure 1 depicts the input I-V characteristics of an Agilent N3304A DC electronic load.

Figure 1: Agilent N3304A DC electronic load input I-V characteristics

An effective solution for low voltage electronic load operation, right down to zero volts, for the electronic load’s full rated current, is to connect a low voltage boost power supply in series with the electronic load’s input. An example of this set up is depicted in Figure 2.

Figure 2: Zero volt DC electronic load set up

The electronic load now sees the sum of the boost supply’s and DUT’s voltages. Selecting a boost supply having adequate voltage will assure the electronic load will be able to operate at full performance at full current, even when the voltage at the DUT is zero. There are a few things that need to be paid attention to:
• The electronic load needs to dissipate the total power of both the boost supply and DUT
• The DUT needs to be adequately safeguarded against reverse polarity if the electronic load is inadvertently turned on too hard
• The electronic load’s voltage sensing must be able to accommodate the extra voltage difference between the electronic load and DUT, due to the boost supply voltage
• The boost supply ripple and noise (PARD) can contribute to noise measurements made on the DUT

Due to these considerations not all electronic loads may be well suited for zero volt operation with a boost supply so it is necessary to validate if a particular electronic load under consideration can be applied in this manner first.  Further details about zero voltage load operation as well as using Agilent N3300 series DC electronic loads for this purpose are described in product note “Agilent Zero Volt Electronic Load”, publication number 5968-6360E. Click here to access