Monday, July 30, 2012

How to get Your Electronic Load Down to Zero Volts

Typically a DC electronic load (eload) cannot go down to zero volts (they cannot sink current at zero volts) and their current handling and other performance specs begin to derate below 3 volts. This can be a big problem when you need to test low voltage power sources, like photovoltaic panels and power supplies for FPGAs. In this post we will look at why eloads begin to derate at low voltages and cannot go down to zero volts and look at simple configuration that will let you test power sources with an eload down to zero volts.

Most eloads have limited operation below three volts. To understand these limitations lets look at a simplified diagram of a typical eload in the below figure.

The FET acts like a shunt resistor across the power supply under test. As the transistor turns on harder it draws more current from the power supply under test. The power generated by the power supply is therefore dissipated in the load transistor. As long as the power supply output voltage is sufficient to bias the load transistor everything works fine. However, if the power supply voltage across Vds is low, about 3 volts or less, the load transistor can no longer regulate the current. At the point Vds minimum is reached, the load transistor is turned on to full saturation and the load it presents to the Power Supply under test is simply its saturation resistance, Rdson. As an example the below figure shows the operating curve for Agilent's N3304A Electronic Load.

Notice in the figure, below 3 volts the load can be used at reduced current but it will have poor dynamic (transient) response due to the fact that the transistor is in saturation.

A solution to the low voltage dilemma is to insert an auxiliary boost power supply in series with the electronic load and the power supply under test as shown in the below figure. The auxiliary supply "boosts" the supply under test's floating low such that it always appears to be at a voltage potential at or above the voltage potential of the auxiliary supply to the eload.

Adding the sense lines connected across the supply under test compensates for the boost supply voltage and the cabling in the measurement. Without the sense lines you would just have to subtract the boost supply voltage from the eload's voltage measurement. The boost supply can be a low-cost fixed output 3V to 5V power supply with current rating at least as high as the maximum peak load current needed. While this configuration will compensate for the load minimum voltage requirement and voltage drop in the power leads it has some disadvantages explained next.

Three factors must be considered when using the above configuration:
  1. Any voltage inaccuracies (when sense is not used) or current noise introduced by the boost supply will affect the measurement accuracy of the eload. 
  2. The eload must have a high enough power rating to dissipate the power from the supply under test and the boost supply.
  3. There is a possibility that the boost supply could reverse bias the power supply under test as the voltage across the load decreases. This can occur, for example, when the power supply under test can no longer maintain its output voltage because it is in overcurrent protection mode.
Fortunatly high performance eloads like Agilent's N3300 eload series employ detection circuits to prevent the third factor from occurring.

In this post we looked at why eloads cannot operate at full current and performance levels at low voltages. We also covered a configuration using a boost supply that allows you to overcome an eload's low voltage limitations and to test a power source all the way down to zero volts. If you have anything to add to this post use the comments section below.