2018

March 2018

# Power supply current source-to-sink crossover characteristics

Posted by StevenLee Mar 29, 2018

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. 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. 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: Keysight 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: Keysight 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!

# Simplify Your Life with Autoranging Power Supplies

Posted by titbin Mar 23, 2018

Don’t you hate it when your power supply can’t provide enough current, even though you are pulling power well within the power supply’s maximum power output? You have to disconnect all your cables from your power supply, which you have meticulously connected, find another supply with enough current output, and reconnect everything again. It’s very frustrating, especially when you have a deadline looming. I share your pain. I have been through it. That’s why I’m sharing a trick I learned to overcome this frustration.

We’re going to look at how an autoranging power supply helps alleviate the pain and gives you more capability. An autoranging power supply is also fondly called an autoranger.

Single-range and Multi-range Power Supplies

Often a basic power supply is a single-range power supply. I’ve plotted a single-range power supply’s output characteristic in Figure 1 below. Pmax is the power supply’s maximum output power. This power supply only outputs Pmax at maximum rated voltage, Vmax, and current, Imax. This single-range power supply has a single range for both output voltage and current.

Figure 1. Single-range DC power supply output characteristic.

In a multi-range power supply, we have wider output voltage and current ranges, as shown in Figure 2 below. This multi-range power supply is also called a dual-range power supply since it has only two ranges for voltage and current output.

Figure 2. Multi-range DC power supply output characteristic.

This dual-range power supply is able to output a much higher Vmax or Imax within the same maximum-rated power output as our single-range power supply. However, the dual-range power supply can only supply Vmax when the output current range is limited to I1. Imax can only be reached if voltage range is limited to V1. Both voltage and current outputs have two operating ranges within the same Pmax power envelope. For most power supplies, you will need to manually select the correct range.

The Wonderful Autoranger

A multi-range power supply has an infinite number of ranges. Even better, it automatically selects the correct range. This type of multi-range power supply is known as an autoranging power supply. The output characteristic of an autoranging power supply is shown in Figure 3.

Figure 3. Autoranging DC power supply output characteristic.

With an autoranger, the voltage and current output is automatically limited to ensure the power output does not exceed its rated maximum power output.

Let’s use the N6755A as an example. The N6755A is a 500W autoranging DC power supply with Vmax = 20V and Imax = 50A. If you set the output voltage to 15V, the output current is automatically limited to 33A, and if you reduce the output voltage to 10V, the current output is limited to 50A.

The same happens for current. The 500W N6755A has a voltage and current range that equals a 1000W single-range power supply. An autoranger has significantly more output voltage and current range combinations compared to a multi-range power supply.

Figure 4. The high-performance N6700 family autoranging DC power supply.

Why Do I Need an Autoranger?

• You Need High Voltage and High Current, But Not High Power

Autoranger is not for everyone. But if you need high voltage and current, but not high power, an autoranger is perfect for you. DC/DC converter testing is a perfect example of this need. A DC/DC converter accepts a wide range of input voltage and is able to output a constant amount of power. During testing, an autoranger is able to supply a wide range of voltages to the DC/DC converter while still providing enough power. Figure 4 below illustrates this point. As input voltage decreases, the DC/DC converter pulls more current to maintain its output power. The autoranger is able to decrease its output voltage and increase available current to the DC/DC converter.

Figure 5. DC/DC converter voltage and current draw from an autoranging DC power supply.

• You Need Flexibility in Your Testers

An autoranger gives you flexibility. Your test station is often set up to test a wide range of product families. Your test station has a wide range of voltage and current needs. Imagine stuffing your test rack with multiple power supplies and the complexity of connecting them together. An autoranging power supply saves you space and keeps your setup simple.

• You are Protecting Mother Nature

An autoranger is more efficient. To cover wide voltage and current demands, you can simply get a high-power single-range power supply that covers the entire voltage and current range you need. While this solution can work, it is not energy efficient. Generally, a power supply’s efficiency reduces as its output power reduces. Therefore, using a high-power single-range power supply at half its rated maximum power output is not only a waste of money, it is also a waste of precious energy resources. Always get an autoranging power supply with just enough power for your application and you will save money and help the environment.

Using a high-power single-range power supply to provide low power is not only a waste of money, it is a waste of precious energy resources.

Conclusion

Autoranging power supplies provide flexibility with the right amount of power, voltage, and current. Getting an autoranging power supply with just enough power saves you money and protects our environment. Let’s do our bit to help mother nature. Our autoranging power supplies can help you. Check them out at www.keysight.com/find/power

Posted by Seok Jin Wong Mar 1, 2018

Whether you use your frequency counter on the bench or in an automated test system, you want your measurements to be as accurate as possible. There’s a key component in your frequency counter that dictates its accuracy: the timebase oscillator.

Measurement accuracy in frequency counters begins with the timebase because it establishes the reference against which your input signal is measured. The better the timebase, the better your measurements can be. I said ‘can be’ because you need to do scheduled maintenance of your frequency counter. To maximize your counter’s accuracy, you need to calibrate it. Here’s why:

The frequency at which quartz crystals vibrate is heavily influenced by ambient temperature.

There are three main categories of timebase technologies, divided up based on how they address thermal behavior: room temperature crystal oscillators (RTCO), oven controlled crystal oscillators (OCXO) and temperature compensated crystal oscillators (TCXO).

Standard Timebase – A Room Temperature Crystal Oscillator

The first category is the standard timebase, also known as room temperature crystal oscillator (RTCO). A standard timebase doesn’t employ any kind of temperature compensation or control. While this has the advantage of being inexpensive, it also gives the largest frequency errors. The curve in Figure 1 shows the thermal behavior of a typical crystal. As the ambient temperature varies, the frequency output can change by 5 parts per million (ppm) or more. This works out to ± 5 Hz on a 1 MHz signal, so it can be a significant factor in your measurements.

Figure 1. The frequency output of an unprotected crystal can vary widely in response to ambient temperature. Putting the crystal in a controlled thermal environment (an oven) helps maintain a stable output frequency.

The obvious solution to this temperature-induced variance is to control the temperature, which leads us to our next type of oscillator.

Oven-controlled Crystal Oscillators (OCXO)

For an OCXO, the crystal oscillator is housed in an oven that holds its temperature at a specific point in the thermal response curve. Surrounding the crystal with temperature-control circuitry gives it better timebase stability. Typical errors are as small as 0.0025 ppm (±0.0025 Hz on a 1 MHz signal). Additionally, oven-controlled timebases also help minimize the effects of crystal aging. This means you don’t have to calibrate your frequency counter as often.

OCXOs are very accurate but more expensive and have a bigger footprint than other timebase options. So, many engineers opt for temperature-compensation circuitry instead of temperature-controlled circuitry.

Temperature Compensated Crystal Oscillators (TCXO)

Temperature compensated crystal oscillators (TCXOs) are designed to account for temperature changes instead of trying to hold a fixed temperature. One method of compensating for frequency changes due to temperature variation is to add external components with complementary thermal responses.

This approach can stabilize the thermal behavior enough to reduce timebase errors by an order of magnitude relative to RTXO (approximately 1 ppm, ±1 Hz on a 1 MHz signal).

Choose a Timebase That’s Right for You

Now you have learned that different types of timebases bring a different response to a counter.

The quality of your frequency counter’s timebase will affect your measurement accuracy. Depending on the accuracy that you need, you can choose the right one.

It’s also worth pointing out that the timebase does not need to be housed within the frequency counter. You can connect a precision source or house-standard external source to the counter to improve measurement accuracy. You should also note that, no matter what timebase you select, leaving your timebase powered up will provide the most accurate results.