Friday, January 28, 2011

Today's Power Supplies More Than a Battery with a Knob

In the past the majority of variable power supplies used for testing were viewed as little more than a battery with knob. They were large, heavy, and had little in the way of measurement capability. The measurement capability typically consisted of an analog display or a digital display with only three digits of resolution. The power supply user often did not trust the power supplies measurement capability so the supplies voltage was set and monitored using a DMM connected across the supply's output. If they needed to measure the supplies output current a shunt with a DMM or a scope with a current probe was used.
In the last decade the test and measurement industry has really witnessed the rise of high performance supplies and application specific supplies. High performance supplies and application specific supplies on the market today come with a whole host of advanced features here a quick summary of some of them:

When it comes to power supplies smaller is better
There are two dominate power supply designs used throughout electronics world, linear and switching. In the past in testing linear supplies were dominate because of their clean output power. Switchers on the other hand had a lot of noise on their output power. The downside of linear supplies was they had a huge bulky transformer which made them large and heavy. Although linear supply designs can still be found, the test and measurement industry has really switched to switcher supplies. Why? Because switchers are smaller and advances in electronic filtering has made switcher outputs just as clean as linear outputs. For instance, Agilent's N6700 modular power supply family can deliver up to 1200 W per mainframe in a 1U full rack size (see figure below) with outstanding output noise specs.

Throw away the current shunt and current probe
Today's high performance supplies have built-in high accuracy measurement digitizers in them for capturing voltage and current in parallel with measurement resolutions up to 18 bit. The digitized points can be integrated together for increased accuracy or the points can be used to capture sharp transients which are common occurrence in today's high speed digital electronics. This beats the current measurement methods of the past. For instance you no longer have to deal with the complexity of setting up a measurement shunt with a DMM or external digitizer and you do not have deal with unwanted series resistance that results from shunts. Using a current probe you do not have to worry about adding series resistance to the circuit, but current probes are not very accurate (typically >1% error) and they typically can't measure current below 10 mA. Below is a screen shot from Agilent's N6705B showing digitized current pulses on its scope like display.

Modern power supplies are really making waves
Modern power supplies have high speed output control loops for dealing with sudden output transients. Supply designers have learned how to manipulate these fast output control loops to create high power voltage and current waveform capabilities in high performance supplies. This has given test engineers the ability to simulate engine crank profiles, to simulate a handheld battery powered device being dropped, or to simulate power line noise on a DC level with just a power supply. 

You brake it, you buy it
Prototype cellular base stations and satellite modules are very expensive. If something goes wrong with one of these expensive devices during the test process and they suck too much current from the supply it can lead to some costly damage and design delays. In the past test engineers had to build in expensive protection circuitry between the supply and the device being tested. Today's high performance supplies have made test engineer's job easier by adding a long list of safety features. As an example, Agilent's N6700 modular power supply family has a built-in watchdog timer. The the watchdog timer can be activated and set by the user to start timing after each command received by the system software. If the timer value that the user sets runs down before the next command from the system controller the power supply will shut off its outputs. This protects the device being tested in the event that the system controller or software freezes up or crashes.

There is a power supply for that app
Some of todays advanced applications have unique power requirements. To meet these unique power challenges test and measurement vendors have developed advanced application specific supplies. An example is Agilent's E4360A Solar Array Simulator. This supply is designed to be a high speed and high power current source. Its output I-V characteristics simulate the I-V output curve of a solar panel or an array of solar panels.This type of supply is used to test satellite power systems and terrestrial solar panel max power point tracking devices (for more info check out this post http://gpete-neil.blogspot.com/2010/11/simulating-photovoltaics-with-standard.html). Another example is Agilent's N6781A and N6782A. These advanced supplies were designed for battery drain analysis of handheld devices and low power optimization of handheld devices and the components that go into them. These supplies have many advanced features but the most impressive is there seamless current ranging capability. Meaning they can measure transition from one current range to another without any discontinuities in the output power. This means they can capture a sudden burst of current that goes from uAmps to Amps with 18 bits of measurement resolution throughout the whole pulse (for more info check out this post http://gpete-neil.blogspot.com/2010/06/breakthrough-dynamic-current.html).

The supply features and the application specific supplies I just covered are just examples of the wide range of features that can be found in modern power supplies. Recently power optimization has become a major factor in the design process in the electronics industry. This emphasis on power optimization will continue to drive more advanced power supply features from the test and measurement industry well into the future. One thing is for sure the high performance power supplies of today and tomorrow are really revamping the power supplies image from a battery with a knob to a sophisticated piece of instrumentation.


2 comments:

  1. Has anyone used one of these new usb power analyzers? http://www.power-testing.com/products.php

    The cost looks pretty reasonable but I don't know about the performance.

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  2. Sorry What I have not used those usb power analyzers. When you go click on "Buy Now" it takes you to a page that gives you more details on the product including operating ranges and measurement specs. I would look through the specs and if the specs that are most important to you are missing then I would stay away. If their specs are too good to be true than I would stay away. Otherwise it may be worth it but with a small new company's product beware of bugs.

    Neil

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