Path: EDN Asia >> Design Centre >> Power/Smart Energy >> Enhance power supply reliability
Power/Smart Energy Share print

Enhance power supply reliability

27 Feb 2015  | Arthur Pini

Share this page with your friends

Have you ever seen an oscilloscope screen image that left you wondering what the waveform represented?

Every switching device has a maximum voltage, current and power specified by the device manufacturer, indicated on its technical application note. Reliability of the power supply is dependent on not exceeding these limits. Safe Operating Area plots help confirm operating margins.

In figure 1, the upper trace shows a MOSFET's drain-source voltage, and the centre trace shows the MOSFET's drain current waveform in a switch mode power supply. Amplitude of current vs. amplitude of voltage is displayed as an XY trace known as a Safe Operating Area Plot. The horizontal axis in the X-Y plot is voltage, and the vertical axis is current. The upper-right corner of the XY trace represents maximum power.

Figure 1: A Safe Operating Area Plot for a single power supply switching cycle shown using Teledyne LeCroy's Power Analysis software.

In oscilloscopes with long memories, it is possible to find SOA violations that occur for only a few cycles after aevent, such as a short circuit or start-up. Such low duty cycle events can be problematic. If they go undetected, they can degrade the device over time, which reduces system reliability. Figure 2 shows an SOA plot taken over 20 ms. (Note that the long acquisition memory maintains the same 2.5 GS/s sample rate used in figure 1.)

Figure 2: An SOA plot for a whole cycle of the AC line (mains). Note the use of long acquisition memory to maintain the 2.5 GS/s sample rate.

The primary interest in the SOA plot is to determine if the device exceeds its maximum voltage, current, or power ratings. In figure 3, the steady-state operation of a switched mode supply yields the SOA plot shown. In addition, parameters read the maximum current, voltage, and power for the whole waveform.

Figure 3: Using the maximum parameter to measure the maximum voltage, current, and power.

The SOA plot for the power supply start-up, figure 4, shows a much different characteristic. The shape of the SOA plot shows larger excursions from the origin. This indicates a greater stress on the device. Note that the peak voltage has increased from 252 to 290V. Peak current is 1.79 A compared to 0.436 A during steady state. The maximum power dissipated by the MOSFET increases from 42 to 92 W.

Figure 4: The SOA plot for power supply start-up shows a more severe operational range of voltage, current and power.

Safe Operating Area plots reveal a great deal about the operating conditions of power devices under different operating modes. Combined with parameter measurements for interpretation of maximum voltage, current and power levels, they are unequalled in device characterisation tools.

About the author
Arthur Pini is a technical support specialist and electrical engineer with over 50 years experience in the electronics test and measurement industry. He has supported oscilloscopes, real-time spectrum analysers, frequency synthesisers, digitizers and arbitrary waveform generators for leading manufacturers.

Want to more of this to be delivered to you for FREE?

Subscribe to EDN Asia alerts and receive the latest design ideas and product news in your inbox.

Got to make sure you're not a robot. Please enter the code displayed on the right.

Time to activate your subscription - it's easy!

We have sent an activate request to your registerd e-email. Simply click on the link to activate your subscription.

We're doing this to protect your privacy and ensure you successfully receive your e-mail alerts.

Add New Comment
Visitor (To avoid code verification, simply login or register with us. It is fast and free!)
*Verify code:
Tech Impact

Regional Roundup
Control this smart glass with the blink of an eye
K-Glass 2 detects users' eye movements to point the cursor to recognise computer icons or objects in the Internet, and uses winks for commands. The researchers call this interface the "i-Mouse."

GlobalFoundries extends grants to Singapore students
ARM, Tencent Games team up to improve mobile gaming

News | Products | Design Features | Regional Roundup | Tech Impact