Why You Can’t Put Electronic Loads in Series – By: Bob Zollo

This educational blog post is derived from an excellent article by Bob Zollo, on electronicdesign.com – Full Story.

“As voltages climb to 400 V, 600 V, or even 1000 V, fewer e-loads (electronic loads) are on the market rated for these high voltages. You may consider putting e-loads in series to construct a solution. But most, if not all, e-loads on the market cannot be safely put in series.”

1205PowerOnFig1

1. In CC mode, the e-load controls the amount of current by adjusting the FET’s RDS.

1205PowerOnFig2

2. With two 60-V e-loads in CC mode in series, e-load #1 keeps the current at 9.99 A. E-load #2 cannot reach 10.01 A, goes unregulated, and becomes a near short. As a result, almost all 100 V is applied to e-load #1, causing failure.

1205PowerOnFig3

3. With two e-loads in series, e-load #1, set to CC, keeps the current at 10 A. E-load #2 is set to CV. If either e-load is turned off, e-load #1 will go to a near-short low resistance and e-load #2 will go to a near-open high resistance. As a result, a voltage divider is formed causing nearly all 100 V to be applied to e-load #2, causing failure.

Summary

“It is not safe to operate e-loads in series if the device under test can supply more voltage than the rating of any one e-load, as the whole voltage will eventually appear across one e-load. So, the only safe way to operate in series is to have every e-load be rated for the full voltage of the device under test. But if you have that, you don’t need to put the e-loads in series after all.”

Information and images in this post gathered from “Why Can’t You Put Electronic Loads In Series To Get More Voltage?

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