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Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
I'd agree with those definitions. "Cutoff", "Linear" and "Saturated" are the terms that I always use. My power electronics lecturer at University used to use the terms "Cutoff" and "Linear" but replaced "Saturated" with the term "hard on". Ok, you can stop laughing now - I was an immature student at the time, but I'm all growed up now!
Regarding linear operation of low-cost switch-mode MOSFETs: The main problem is the negative temperature coefficient of the gate-source threshold voltage. Something that is rarely discussed...
For some reason engineers seem to be educated to remember that MOSFETs have a positive temperature coefficient to their Rds(on) value resulting in two consequences. Firstly, paralleled devices will inherently share current, and secondly they are immune from thermal runaway. This is true to some extent, but only for switching circuits! This only applies in the "saturated" or "hard on" region!
The problem in the linear region is that certain areas of the MOSFET die will naturally have a slightly lower gate-source threshold voltage Vgs(th) and these areas will turn on harder and conduct more of the current than other regions with higher Vgs(th). You can guess what happens to the regions of the die that carry the most current - They heat up. Because of the negative temp coefficient of the Vgs(th) these areas turn on more and hog even more of the current. This is thermal runaway, but only in small areas of the die. It is often called hot-spotting or current hogging.
As Neil said it's down to spending too long in the linear region with a device that wasn't designed for this service.
The criteria for switch-mode devices are low Rds(on) and very high AC gain in the linear region. The latter being to get you through the lossy linear region as quickly as possible and minimise switching losses. However, this is exactly what you don't want for linear operation!
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
Just a thought. There are very few published designs for switch-mode current sinks, but a gazillion for switch-mode power supplies. On the basis that it's always easier to ride the horse in the direction it's going in, how about flipping the statement of the problem slightly from a varaible load, into a varaible supply followed by a fixed load? The result will be more expensive than a dedicated generator load design, but that's a tradeoff for using freely available design information.
Consider the following loading Mr. Smooth's 260v DC output. An input filter, a buck power supply, and his 3kW 18ohm resistor. By programming the output voltage of the supply, we control the current that the fixed load takes, and therefore that the supply takes. Therefore it remains only to program the PSU output voltage to be a function of its input voltage to acheive generator limiting. Say 0v output for 260v, rising to 280v output at 280v input. The current drawn by the supply will not be a linear function of the output voltage, but it will be smooth and monotonic. The PSU input filter will be responsible for attenuating the PSU input current ripple.
Now hit the FET and PWM controller supplier sites to get app notes with design equations for offline buck power supplies.
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mosfets: linear range and fully open
