why are fets used in electronic loads? - Page 1
Forum section: electronics / beginners

author: electronics man
author_rank: Frequent Contributor
timestamp: February 11, 2015, 11:21:42 pm
content: hi,I'm trying to build an electronic load and have been using fets all along, but i was wondering why fets are used and not bipolar transistors (in the darlington configuration to up the current gain)could someone explain why this is?

author: tszaboo
author_rank: Super Contributor
timestamp: February 11, 2015, 11:31:01 pm
content: At higher current, the low Rdson means is less voltage than Vbe. Also, FETs have usually less thermal resistance to the heatsink, so they dissipate more. And a big FET is cheaper than a big BJT.These are just general guidelines, there are bunch of exceptions.

author: dannyf
author_rank: Super Contributor
timestamp: February 12, 2015, 12:12:10 am
content: Less chance for 2ndary break-down / thermal run-away, and the ability to get down in terms of ohmic range.Not to mention their lower prices.At high current levels, a BJT or IGBT wins.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 12, 2015, 01:48:29 am
content: Quote from: AcHmed99 on February 12, 2015, 01:27:25 amAlthough probably under 50 of those FETs are rated for linear operation, while most of the BJTs will be.But, the set of FETs that can withstand DC to a higher voltage level than the BJTs (whether targeted for this purpose or not) is probably larger than just those intended for linear operation. The problem is, you have to test them to find out.Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 12, 2015, 02:24:12 am
content: Quote from: electronics man on February 11, 2015, 11:21:42 pmAlmost exclusively for the same reason you did: because everyone else did, without understanding it.Completely independent of the device, mechanism, technology or field something is done in -- this applies not merely to electronics, but mechanics, biology, music and anything else in the domain of human experience -- you can bet something is almost always done, and repeatedly re-done, in some typical manner, because of "monkey see, monkey do". The brain is very good at pattern matching, so it's much easier to match patterns and repeat actions, than it is to learn an immense body of technical knowledge and do it with active, willful understanding.Or, in short: you don't have to know WHY something works, to be able to do it.You might not be doing it as well as possible, but if optimization doesn't matter...Another electronic example: Tesla coils. Basically their only purpose is visual impact, even if the sparks they make aren't as tall as they can be. The standard design (a long, thin secondary, wound with fine wire, closely packed) is not at all an ideal shape for the purpose. But it works well enough.Or, how about driving in traffic. How many times have you seen people take off from a green light, only to stop a block later at a red light, that was clearly red when they started? How many times have you done this yourself? Such a waste of fuel, but without some metric (such as fuel consumption) or understanding, it's simply "the way people drive", a social phenomena with no need or want of an engineered approach....Philosophy aside...As for the proper, technical reason: partly because of current error, as you note; but also because the SOA is generally larger. All silicon amplifying devices exhibit 2nd breakdown*, which is a matter of the localized reduction of Vgs(th) or Vbe at a hot-spot, causing increased current density and eventual failure.*I suppose even non-linear-amplifying ones do, like SCRs and diodes; difference being, once the SCR trips, it latches and stays on, while the diode simply draws current in avalanche breakdown (maybe without also blowing up).The pertinent quantity is power density. At low voltages, you can draw pretty much any current through any device (BJT or FET), because the power density is so low, the local temperature rise is small. At higher and higher voltages, the effect becomes more pronounced.Majority carrier devices, like BJTs and IGBTs, achieve much higher current densities, which is why they are cheaper than MOSFETs are, for a given amperage rating. But that means higher power densities at a given voltage, which means more sensitivity to 2nd breakdown. BJTs designed for linear operation are designed to minimize this effect, but they still exhibit it (however, it may be masked by the fact that actual avalanche breakdown occurs first!).Historically, MOSFETs were very low density devices, so although their performance was poor, they did achieve good linearity and full SOA -- "no 2nd breakdown". But, after decades of process refinements and dozens of new geometries, their current density (per die area) is almost as good as BJTs, and therefore their sensitivity to 2nd breakdown, just as bad. How much depends on the process, with some being particularly poor (e.g., IXYS HiPerFET I think, which begins to poop out even at rather short time scales < 1ms), and some being quite good (the latest super-junction types apparently take it, while also offering better performance at higher voltages than ever).Tim