Why are mosfets used as pass elements in linear power supplies? - Page 1
Forum section: electronics / projects

author: Farrow
author_rank: Contributor
timestamp: March 26, 2021, 02:49:51 am
content: I'm well aware that mosfets have many flaws as pass elements compared to more traditional bjt design but still, I see big-name companies use them in their multi-thousand dollar linear lab supplies? Why is that? Does the complexity and heat issues of a mosfet outweigh its advantage?

author: uer166
author_rank: Super Contributor
timestamp: March 26, 2021, 02:56:13 am
content: BJTs suffer from second breakdown and can't dissipate as much power at high voltages compared to FETs with specified (or tested) DC SOA.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 26, 2021, 03:08:42 am
content: In short, BJTs work better at low voltages, where their higher transconductance improves performance and aids design; and MOSFETs work better at high voltages, where their freedom from 2nd breakdown (depends on type -- both old and modern types do and do not have it -- RTFDS), and perhaps low drive current, makes them preferable.Or MOSFETs might be used for low voltages at very high currents, just because they're economical. Or low voltages and very low voltages (LDO chips!).Incidentally, not just power supplies, but loads as well, or both as in the SMU.Tim

author: Farrow
author_rank: Contributor
timestamp: March 26, 2021, 03:12:36 am
content: Ty for the clarification!

author: Kleinstein
author_rank: Super Contributor
timestamp: March 26, 2021, 08:25:49 am
content: Quote from: uer166 on March 26, 2021, 02:56:13 amThe very early MOSFETs did not have the 2nd break-down problem, but modern one can have it - though under a different name. It very much depends on the MOSFET type: some are really poor (may show SOA limits even below 12 V) and other are good.MOSFETs with a tested DC SOA are very rare. It is even so bad that some data-sheets show wrong DC SOA curves - so one can not always trust the curves. A DC SOA curve that shows just a simple power limit is suspicious. Looking at the DS is required, but not sufficient.There are pros and cons to using MOSFETs for a linear pass element. So it depends which is more suitable.The low drive current can be an advantage, but it can also be misleading as one still needs to drive the gate capacitance and thus may need a comparable driver impedance / current capability.

author: David Hess
author_rank: Super Contributor
timestamp: March 26, 2021, 11:59:03 pm
content: MOSFETs cost more than bipolar transistors for the same power dissipation which depends on die area, but do not suffer from Ft and gain droop at high current, although these are much improved with modern (1) bipolar ring-emitter type transistors. Taking advantage of this in MOSFETs however requires drive circuits which are equally if not more complex than with bipolar transistors. There is no continuous base current to supply but a class-AB driver which can handle the high capacitance at the gate is required. The highest performance bipolar designs require this also but not to the same extreme because of their lower capacitance and higher transconductance.A more practical factor is simply that power MOSFETs are available with larger power ratings. The largest ring-emitter transistors are 230 watts in the TO-264 package while the smallest MOSFETs in the same package are twice the power rating reflecting a die which is twice as large. Cost per watt is not even close though at 0.93 *cents* per watt for bipolar and 1.6 cents per watt for a MOSFET. So you pay almost twice the price for the same total power, but require less than half as many parts assuming that your cooling solution is up to the task.I do not know why larger bipolar transistors are not available in the TO-264 package. It seems that they are limited to the same size dies which are used in the TO-3/TO-204 packages. My guess is that unlike MOSFETs, there is simply no demand for a larger bipolar part.(1) Modern is relative since bipolar ring-emitter type transistors have been around since at least the 1980s.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 27, 2021, 06:56:39 am
content: Hmm, larger BJT dies have certainly been made, or, I would assume so anyway -- has anyone peeked inside a BJT (typically darlington or "trilington") power transistor module?I've seen plenty of IGBT modules, where they use relatively large dies, but also many in parallel. Possible they did the same for BJTs, but Idunno.Not that those were very long lived, AFAIK? They served a somewhat niche market I think, large enough that regular power transistors wouldn't fit the bill (e.g., motor drives of more than a few HP), but not so big that the economy of scale granted by pissoff great SCR pucks was worth the effort? Or also, that BJTs just aren't as efficient, or don't happen to scale up very well. (Not sure which all mattered, but this was relevant design knowledge in what, the 70s, 80s at the latest? Well before my time.)Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 27, 2021, 08:54:37 am
content: Quote from: David Hess on March 26, 2021, 11:59:03 pmHmm, ya know, I haven't surveyed BJTs at all yet... do you have some examples?Off the top of DK I see:(Sorted by price, and only including parts that show DC SOA, so, all those switching parts like MJE13009 are goneski.)https://www.digikey.com/en/products/detail/ween-semiconductors/BUJ303A-127/1155258Even better -- 0.356 $c/W in 1000s, and apparently full rating to 100V, 1A. Not bad for a "switching" transistor!https://www.digikey.com/en/products/detail/on-semiconductor/NJW0281G/1853548Bigger package, 0.947 $c/W, full rating to only... around 3A, 45V? The graph isn't real clear. Down to 100W by ~70V. Very fast.https://www.digikey.com/en/products/detail/on-semiconductor/FJA4313OTU/10564951.47 $c/W, full rating to only 60V (2.17A). Pretty fast.https://www.digikey.com/en/products/detail/on-semiconductor/FJL4315OTU/1057395Or in the other package, 1.49 $c/W and 2.5A. Surprisingly, the bigger package isn't a big cost adder?Note that, to beat the first price at this point, the next option has to be over 500W, at the same price as the last one... but the list only goes up to 250W, so that's not happening. Possibly others have wider SOA.For flavor, the MJL21196G has a 200W rating, and sustains 200V 0.5A -- but only for 1s, a DC curve is not given(!). So I technically can't include it here.And the classic MJ15024 is 1.77 $c/W, topping out at 70V full ratings (250W), and still doing 100W at about 100V.In comparison, the last high voltage champion I found was FQA9N90C, clocking in at 0.625 $c/W, competitive in the above list, and not a bad deal considering it handles over ten times more voltage at full ratings!Mind, there may be others now, or at lower voltages, as far as MOSFETs with competitive ratings. I was looking for >500V last time, and this one looked to be the best in that range.Tim

author: strawberry
author_rank: Super Contributor
timestamp: March 27, 2021, 07:14:27 pm
content: SOA

author: David Hess
author_rank: Super Contributor
timestamp: March 27, 2021, 09:30:15 pm
content: Quote from: T3sl4co1l on March 27, 2021, 06:56:39 amLarger bipolar transistors existed in the past but only slightly larger in TO-3 packages. My old Motorola book shows 300 watt TO-3 darlingtons and 500 watt MO-040 parts with a separate emitter connection.Quote from: T3sl4co1l on March 27, 2021, 08:54:37 amI limited my survey to the largest through hole package, TO-264, and ring-emitter transistors which are higher performance and well characterized for linear operation albeit more expensive. There are some slightly larger TO-3 and non-ring-emitter parts but even so, nothing close to the power of MOSFETs in the TO-264 package.This reflects current availability and what I would look to use in a new design. If anything it is optimistic about the difference in price because linear rated MOSFETs are more expensive than I indicated, but many linear designs will be power limited while not operating in the bipolar secondary breakdown or MOSFET thermal instability regions.

author: schmitt trigger
author_rank: Super Contributor
timestamp: June 21, 2022, 09:26:55 pm
content: Quote from: strawberry on March 27, 2021, 07:14:27 pmWow, I had to look up this bad boy.Ic = 300 ampVcebt = 700 VHfe = 5Icbo = 10 mA

author: Marco
author_rank: Super Contributor
timestamp: June 21, 2022, 09:40:07 pm
content: Quote from: Kleinstein on March 26, 2021, 08:25:49 amSpirito effect to put a name to it, for easier googling.

author: Benta
author_rank: Super Contributor
timestamp: June 21, 2022, 11:20:09 pm
content: I don't really see atechnicalreason for the focus on MOSFETs in linear applications. As always, it's function dependent.Idosee a marketing reason, though. When I see the posts on this forum, almost everyone selects a MOSFET for some design. Although a BJT or JFET might be the much better choice.Why?Because the semiconductor manufacturers over the past 25 years have only pushed and marketed MOSFETs, always competing on on-resistance or gate charge/capacitance.There are suppliers offering wonderful power BJTs, onsemi is one, eg, MJ15004. Or MJ21194.Or for audio amplifiers, the legendary Sanken 2SA1216/2SC2922. I'd use those again anytime for a power amp.BJTs are mature, and do not lend themself well to marketing campaigns. That's the issue.

author: magic
author_rank: Super Contributor
timestamp: June 22, 2022, 08:46:06 am
content: Quote from: Benta on June 21, 2022, 11:20:09 pmNot too bad, but no SOA rating at DC and cooling metal cans sucks. Are there versions of these in plastic?

author: Kleinstein
author_rank: Super Contributor
timestamp: June 22, 2022, 09:59:24 am
content: The old motorola DS for the MJ15004 has an SOA curve (DC only). It even states that the parts are SOA tested, which may add to the costs.Anyway the transistor is relatively slow and may not be a good chioce of a high performance supply. At low current the transistors tend to be even slower and a slow transistor makes it more tricky to stabilize the control loop without getting sluggish.The obvous choices are the BJTs targeted for high power audio, like 2SC5200, 2SC3281 and similar. The power rating is a bit lower in TO247 case, but still good up to some 50 V. Above some 50 V BJTs are often not that great. In power supply one should than consider using mulitple transformer taps and things like a series configuration.A slight problem can be fake parts if not from a reliable source.

author: H713
author_rank: Regular Contributor
timestamp: June 29, 2022, 06:27:17 pm
content: Even switching MOSFETs can have really good linear characteristics.Part of the problem is that it's hard to find good BJTs rated for more than about 250V. The xxx21194 or the xxx3281 are probably the best value for this sort of application, but above 80V their SOA drops off like a rock.Higher voltage BJTs tend to have extremely low current gain, so they need a lot of drive current. In general, a PITA.Some of the 900V MOSFETs (depending on which one we choose) have monstrous SOA where power supplies often would use them. I did some testing of FETs for my current project, and below about 80 V, the linear operation of the FQA9N90C is pretty much limited by its 280W dissipation rating. The problem is that gate threshold voltages vary a lot, so you need really large current sharing resistors, so ultimately I moved to NJW21194s.