Author Topic: Multiple LL N-Channel MOSFET's in Series  (Read 5254 times)

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Offline TheDoodTopic starter

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Multiple LL N-Channel MOSFET's in Series
« on: August 03, 2019, 05:47:09 pm »
Hey guys, I was wondering if is it possible to reduce power dissipation by using multiple LL MOSFET's (VDSmax ~55V) in series to replace a single higher ΩDSon power MOSFET (VDSmax ~200V)?  Are there special considerations to design for or take into account?

As an example:
(4) IRLZ44N : total RDSon 0.088Ω

(1) IRF640N : total RDSon 0.150Ω

Thanks!
« Last Edit: August 03, 2019, 08:45:20 pm by TheDood »
 

Offline nigelwright7557

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Re: Multiple LL N-Channel MOSFET's in Series
« Reply #1 on: August 04, 2019, 12:02:55 am »
Hey guys, I was wondering if is it possible to reduce power dissipation by using multiple LL MOSFET's (VDSmax ~55V) in series to replace a single higher ΩDSon power MOSFET (VDSmax ~200V)?  Are there special considerations to design for or take into account?

As an example:
(4) IRLZ44N : total RDSon 0.088Ω

(1) IRF640N : total RDSon 0.150Ω

Thanks!
You could put the mosfets in parallel to reduce power in each  but together they would still generate same amount of heat.
If its just a switching mosfet application why not use a lower RDSon mosfet ?

 
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Online Circlotron

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Re: Multiple LL N-Channel MOSFET's in Series
« Reply #2 on: August 04, 2019, 12:13:22 am »
If it is a fast switching application like in a power supply then the total energy stored in the drain-source capacitance and dissipated every switching cycle might be a whole lot greater with series low voltage mosfets. Driving all the non ground referenced gates will be a nuisance. So will making sure the max voltage across each mosfet is not exceeded. IRF640 is not a modern mosfet. There are much better ones available today.
 
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Offline digsys

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Re: Multiple LL N-Channel MOSFET's in Series
« Reply #3 on: August 04, 2019, 12:34:11 am »
Without taking into account as to what it is used for - the answer is YES of course - BUT there are considerations to address, some have been pointed out -
IF it is just a switch, and you're worried about losses, and multiple parallel is not an option (which it is) - then you have 2 major questions -
1/ Protection of each FET, should one FET go O/C or short, leaving the other(s) with too high voltage
2/ Extra complexity of drive circuits, as these are all at different levels and protection for 1/
Hello <tap> <tap> .. is this thing on?
 
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Offline Whales

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Re: Multiple LL N-Channel MOSFET's in Series
« Reply #4 on: August 04, 2019, 12:44:03 am »
> 2/ Extra complexity of drive circuits

This would be my main point here.  The lower mosfet can be driven normally, but the upper mosfet must be driven relative to its source pin.  This requires much more complex driving circuitry.

If at all possible: try and get a better mosfet, it will make things a lot simpler.  Are there any reasons you would prefer to do otherwise?
 
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Offline Jwillis

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Re: Multiple LL N-Channel MOSFET's in Series
« Reply #5 on: August 04, 2019, 01:02:58 am »
Having two MOSFETs with different RDS(on) can cause current unbalance.This means that one of the MOSFETs would carry more load than the other creating unbalanced heat distribution between the 2.

Edit: I should also mention that Mosfets in series can have unequal voltage sharing and with different RDS(on) can exasperate this problem even further.This can destroy the components.
« Last Edit: August 04, 2019, 01:13:55 am by Jwillis »
 
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Online Circlotron

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Offline TheDoodTopic starter

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Re: Multiple LL N-Channel MOSFET's in Series
« Reply #7 on: August 04, 2019, 05:03:16 am »
Thanks for the responses. The question is asked with SMPS in mind. Was curious what complications would arise. It seems there's easier ways to achieve a lower RDSon then using LL FETs in series.

Feel free to list any low RDSon power FETs you guys use. That infineon is more what I'm after, thanks @circlotron, do you know of any that are a bit cheaper but still low RDSon like that?

**
I've been trying to understand Xc as well. I set up a 47nF film cap in series with an LED pair (1 LED faces 1 way the other LED is in parallel & faces the other way), 1k POT, and a 60Hz 120VAC potential. I calculate a Xc of ~56.4kΩ, this means only ~2mA would flow through LED pair? When I turn the POT to 1kΩ I get a 2.0V drop across it (though LEDs are uneffected regardless of POT rotation, LEDs seem to shine same intensity??), this correlates to 2mA. The LEDs read a 2.2V drop across them and the film cap reads 120V across it. All I've heard and read says that a capacitor in series with AC load drops V, but it seems like really it's current limiting rather than V dropping? Impedence deals with opposition to current flow, right? Is it capacitive resistance or capacitive impedence, and are they the same thing? (Pic is showing only testing ccts that I'm using to confirm an understanding, not using for anything in reality, bottom loop is Xc cct previously described)
Thanks!

EDIT:
I'd throw the multimeter in series with the Xc cct to determine amperage, but my multimeter doesnt measure AC current :/ I have a single channel scope with a max sensing input of 50V, so I may try to throw that on there but its my first and only scope and Im pretty green so just double checking everything before I try to use it on the unisolated mains Xc cct. Was going to attach to either side of the LED pair and see what the wave form looked like and then see if messing with the POT is effecting LEDs or not.
« Last Edit: August 04, 2019, 05:14:49 am by TheDood »
 


Offline Jwillis

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Re: Multiple LL N-Channel MOSFET's in Series
« Reply #9 on: August 04, 2019, 06:02:36 am »
IRF3205 http://www.irf.com/product-info/datasheets/data/irf3205.pdf
Good all around for most switching applications up to 55 volts
 
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Offline T3sl4co1l

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Re: Multiple LL N-Channel MOSFET's in Series
« Reply #10 on: August 04, 2019, 09:08:40 am »
STW70N60M2, better Rds(on) than the examples you've brought up.

Qg(tot), Coss, package size, cost?  What are those? ;D

Tim
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Offline TheDoodTopic starter

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Re: Multiple LL N-Channel MOSFET's in Series
« Reply #11 on: August 04, 2019, 09:35:04 am »
Thanks Tim, MOSFET Coss is something I did not know about and will need to study up on, Ill add it to "ferrororesonant transformer." Very informative.

Whales, nope just curious if it was possible and what special design characteristics would be nessecary. I was thinking of trying to reduce power dissipation. I will take your advice.


**
EDIT:
If I used N MOSFETS, to replace schottkys in a full bridge, wouldnt my gate V need to be greater than the load V? 
« Last Edit: August 04, 2019, 10:14:13 am by TheDood »
 

Offline TheDoodTopic starter

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Re: Multiple LL N-Channel MOSFET's in Series
« Reply #12 on: August 04, 2019, 09:47:41 am »
IRF3205 http://www.irf.com/product-info/datasheets/data/irf3205.pdf
Good all around for most switching applications up to 55 volts
Can eBay be trusted on MOSFETS? Any components?

Looks like the Vishay 200V FET (see attached .pdf thanks Ctron) @ $2.50/ is my cheapest low RDSon higher VDS option thus far. I'm liking the irf3205 for my LL goto, thanks Jdub

EDIT:
I forgot to add the .pdf I was referencing, but it's there now, oops   :wtf:
« Last Edit: August 04, 2019, 10:20:35 am by TheDood »
 

Online Circlotron

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Re: Multiple LL N-Channel MOSFET's in Series
« Reply #13 on: August 04, 2019, 10:03:12 am »
Looks like the Vishay 200V FET (see attached) @ $2.50/ is my cheapest low RDSon higher VDS option thus far. I'm liking the irf3205 for my LL goto, thanks Jdub
That's 55V not 200V.
 
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Offline TheDoodTopic starter

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Re: Multiple LL N-Channel MOSFET's in Series
« Reply #14 on: August 04, 2019, 10:33:34 am »
Also, looking for a rail to rail comparator.
 

Offline T3sl4co1l

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Re: Multiple LL N-Channel MOSFET's in Series
« Reply #15 on: August 04, 2019, 11:50:37 am »
What voltage, speed?

Tim
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Offline TheDoodTopic starter

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Re: Multiple LL N-Channel MOSFET's in Series
« Reply #16 on: August 04, 2019, 07:55:00 pm »
What voltage, speed?

Tim
Was thinking single shottky for half wave rectification and a cap in parallel across comparator so that it had charge to output when supply V was 0. So I guess up to 120V? Also not sure on speed, 20kHz?

I've some lm358s and was looking at the lm339 for true comparator but both opamps have lower supply V's than I was expecting. Trying to get acquainted with a few different "goto" generic choices with maybe some higher V supply maxes
« Last Edit: August 06, 2019, 05:25:45 pm by TheDood »
 

Offline T3sl4co1l

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Re: Multiple LL N-Channel MOSFET's in Series
« Reply #17 on: August 05, 2019, 01:11:22 am »
What?

What, exactly, overall, are you doing?  This appears to go well beyond single MOSFETs and comparators...

Tim
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Offline TheDoodTopic starter

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Re: Multiple LL N-Channel MOSFET's in Series
« Reply #18 on: August 05, 2019, 03:37:16 am »
What?

What, exactly, overall, are you doing?  This appears to go well beyond single MOSFETs and comparators...

Tim
I'm just tinkering trying to develop an understanding so that I can build XYZ in the future. When I say "Trying to get acquainted with a few different "goto" generic choices with maybe some higher V supply maxes," that is exactly what I'm after. I'm really big into effeciency and just starting out into electronics so looking for parts that are currently used (ie the IRF640N is outdated) that offer lower power dissipation than what I've been exposed to. Idk brands or who to buy from or anything, so just trying to learn.

I'd like to make a variable CC/CV PS (what I had in mind when talking about the cap in parallel) that I can use for my bench, and also interested in audio amplification. I'd like to design a drone at some point and effeciency would be very important to me then as well. All projects seem like comparators would be nessecary, the higher voltage comparator more so in the PS, but I'm not even sure how accurate that is lol. I'd like to increase my comprehension so that I can modify cheap projectors to increase lumen count, make effecienct small wattage AC/DC PS's (95%+), create a drone + receiver, ec, just typical nerd stuff haha
 

Offline BrianHG

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Offline T3sl4co1l

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Re: Multiple LL N-Channel MOSFET's in Series
« Reply #20 on: August 05, 2019, 08:57:30 am »
Okay, yeah... comparators and other control logic stuff, do it at low voltage.  Add level shifters into and out of the block.  So, a 100V buck converter made with a discrete controller, use a bootstrap gate driver.

I tend to reach for LM393 (5-30V) or MCP6562 (2.7-5V) for jellybean comparators.

Tim
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Offline TheDoodTopic starter

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Re: Multiple LL N-Channel MOSFET's in Series
« Reply #21 on: August 18, 2019, 08:17:05 pm »
Hey guys back with more questions  :-//

Can someone look over this block diagram and help me confirm my understanding or correct it? Thanks in advance..

Trying to reduce power dissipation in the rectification part of PS cct, so I can isolate effeciency loss to the regulation part. In terms of total efficiency is the power required by the op amp offsetting any localized effeciency gain noticed in a reduced diode turn on time?

If I'm using opamps to slam diode on fast, am I using less power? Or am I just turning diode on faster, or both?  There's still a V drop across diode, its just that the op amp is facilitating the V needed so that the load V isn't disturbed?
« Last Edit: August 18, 2019, 08:54:30 pm by TheDood »
 

Offline T3sl4co1l

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Re: Multiple LL N-Channel MOSFET's in Series
« Reply #22 on: August 18, 2019, 09:26:52 pm »
What's the opamp powered from?

What about the voltage drop in the opamp output stage?

Tim
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Offline TheDoodTopic starter

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Re: Multiple LL N-Channel MOSFET's in Series
« Reply #23 on: August 19, 2019, 04:41:58 am »
Thanks for the reply Tim, you've been very helpful.

C1

Depending on the load, I would have to add or remove R's, my thought was perhaps a 5V-10V max op amp output?


The V drop across the diode? Or I'm not sure I'm comprehending? To me it seems the OP amp turns the diode on, but that there's still a 0.7V drop across it, what I'm trying to understand, is, P=V*I, so really, I'm not gaining any in effeciency turning the diode on quicker because the load current is still flowing through a 0.7V drop regardless the origin of the voltage (turned on diode is like (loadV + 0.7V), still flowing same current just a bit more V to calculate power dissipation from)? Also, because barely no current flows in an OP amp (right?), how much power are they consuming, generating? Is this offsetting any effeciency (if I happen to not be comprehending the power dissipation across diode correctly)? 

If I were to switch schottkys out for FETs, so that the load current doesn't get dropped by 0.7V, just by the RDSon of the FET (assuming current produces less V drop than 0.7V), would that increase effeciency?

For calculations let's assume a 30VAC supply.
 

Offline T3sl4co1l

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Re: Multiple LL N-Channel MOSFET's in Series
« Reply #24 on: August 19, 2019, 07:41:43 am »
Just that the op-amp powered from a "sky hook" is the electronic version of the mechanical beginner's shaft passer.

The crux of it is this: it's easy to forget that the op-amp is itself made of transistors.  It cannot generate more voltage than supplied with; the full load current (plus a little extra!) is drawn from those supplies; and the full voltage difference between supply and output is burned as power dissipation.

Op-amp input pin current is pretty low, right, but the output current has to come from somewhere.

So, making a synchronous or precision rectifier, with op-amps, and expecting any efficiency out of it, is a fairly early and simple mistake to make. :)

Once this is realized, it's easy to see why this can never work, but also that one must explore circuits a bit more clever to actually implement it.


So... not to beat you to death about it, just to be perfectly clear what it is, and that we've all been there at some point. :P

Using transistors is indeed one such case; they must be controlled to turn on and off at the appropriate voltages/currents, but one should also be careful not to slam them on and off (e.g. using a comparator and gate driver), because that will lead to oscillation and ugly recovery* effects!  A modest gain factor, between Vds and Vgs, is probably best.

*Recovery is effectively the time a diode needs to "decide" when being turned on or off.  There is forward and reverse recovery.  Forward recovery is a relatively high forward voltage drop, for the first however many, usually nanoseconds, as the forward current rises to reach its nominal value; this manifests effectively as a series inductance (higher dI/dt --> shorter t_fr, higher peak V_fr).  Forward recovery usually isn't very substantial.

Reverse recovery is the time taken for the current to go through zero, to some negative peak I_rr, during which time the voltage drop remains near Vf; this manifests effectively as a parallel capacitance (higher dV/dt --> shorter t_rr, higher peak I_rr).

Diagrams of reverse recovery and more discussion can be found here: https://www.allaboutcircuits.com/technical-articles/switching-losses-effects-on-semiconductors/

In an emulated or active diode like we're discussing here, the recovery is determined by the speed of the op-amp and transistors.

Here's a fairly simple example as a single diode, although, I guess I don't know how explanatory it will be since it uses a handful of BJTs -- I may be inviting something more complicated than you're prepared for..?  Explanation below:



The leftmost transistor is used as an inverted* diode (base strapped to emitter).  This is done for two reasons: one, the forward voltage drop matches that of the next transistor; two, the breakdown voltage is higher (about Vceo, say 60V for 2N3904).  The second transistor is also inverted, in a common-emitter configuration (except the collector is common, because inverted; but again, it works the same way!), so if the "Cathode" voltage is high, the first diode is reverse biased and the 470 ohm resistor turns on this transistor, pulling its output (the schematic emitter) down towards ground.

*BJTs are always introduced like a Tetris piece, a stack of N-P-N or P-N-P blocks.  But this is really pretty disingenuous as to how they work, or are made.  Well, most BJTs aren't symmetrical, one end is made stronger than the other, giving that side has a lower voltage rating (typically 2-10V) than the other (typically 20-100V+), but also better "emission" of electrons/holes into the transistor, hence it's called the "emitter" and is typically used as the common (grounded / reference) terminal.  It still works just transistorly if we swap E and C -- but, the performance is very different, hFE(inv) typically being quite low, under 5 say.  So that will be one drawback that applies here.

The PNP and three other resistors is a simple current source, about 0.6mA.  This biases the second transistor, giving it voltage gain and a working range of about 0-5V.

Finally the two rightmost transistors are a complementary emitter follower; they boost the gain node's output current from ~0.6mA to >60mA.  This is able to drive the MOSFET gate reasonably quickly.  The follower is not biased, which is kind of unfortunate (this introduces another nonlinearity, probably making this circuit awkward to use on arbitrary AC waveforms, say), but probably not worth addressing when used with most power circuits (like mains or switching supply rectification).

So, conversely, when the "Cathode" voltage dips negative, the first transistor turns on, shunting current away from the second one, turning it off, and allowing the gate voltage to rise.  This limits the FET voltage drop (Vds) to perhaps -60mV at low currents (the "decision range" is driven by the BJT's exponential gain, so a few multiples of 26mV goes from reasonably-fully-on to reasonably-fully-off), and I_load * Rds(on) at higher currents.

Two or three major downsides to this circuit are: the supply requirement (5V referenced to "Anode"); the relatively high bias requirement (mainly the 470 ohm resistor, or ~11mA; this is due to the low hFE(inv)); and probably the switching speed, say for switching supply application (this will take some 100s of ns, which is worse than a high speed PN diode, and despite the lower voltage drop, it may not turn out to perform better than a schottky diode).  For mains rectification it would be okay, but the first two issues are still unfortunate.

Note another characteristic of this: the transistors act as an amplifier, with a limited output voltage range -- 0-5V.  The input might be 40V.  It's not enough to have a gain function here: there also must be a limiting function.  If we used an ideal op-amp (one with a true skyhook for supplies!) to set Vgs = Vds * -100 say, we'd end up applying a tremendous voltage to the gate, blowing it in no time.  We need a limited output, so Vgs remains in a reasonable range.

Also, since it's a 5V supply, we would want to use a logic-level FET, which is fine, they're plentiful. :)  The supply can of course be increased to use others, but the bias current will be even more annoying then, and a better solution would be attractive.


There are of course ICs for this; LT for instance makes one, if you don't mind paying for it of course: https://www.analog.com/media/en/technical-documentation/data-sheets/4320fb.pdf
Which, despite the multi-dollar price tag, is really a pretty good deal when you're tight on space and can't afford the heatsinks or the temp rise (let alone the raw efficiency points) of a passive (diode) solution.  The knock-on savings are heatsinks, mounting hardware, fans, all those sorts of things.

Tim
« Last Edit: August 19, 2019, 07:43:44 am by T3sl4co1l »
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