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FET's Issue
3

FET's Issue

FET's Issue

(OP)
Anyone has an idea on:
For high current applications which choice is more reliable:
1- Single FET with high current rating.
2- Several FET's with lower current rating connected in  parallel.
Thanks in advance

RE: FET's Issue

I would always opt for a single...
Unless there is some unique condition.

Much simpler.
More reliable.
Less labor.
Less expensive for equivalent power.

RE: FET's Issue

Depends somewhat on what size FET you are talking about:
A single FET has no redundancy, and probably higher RDSon compared with parallel group. Multiple TO-218 plastic package devices can be easier to PCB-mount in a manner suitable for heatsinking than single TO-3 metal cans. Large single isolated base FET modules are easier to deal with than multiple smaller modules.

When the voltage / current / power starts rising to high levels, why not consider using IGBTs unless you are working with really high frequency?

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  Your body might be a temple. Mine is an amusement park...

RE: FET's Issue

Scotty... are you serious about the redundancy??!?!?

What product have you ever seen that used multiple FETs for redundancy?

What about the failed shorted FET?

RE: FET's Issue

ItSmoked,

Yes, definitely seeen it in professional amplifiers from a large-scale PA. I dallied working in professional sound while at college (too many holidays) and decided that it was a great way live like a hobo and make no money. The amp had individually fused FETs, and the amp was rated for full output with one of the three output FET's gone. Not sure how effective it was because I never knew of one that blew - but an interesting design none the less. The only things we ever replaced were fans and filters.

----------------------------------
  Your body might be a temple. Mine is an amusement park...

RE: FET's Issue

I guess that would be a major league amp to rate individual fuses per FETs.  Course I suppose the show-must-go-on! :)

RE: FET's Issue

Two channels @ 1.5kW apiece, but they never worked flat out. Reliability took priority.

On the subject of reliability, losing a large LF enclosure is immensely irritating to the sound engineer (my boss at the time) but is usually masked to a large degree by its neighbours; losing an HF bank is much more awkward to deal with because HF sound is much more directional and you get noticeable 'dead' areas where there is a gap in the HF coverage. That said, most of the kids probably wouldn't notice the difference!

Have we dragged Zacky's thread sufficiently off-track yet?

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  Your body might be a temple. Mine is an amusement park...

RE: FET's Issue

zacky, you are generally better off with a single FET, since
gate control is simpler.  Multiple gates can be hard to drive
properly if the rise/fall times are short.
The only time I use more than one is when the power
requirements are greater than a single FET can handle.

smoked, scotty - I have always been under the impression
that an FET was constructed of lots of tiny internal
paralleled devices.  But maybe that is only certain parts.
I have built a small induction heater using 6 in each leg
of an H-bridge; works great. After years of finding places
to mount emitter resistors, and trying to match beta on the
driver transistors, I /love/ FET's. :)
<als>

RE: FET's Issue

Well, crap.  Should'a added the obvious use of paralleled
FET's - smaller (1-3 kVA) backup power systems.  Got about
20 that use 4 banks of 5 ea. RFP45N06.
<als>

RE: FET's Issue

The rest of the guys have already beat me to the punch... the true answer is "It depends on the application".  Multiple parallel FETs will lower the Rds and provide redundancy, but require a more powerful base driver.

If you need reliability, multiple FETs would be the way to go, and give yourself some breathing room spec-wise if one or more of the FETs should fail.  Don't forget to include some sort of indicator for an individual failure, else you could run with a failing unit for a long period of time, only to find out  when another failure drops you below spec at the worst of times.

Dan
Owner
http://www.Hi-TecDesigns.com

RE: FET's Issue

Additionally, for reliability, you need to consider the percentage of maximum rating that you are attempting to use.  Let's say that your analysis shows that reliability requires 50% margin on current rating.  It would be easier to find smaller FETs that provide 50% margin than a single FET to provide that margin.

Ultimately, most reliability models consider temperature as the driving force.  To lower operating temperature, you need to have less dissipation in the FET relative to its maximum ratings.  Therefore, you need to have the operating margin.

TTFN



RE: FET's Issue

(OP)
Regarding to power MOSFET Failure ,is it probably fail shorted or opened?
What will be the casa for IGBT ?

RE: FET's Issue

2
The very large "block type" single FETs are actually multiple FETs if you pull one apart and have a look inside. The question is, what is better, a big single, or do it yourself and parallel your own ?

My biggest concern would be current sharing. With dc or low frequencies there is no real problem. But if extremely high di/dt currents are anticipated, the very compact low inductance package of a large single would be preferred.

If it was an audio amp, 60Hz inverter, or dc regulator I think I would opt for multiple small FETs, the heat into the heatsink would be better distributed, and the parts cost may be less.

If it was a monster >20Khz high frequency switching supply, the big single would be my choice. Cost of repair may come into this too. A big single is easy but expensive to replace. Multiple cheapies may be a real pain to repair, but the parts cost to do so may be lower.

Reliability with FETs mainly has to do with NEVER exceeding their ratings. Easier said than done, but with care and some inbuilt protection either approach should be equally reliable.

RE: FET's Issue

There are several possibilities:
> gate short
> gate open
> junction short -- metallization spikes

Of the three, the most likely is junction short, which is prevented by minimizing the current density, i.e., running at well below maximum rating.  As stated above, even a single FET is composed of a paralleled array of smaller FETs, so junction shorts affect either configuration equally.

TTFN



RE: FET's Issue

One thing that's slightly more likely to happen with a larger FET is that the load current may not distribute uniformly within the die, thereby aggravating the current densities, precipitating earlier junction fails.  On a smaller die, the current non-uniformity is more likely to have been accounted for in the device rating.

TTFN



RE: FET's Issue

According to legend, if you try really, really, really hard, you can turn on the parasitic bipolar transistor and then have all the joys of 2nd breakdown.

It's a dv/dt or di/dt thing from what I can recall from 15 - 20 years ago...

RE: FET's Issue

(OP)
Warpspeed, I am fully agree with what you said, but I have seen some scaring articles such as  "Turn-off Failure on Power MOSFETS" & "Dynamic stress might cause Power MOSFET Failure" &.......
So extra precautions my be required for selecting the suitable switching device specially for high frequecy applications.
Actually, I am working on DC/DC Flyback converter, 120V/250V-25Amps output at 50Khz switching frequecy, and I am looking for a heavy reliable single switching device, if you can suggest specific one ( FET or IGBT ) - cost is not the subject.

RE: FET's Issue

Yes, I think that is probably the whole point.  Static dc maximum ratings are one thing, you do your thermal heatsink calculations, and all is well.

But once you start really punishing the thing with extreme speed and violence, unequally distributed parasitic effects can become most serious. Localised voltage and current spikes can sometimes seriously overload individual devices in a large distributed array.

The manufacturers of the large singles are very careful to form all the internal connections and bonding wires into a compact "pyramid" formation with very short internal interconnections. But with dc or relatively low frequencies, hardly any of that really matters.

RE: FET's Issue

zacky,  For flyback, fast clean turn off is very important, which really rules out IGBTs. The tail current of an IGBT may create significant switching losses in a flyback circuit. Snubbing a flyback supply is counter productive and not really possible.

I would definitely go the big single mosfet route. Fastron and Semicron and possibly other sources should have exactly what you need.

Another approach to really high powered flyback systems is to run two or more flyback supplies in parallel, but out of phase. That can significantly reduce the peak currents both at the source and at the output capacitor. Ripple frequency will then be a multiple, and much easier to filter. It has a lot of other advantages too.

Another useful trick would be to use an IGBT and MOSFET in parallel to switch your flyback converter.

Turn on both together, the low conduction voltage of the IGBT will ensure it carries most of the rising current with fairly low conduction loss. Turn off the IGBT first so that all the load then transfers briefly to the MOSFET.  The IGBT can then turn off relatively slowly while the MOSFET maintains the peak current.  Then switch off the MOSFET to perform the actual flyback function. The IGBT will be well off, and tail current will not then be a problem.

THe IGBT carries most of the conduction current, The FET does the actual switching.  With careful design the whole thing can be made far more efficient than by using either a MOSFET or IGBT by itself. The particular advantages of each type of device can be exploited to the full, and the disadvantages of each device minimised. They compliment each other so well when used that way.
 

RE: FET's Issue

Warpspeed, IGBT's are used to generate the spark in automotive applications (a simple flyback cicuit).  I know this to be 100% accurate b/c I spent several years designing automotive engine controllers.

RE: FET's Issue

Zachy,

I've just got to ask;  what's the input voltage, output voltage and the output current.  The reason I ask is that the flyback "Transformer" can get large and expensive for high power outputs.

RE: FET's Issue

(OP)
It is 120V DC input & 250 V DC - 25 A output.

RE: FET's Issue

At 6.25 kVA, you are venturing into SCR and TRIAC areas.
FET's with a 200v rating (the minimum I would consider IF
everything else was right), are still premium devices.
At 120vdc in, you will be switching over 60A, even with
a very efficient design.  IGBT's with a sufficient voltage
rating are readily available, but suffer from the need to
use emitter balancing similar to BJT's.
<als>

RE: FET's Issue

Zachy,

One more question; is this an isolated flyback topology or a non-isolated boost topology?

RE: FET's Issue

(OP)
Yes,it is insulated Flyback transformer
fsmith, I think SCR's will need bulky commutation components.

RE: FET's Issue

For an isolated flyback at 50 kHz, a peak current of 250 plus amps will be required.  I think that a phase shifted full bridge would be much more appropiate.

RE: FET's Issue

O/k guys plenty of meat here to chew on.

melone, yes most automotive ignition modules certainly do use IGBTs these days in flyback mode. But ignition flyback is a fairly gentle affair !!! First, turn off will be relatively slow due to the low self resonant frequency of a typical automotive ignition coil, and the turn off spike on the primary will be relatively low voltage, perhaps only 200v maximum. Remember it is basically a 12v system being switched.

An ignition system running at 100 Hz cannot really be compared with a 20KHz high voltage switching power supply. Even if the mJ losses per cycle were identical, the total switching loss would be twenty times higher at the higher operating frequency.

zacky, wow that is a lot of power.  The average dc input current may be around 50A, and peak current in the switching device four times that, say 200A.  Probably closer to 250A in a real supply (worst case).

I think the way I would do it would be to use four separate flyback supplies phased ninety degrees apart. Each would then only see 60 Amps peak current. They could run from a master oscillator and some flip flops to frequency lock the whole thing.

If each flyback supply runs at 20Khz, input and output ripple frequencies would then be 80Khz, but of much lower amplitude than one huge single flyback supply.  That will be far easier to filter, and capacitor size, cost, and ripple rating will be much reduced.

Design of the magnetics will be less of a problem too. Skin effect means you cannot just make things bigger, and 250 peak amps is still 250 peak amps. But four independent foil wound flyback transformers (chokes actually) that only see 60 primary amps peak each, is doable. Current sharing between the four supplies will be automatic.

Conduction losses of higher voltage FETs will be a problem for you, but parallel FETs and IGBTs would be one possible way to overcome that.  An interesting project to be sure.

Don't overlook the advantages of the diagonal half bridge flyback topology. For reasonably high input voltage, and high power it can be far more reliable. The peak drain voltage on the switching FETs will be hard clamped to the maximum dc input voltage, and that solves a whole lot of problems. The disadvantage is twice the conduction loss, but I think it well worth it for relatively high input voltage.

Ah, this all stirs memories.  I used to do a lot of this sort of thing once.  Now, being retired, it all seems so very long ago.

RE: FET's Issue

So warp... You just run them all as independent supplies each putting out 250VDC but phase locked?

You say that sharing is automatic...

This is because each one will be at it's output voltage peak at a different time?

You don't need any diode isolation between them?  Or is it a give by their output rectifiers?

RE: FET's Issue

One way to do it would be to use a standard dual output push pull PWM control chip, each output could drive a separate independent flyback supply. These would operate alternately, with each having an "on" duty cycle variable from zero to half the total period. They would then operate 180 degrees out of phase if you like.

So while one is charging up, the other is dumping into the output capacitor. They will load share exactly because the conduction times will be identical, and the load dumps will be into the same output capacitor.

This whole thing could be duplicated to make a total of four independent supplies all operating in phase quadrature. Each of the two PWM control chips need to be synchronised to a master oscillator so they will run ninety degrees out of phase. That will then give you 0, 90, 180, and 270 degree outputs.

The concept can be expanded to have any number of phases.  But the beauty of it is that the total supply and load currents become much smoother. Provided all the "on" times are similar, all the supplies will load share exactly. Getting similar "on" times is just a case of paralleling the control voltages to the PWM chips.

There may also be a certain amount of redundancy if one circuit fails.

RE: FET's Issue

That is sweet!  I like it.  You could save bucks and size of final output caps and get faster response, current pulse size would drop a lot generating far less EMI and reducing ripple currents.  You essentially get to build one smaller cheaper unit, debug it without as much fanfare, then step and repeat.  Very nice.

RE: FET's Issue

Where does power factor correction come into all of this?

Or are the flyback converters just operating off the raw 120Hz bridge rectified AC as a sort of do it yourself pf correction circuit?

This sort of power level is way out of my league,so I've just got to ask :) (curiosity killed the cat etc).

RE: FET's Issue

zacky; your "bulky" commutation components could be as simple as a resistor/cap combination, possibly in parallel
with a diode/zener pair or MOV.  The higher the switching
frequency, the smaller the surrounding components will be,
but the semiconductors must be up-rated accordingly.

smoked; current sharing between paralleled supplies is NOT
automatic.  Some designs have in-built sensor loops to allow
feedback to paralleled supplies, and there are other schemes
to prevent current-hogging.  It is almost never as simple as
hooking multiples and expecting the sources to be balanced,
whether analog or switching.  (Analogs are more forgiving).

zeitghost;  PFC and harmonic reduction is a whole 'nother
subject, and is of great concern in line-operated switchers.
Much of the same problems encountered with SCR/TRIAC input
drives and supplies are reflected in SMPS's - the conduction
period of the input device(s) are not generally the full
cycle, and the results are often ugly and hard to suppress.
<als>

RE: FET's Issue

Warpspeed,

Actually, the spikes are in the 250-500V range.

RE: FET's Issue

(OP)
fsmyth, how simple resistor/cap combination can do the commutation task ? Do you mean by using SCR's with turn off gate?

RE: FET's Issue

I'm a little curious why a conventional full or half bridge can't be employed for this application? The magnetics would be used far more effectively, and the magnetics are going to be expensive. Generally the higher you can push the frequency the better from the point of view minimising size and cost of the wound components, providing you can design a winding to suit. At the frequency you're proposing, use of one of the high saturation ferrites such as 3C85 should be possible which will also help keep the core size down. You could probably get up as high as 100kHz, but the winding design will become trickier. You're almost certainly going to be using tape windings at these frequencies and current ratings.

The suggestion to use multiple phase-locked modules is a very good one - well worth investigating further.

----------------------------------
  Your body might be a temple. Mine is an amusement park...

RE: FET's Issue

Scotty, what you say about the full bridge forward converter being the traditional route to very high power is true. Theoretically it makes the most efficient use of the magnetics, but it is not usually that simple to put into practice.

There are problems of flux doubling and staircase saturation to think about in the ungapped magnetics. Current mode control is not easy to apply because the input current is a squarewave, and if magnetising current is made deliberately low, it will almost be perfectly square.  So protecting it from overload is not a simple task.

I have seen a lot of high powered forward converters run for hours and abused in all sorts of imaginative ways, and then spontaneously go *BANG* at quite low load, leaving a pile of smoking parts, with little clue as to why it actually failed.

Another difficulty is the requirement for an output choke/capacitor combination which makes designing the frequency/phase characteristics of the control loop a far a from trivial task.

On the other hand flyback supplies run in current mode are beautiful things and very easy to apply. Control loop design is simple, and current mode operation is inherently protective.  The only real disadvantage of flyback is the horribly high peak cyclic current drawn from the source. But multiple phased flyback supplies can solve that particular disadvantage rather neatly.

The easiest way to get the phasing is from a Johnston (twisted ring) counter. Two flip flops give you four 90 degree phases, and three flip flops six sixty degree phases, and so on.

RE: FET's Issue

(OP)
Warpspeed, 15 years ago, 500W was the maximum power could be obtained from flyback supplies, what you think about the maximum limit today ?

RE: FET's Issue

I have not looked at all this for a very long time, but basically it depends upon the availability of the magnetics and the switching devices.

For 1.5Kw of transmitted power, that would be 75mJ per cycle at 20Khz. So you need to design a choke that will store 75mJ worth of energy, (assuming that the flyback supply will be discontinuous). That is rather a lot, but the larger U cores, or U and I cores should be well up to the job. Inverter grade ferrite being the material of choice.

Just design the number of turns and wire gauge as for a normal transformer using Faraday's laws, and then gap it to reduce the inductance down to the reqired inductance.

Foil windings may be best, because skin effect will limit you to about 2mm thickness of whatever you decide to use anyway.

The choice of switching device looks a lot brighter these days than it did 15 years ago. And the wider choice of control and driver chips makes a high powered flyback design more feasible too.

I would start off and do a trial design of either a 1Kw module (six required) or a 1.5Kw module (four required), or even a 3Kw module (two required) and see what sort of numbers come up.  Then look at the costs and availability of parts, and then decide which way to go from there.

Probably the biggest headache will be getting ferrite sample cores to play with. Ordering several thousand is easy, getting a single sample pair is hell. All you need to do is get one module fully tested, and then you can scale up the whole thing by using as many modules as you need.

In the end it will take X number of ferrite cores, and Y number of power transistors.  It really does not matter if you build one huge single flyback inverter, or several smaller modular ones. Going to a number of uniform smaller modules has a number of advantages.

RE: FET's Issue

Warpspeed,

Excellent points you raise in your last couple of posts, a lot of real world experience in there which I suspect Zacky has yet to meet. Well worth a star. How long were you in the power converter design game for? Longer than I was, I suspect - I had a major career change about 8 years ago into an industry that wasn't leaving England's shores bound for China.

Quote:

"I have seen a lot of high powered forward converters run for hours and abused in all sorts of imaginative ways, and then spontaneously go *BANG* at quite low load, leaving a pile of smoking parts, with little clue as to why it actually failed."
brought a wry smile to my face. I have similar recollections!

----------------------------------
  Your body might be a temple. Mine is an amusement park...

RE: FET's Issue

Yeah a bunch of smaller ones would also crank up the quantities more quickly for a quantity/price part.

RE: FET's Issue

Ah yes Scotty, I too love the smell of burning silicon in the mornings !

I have been in and out of the power electronics game for many years. The highest powered switchers I had to design were for charging the pulse forming network of a pulsed solid state laser in the megawatt range. It was all hush hush for the government, so cannot say much about it.

Another rather interesting application for a very high voltage, high power switching power supply actually used a radio transmitting valve, believe it or not.

Later, I was designing really small switching supplies for telecommunications equipment. Another interesting job was designing fairly large no break computer supplies that used ferro-resonant transformers. All interesting stuff.

I have to agree with you, in that what at first looks rather simple, can quickly turn out not to be, and there is no substitute for experience.  The only way to get that is the hard way, unfortunately.

RE: FET's Issue

(OP)
Warpspeed, I can see your wide range experience includes ferro-resonant transformers.
If you don't mind, could you please point me to a good reference book about ferro-resonant transformer design.

RE: FET's Issue

I am not sure if there are any good books available. My connection with ferroresonance was when I worked at Sola in Melbourne, and my duties were just the design of the electronics and control system for the beast. The actual ferroresonant transformer itself was an existing design.

You could try looking up the original patents applied for by Mr Sola. I looked through some of those many years ago, there is a lot of very good information contained in there. Apart from that, I have seen very little published information in books. I think they are a bit of a dark secret that not too many people truly understand, it is a rather obscure field.

RE: FET's Issue

(OP)
Warpspeed, another question please, when it will be a good idea to use Foil coil istead of Wire coil ?
Also, regarding to the wire coil, if all the turns of one layer are shorted together, but we have a good insulation between each layer,does that affect the transformer operation?
 

RE: FET's Issue

I suggest you read up on "skin effect". At high frequencies the magnetic field around each wire forces the current to travel only along the outer surface of the wire, along the wires "skin" if you like.

This means you cannot just use a thicker wire to carry more current. The solution is to use a bundle of very thin wires (all insulated from each other), that is called Litz wire.  Another often much better solution, is to use copper foil which has a very high surface area.

The design of the magnetics for switching power supplies is a whole very specialised field in itself. One really excellent hard cover book on all of this is:

Switchmode power supply handbook by Keith Billings, published by McGraw-Hill.  ISBN 0-07-005330-8  

RE: FET's Issue

Another excellent book on HF magnetics is 'Soft Ferrites' by Edgar Snelling. It is probably getting hard to find these days. The Ferroxcube databook is worth a look too - it's a free download off their website. Not anywhere near as good as Snelling's book, but useful none the less.

----------------------------------
  Your body might be a temple. Mine is an amusement park...

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