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Old 21st September 2004, 11:58 PM   #21
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Quote:
Originally posted by Konrad
Driver, note the reverse diodes for the mosfets are important!
Hello,

Hi Konrad, diode D220 might actually have the adverse affect of holding the gate higher than the transistor would keep it on its own. Say it's clamping it to ~.7V when the transistor would clamp it to ~.3V. You could reduce that by using a schottky perhaps.

Regarding the parallel body diode, I fully agree with everyone there.

I've learnt that it helps the reverse recovery of the body diode simply by not allowing it to become fully saturated, which lets it recover much faster.

Forward current of a few amps is actually sufficient as the body diode handles the full current, but it should be rated to block full rail to rail voltage, same as the mosfets Vds. Rectifier schottkys are perfect for the job. Of course it's no substitute for proper timing.

I've found MOSFETs with "fast body diodes", IXYS for example, has a good variety of them, but looking at the rest of their specs and from the way they simulate, it would take one of their monster current gate drivers to get them to switch half decent. I trust the "fast diode" is nothing more than an internal schottky.

Anyone know of other MOSFET lines that incorporate such a thing worth looking at?


Cheers.
Chris
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Old 4th October 2004, 07:13 PM   #22
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Quote:
Originally posted by classd4sure
I've found MOSFETs with "fast body diodes", IXYS for example, has a good variety of them, but looking at the rest of their specs and from the way they simulate, it would take one of their monster current gate drivers to get them to switch half decent. I trust the "fast diode" is nothing more than an internal schottky.

Anyone know of other MOSFET lines that incorporate such a thing worth looking at?
Sorry just dropping in.

The body diode of a MOSFET is an inherent "parasitic" device. As such it can have any of the recovery characteristics other diodes have. The opportunities for optimisation are much less unfortunately, e.g. the doping profile required to obtain softer recovery characteristics of the diode adversely affects the on-resistance. Because of these tradeoffs, the body diode of a MOSFET will always be somewhat lower in performance than similarly sized dedicated diodes.

So far, MOSFET/Schottky combos are co-packaged devices. Integration of schottky diodes onto the MOSFET die has been experimented with (simple extension of the source metal should do) but with only limited success, largely because diode current would make quite a detour through the epi layer, resulting in a less than optimum resistance.

The use of separate schottky diodes is not very efficient. Suppose even a co-packaged diode (IR Fetky). Total wiring inductance is on the order of 4nH.

Now suppose you're conducting 10A in reverse though the channel. Next you turn off the FET. Under 10A the body diode will develop 0.8V, the schottky would develop 0.4V. The schottky won't be seeing any of this current for a while though. It was flowing through the MOSFET bond wires and due to the inductance it will continue to do so for a while.
At a 0.4V difference across 4nH current will transfer at dI/dt=U/L=0.4/4nH=0.1A/ns. It would take a bl**dy one hundred nanoseconds! You'll guess that waiting 100ns is the ideal way of getting lots of stored charge into that diode.

The schottky is effective in preventing storage of charge during reverse conduction through the channel when the drop across the on resistance exceeds the diode voltage. Charge storage in the diode when the gate is positive is relatively minimal, so unless you're using very old FETs (like the IRF9540 in the 100W SODA...) adding a schottky does positively nothing.

How To Prevent Stored Charge:
1) Minimise dead time. That's when charge gets stored. This is practically feasible and the effect on efficiency is very measurable.
2) Hold the gate sub-threshold instead of driving it to zero when you are turning the FET off. This is impractical and verges on the impossible, but is an interesting detail in MOSFET physics.
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Old 5th October 2004, 07:27 PM   #23
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Default MOSFET on ucd180

Hello Bruno,

can you disclose to us which MOSFET's you used on the UCD180 modules?

thanks,

Ludo
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Old 5th October 2004, 08:19 PM   #24
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Great reply Bruno, by all means, drop in more often
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Old 6th October 2004, 07:30 AM   #25
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Default Re: MOSFET on ucd180

Quote:
Originally posted by lbruynseels
Hello Bruno,

can you disclose to us which MOSFET's you used on the UCD180 modules?

thanks,

Ludo
STP14NF12FP. At the time when we selected them (2000) they were the best compromise between switching, soft recovery and cost. Actually they were the lowest cost fets available anywhere for the given current and voltage rating, and they happened to have way better than average performance
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Old 7th October 2004, 03:20 PM   #26
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So my conclusion is: Don't use any external diodes, it doesn't help much, but keep deadtime as low as possible while turning the FETs on at significantely slower speed than turing them off.

Is this right then ?

Regards

Charles
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Old 7th October 2004, 03:57 PM   #27
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Bruno: Very generous of you to disclose your choice of MOSFET's.
This reminds me of an old speculation: how to calculate safety margins for the MOSFET's ???

What is your opinion about or method for this? I can see from the datasheet of STP14NF12FP that they have extremely low Cfb of just 30 pF, and can (safely) deliver 6 Ampere, (at Tc 100 C) . But to produce the spec'ed power of 180 Watts in 4 Ohms, you need a peak current of ~ 10 Ampere. (Vrail = 40 Volts and not taking any significant rail loss in to account).
Does this mean that your module has to be limited from reproducing full power at low frequencies, so to prevent overloading the MOSFET's?

What is the safe loadability of the UcD400?

All the best from

Lars

EDIT: Perhaps i should disclose my own method of calculating the safety margin:
Maximum specified load should not exceed the DC load capacity of the output devices at 100 deg. C.
If bandwidth is limited from 20 - 20.000 Hz, (should be clearly stated in specs), current can be shared between two devices, and divided by sqrt2.

A IRFB38N20D (of the ZAPpulse) can carry 32 Ampere DC at 100 C.
This gives the amplifier a max loadability of (((2 x 32) / 1.41)^2) / 2 = 1030 Watts in 2 Ohms. (We spec. max. 1 kW). Limited bandwidth gives a loadability of 1 kW in 1 Ohms.

The UcD in the same calculation:

STP14NF12FP can carry 6 Ampere DC at 100 C. This give the amplifier a max loadability of (((4 x 6) / 1.41)^2)/4 = 72.4 watts in 4 Ohms. You spec 180 Watts in 4 Ohms.
At limited bandwidth you get 1.41 times 6 = 8.46 Ampere.
Then at my safety limits (calculated the same way as above) gets you a safe output power of: 144 Watts in 4 Ohms.
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Old 7th October 2004, 05:01 PM   #28
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When I browsed the ST Microelectronics I was impressed by the difference between their MOSFETS AND HEXFETS.

For example, in 1996 the company (STMicroelectronics) announced a new technology known as Mesh Overlay that significantly improves the performance of power MOSFETs, leading to smaller die sizes, lower costs and higher performance.

The parasitic capacitances are greatly lowered permitting much faster switching. Channels are placed adjacent to each other in a row. That way, they can be shorter and smaller and still maintain the same d-s voltage rating. Yet one must be cognizant that HEXFETS possess greater power handling since their bigger channels have more thermal mass and more surface area to transfer heat.
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Old 7th October 2004, 07:15 PM   #29
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subwo1: Thanks for this tip

I checked out ST's homepage, and found the closest alternative for our use, the STP40N20. Even though the rds on is slightly lower on the ST device, it can only withstand a load of 25 Ampere, compared to IRFB38N20D's 32 Ampere (both at 100 C).

The capacitances are :

STP : 130, 580 and 3500 pF
IRFB: 73, 450, and 2900 pF.

So the IRF is better in every aspect except the rds on.
What does this mean in real life?

At full load 580 Watts in 4 Ohms, we have an RMS current of: 12.06 Ampere. With the IRF's 9 millioms higher rds on, we get a serial loss of:

12.06 x 0.009 = 0.11 V * 12.06 A = 1.3 Watt serial loss.

I think this loss is more than compensated for by the IRF's lower capacitance, and hence lower idle loss.

On the other hand the ST devices would limit our safe operating area to: 625 Watts compared to the IRF's 1030 Watts in 2 Ohms. So in effect 2 Ohms loadability would be off the table with the ST's where the IRF's can make it possible.

No i think all in all the IRF devices are better than the ST's. The ST's may be cheaper though, i haven't checked. No matter what we stick with IR
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Old 7th October 2004, 08:52 PM   #30
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Lars, you're welcome.
IR has striven for ruggedness and reliability. I have actually let smoke out of some HEXFETs during test without failure. I would like to point out that ST has many classes of MOSFETs that vary in properties. The suffix at the end of the part number is often indicative.

Description of ST MOSFET types
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