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Old 8th October 2004, 10:16 AM   #31
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Hi Lars,

I think most can agree that the "180W" value tagged onto the small module is slightly optimised, given the commercial nature of the venture. The same circuit is known here as a 150W/6ohm amplifier. Nevertheless, since audio circuits do not as a rule process continuous sine waves, the 100 degrees C are unlikely to be achieved on music.

Now, I would have taken action to keep JP from publicising a 180W number if it was unsafe. One should keep in mind that the 6A figure is for an overmold package and 9A for an uninsulated package. The reason is that "case" temperature for one is the metal surface for the TO220 and the plastic surface for the TO220FP, both being the first "externally accessible temperature".

Curiously, if one uses noninsulated TO220's with impregnated glassfibre thermal washers, the situation is very similar. The thermal resistance of these is typically 5K/W (Bergquist Sil-Pad 400) to 2K/W (Bergquist Sil-Pad 2000), on average very comparable to the 3.5K/W that the overmold (full-pack) TO220's add compared to the noninsulating ones. To compare apples to apples we can elect to measure temperature at the heatsink (accessible in both cases) or at the TO220 metal plate (accessible if you chip off some of the plastic on the face side).
You’d find the temperature is roughly the same for both (which is better depends on whether you’re using standard or good sil-pad material).

We may wish to do the same exercise with the IRFB38N20, and see what the current handling will be for a heatsink temperature of 100 degrees, presuming normal mounting practics.
The 32A figure corresponds to the maximum junction temperature of 175 degrees (75 deg above 100 deg case). Cross-check (never trust a data sheet if the numbers don’t jive): Ron at 175deg becomes 155mOhm. At 32A this is a power dissipation of 159W. Rated Rth,j,c=0.47K/W, yielding 75 deg temp rise. Spot on. Good data sheet.
Now insert a 3.5K/W thermal washer (I do not know what insulator you are using, fill me in on the exact numbers). Thermal resistance to the heat sink becomes 3.97K/W. At 100 degrees heat sink temperature we can dissipate 19W before junction temperature goes out of spec. This corresponds (again at 155mOhm Ron) to 11 (ELEVEN) amperes, rather a far cry from the promised 32A.
Your own suggested rule for wide-bandwidth would give us no more than 242W at 4 ohm or half that at 2 ohm.

Luckily for both of us, it doesn't work that way.

The metal tab (external on the TO220, internal on the TO220FP) does not respond to 20Hz in a significant manner. You will find this info on the thermal response graphs on page 4 of the data sheet (mind the horizontal scales are different). Otherwise put, the currents found (6A for the STP14NF12FP, 14A for the IRFB38N20 using Silpad 2000) may be used with presumption of "power sharing", even at 20Hz.

Under this rule, at a heat-sink temperature of 100 degrees, the UcD module remains within spec up to 144W (also at 20Hz) and the Zappulse modules will do 484W.

Nevertheless, it is possible to get most from the TO220 uninsulated package, without having to forgo electrical insulation. This is done using ceramic (alumina) insulators. A typical 1.5mm thick TO220 Al2O3 insulator has around 0.4K/W worth of thermal resistance. Counting on this requires excellent planarity of the TO220 and the heat-sink in order to allow the thinnest of grease films to be applied. It shall be no surprise that the UcD400 modules are constructed in this manner. Recalculation of the DC current rating at 100 deg heatsink temperature and a combined Tj,sink of 1.6K/W produces 21A. By anyone's rules this is more than sufficient for 400W into 4 ohms. The 400W rating of the UcD400 is therefore no hyperbole (the voltage restriction of 63V is produced by the use of 63V elcaps – moving to higher voltages to match the +/-75V rating of the FETs is under consideration).

It may be of comfort to you that there is no standard requiring an amplifier (for consumer use) to deliver full rated power continuously at a heatsink temperature of 100 degrees C.
The most stringent test used to verify commercial power ratings of end-user equipment is the IHF (FTC) power test. The amplifier was first run at 1/3rd rated power for an hour, then full power for 5 minutes ("was" because the test now specifies the pre-heating at 1/8Pr, who said there was no audio industry lobby). During this test, there is no requirement for all parts to remain within their ratings (!) as long as the amplifier delivers its rated power at the distortion quoted for this power. Under all circumstances, music is a much more relaxed condition, even in this age of hypercompressed CDs.

Cheers,

Bruno
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Old 8th October 2004, 11:45 AM   #32
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Hi Bruno

Thanks for your thorough answer Just to exact the details, here is a short recalculation:

We use the st. Gobain typ. 1877 insulator which has a thermal resistance of 1.4 K/W for a TO220 package. (0.24 K In2/W).

The total t resistance from junction to sink is: 1.87 K/W.

Using your method of calculation we have 15.8 Ampere
to work with, at 100 C in each device. Corresponding
output power (without extending the power according to your
interpretaions of the graphs on page 4) is 499 Watts in 2 Ohms, and 998 Watts in 4 Ohms.

About UcD400, the ceramic insulators are a really nice touch!
Just another question, can the supply voltage be raised if the capacitors were to be exchanged to 100 V types?

All the best from

Lars
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Old 8th October 2004, 12:39 PM   #33
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Quote:
Originally posted by Lars Clausen

About UcD400, (...), can the supply voltage be raised if the capacitors were to be exchanged to 100 V types?
The FETs are 150V devices so the overvoltage detection would be set to trip at 75V.
For a normal PSU design, you'd go for an idle voltage of 67V under nominal mains condition so it doesn't exceed 74V at 10% overvoltage.
That makes it practical to get the actual 400W out of the modules with a realistic supply (with the current 63V setting this is difficult).
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Old 12th January 2005, 04:19 AM   #34
Kenshin is offline Kenshin  China
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How can I get a shemetic of UcD?

Quote:
Originally posted by classd4sure
There's a good choice of high end drivers out there that will allow you to do it easily enough (non you can actually simulate with as far as I know), and if you look at the UCD circuits around here you'll see a discrete method which should provide some good insight as to how it's done discretely, and which do simulate easily enough.

Cheers,
Chris
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Old 12th January 2005, 05:14 AM   #35
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Hi,

Welcome to the forums.

You can find some in here:

Development of a "reference" class D starting point

If you dont' want to read the whole thing, skip to the end there's a fairly simplistic, low power one there, made for parts I had around here, I built it on a protoboard.

Here is an example at a full bridge attempt, I think it still needs improving, ie. the comparator outputs are still susceptible to current hogging. I think I negated the benefit of the cascode in my revised version, but it created some time delay problem with other configuations for some reason.

I'd like to do away with the summing op amps, or preferably, all of them, having only a discrete class A input stage. This circuit has not been built. With some tweaks to the input stage (which were found to be meaningless in spice) it gets down to the mid 0.00* THD range. As it is it simulates very well. I might work on it more some day. Better gate drivers would be a big help too, it is too easy to load these ones down!~

UCD Based Fully Differential Full Bridge 450W RMS Amp

Non of these schematics are good for AC analysis unfortunatly, but work well enough in transient. If you have orcad and want to try one of the attached .dsn files it won't open it with pspice. Best workaround is to open the .dsn file in orcad, create a new project, copy and past it into the new project, then you can simulate it.

Feel free to add your feedback to those threads if you like.
Regards
Chris
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Old 12th January 2005, 08:08 AM   #36
Kenshin is offline Kenshin  China
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Quote:
Originally posted by classd4sure
Hi,

Welcome to the forums.

You can find some in here:

Development of a "reference" class D starting point
Chris
this includes the discrete high-side drive?
following the link I can't find it.
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Old 12th January 2005, 08:39 AM   #37
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Quote:
Originally posted by Kenshin


this includes the discrete high-side drive?
following the link I can't find it.
This one works. Yes it has a bootstrapped high side supply.

http://www.diyaudio.com/forums/showt...588#post472588
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Old 12th January 2005, 11:37 AM   #38
Kenshin is offline Kenshin  China
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Quote:
Originally posted by JohnW
OOOOPS,

Sorry I sent the wrong version of the PDF file for the Real Budget 100W OPS above, here's the correct one.

Iq with 15R shown, +/-22mA @ 384KHz, +/-25V

when the PSU is turned on, the couple capicitors needs charging. Will the charging current turn on the BOTH MOSFETS and cause short circuit?
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Old 12th January 2005, 11:43 AM   #39
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something looks complicated -- more complex than what I had expected (open collector output stage & voltage follower & boost at the high side to power the voltage followers).

Quote:
Originally posted by classd4sure


This one works. Yes it has a bootstrapped high side supply.

http://www.diyaudio.com/forums/showt...588#post472588
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Old 12th January 2005, 12:00 PM   #40
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Quote:
Originally posted by Kenshin



when the PSU is turned on, the couple capicitors needs charging. Will the charging current turn on the BOTH MOSFETS and cause short circuit?
I'm pretty sure you'll see some pull down resistors on the gates to prevent this from happening. In fact I think it's mentioned in one of the posts here.

Quote:
Originally posted by Kenshin
something looks complicated -- more complex than what I had expected (open collector output stage & voltage follower & boost at the high side to power the voltage followers).

It might seem complicated but look at this way, where's the level shifters?
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