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FSAM75sm60A Fault Protection Timing Circuit

Discussion in 'General Electronics Discussion' started by Fish4Fun, Aug 28, 2016.

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  1. Fish4Fun

    Fish4Fun So long, and Thanks for all the Fish!

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    Aug 27, 2013
    I am likely making this too hard, but a bit of help sure would be appreciated! I picked up some FSAM75sm60A "NOS" ICs off ebay a while back for cheap....and I thought I might design a test bed for one of them....I was trying to "rough in" some component values from the data sheet, and things were going quite nicely until I got to the short-circuit-protection segment.....In the 2013 version of the PDF there is very little data on the selection of "Rsc, Rf, Rcsc and Csc"....In the 2014 version they suggest using 0 ohm current sense resistors and a value of 26ohms for Rsc on page 7 in "2nd Notes #5" .....later, on page 13 in "4th Notes #8" they go onto suggest: "In the short-circuit protection circuit, please select the RfCsc time constant in the range of 3 ~ 4uS." But no matter how many times I calculate the voltage drop across a Zero ohm current sense resistor, I simply can't make the drop across a 26 ohm resistor greater than Zero which, of course begs the question, "How do I calculate the time constant created by Rf, Csc and Rcsc when the drop across Rsc (26 ohms) calculates to 0V when placed in parallel with 0 ohm sense resistors?"

    A link to the Data Sheet is here:

    https://www.fairchildsemi.com/datasheets/FS/FSAM75SM60A.pdf

    Obviously I am missing SOMETHING.....but if they have gone to the trouble of updating the PDF to suggest using 0 ohm sense resistors and a value of 26ohms for Rsc, why didn't they just go ahead and give the values for Rf, Csc and Rcsc and be done with it? I realize there is a wide range of values that **would work** if there were a voltage present (and obviously there must be some voltage on Pin10 when the current reaches 125A, but I am at a loss as how to figure out what it might be....).

    I know it is a big ask, but if anyone has a few moments to look @ the PDF and can figure out what I am missing, I sure would appreciate it.

    Thanks in Advance!

    Fish
     
  2. hevans1944

    hevans1944 Hop - AC8NS

    4,606
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    Jun 21, 2012
    On page 8, Figure 6, the values of Rsu, Rsv, and Rsw vary from 0 to 0.030 Ω with appropriate current limiting Rsc values diminishing from 27 Ω down to about 2 or 3 Ω as the "ground" resistance in each of the three phases increases. The Rsc resistor connected to pin 10 is NOT in parallel with the three current sense resistors. It is connected to the "current sensing" emitters of the three IGBTs and samples a small fraction of the output emitter current from each of the IGBTs. This is a "feature" for this type of IGBT construction. As the "ground" resistance gets larger (up to 0.030 Ω) less resistance is required for Rsc to achieve current limiting at either 75 A or 100 A as shown in the two curves in Figure 6 on page 8.
     
  3. Fish4Fun

    Fish4Fun So long, and Thanks for all the Fish!

    464
    105
    Aug 27, 2013
    Hey Hop!

    Ok, I "see" the extra arrow in the IGBT....so there is some voltage on Pin10.....and that voltage is a function of Rsc and the current sense resistors.....I missed that, and it certainly 'helps'....But, I still don't understand how to define values for Csc, Rf and Rcsc such that: "the RfCsc time constant in the range of 3 ~ 4uS." I guess I could guesstimate the current sense emitter voltage with respect to primary emitter current from figure 6 on page 8 and then select Csc, Rf and Rcsc based on the guesstimate, but that seems like a very imprecise way to arrive @ the require time constant of 3 ~ 4uS....Am I still missing something, or is that really the way I should proceed?

    ****For the record, I am NOT planing on driving a motor from line voltage with my "test bed", or even a particularly high voltage BLDC....likely start with a 24Vdc supply with a max current capability ~ 20A, and, if things go well, might work my way up to an 80V analog supply I have on hand (massive 60hz line transformer regulated output 60V, 8A .... but the regulator and Rectifier circuits are modular and are designed so that the output terminals can be connected to either the un-regulated DC or simply the transformer output.....I would guess the transformer could output > 30A without breaking a sweat....weighs > 50lbs, LoL.) .....So over current protection is not strictly requisite at this stage, or even in the near future, but since I have some number of these modules, if testing at lower power levels goes well, I **might** eventually want to design/build a line driven driver for a 2kW to 6kW spindle motor...in which case I would absolutely want over current protection....so I thought it would be a good idea to build it into the initial test bed...****

    Oh CRAP, LoL....I am definitely OVER THINKING this! If the "Current Sensing Emitter" is an analog voltage AND the voltage is a function of the Emitter current (as obviously it is), then I don't need a timing circuit at all, I could simply connect Pin10 to an ADC and monitor the real-time current via the ADC and if there is an "Over Current Condition", it can be handled in firmware.....that would be MUCH better for my purposes...obviously if I were designing a BLDC driver for a washing machine or an HVAC system (a couple of the suggested purposes for these modules), then an ADC would be over-kill, but for a CNC spindle for an extreme DIYer who loves whistles and bells, having real-time access to the output current would be like icing on cake!!!

    THANKS Hop! I was so focused on "over-current protection" I missed the more salient benefits of "real time current monitoring"....As long as I start with a supply that is not capable of creating an "over current" situation, I can adjust ***Rcsc/Rf/Rsc/Csc*** to suit a dedicated ADC/uC that would cover the full range of output current and "display" the real-time current....and it would be easy enough to add an "adjustable" analog comparator as a binary over-current signal to the uC providing the switching signal so that "over current" could be adjusted to suit the particular BLDC motor being driven....

    Again, THANKS!

    Fish
     
  4. hevans1944

    hevans1944 Hop - AC8NS

    4,606
    2,151
    Jun 21, 2012
    I like this chip and was considering it for large stepper motor drivers. If I understand it correctly, you can monitor the voltage on pin 10 to get a measure of output load current, but its main purpose is feedback into the chip to shut it down if there is a current overload condition. Nice feature that. The RC network does not appear to be critical to the operation. It's just a filter to prevent "false positive" tripping of the overcurrent shutdown mode. Note there is NO short-circuit protection for the high-side IGBTs, but I guess some protection is better than no protection.

    This chip is apparently designed for DC-to-AC inverter applications, which would come in handy for a grid-tied solar panel installation. Not sure how or why you need three phase outputs for that.

    I am also not sure what I have to do to heat-sink this puppy properly, or if a heat-sink is even required for low current operation at just a few amperes. I have zero experience with IGBTs but seem to recall that for a given current they dissipate more power than a MOSFET.

    So, how much did you have to pay for this on eBay?
     
  5. Fish4Fun

    Fish4Fun So long, and Thanks for all the Fish!

    464
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    Aug 27, 2013
    Hey HOP!

    This family of Chips are designed primarily for driving 3 phase motors for low to medium power applications (this particular chip is for 2kW to 6kW motors)....suggested uses are HVAC fans/compressors/pumps, Consumer Washing Machines etc, etc....

    As I understand it, IGBTs are designed for relatively high voltages where a forward voltage drop is preferable to RDSon..... IGBTs are typically rated @ 500V and up; so,switching 10 Amps with a forward voltage drop of 1V gives a fairly constant device power dissipation of I * Vf --> 10W at Vcc = 50V-500V while a 500V mosfet would have a device power dissipation of I^2 * RDSon.....But before we get all excited about IGBTs, they are pretty slow compared to mosfets....typical switching frequencies are 1kHz to [email protected] lower voltages mosfets are clearly superior in both power dissipation and switching speed....(not to mention relative immunity to "latch-up")...but in the case of line driven BLDC motors where typical "on times" are in the mS time domain, switching speed is not as critical as it might be in low voltage//high current BLDC motors typical in the RC hobby industry....And all that being said, in the last decade there have been huge strides made in 400V-500V rated mosfets....primarily for the lighting industry.... but certainly well enough suited for driving line-powered BLDC motors in the 2kW-6kW power range.

    @Cost.....I think I paid ~$50 for 6 of these chips (including shipping....I am thinking they were $5 each + $20 shipping regardless of quantity, and I bought the last 6...but that was two or three years ago and I might have the numbers a bit jumbled...regardless, the lot of 6 was cheap enough I didn't think twice about ordering all of them)....I had an interest in eventually designing/building a line-powered variable speed spindle in the 3Hp range ..... I discovered them on Digi-Key searching HS drivers and on a lark I did an eBay search.....On Digi-Key I had been looking @ the 30A chips, and they were in the $60 each range.....When I found these on e-Bay for < $10 each I just ordered them figuring I couldn't really go wrong....6 * discrete 75A IGBTs would have cost about the same as all six modules....

    Just to prove that sometimes patience is more prudent than DIY....I was investigating building a 3Hp spindle driver because @ the time 2.2kW spindle & driver combos were just emerging from China @ a bit North of $2k....which was a fraction of the price for a US made spindle, but still a bit too much for my CNC Router Hobby Budget......Over a year ago I bought a 2.2kW Spindle and matching driver for $450, LoL.... So at this point I am not really in need of a 2kW-6kW BLDC driver, but since I have the chips, I thought I would play with them....I do eventually plan on building a CNC lathe, and at some point I would like to re-power my step-pulley driven Mill....both of these projects would require a somewhat lower RPM motor than the ones currently available for cheap from China.....but I suspect long before I finish exploring these chips (and find suitable motors) I will be able to buy exactly what I want for less than I can DIY them, LoL!

    Again, Thanks for the help!

    Fish
     
  6. hevans1944

    hevans1944 Hop - AC8NS

    4,606
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    Jun 21, 2012
    Yep, a lot of my "someday" DIY projects have fallen by the wayside since the Chinese started making stuff dirt cheap. But I will continue to piddle around with things, simply because I enjoy doing so. I bet you will too!

    Thanks for the info on three-phase inverters for motor drivers. When I was working, we purchased and installed two of those to drive variable-speed motors for two bearing and lubricant test rigs. We simply copied an existing installation of some twenty rigs that were already in use at a lab our company ran under contract for the Air Force at Wright-Patterson AFB. The product was made in Germany and performed flawlessly. I don't remember where the motors came from. You might be able to buy an equivalent product today, made in China, for a tenth of the price we paid.

    I did a little research and discovered you mount the IGBT controller package upside down on a heat sink. There is an accessory thingy that bolts on top of it to make attachment of the heavy current-carrying leads easy. Presumably all the support components are molded inside and potted with mating DIP connector pins because only the control and power leads were visibly present on the outside. I think the whole design is obsolete now, and probably unobtainable at reasonable cost, unless someone like you stumbles onto "New Old Stock" for sale at pennies on the dollar of the original manufacturer's suggested retail price. So, your six devices are good for one-off projects only. So much of my hoard of old parts is that way, but I can't bear to just throw them all away... and sometimes I succumb to the temptation to purchase historical parts at bargain prices, knowing I will never build them into a project. <sigh>

    Hop
     
  7. Fish4Fun

    Fish4Fun So long, and Thanks for all the Fish!

    464
    105
    Aug 27, 2013
    Hey Hop!

    @ "I think the whole design is obsolete now..." Assuming you were talking about the FSAM75sm60A chips.....not doubting your findings, but Digi-Key currently has 48 of them in stock ( http://www.digikey.com/product-search/en?keywords=FSAM75SM60 ).....Granted Digi-Key wants $78.20 each or $742.90 for 10. The Fairchild website lists the current status of the FSAM75S60A as "Full Production" ( https://www.fairchildsemi.com/produ...m-modules/motion-spm-modules/FSAM75SM60A.html ).....There are currently a few e-Bay offerings, but they certainly aren't a "deal".....

    Regardless of status, I think 6 of these chips will turn out to be a "lifetime supply" for this DIYer! (And likely they will be VERY obsolete before I use the last one, hehehe.) I do have LOTs of obsolete parts on hand...some in fairly large quantities....some other parts I have absurd quantities of simply because I needed a few and "found a deal"....case in point, I recently needed ~20 x 0.1uF/50V ceramic decoupling capacitors in an 805 footprint (I generally use 1205 footprint passives for my projects....they are easier to see....)....sure, I could have ordered 100 for a ~$1 or so, but I ran across 10,000 of them for $7....will be a while before I have to scratch that itch again ;-) LoL I think the popular term for it is "hoarding" :) Back when California decided to "Ban Lead" (how can you ban an element?), I found a deal on some premium 63/37 made in USA solder.....~50lbs of it...in theory it was $0.50/lb ( @ the time I thought they were trying to get rid of some 1# ingots like they use to use on big plumbing and roofing jobs....I was planning on making fishing sinkers out of them, but when I went to pick it up, I discovered the solder was premium electronics grade spools)......to top it all off, the company didn't have the infrastructure to make a "cash sale" so rather than "set up an account" so I could pay them, they just gave me all they had on hand....I have only used a couple pounds so far, so that too is likely a lifetime supply, LoL. Hrmmm, sure wish I had bought some assorted sizes/colors of solid and stranded copper hook-up wire "back in the day"....

    Sorry....I am rambling....bit lost in nostalgia....

    Have a Great Day!

    Fish
     
    hevans1944 likes this.
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