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#21 | |
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![]() Quote:
![]() And what about this configuration, wouldn't this increase transistor protection all the more? ![]()
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Last edited by EasyGoing1; 04-25-2022 at 12:41 AM.. |
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#22 |
Solder Sloth
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![]() first one is sufficient, D2 is superfluous.
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#23 |
Shock Therapist
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![]() Last edited by EasyGoing1; 04-25-2022 at 04:12 AM.. |
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#24 |
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![]() What are you keeping in the box inquiring minds want to know
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9 PC LCD Monitor 6 LCD Flat Screen TV 30 Desk Top Switching Power Supply 10 Battery Charger Switching Power Supply for Power Tool 6 18v Lithium Battery Power Boards for Tool Battery Packs 1 XBox 360 Switching Power Supply and M Board 25 Servo Drives 220/460 3 Phase 6 De-soldering Station Switching Power Supply 1 Power Supply 1 Dell Mother Board 15 Computer Power Supply 1 HP Printer Supply & Control Board * lighting finished it * These two repairs where found with a ESR meter...> Temp at 50*F then at 90*F the ESR reading more than 10% 1 Over Head Crane Current Sensing Board ( VFD Failure Five Years Later ) 2 Hem Saw Computer Stack Board All of these had ![]() All of the mosfet that are taken out by bad caps ![]() |
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#25 | |
Shock Therapist
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#26 |
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![]() you put drugs in it and link it to an rfid reader so you can supply your "customers"
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#27 |
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#28 |
Solder Sloth
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![]() Refill every after customer? They should only be able to open it if it's their turn...
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#29 |
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![]() I suggest adding a resistor base-emitter to reduce sensitivity to leakage and noise. This is normal practice with regular transistors, and I would think the high beta of a Darlington would make doing so even more important.
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PeteS in CA Power Supplies should be boring: No loud noises, no bright flashes, and no bad smells. **************************** To kill personal responsibility, initiative or success, punish it by taxing it. To encourage irresponsibility, improvidence, dependence and failure, reward it by subsidizing it. **************************** Anti-Covid-Vaxxer pig crap claim/prediction, Doctor: Heart Failure from mRNA Jabs "Will Kill Most People" | Principia Scientific Intl. ; Dr. Geert Vanden Bossche Warns COVID-19 Jab Injuries and Deaths Will Soon "Collapse Our Health System" (VIDEO) ; Fully Vaxxed May 2021; Since that time I've done 7 5Ks, 1 8K, 8 10Ks, and 4 half marathons |
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#30 |
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![]() The TIP120 has them built in, otherwise it would be even slower switching. Plus you get a free diode too.
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#31 |
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![]() I sit partially corrected about the TIP120 incorporating B-E resistors. However I think those are to prevent leakage or noise from turning on the Darlington. IIRC, it's a C-B resistor that keeps a transistor or Darlington pair from saturating. Saturating would slow the turn-off time.
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#32 |
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![]() As I know, the E-B resistors are mainly to dissipate stored charge and thus speed up turn-off times. If you build a Darlington on the breadboard out of two transistors, they are super slow switching without them. But they also work for keeping the transistors off due to leakage or an Arduino pin floating during RESET.
I just noticed the datasheet switching time test is a cheat - they apply a -4V off-bias and +12V pulse to get (force?) the ~1usec rise/fall times. Darlingtons are slow, and lossy because of their high VCE sat. 2V at 4A and 4V at 5A means they dissipate much more heat that a mosfet. I think OP is fine using them for the solenoid driver, but PWM with an LED or running the TIP120 at a few amps it would just run hot and need a heatsink. I haven't seen a C-B resistor used to help with saturation, or a Baker Clamp. I tried making SMPS with Darlingtons and it was awful compared to mosfets. |
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#33 |
Solder Sloth
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![]() Compare a TIP120 C-E drop with an IRF620 D-S drop at 5A load ...
![]() Yes these things are slow but there have been plenty of SMPS with BJTs, though I can't say, other than the MC34063, many Darlington designs... and it too wastes power to speed up the Darlington. |
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#34 |
Shock Therapist
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#35 |
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![]() redwire is correct that I was confusing a C-B resistor with a Baker Clamp, which uses a diode to reduce storage time. Lo siento.
Basically, transistors' rise, fall, and storage time combine to limit the practical switch frequency of a transistor-based SMPS to less than ~50KHz. Above that practical frequency a transistor is either off or in its dissipative linear region for too much of the potential "On" time to allow the necessary input voltage range for an off-line SMPS. |
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#36 |
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![]() TIP120 switching times are about 0.5usec on and ~3usec off with a -ve voltage drive to force it off faster. If you're not doing that or saturating it, it's much slower I would say up to 10usec to turn off in the real world. You could not use it to pulse an LED at 1usec say. Note the datasheet complicated test circuit is using +8V and -12V drive, which is unrealistic.
Power MOSFETS are 100x faster and less losses when fully on, so less heat. This is why they are king in SMPS use. TIP120 is OK as a power switch as long as you are OK with the losses/heat VCE(sat)=2V and not switching really fast. |
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#37 |
Solder Sloth
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![]() IRF620 is only around 10x faster than the TIP120
![]() BTW, this is specific to Darlingtons. I'm pretty sure you can build custom BJT circuits faster than off the shell Darlingtons, after all people have used BJT output transistors for RF circuitry -- note that the gain per stage is very low and separate from each other to keep the speed up. |
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#38 |
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![]() IRF620 switches in nanoseconds, not microseconds. FAST.
I've tried building SMPS using Darlingtons and it was basically terrible to get past even 30kHz out of them. It's because their gain is so high, bandwidth is low. Turn-off takes forever. I suppose you could add circuitry to lessen charge storage. But the last power transistor having a hFE of 10-20 is no fun either. Back in the old days of the first SMPS i.e. Apple and others in the 70's didn't even use them as I recall. Today, it's a single BJT with transformer drive in SMPS. |
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#39 | |
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The faster switching times of MOSFETs took the practical maximum switch frequency way above 100KHz, which shrinks magnetic components, and probably were what made zero-voltage-switching quasi-resonant phase modulation designs practical (add in synchronous rectification, and welcome to >90% efficiency). |
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#40 | |
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The "big" goal when I was at Boschert (Bob Boschert was still there, though as the VP of Engineering; Ray Noorda was the CEO) was to get their flyback models down to or below $1 per watt and efficiency above 70%. At 150W and above, Boschert used a two stage design, a primary side buck regulator followed by a square wave inverter. The controller for the buck regulator was a 723C, a linear regulator adapted to be a switching regulator. Boschert did current mode designs. The then-popular SG3524 series was voltage mode, and the current mode UC384# series was not yet available. There still were occasional wooly mammoth sightings in our part of Sunnyvale. ![]() Last edited by PeteS in CA; 05-13-2022 at 12:30 PM.. |
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