| Electronics > Projects, Designs, and Technical Stuff |
| Power bipolar transistor die -> parallel??? |
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| T3sl4co1l:
--- Quote from: exe on November 13, 2019, 10:07:06 am ---gate will conduct if reverse biased. Anything else? --- End quote --- Well, forward biased. Well, both I suppose. Well... so will MOSFETs, in fact you get increased leakage near Vgs(max), it's just that it's not enough current to prevent it from going pop (it's not an avalanche breakdown phenomenon), and then you have a MOSFET that's very leaky and doesn't turn off all the way. :P On that note, never tried JFETs (of any sort) in avalanche breakdown. Wonder if it's okay for small amounts, or large amounts too, if it has a breakover effect like BJTs, or if's just destructive. Tim |
| David Hess:
The 2SC2922 is a a ring or perforated emitter transistor. This is not quite the same thing as multiple smaller ballasted transistors in parallel which is common for RF transistors and integrated power transistors. The special emitter structure prevents current crowding so bandwidth and current gain are improved at high currents. The safe operating area is also improved and they have a reduced or sometimes no secondary breakdown region. These characteristics can be used to identify them if the manufacturer does not otherwise say. Apparently they also have very low base and emitter resistances making them low noise and low saturation. What is not to like? They are very popular for audio output stages but also work well in switching applications and high performance linear regulators. On Semiconductor makes a lot of them. I think the old Zetex Super E-Line transistors were constructed this way which is why TO-92 sized parts can be found which can switch several amps. |
| Noopy:
--- Quote from: David Hess on November 13, 2019, 12:28:21 pm ---The 2SC2922 is a a ring or perforated emitter transistor... --- End quote --- Thanks for the explanation! :-+ --- Quote from: T3sl4co1l on November 13, 2019, 10:47:02 am ---On that note, never tried JFETs (of any sort) in avalanche breakdown. Wonder if it's okay for small amounts, or large amounts too, if it has a breakover effect like BJTs, or if's just destructive. --- End quote --- I did some research regarding the LF355 in the Input circuit of the Gould 4074 (https://www.richis-lab.de/Gould407X.htm). The LF355 has J-FET-Inputs. The Application Note 447 from National Semiconductors (http://www.ti.com/cn/lit/an/snoa736/snoa736.pdf) says: "If either input is pulled above V+, nothing happens until the difference between the input and V+ gets near the breakdown voltage, typically 50V. At this point, the FET’s gate-source junction avalanches and will draw all the current it can. Limiting this input current to something less than 3 mA helps prevent damage." and "Failure to clamp the voltage or limit the current adequately may not destroy the part, but the offset voltage and bias current will be permanently degraded." |
| T3sl4co1l:
--- Quote from: Noopy on November 13, 2019, 10:58:49 pm ---"If either input is pulled above V+, nothing happens until the difference between the input and V+ gets near the breakdown voltage, typically 50V. At this point, the FET’s gate-source junction avalanches and will draw all the current it can. Limiting this input current to something less than 3 mA helps prevent damage." and "Failure to clamp the voltage or limit the current adequately may not destroy the part, but the offset voltage and bias current will be permanently degraded." --- End quote --- Fascinating, an unusually candid explanation. I would guess IC FETs to be perhaps a bit more consistent than discrete ones; so, not that they'll necessarily handle any more current (discrete ones are typically rated for ~mA as well, under forward bias that is), but that they give a hand-waved chance of failure (rather than "outside of warranty / you're on your own", say) beyond that is unusual. Tim |
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