6 ms·

FET: The Friendly Efficient Transistor

55 points by l8rlump 3y ago

Animats 3y ago
That's a good explanation. Covers all the basics, including mentioning that FETs fail in the ON direction. This is why switching power supplies badly need protection circuitry, and why you don't want wall warts and battery chargers that didn't pass UL testing. He mentions electrostatic discharge sensitivity. Yes. It's very easy to blow out the gate of an FET just by touching the gate lead. Once it's attached to a driver transistor, it's protected, but loose FETs need to be stored in conductive foam and handled while wearing a wrist strap. FETs are really amazing. ON resistances in the milli-ohm range, and OFF resistances in the megohm range. It's surprising that's physically possible.
- Sharlin 3y ago
  (Based on the name, the author's probably a she.)
- nunuvit 3y ago
  Proper galvanic isolation is most of what protects you from mains. The YouTube channel Diode Gone Wild does a lot of charger teardowns explaining what makes them safe or dangerous. Unfortunately, we often rely on trusting secretive vendors with insulation specs, trusting assemblers with following instructions and not damaging hard to inspect wiring and insulation, and trusting brand and certification markings on the product since you can't open it without destroying it.
cellularmitosis 3y ago
> Speaking of the Pinecil, it uses an unorthodox driving circuit – it has an NPN transistor, but its base driven through a capacitor, so that only the AC component of the driving signal gets through. As a result, if the main MCU hangs and the control GPIO is stuck high, the FET won’t stay enabled! Clever!
- adql 3y ago
  That kinda begs the question "why not watchdog?". Most MCUs have one
  - ssl232 3y ago
    The Pinecil implementation is essentially a watchdog, it just operates proactively instead of reactively. The end result is the same but relies on simple hardware. See also: dead man's switch [1] used in trains and other safety critical infrastructure. [1] https://en.wikipedia.org/wiki/Dead_man%27s_switch https://en.wikipedia.org/wiki/Dead_man%27s_switch
    - adql 3y ago
      If the end result is the same you're just wasting components that already are builtin into most microcontroller. And you'd need to remember to not use autonomous part of microcontroller (like a timer output) to implement it. Considering the connected net is named "tip_control" I'd assume its PWM that is fed into MOSFET controlling the heater... and in vast majority of cases that PWM will happily run when core is hanged. Looking at datasheet of the microcontroller PA6 does indeed go to tip_control and one of alternate functions is TIMER2_CH0, and at least glancing at code /* enable TIMER1 - PWM control timing*/ timer_enable(TIMER1); timer_enable(TIMER2); ... https://github.com/Ralim/IronOS/blob/1fbcdcdf987a28541524e4f23ddc60fc942633ea/source/Core/BSP/Pinecil/Setup.cpp#L298 https://github.com/Ralim/IronOS/blob/1fbcdcdf987a28541524e4f... ... timer_init(TIMER2, &timer_initpara); /* CH0 configuration in PWM mode0 */ timer_channel_output_struct_para_init(&timer_ocintpara); timer_ocintpara.outputstate = TIMER_CCX_ENABLE; timer_ocintpara.outputnstate = TIMER_CCXN_DISABLE; timer_ocintpara.ocpolarity = TIMER_OC_POLARITY_HIGH; ... they are using it for PWM. The watchdog itself also appears to be enabled tho, so there is a chance it will kill itself the normal way
  - nunuvit 3y ago
    They need both. Watchdog can't help if the GPIO hardware is stuck high or if the MCU is browned out and doing undefined behavior. The npn can't help if the code is stuck.
deleted 3y ago
[deleted]
dvh 3y ago
Jfets are used a lot in guitar pedals (j201, 2n5457, 2sk30a, ...). Unlike MOSFET they only need one gate resistor (10M gate to ground) in a input buffer configuration. The drain and source resistors are same as MOSFETs.
- midoridensha 3y ago
  I feel like guitar pedals are obsolete because of modeling amps now. Why bother messing around with a bunch of separate pedals with 9V batteries and interconnect cables when you can just program the effects you want all in software (using your phone, no less), and instantly switch between completely different configurations (sound profiles) by pressing a button?
  - ramenbytes 3y ago
    Because with physical gear I own the means of sound production.
    - pohl 3y ago
      Is that true in some way that is not also true about a Line6 Helix, though, for example?
      - ramenbytes 3y ago
        Yes, with some qualifications. Below, with some edits, is what I originally wrote for the comment you replied to, before scratching it for being too wordy. Some personally compelling reasons for myself as a guitarist: 1) With physical gear, I own the means of sound production. Less so for pedals that integrate closed computing elements into their circuits though, and even less if it has IOT crap (why??). Analog devices are generally best at this aspect. 2) Along those lines, longevity and repairability. Again this applies most strongly to analog pedals. I can keep using a well-designed pedal for decades, and keep it going with occasional repairs and maintenance. Digital modelers (including digital pedals to an extent), not so much, for a number of reasons. 3) In general, physical signal chain elements, particularly analog ones, strike me as more malleable as an end-user. I can do tweak the gain of individual stages, rework the inter-stage EQ and attenuation, etc. with a physical tube amp, and do similar things with analog pedals. Perhaps to a lesser extent with digital pedals too. I'm not aware of a modeling system with comparable flexibility, though I'd be interested in being proven wrong. 4) Some analog stuff, to my limited knowledge, is still difficult to model.
adql 3y ago
I'm annoyed that they just call MOSFETs FETs. There are different types dammit!
- hulitu 3y ago
  Amateurs. But maybe one day they will learn the differences between MOS(FET) and (J)-FET.
l8rlump 3y ago
Also see part 2: https://hackaday.com/2023/05/02/fet-fun-endeavors-together/#more-587368 https://hackaday.com/2023/05/02/fet-fun-endeavors-together/#...

55 points by l8rlump 3y ago

That's a good explanation. Covers all the basics, including mentioning that FETs fail in the ON direction. This is why switching power supplies badly need protection circuitry, and why you don't want wall warts and battery chargers that didn't pass UL testing. He mentions electrostatic discharge sensitivity. Yes. It's very easy to blow out the gate of an FET just by touching the gate lead. Once it's attached to a driver transistor, it's protected, but loose FETs need to be stored in conductive foam and handled while wearing a wrist strap. FETs are really amazing. ON resistances in the milli-ohm range, and OFF resistances in the megohm range. It's surprising that's physically possible.

Sharlin 3y ago
(Based on the name, the author's probably a she.)
nunuvit 3y ago
Proper galvanic isolation is most of what protects you from mains. The YouTube channel Diode Gone Wild does a lot of charger teardowns explaining what makes them safe or dangerous. Unfortunately, we often rely on trusting secretive vendors with insulation specs, trusting assemblers with following instructions and not damaging hard to inspect wiring and insulation, and trusting brand and certification markings on the product since you can't open it without destroying it.

cellularmitosis 3y ago

> Speaking of the Pinecil, it uses an unorthodox driving circuit – it has an NPN transistor, but its base driven through a capacitor, so that only the AC component of the driving signal gets through. As a result, if the main MCU hangs and the control GPIO is stuck high, the FET won’t stay enabled! Clever!

adql 3y ago
That kinda begs the question "why not watchdog?". Most MCUs have one
- ssl232 3y ago
  The Pinecil implementation is essentially a watchdog, it just operates proactively instead of reactively. The end result is the same but relies on simple hardware. See also: dead man's switch [1] used in trains and other safety critical infrastructure. [1] https://en.wikipedia.org/wiki/Dead_man%27s_switch https://en.wikipedia.org/wiki/Dead_man%27s_switch
  - adql 3y ago
    If the end result is the same you're just wasting components that already are builtin into most microcontroller. And you'd need to remember to not use autonomous part of microcontroller (like a timer output) to implement it. Considering the connected net is named "tip_control" I'd assume its PWM that is fed into MOSFET controlling the heater... and in vast majority of cases that PWM will happily run when core is hanged. Looking at datasheet of the microcontroller PA6 does indeed go to tip_control and one of alternate functions is TIMER2_CH0, and at least glancing at code /* enable TIMER1 - PWM control timing*/ timer_enable(TIMER1); timer_enable(TIMER2); ... https://github.com/Ralim/IronOS/blob/1fbcdcdf987a28541524e4f23ddc60fc942633ea/source/Core/BSP/Pinecil/Setup.cpp#L298 https://github.com/Ralim/IronOS/blob/1fbcdcdf987a28541524e4f... ... timer_init(TIMER2, &timer_initpara); /* CH0 configuration in PWM mode0 */ timer_channel_output_struct_para_init(&timer_ocintpara); timer_ocintpara.outputstate = TIMER_CCX_ENABLE; timer_ocintpara.outputnstate = TIMER_CCXN_DISABLE; timer_ocintpara.ocpolarity = TIMER_OC_POLARITY_HIGH; ... they are using it for PWM. The watchdog itself also appears to be enabled tho, so there is a chance it will kill itself the normal way
- nunuvit 3y ago
  They need both. Watchdog can't help if the GPIO hardware is stuck high or if the MCU is browned out and doing undefined behavior. The npn can't help if the code is stuck.

deleted 3y ago

[deleted]

dvh 3y ago

Jfets are used a lot in guitar pedals (j201, 2n5457, 2sk30a, ...). Unlike MOSFET they only need one gate resistor (10M gate to ground) in a input buffer configuration. The drain and source resistors are same as MOSFETs.

midoridensha 3y ago
I feel like guitar pedals are obsolete because of modeling amps now. Why bother messing around with a bunch of separate pedals with 9V batteries and interconnect cables when you can just program the effects you want all in software (using your phone, no less), and instantly switch between completely different configurations (sound profiles) by pressing a button?
- ramenbytes 3y ago
  Because with physical gear I own the means of sound production.
  - pohl 3y ago
    Is that true in some way that is not also true about a Line6 Helix, though, for example?
    - ramenbytes 3y ago
      Yes, with some qualifications. Below, with some edits, is what I originally wrote for the comment you replied to, before scratching it for being too wordy. Some personally compelling reasons for myself as a guitarist: 1) With physical gear, I own the means of sound production. Less so for pedals that integrate closed computing elements into their circuits though, and even less if it has IOT crap (why??). Analog devices are generally best at this aspect. 2) Along those lines, longevity and repairability. Again this applies most strongly to analog pedals. I can keep using a well-designed pedal for decades, and keep it going with occasional repairs and maintenance. Digital modelers (including digital pedals to an extent), not so much, for a number of reasons. 3) In general, physical signal chain elements, particularly analog ones, strike me as more malleable as an end-user. I can do tweak the gain of individual stages, rework the inter-stage EQ and attenuation, etc. with a physical tube amp, and do similar things with analog pedals. Perhaps to a lesser extent with digital pedals too. I'm not aware of a modeling system with comparable flexibility, though I'd be interested in being proven wrong. 4) Some analog stuff, to my limited knowledge, is still difficult to model.

adql 3y ago

I'm annoyed that they just call MOSFETs FETs. There are different types dammit!

hulitu 3y ago
Amateurs. But maybe one day they will learn the differences between MOS(FET) and (J)-FET.

l8rlump 3y ago

Also see part 2: https://hackaday.com/2023/05/02/fet-fun-endeavors-together/#more-587368 https://hackaday.com/2023/05/02/fet-fun-endeavors-together/#...