Electronics > Beginners
BJT/MOSFET as switch
Zero999:
BJTs also need to have the energy sucked out of them to turn off quickly. Just open circuiting the base connection will cause a BJT to turn off very slowly. To turn a BJT off quickly, the base must be shorted hard to the emitter, thus discharging the base-emitter junction. At low speeds this is a non-issue, but if something needs to be switched quickly it can ruin your day. There are various tricks to achieve this such as: bypassing the base resistor with a small capacitor, using another transistor to short the base-emitter junction to turn it off or making the base voltage slightly negative and making sure the BJT never saturates (turns fully on).
The main reasons for choosing a BJT over a MOSFET are: cost and ESD resistance.
MOSFETs are extremely vulnerable to ESD because the gate is insulated from the channel by an oxide layer, which will be destroyed when subject to over-voltage. BJTs can also be damaged by high voltages, but unless the energy is high or repeated, it's normally non-destructive. The reason for this is BJTs contain no insulating oxide. They're composed entirely of PN junctions which non-destructively breakdown and start conducting when subject to over-voltage and recover back to the previous state, when the current flow stops. Of course there's a limit to this mechanism and the device can be destroyed or degraded (reduction in Hfe and increased leakage current) if the energy level is high enough or it's repeated many times. Some MOSFETs include built-in protection diodes on the gate, but this increases the cost of the device and there's a limited to how much energy they can take.
BJTs are generally cheaper, especially in low current and high voltage applications, but at much higher currents and low voltages, MOSFETs generally become the more economical option.
Speed wise, MOSFETs are generally faster than BJTs, especially when turning off and at higher currents. As devices get larger, they generally also become slower, yet MOSFET speed declines less than BJT speed, with increasing scale.
One reason for this is, low voltage rating MOSFETs drop less voltage, than BJTs, so a smaller heatsink can be used. In a high voltage application, a high voltage BJT is often connected in series with a low voltage MOSFET.
At high voltages, above around 500V, the situation reverses: BJTs drop less voltage, than MOSFETs. Often both a MOSFET and a BJT are used together. The IGBT is a device which combines both a MOSFET and a BJT and can switch high voltages, with less voltage drop than a MOSFET, without needing the large base current of a BJT. A high voltage BJT is often connected in series with a low voltage MOSFET, forming a cascode, or emitter switched BJT (ESBJT) which combines the speed of a MOSFET with the low loss of a BJT.
https://electronics.stackexchange.com/questions/15677/when-do-you-want-an-esbt-emitter-switched-bipolar-transistor
Moriambar:
--- Quote from: Zero999 on January 22, 2019, 12:46:57 pm ---BJTs also need to have the energy sucked out of them to turn off quickly. Just open circuiting the base connection will cause a BJT to turn off very slowly. To turn a BJT off quickly, the base must be shorted hard to the emitter, thus discharging the base-emitter junction. At low speeds this is a non-issue, but if something needs to be switched quickly it can ruin your day. There are various tricks to achieve this such as: bypassing the base resistor with a small capacitor, using another transistor to short the base-emitter junction to turn it off or making the base voltage slightly negative and making sure the BJT never saturates (turns fully on).
The main reasons for choosing a BJT over a MOSFET are: cost and ESD resistance.
MOSFETs are extremely vulnerable to ESD because the gate is insulated from the channel by an oxide layer, which will be destroyed when subject to over-voltage. BJTs can also be damaged by high voltages, but unless the energy is high or repeated, it's normally non-destructive. The reason for this is BJTs contain no insulating oxide. They're composed entirely of PN junctions which non-destructively breakdown and start conducting when subject to over-voltage and recover back to the previous state, when the current flow stops. Of course there's a limit to this mechanism and the device can be destroyed or degraded (reduction in Hfe and increased leakage current) if the energy level is high enough or it's repeated many times. Some MOSFETs include built-in protection diodes on the gate, but this increases the cost of the device and there's a limited to how much energy they can take.
BJTs are generally cheaper, especially in low current and high voltage applications, but at much higher currents and low voltages, MOSFETs generally become the more economical option.
Speed wise, MOSFETs are generally faster than BJTs, especially when turning off and at higher currents. As devices get larger, they generally also become slower, yet MOSFET speed declines less than BJT speed, with increasing scale.
One reason for this is, low voltage rating MOSFETs drop less voltage, than BJTs, so a smaller heatsink can be used. In a high voltage application, a high voltage BJT is often connected in series with a low voltage MOSFET.
At high voltages, above around 500V, the situation reverses: BJTs drop less voltage, than MOSFETs. Often both a MOSFET and a BJT are used together. The IGBT is a device which combines both a MOSFET and a BJT and can switch high voltages, with less voltage drop than a MOSFET, without needing the large base current of a BJT. A high voltage BJT is often connected in series with a low voltage MOSFET, forming a cascode, or emitter switched BJT (ESBJT) which combines the speed of a MOSFET with the low loss of a BJT.
https://electronics.stackexchange.com/questions/15677/when-do-you-want-an-esbt-emitter-switched-bipolar-transistor
--- End quote ---
wow, nice explanation.
Basically if I do not fear the ESD and do not consider the cost as an issue (as it is in my one-off circuits) than I can safely stick to MOSFETs, although learning BJTs could always be beneficial (still finding them way harder than mosfets to use as switches). Right?
Cheers
Doctorandus_P:
Both BJT's and Fets are usefull devices and you should know how to work with both of them.
So start plugging those holes in your knowledge.
Also Fet's aren't always that simple.
It starts with J-Fets and MOS-Fets.
With those Fets you also have Enhancement and Depletion variants.
A somewhat hidden defficiency of MOS-Fets is that they tend to have a relatively high leakage current, even when there is no Gate voltage, which can easily be 10uA or more (@ 55 Celcius).
This can be a limiting factor when designing low power battery powered circuits.
And when you get into the higher power stuff:
Don't forget the IGBT's, which are somewhere between a MOS-Fet and a Transistor.
Read some tutorials & books. Do some experiments on breadboads to consolidate & confirm what you've learned.
Also:
On Ali / Ebay / China there is a very popular "transistor tester".
I highly reccomend you buy at least one of them.
They are very handy to snif out the connections of unknown salvaged components and can identify if they still work.
As a beginner in electronics it is also a usefull project to study the power supply of this circuit.
When off it consumes 100nA or less (Thanks to BJT's) which is below my measurement limit (without getting complicated), it has a single momentatry push button to turn it on, and when done it turns itself off. And I mean OFF, not some whacky stand-by mode which consumes several uA.
And if you decide you do not like it you still have an ATMEGA328 development board with LCD.
https://hackaday.com/2019/01/10/transistor-tester-becomes-car-display/
Schematics and source code of (the many variants) of this thing are on github, and you can find a link in the Hackaday article.
Moriambar:
--- Quote from: Doctorandus_P on January 23, 2019, 12:58:59 am ---Both BJT's and Fets are usefull devices and you should know how to work with both of them.
So start plugging those holes in your knowledge.
--- End quote ---
yup, that's what I'm here. Been using only mosfets for years and still am not convinced that BJTs can be valid alternatives except some very specific cases. So I think I'm wrong due to my ignorance and I have to study more. But BJTs are quite hard to use imo.
--- Quote ---Also Fet's aren't always that simple.
It starts with J-Fets and MOS-Fets.
With those Fets you also have Enhancement and Depletion variants.
A somewhat hidden defficiency of MOS-Fets is that they tend to have a relatively high leakage current, even when there is no Gate voltage, which can easily be 10uA or more (@ 55 Celcius).
This can be a limiting factor when designing low power battery powered circuits.
--- End quote ---
this is interesting: I always thought (logic level) MOSFETS were more power efficient than BJTs. I did not know about this issue. Thanks!
--- Quote ---And when you get into the higher power stuff:
Don't forget the IGBT's, which are somewhere between a MOS-Fet and a Transistor.
Read some tutorials & books. Do some experiments on breadboads to consolidate & confirm what you've learned.
--- End quote ---
I do not plan to do high power stuff but it's good to know
--- Quote ---Also:
On Ali / Ebay / China there is a very popular "transistor tester".
I highly reccomend you buy at least one of them.
They are very handy to snif out the connections of unknown salvaged components and can identify if they still work.
--- End quote ---
it costs nothing and I'm buying it, I never thought it could be useful. I do not salvage many parts unfortunately. And I thought I was set with the test equipment. But for 5€ I don't mind having one more.
--- Quote ---As a beginner in electronics it is also a usefull project to study the power supply of this circuit.
When off it consumes 100nA or less (Thanks to BJT's) which is below my measurement limit (without getting complicated), it has a single momentatry push button to turn it on, and when done it turns itself off. And I mean OFF, not some whacky stand-by mode which consumes several uA.
--- End quote ---
which circuit are you talking about? Incidentally I did something similar using an LTC6990 kinda as described in the art of electronics (I'm at work and I do not have the text here, it's in the chapter about monostable multivibrator)
Thanks
Zero999:
--- Quote from: Doctorandus_P on January 23, 2019, 12:58:59 am ---Both BJT's and Fets are usefull devices and you should know how to work with both of them.
So start plugging those holes in your knowledge.
Also Fet's aren't always that simple.
It starts with J-Fets and MOS-Fets.
With those Fets you also have Enhancement and Depletion variants.
A somewhat hidden defficiency of MOS-Fets is that they tend to have a relatively high leakage current, even when there is no Gate voltage, which can easily be 10uA or more (@ 55 Celcius).
This can be a limiting factor when designing low power battery powered circuits.
And when you get into the higher power stuff:
Don't forget the IGBT's, which are somewhere between a MOS-Fet and a Transistor.
--- End quote ---
A MOSFET is a transistor. It stands for Metal Oxide Semiconductor Transistor, so the last sentence makes little sense.
I know what you meant: IGBTs are somewhere between a MOSFET and BJT.
Many people just assume BJT when one refers to transistors, but MOSFETs are actually the most common form of transistor nowadays, especially in digital ICs. Perhaps it's because most discrete transistors are still BJTs, especially the smaller ones.
Regarding leakage current: yes MOSFETs are leaky, compared to BJTs, but circuits composed entirely of MOSFETs normally use less power, compared to the equivalent circuit made with BJTs, especially in low frequency, low power applications, because a MOSFET doesn't need any current to remain on, whilst a BJT requires a base current to stay on. A classic example is digital logic: compare the power consumption of a CMOS quad NAND gate IC such as the CD4011 or 74HC00 with a TTL IC such as the 74LS00 and note the huge difference in current draw.
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