Understaing parameters of transistors for swtich mode

Both bipolar transistors and FETs are proportional devices.

The collector current in a bipolar transistor will be the base current * the gain (hFE) of the device.
(The hFE is given in the data, for a typical collector current. It tends to fall at high currents).

If that level of current is great enough to apply close to the full supply voltage across the load, the transistor will "saturate", so more base current will have little to no effect.

eg. If a transistor had a gain of 10 at 100mA, operating on a 10V circuit with a 100 Ohm collector load:

With 5mA base current, the collector current would be around 50mA, so about 5V across the load resistor and 5V between emitter and collector.

For switching applications, such as in switched-mode PSUs or logic, using anything up to 10x more base current is not uncommon, to ensure the lowest voltage drop across the transistor while it is conducting - though a saturated transistor switches off slower than one operated in its linear range.

Welcome to ETO!

anso-engineer said:

where in datasheet the parameter that allows to understand at which point transistor goes opened?

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There is no single point. For a BJT, base current, and hence collector current, vary non-linearly as Vbe changes. For a FET, drain and source current vary non-linearly as Vgs changes.

Saturation behavior in bipolar looks similar to this, varies transistor to transistor,
but general behavior as follows :




A bipolar transistor is fundamentally a V controlled device, although most often used model
is as a current controlled device.

The defining equation for collector current is :

anso-engineer said:

2) MOSFET, mainly the same question in respect that they are voltage controlled. How I can understand how much voltage should be applied to the gate for it opens?

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For the MOSFET to be fully conducting as a switch you want the minimum on-resistance.
This is shown below (bottom row) for the IRF540 as 0.077Ω with a Vgs (gate-source) voltage of 10V, so 0V and that voltage are typically used as a switching control voltage.
Logic-level type MOSFETs that have a lower Vgs(th) can be switched with a lower voltage (typically 5V or less).

crutschow said:

This is shown below (bottom row) for the IRF540 as 0.077Ω with a Vgs (gate-source) voltage of 10V, so 0V and that voltage are typically used as a switching control voltage.
Logic-level type MOSFETs that have a lower Vgs(th) can be switched with a lower voltage (typically 5V or less).

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ah, I see, but there is indicated the max resistance, so it's a max possible value or resistance at this Vgs voltage and load consumption?
Additional question, is 5V is standard voltage for this purposes, or there is some equation?

danadak said:

Saturation behavior in bipolar looks similar to this, varies transistor to transistor,
but general behavior as follows :

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yeah, I saw something similar in some datasheets.
Is it carry the same maninning as your plots?

Btw, how it should be propely interpret? I understand it like this, that when base current will be 50mA, and load current located on C-E line will be 500mA, we will have a voltage drop on BE equals to 2,6V and on CE 1,6V?

rjenkinsgb said:

eg. If a transistor had a gain of 10 at 100mA, operating on a 10V circuit with a 100 Ohm collector load:

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jsut to clarify... so tehcnically, Ic is the maximum value of current that is able to be utilized by load?

rjenkinsgb said:

For switching applications, such as in switched-mode PSUs or logic, using anything up to 10x more base current is not uncommon

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yes, I remeber something like this from experienced guys. Then I understood it the following way, x10 coefficient allows to simplify calculations and in the same way it guarantee that you did it right, because most of transistors have H_fe bigger than 10.

anso-engineer said:

ah, I see, but there is indicated the max resistance, so it's a max possible value or resistance at this Vgs voltage and load consumption?
Additional question, is 5V is standard voltage for this purposes, or there is some equation?

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5V is just the standard TTL power supply voltage, so is often used for micro-controllers etc. - although many later ones are now only 3.3V, and 5V could kill them.

Btw, how it should be propely interpret? I understand it like this, that when base current will be 50mA, and load current located on C-E line will be 500mA, we will have a voltage drop on BE equals to 2,6V and on CE 1,6V?

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Not sure your line references but curves essentially show, for a load of 500 mA, if base current << Ic/10
that transistor is in active region, not saturated, so Vce >> Vcesat. But once we drive Ibase hard enough
the Vce will bottom out at Vcesat. Vbesat or Vbe are roughly -




Further insight -



Regards, Dana.

anso-engineer said:

there is indicated the max resistance, so it's a max possible value or resistance at this Vgs voltage and load consumption?

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Yes, that's the maximum.
Typically it would be somewhat less.

anso-engineer said:

is 5V is standard voltage for this purposes, or there is some equation?

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10V is common for standard MOSFETS.
5V or less is used for those designated as "logic-level" types which have a lower Vgs(th) voltage.

ah, I see, but there is indicated the max resistance, so it's a max possible value or resistance at this Vgs voltage and load consumption?

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In that case its max guaranteed value, done by testing in final test of part. Note its for
specd conditions, T = 25C. Rdson versus T (representative enhancement mode MOSFET) -



Note test engineers typically have a margin in test program to manage guarantees, but for
design purposes use that max, excepting of course T variation which you have to get a handle
on in critical designs. As you can see from above T variation significant. @5Vgs its ~15% higher
due to heating.

anso-engineer said:

ah, I see, but there is indicated the max resistance, so it's a max possible value or resistance at this Vgs voltage and load consumption?
Additional question, is 5V is standard voltage for this purposes, or there is some equation?

Click to expand...

When you see parameters in a datasheet in a MIN or a MAX column it is a statement about the statistical variation of that parameter from the mean or expected value. I usually associate a MIN or a MAX value as being ±3σ away from the mean. In layman's terms this represent a 0.5% chance you will see a value greater than the MAX or a value lower than the MIN. So not impossible, but very unlikely. If the mean value is specified it will be listed as TYP (Typical).

Is that true in todays manufacturing ? That one can infer statistical characteristics
from a min/max spec in a datasheet ? That the production engineers gather
and characterize the statistical manufacturing results for the given part and
establish the MIN and MAX to behave to your ±3σ as well as the limits ?

Curious, as I am an old retired Production/Test/Applications EE, and our limits were
not derived from a real statistical study, hence never discussed in a datasheet
as distribution was not a sought or published spec. That being said one could
ask for characterization data, however on integrated parts (generally) that was
a one off in time data gathering event. Note we would have lots / runs that
were skewed to limits. So the characterization data had limited usefulness.

Lastly chance design of a product seems a tad risky, especially in life support and
human safety applications.


Regards, Dana.

danadak said:

Is that true in todays manufacturing ? That one can infer statistical characteristics
from a min/max spec in a datasheet ? That the production engineers gather
and characterize the statistical manufacturing results for the given part and
establish the MIN and MAX to behave to your ±3σ as well as the limits ?

Curious, as I am an old retired Production/Test/Applications EE, and our limits were
not derived from a real statistical study, hence never discussed in a datasheet
as distribution was not a sought or published spec. That being said one could
ask for characterization data, however on integrated parts (generally) that was
a one off in time data gathering event. Note we would have lots / runs that
were skewed to limits. So the characterization data had limited usefulness.

Lastly chance design of a product seems a tad risky, especially in life support and
human safety applications.


Regards, Dana.

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For mission critical applications I agree with a more cautious approach to characterization data. For experimental and hobby purposes the understanding of "not impossible, but unlikely" is a useful standard IMHO.

alec_t said:

There is no single point. For a BJT, base current, and hence collector current, vary non-linearly as Vbe changes.

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if I understand correclty, from those 4 plots danadak posted, techincally, C-E goes opened from the point of 0,1 but with very big limitations.

No, the CE is approaching a maximum Ic :

I find there’s alot to these transistors and that my pursuit into this knowledge base and field is a “little big for my britches.”

I am reading through sort of a “zero to hero” dc / ac electronics engineering text. While at same time tinkering with microbit, esp32, an electronics kit from online and one from radio shack (from maybe 10-15 years ago).
While Im already building circuits and sorta going one by one through learning each component, the transistor has alot goin on.
I looked at my text, where Im currently in like Ch. 3. It’s still telling you how meters work. It appears, chapter TWENTY TWO is where transistors get intro’d.
Im anxious to mess with transistor, but already blew a red LED cuz Im not grasping all that is going on.

Actually, I had set up a circuit where 5V is coming in from apple adapter. I fashioned a usb-cable to have just +/- wires exposed, in order to plug into breadboard. So 5V comin in up to an Amp of course.

I was gonna light red LED in line with collector and emitter, and set up a rheostat to dial up the current into base… thinkin I could open and close transistor, thus blinking light.

I used 220 ohm resistor for LED and put a 1k resistor before base.
Also, beforehand I adjusted the pot (10k) so that .6V was being allowed through, so I would know kind where this “switch” was. Somehow I remembered it takes “.6V” to open switch.

Sure enough the light kinda did light up at the spot on dial, but I also burned led later when trying again. I. ightve turned it up to high I guess?

I realized I was messin with stuff before knowing WHAT I was messin with.

PS anyone looking for comprehensive transistor playlist. I found his guy Aaron Danter on youtube. Here is his playlist on subject.

Im thinkin all will,be understood following this dude’s breakdown.

qb748 said:

I was gonna light red LED in line with collector and emitter, and set up a rheostat to dial up the current into base… thinkin I could open and close transistor, thus blinking light.

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If you are using transistors as simple switches, then the circuit being controlled (eg. the LED and resistor in your example) must be OK to operate continuously on the full circuit supply voltage, and be switched on and off - so eg. a relay should have a flywheel diode or some form of back EMF protection.

If that is all OK, then nothing you can do by controlling the transistor can damage that circuit.

Bipolar transistors are current controlled at their base-emitter junction, like LEDs. Never try to use voltage control directly, as there is next to no range between doing nothing and burning it out.

You use a series resistor of a suitable value to get and appropriate current at the "on" voltage from the switch control input, again similar to calculating the resistor for an LED (except using a voltage drop of 0.6 or 0.7V).

The base current in a switch application is generally around 10x (the load current divided by the transistor gain [Hfe] at that load current).

eg. For a 20mA load, and a transistor with a gain of 150, a base current around 1.3mA - or something between 1 - 1.5mA - should be fine.

So with a 5V input, that is 4.4V across the resistor so call it 3k9 as the base resistor.

(The 10x minimum base current requirement is far more important at high currents; if the current is too low to overheat the transistor if it's not fully switched on, you can experiment with rather higher values; eg. 10K would likely be fine).