High-side vs. low-side NPN transistor

[rstofer]

Logic Level MOSFETS might be better than BJTs but it sometimes takes 10V between the gate and source (equivalent to base and emitter) to turn a MOSFET fully on.  You want to drive it to saturation to reduce heating.

There can be enormous gate capacitance and this slows the transition to whatever the final gate voltage is going to be.  During the transition period, the MOSFET is heating.  Add two transitions (rising and falling edge) and that is the  reason that PWM is typically at a low frequency.  The designers try to make the transition time a small percentage of the total period.  Take 1 kHz = 1 mS.  If the rise time and fall time are 250 ns, then half the time is spent in the transition region where heating occurs.  I just grabbed those numbers, they aren't real, but it is important to understand that uCs can't always deal with conventional MOSFETs and may have a problem with Logic Level MOSTFETs as well.  You just have to work the numbers!

Which leads us to why they invented MOSFET driver chips.  These puppies can dump AMPS into the gate to get through the transition in a great big hurry.

MOSFETs are great devices but the user better read the datasheet and understand what is going on.  Vgs is a big deal, so are all the capacitances.

[spec]

+ permal

Here is a mini teach-in covering Bipolar Junction Transistors (BJTs) (normal transistors) and Metal Oxide Silicon Field Effect Transistors (MOSFETS). To keep it simple, this teach-in only covers the two transistor types in their linear mode (amplifiers) and omits the switching mode. But have no fear, once you have got the linear mode, the switching mode is pretty straight forward.

BJTs are available in complimentary types, NPN and PNP. Likewise MOSFETs are available in complimentary types, N type and P type. The only difference between the two types is that all voltages and currents are the opposite polarity. To simplify matters only the N versions of BJTs and MOSFETs will be covered further. So there will be NBJTs and NMOSFETs.

In most fields of engineering you can form simple models of components that describe their fundamental operation, and it is very  important not to let any of the more esoteric aspects cloud your view.

NBJTs
An NBJT has three terminals, emitter, base, collector.
In a linear circuit the collector is always more positive than the emitter, 5V to 50V say.
The  base is like a diode connected to the emitter.
To make a diode conduct current you put 600mV across it, with the base being the more positive, If you put less than 600mV across the base emitter no current will flow, but if you put more than 600mV across the base emitter a large current will flow.
The current (Ib) flowing from the base to the emitter will be amplified by the NBJT and will cause a higher current to flow from the collector to the emitter.
The ratio of the collector current(Ic) and the base current (Ib) is the current gain of the NBJT, called HFE in data sheets.
And that completes the basic model for an NBJT.

But there is one very important characteristic to remember about NBJTs: the collector current will always be HFE * Ib, regardless of what the collector/emitter voltage is. This characteristic has some very important consequences, and is fundamental to many linear NBJT circuits.

NMOSFETs
An NMOSFET has three terminals, source, gate, drain. These are analogous to the emitter, base, collector of an NBJT
In a linear circuit the drain is always kept more positive than the source, say 5V to 50V.
The  gate is completely insulated from the rest of the NMOSFET and cannot pass any current. This is radically different to the BJT base.
If the gate voltage is the same as the drain voltage no current will flow from the drain to the source.
As you make the gate more positive than the source a voltage will be reached when a small current starts to flow from the drain to the source, say 100uA. This gate/source voltage is the threshold voltage (Vth) of the NMOSFET.
As you increase the gate/source voltage so the drain/source current will increase, exponentially.
And that is it for NMOSFTs

But there is one very important characteristic to remember about NMOSfets: provided that VGS is kept constant the drain current will always remain constant regardless of what the collector voltage is. This characteristic has some very important consequences and is fundamental to many NMOSFET linear circuits.

Here endeth the first leson.

[T3sl4co1l]

Uhm, can you repeat that in English please? Are you saying my measuring is causing side effects?

What do you want?

I want to understand why I get such a low voltage reading in this particular scenario - 2.7 < 4.4. So yes, an explanation is what I'm asking for.

I can't explain it much better than that; I know it's written in jargon, so do take the time to find out (or ask) about what the terms mean.  Bias, forward and reverse, current flow, voltage difference...

I have no idea what your level is, aside from having posted in "beginners".  Unless I know better, I'm just going to go with what comes to mind.  Help me help you, as they say.

Tim

[permal]

Thank you all so much for these explanations and extra kudos to @Spec for his/her summary. I will be reading these thoroughly and will surely be back with more questions.

Again, thank you!

[spec]

Thank you all so much for these explanations and extra kudos to @Spec for his/her summary.

Last time I looked I was a his

[spec]

Logic Level MOSFETS might be better than BJTs but it sometimes takes 10V between the gate and source (equivalent to base and emitter) to turn a MOSFET fully on.  You want to drive it to saturation to reduce heating.

There can be enormous gate capacitance and this slows the transition to whatever the final gate voltage is going to be.  During the transition period, the MOSFET is heating.  Add two transitions (rising and falling edge) and that is the  reason that PWM is typically at a low frequency.  The designers try to make the transition time a small percentage of the total period.  Take 1 kHz = 1 mS.  If the rise time and fall time are 250 ns, then half the time is spent in the transition region where heating occurs.  I just grabbed those numbers, they aren't real, but it is important to understand that uCs can't always deal with conventional MOSFETs and may have a problem with Logic Level MOSTFETs as well.  You just have to work the numbers!

Which leads us to why they invented MOSFET driver chips.  These puppies can dump AMPS into the gate to get through the transition in a great big hurry.

MOSFETs are great devices but the user better read the datasheet and understand what is going on.  Vgs is a big deal, so are all the capacitances.

Of course you have to look at the data sheets. And it is obvious that you have to take note of all the device characteristics. That is what design is all about.

[spec]

I want to understand why I get such a low voltage reading in this particular scenario - 2.7 < 4.4. So yes, an explanation is what I'm asking for.

Re the low/high side switches, as long as I put the transistor on the right side, I should be good if I've understood things correctly.

In normal operation, an NPN transistor base will be about one diode junction drop higher than the emitter.  It's 0.5V in the case of the 2N3055.

So, if the base is 3.1V, I wouldn't expect the emitter to be higher than 2.6V.


See V_be(on) - second page here:
https://www.onsemi.com/pub/Collateral/2N3055-D.PDF

2N3055 

[permal]

I'm starting to like MOSFETs

Question re. them: In the SOA-diagrams a pulse length is mentioned, but it is assumed you know of *what*. I understand it as the pulse length of where V_gs is larger than its threshold voltage, i.e. V_gs > V_gs(th), is that correct?

What if you have a high duty cycle, would that effectively be considered as a longer pulse? Am I correct in that the the rise time of V_gs from 0 to lowest voltage where R_DS(on) is specified [N-channel] matters, i.e. if it is short enough the MOSFET would pass through the linear mode very quickly, effectively reducing the power dissipation and thus allowing for a longer pulse?

@Spec: In your example schematics (thanks) you put a R120 in series with the gate - why is that needed if the gate of an NMOSFET already is so high (10¹⁴Ohms?) Is it purely as a protection for the MCP23017?

[rstofer]

I want to understand why I get such a low voltage reading in this particular scenario - 2.7 < 4.4. So yes, an explanation is what I'm asking for.

Re the low/high side switches, as long as I put the transistor on the right side, I should be good if I've understood things correctly.

In normal operation, an NPN transistor base will be about one diode junction drop higher than the emitter.  It's 0.5V in the case of the 2N3055.

So, if the base is 3.1V, I wouldn't expect the emitter to be higher than 2.6V.


See V_be(on) - second page here:
https://www.onsemi.com/pub/Collateral/2N3055-D.PDF

2N3055 

Just a common pass transistor.  Not the one in the OP, just an example I am familiar with.  Nevertheless, I would expect the datasheets to have the same information.  Probably not in the same order but essentially the same.  The numbers will be slightly different, of course, but the ideas are identical.