How to find equivalent power MOSFET?

[knicklicht]

I am trying to recreate an amplification circuit for a waveform generator similar to the one I have described in https://www.edaboard.com/threads/trapezoidal-waveform-generator.398847/. I figured out the power transistors I need: https://www.digikey.de/products/en?t=278&pv606=378999&pv570=377049&pv608=247737&pv1236=176525 . However, they are only sold in bulk and I don't want to order 5000 pieces. How do I determine which characteristics of the MOSFET are important and how can I find an equivalent one that is available in small quantities. The package type does not matter to me. I tried digikey's "similar products" feature but it doesn't find anything.

P.S.: I created a new thread for this, as I thought it might be a helpful resource for others.

 

[barry]

You say you “figured out the power transistor I need”. Then you say “how do I determine...”.

Didnt you you just say you figured it out? How did you figure out what you needed to begin with???

 

[KlausST]

Hi,

I guess it´s impossible for us to find out the specifications for your transistor.
I don´t know how we can help in this regard.

So somehow you have to find out on your own.
Usually on the first page of a transistor datasheet there are the key features and key specifications.
This is a good starting point.
Use your transistor´s datasheet and go step by step through the items and ask yourself hat your own "limiting value" is.

Klaus

 

[dick_freebird]

Many vendors' web sites will have "selection guides"
and these may accept competitor P/Ns as one input,
if you don't want to go by electrical / mechanical
attributes but just find their best alternative to what's
in hand.

 

[d123]

Hi,

If you mean yourself thinking about it, a few things to consider all go back to the original circuit I think you are emulating (?) and the parameters of your circuit more importantly. ...Not knowing much about MOSFETs and finding selection a bit cumbersome as a task because it's bewildering with so many to choose from, I'd ask myself stuff like:

What do I need this MOSFET to do?
What supply and gate on/off voltages do I need/have at my disposal, if they are not the same. / Will the ones' I'm considering VGS requirements fit in with the supply voltage and input signals or need to be limited? Will the one available in a pack of ten have a useless VGS(min) or (max) for the voltages I have at my disposal and the currents I need it to pass? i.e. it'll never turn off or it'll never turn on with the circuit I need it for or the voltage I have will get about 250uA out of it, according to the graphs.
How fast does it need to turn on and off or how fast does it have to be able to turn on and how fast does it need to turn off - add up the rise, fall, etc. times; and if you like maths, either wade through device capacitances or take a shortcut and just look at the gate charge nC graphs)?
How much does RDSon matter for what they have to do?
How much current do I have to pass through them (and what supply voltage)?
Ssould I bother to look closely at the SOA curves, and the peak and continuous power ratings, in datasheets to see if there's some correlation between the one I think I need and the one I want to buy?
Can I use a general purpose high voltage MOSFET for this? Am I looking at something expensive when a (much) cheaper one will be of equal quality?
If simulating needs to be done with it: Is there at least one (Spice) model available on one manufacturer or another's website? And, is the first model I found dismal/annoying to use and should I check if there is another model provided by another manufacturer that is 'better' suited to my needs?

Other stuff, too, depending on how precise/fussy requirements are. As stated above, compare datasheets of likely suspects and go refining search from there.

Good luck.

 

[knicklicht]

You say you “figured out the power transistor I need”. Then you say “how do I determine...”.

Didnt you you just say you figured it out? How did you figure out what you needed to begin with???

Sorry for the confusing question. I basically want to know what characteristics of an amplification circuit matter so that I can find a transistor "similar" to the one that I want to emulate.

Hi,

If you mean yourself thinking about it, a few things to consider all go back to the original circuit I think you are emulating (?) and the parameters of your circuit more importantly. ...Not knowing much about MOSFETs and finding selection a bit cumbersome as a task because it's bewildering with so many to choose from, I'd ask myself stuff like:

What do I need this MOSFET to do?
What supply and gate on/off voltages do I need/have at my disposal, if they are not the same. / Will the ones' I'm considering VGS requirements fit in with the supply voltage and input signals or need to be limited? Will the one available in a pack of ten have a useless VGS(min) or (max) for the voltages I have at my disposal and the currents I need it to pass? i.e. it'll never turn off or it'll never turn on with the circuit I need it for or the voltage I have will get about 250uA out of it, according to the graphs.
How fast does it need to turn on and off or how fast does it have to be able to turn on and how fast does it need to turn off - add up the rise, fall, etc. times; and if you like maths, either wade through device capacitances or take a shortcut and just look at the gate charge nC graphs)?
How much does RDSon matter for what they have to do?
How much current do I have to pass through them (and what supply voltage)?
Ssould I bother to look closely at the SOA curves, and the peak and continuous power ratings, in datasheets to see if there's some correlation between the one I think I need and the one I want to buy?
Can I use a general purpose high voltage MOSFET for this? Am I looking at something expensive when a (much) cheaper one will be of equal quality?
If simulating needs to be done with it: Is there at least one (Spice) model available on one manufacturer or another's website? And, is the first model I found dismal/annoying to use and should I check if there is another model provided by another manufacturer that is 'better' suited to my needs?

Other stuff, too, depending on how precise/fussy requirements are. As stated above, compare datasheets of likely suspects and go refining search from there.

Good luck.

Thanks for elaborating d123. Since I want to build an amplification circuit for a function generator I was hoping that there are certain characteristics, that are always going to be important. Also since I am a novice in designing analog circuits someone could point me to resources that help me figure out what limits I should not break, e.g. some kind of current that could harm the components, so that I can buy a couple of candidates and test them in my circuit if they work.

Since I have the original circuit that I am trying to emulate here are there any measurements that I can take to find/limit the characteristics that I need to consider?

 

[d123]

Hi,

Oofff, what would I know... You ask wide questions there. I don't know your circuit and I don't really understand the schematic you put in post #17 there. What do you mean by amplification circuit for a function generator? I associate amplification with BJTs (small input voltage and large output voltage, or as textbook description goes: small input current and large output current), same goes for MOSFETs: low VGS on (fully on) and large drain source ability - not needing 10 to 20 volts to turn on a MOSFET with a supply voltage of 10 to 20 volts. Assuming we're still talking about MOSFET selection:

Device current
Device voltage
Gate voltage
Package power dissipation
Operating temperature range
Can't think of anything else right now.

The datasheet will tell you '(absolute) maximum ratings' - which basically you can't use, they are the far limits, the same as noone drives a car at full speed until it breaks. You shouldn't need to understand more than this - the device ratings are xyz and you want to use it for abc, and if your abc do not exceed its xyz then it is a suitable part. As you know, trying to pass 1,000 volts and/or 50 amps continuously through a MOSFET rated for 75 volts and/or 10 amps as a 1 millisecond pulse isn't going to end well for the MOSFET.

Your other thread mentions 1us to 9us either transition times or durations for the waveform segments. If you need a MOSFET that switches fully on and/or off well within a minimum of 1us or MOSFET on + off has to be included within that 1us, you want to check the datasheet fields of 'turn-on delay time' + 'rise time' + 'fall time' + turn-off delay time' = e.g. 150 nanoseconds, then subtract that total each time from the time each of your trapezoidal waveform segments are high to see if the MOSFET can turn on and off and leave the time necessary for the rest of the signal to look how you need it to/last as long as it should. Expect to do uncertainty and guesstimating as your circuit may not match the parameters used in the datasheet to obtain those numbers.
Coupled to that is - bad description - see the MOSFET as a three terminal device with a capacitor that needs to be charged as the on/off signal pin: you are more charging a capacitor quickly or slowly to open the doorway between drain and source than simply turning it on or off like a light switch. And there the nC business of yesterday: Look at the Qg ('total gate charge') field of the datasheet, if it has one, and use the simple formula in the attached document to decide how much current you need to turn the thing on at such and such a speed and therefore what gate resistor will be necessary. This also has its own graph to help guesstimate the nC number you will- I promise you - be feeling uncetain about and guesstimating from the data available in it.

If you don't want to damage the MOSFET, use the SOA graph - it shows you how long you can pass x current at y voltage through the device for z amount of time. SOA graphs are quite simple to understand and very useful. Another non-precise and approximate way, for datasheets that provide maximum continuous and pulsed power dissipation in Watts, is to feel uncertain from the test parameters which will be unlike your circuit values to guesstimate the actual RDS(on) your MOSFET will have when on, and use current squared multiplied by the MOSFET ON resistance to see if you are within its permitted power dissipation values.

If you are comfortable taking current and voltage measurements with a DMM and have an oscilloscope to see waveforms and voltage levels, you would want to know what the mystery MOSFET gate voltage is, what current it is passing. I really don't know what else you may want to measure as I don't know what it does in the trapezoidal waveform generator.

And maybe you know if the MOSFET has to pass power/a large current and a large voltage or is just for a signal/a tiny current and has to level shift a small voltage to a large voltage.

You could look for design notes and application notes and tutorials (manufacturer's usually have a wide range) about each component's datasheet fields and what they mean, about their absolute maximum ratings, maybe stuff on changes due to temperature rise, MOSFET gate drive requirements and techniques, derating, PCB design for this kind of circuit. ..It's too much stuff I think to do here and now, it would take months to read all the things I suspect you may be interested in, and you may want to just focus on this circuit you are doing and deal with topics as they come up during the design, otherwise you'll never get anywhere with it. I personally feel that every component and circuit building block and so on is a topic that is composed of sub-topics that are themselves composed of sub-topics and it is sometimes better to know the minimum necessary to actually get round to making something.

A good use of time could be to look at a MOSFET datasheet and just ponder what you come across for a while, comparing it's electrical specifications fields and graphs to what you think the MOSFETs are doing in the circuit you are copying.

--- Updated Sep 10, 2021 ---

Hi,

Oofff, what would I know... You ask wide questions there. I don't know your circuit and I don't really understand the schematic you put in post #17 there. What do you mean by amplification circuit for a function generator? I associate amplification with BJTs (small input voltage and large output voltage, or as textbook description goes: small input current and large output current), same goes for MOSFETs: low VGS on (fully on) and large drain source ability - not needing 10 to 20 volts to turn on a MOSFET with a supply voltage of 10 to 20 volts. Assuming we're still talking about MOSFET selection:

Device current
Device voltage
Gate voltage
Package power dissipation
Operating temperature range
Can't think of anything else right now.

The datasheet will tell you '(absolute) maximum ratings' - which basically you can't use, they are the far limits, the same as noone drives a car at full speed until it breaks. You shouldn't need to understand more than this - the device ratings are xyz and you want to use it for abc, and if your abc do not exceed its xyz then it is a suitable part. As you know, trying to pass 1,000 volts and/or 50 amps continuously through a MOSFET rated for 75 volts and/or 10 amps as a 1 millisecond pulse isn't going to end well for the MOSFET.

Your other thread mentions 1us to 9us either transition times or durations for the waveform segments. If you need a MOSFET that switches fully on and/or off well within a minimum of 1us or MOSFET on + off has to be included within that 1us, you want to check the datasheet fields of 'turn-on delay time' + 'rise time' + 'fall time' + turn-off delay time' = e.g. 150 nanoseconds, then subtract that total each time from the time each of your trapezoidal waveform segments are high to see if the MOSFET can turn on and off and leave the time necessary for the rest of the signal to look how you need it to/last as long as it should. Expect to do uncertainty and guesstimating as your circuit may not match the parameters used in the datasheet to obtain those numbers.
Coupled to that is - bad description - see the MOSFET as a three terminal device with a capacitor that needs to be charged as the on/off signal pin: you are more charging a capacitor quickly or slowly to open the doorway between drain and source than simply turning it on or off like a light switch. And there the nC business of yesterday: Look at the Qg ('total gate charge') field of the datasheet, if it has one, and use the simple formula in the attached document to decide how much current you need to turn the thing on at such and such a speed and therefore what gate resistor will be necessary. This also has its own graph to help guesstimate the nC number you will- I promise you - be feeling uncetain about and guesstimating from the data available in it.

If you don't want to damage the MOSFET, use the SOA graph - it shows you how long you can pass x current at y voltage through the device for z amount of time. SOA graphs are quite simple to understand and very useful. Another non-precise and approximate way, for datasheets that provide maximum continuous and pulsed power dissipation in Watts, is to feel uncertain from the test parameters which will be unlike your circuit values to guesstimate the actual RDS(on) your MOSFET will have when on, and use current squared multiplied by the MOSFET ON resistance to see if you are within its permitted power dissipation values.

If you are comfortable taking current and voltage measurements with a DMM and have an oscilloscope to see waveforms and voltage levels, you would want to know what the mystery MOSFET gate voltage is, what current it is passing. I really don't know what else you may want to measure as I don't know what it does in the trapezoidal waveform generator.

And maybe you know if the MOSFET has to pass power/a large current and a large voltage or is just for a signal/a tiny current and has to level shift a small voltage to a large voltage.

You could look for design notes and application notes and tutorials (manufacturer's usually have a wide range) about each component's datasheet fields and what they mean, about their absolute maximum ratings, maybe stuff on changes due to temperature rise, MOSFET gate drive requirements and techniques, derating, PCB design for this kind of circuit. ..It's too much stuff I think to do here and now, it would take months to read all the things I suspect you may be interested in, and you may want to just focus on this circuit you are doing and deal with topics as they come up during the design, otherwise you'll never get anywhere with it. I personally feel that every component and circuit building block and so on is a topic that is composed of sub-topics that are themselves composed of sub-topics and it is sometimes better to know the minimum necessary to actually get round to making something.

A good use of time could be to look at a MOSFET datasheet and just ponder what you come across for a while, comparing it's electrical specifications fields and graphs to what you think the MOSFETs are doing in the circuit you are copying.

 

[D.A.(Tony)Stewart]

Rochester NY has a slightly smaller MOQ but no stock. Don't count on other sources either, https://german.alibaba.com/product-...on-mg-wdson-2-bsf450ne7nh3-1600190729166.html

but if you need Eng help define all design specs for acceptance.

 

[FvM]

I tried digikey's "similar products" feature but it doesn't find anything.

In other words, you don't yet understand how it works. The "similar products" search and even more the general parametric search at Digikey are powerful tools, I'm often using it to identify usable parts.

Go to "Product Index > Discrete Semiconductor Products > Transistors - FETs, MOSFETs - Single" https://www.digikey.de/products/en/...-products/transistors-fets-mosfets-single/278 and make a few appropriate selections, e.g. N-Channel, surface mount etc.

 

[D.A.(Tony)Stewart]

I am trying to recreate an amplification circuit for a waveform generator similar to the one I have described in https://www.edaboard.com/threads/trapezoidal-waveform-generator.398847/. I figured out the power transistors I need: https://www.digikey.de/products/en?t=278&pv606=378999&pv570=377049&pv608=247737&pv1236=176525 . However, they are only sold in bulk and I don't want to order 5000 pieces. How do I determine which characteristics of the MOSFET are important and how can I find an equivalent one that is available in small quantities. The package type does not matter to me. I tried digikey's "similar products" feature but it doesn't find anything.

P.S.: I created a new thread for this, as I thought it might be a helpful resource for others.

This excessive dialog of insufficient scope and lack of specifications demands more attention to defining the end-use of power, frequency tolerance, load capacitance, heat rise, desired efficiency, application use. It is commonly referred to an XY question when you ask for answers to X when Y is the real problem.

You may want a trapezoid generator, but might really need Mux'd bridge with 42V 2A slew rate limited current regulated class C pulse driver.

Which print head are to trying to drive?
Pls include specs, model p/n etc. in a concise list. This thread makes me feel like I'm on a raft down a winding river, and it's easy to get lost.

There are many ways to solve most problems, understanding the reasons and converting them to design specs is the designer's job. When you don't have specs, one must try to measure parameters to make them.

e.g., print head impedance at driving frequency, or f, C and ESR.
Can you measure this with an LCR meter at 100kHz or equivalent method?


Also FYI

“For quality printing, it is necessary to know and control the temperature of the print head. Thus,
the latter has a built-in aluminum resistor, whose value changes slightly with the temperature.
The temperature determination is done by injecting a constant current in the resistor, and
measuring the voltage drop across it. Since high-end printers have two heads, it must also be
possible to switch quickly the measurement process from one to the other.

ref https://www.st.com/en/power-management/l6452.html

 

[BigBoss]

Mouser sells small quantities..

https://www.mouser.fr/ProductDetail/Infineon-Technologies/BSF450NE7NH3XUMA1/?qs=LxJ0xX%2FWJR4DWAv%252BmN9YTA==

Also, SPICE Model is Available at Infineon's Website..