Hi NANDBlog,
You are in write only mode
Oooo, please give me the spec's of what you build, and how it is used fo years...
I have no problems with a IRF540 on relative low DC voltages and high power.
If you look at the datasheets, if there is a DC spec, its at 25C case temperature, that wil be a hell of a heatsink.
And yes, i will give you that the IRF540 is better than most of the modern parts voor DC. :-)
Dit you read my reply #24 about the IXAN0061 application note?
Kind regards,
Blackdog
that wil be a hell of a heatsink.
But for a FET to fail, you need to have 1) high temperature 2) drive it above the inflection point* 3) high power 4) usually high voltage
But for a FET to fail, you need to have 1) high temperature 2) drive it above the inflection point* 3) high power 4) usually high voltage
Isn't driving below the inflection point the critical situation?
According to ON Semiconductor AND8119/D "The thermal-runaway situation occurs when you use large devices at low current-limit settings."
According to ON Semiconductor AND8119/D "The thermal-runaway situation occurs when you use large devices at low current-limit settings."I dont think that is the right document.
The quote is right though. One FET is a lot of small FETs in parallel. If they get warm, the Rds of each small fet will change, based on their temperature. If you increase the temperature, below a certain Vgs, they will conduct less, above a certain Vgs they will conduct more. If you operate the FET above that Vgs, then it could thermal runaway.
It is hard to explain these, since these are 5 parameters, Vgs, current, temperature,Rds, Time, and it is different for the small FETs. Basically all the current and the dissipation will be concentrated into a small region, because the Rds changes. And then that part of the silicon will overheat, and fail. And then the rest will take over, and fail too.
blackdog is right about this, and yes, Figure 10 of his appnote describes this.
http://www.ixys.com/Documents/AppNotes/IXAN0061.pdf
According to ON Semiconductor AND8119/D "The thermal-runaway situation occurs when you use large devices at low current-limit settings."I dont think that is the right document.
What is wrong with this document?
But the figures in the app-note i've listed shows VGS and the behaviour is described in the text. From what i can read there, the behaviour seems to be the opposite than the one you described. At a gate-to-source Voltage greater than that of the inflection point an increase of the temperature will decrease the drain current.
But this looks like a serious limitation for the battery app. In the video it sounded like it can do only a constant current discharge test in this mode?
But this looks like a serious limitation for the battery app. In the video it sounded like it can do only a constant current discharge test in this mode?
That sucks, yes. But its something fixable in firmware, which I think they are going to fix because a lot of people will complain.
What is wrong with this document?AND8119 is a 2W bias power supply design, and there is nothing in it about thermal runaway.
In my Siglent SPD3303D power supply there is the same problem for an otherwise good device: no velocity for the knob ...
In my Siglent SPD3303D power supply there is the same problem for an otherwise good device: no velocity for the knob ...
to be honest: i really hate devices with velocity in the knobs. It happens so often that i want to increase some value by a small amount - and when turning just a little bit too much it makes huge steps into an area of damage.
In my Siglent SPD3303D power supply there is the same problem for an otherwise good device: no velocity for the knob ...
to be honest: i really hate devices with velocity in the knobs. It happens so often that i want to increase some value by a small amount - and when turning just a little bit too much it makes huge steps into an area of damage.It's good if it's implemented well, unfortunately it often isn't.
If you want real battery testing, one would use a different gear in most cases anyway. More like multi channels and usually lower power / lower voltage.
It might be interesting how good / fast the current read back is.
In the video Dave measures it out of spec (around 30 minutes). No wonder why.
You made me take a second look at it.
The shunt is isabellenhütte BVS 0.0005Ohm, which has a typical 50ppm/K tempco, but the 0.0005Ohm version has a typical 70ppm tempco. And the spec for the rigol is 50ppm/K.
And they dont include the tempco of the AFE. No way for it to be within that spec.
They also send 40A through that shunt, so self heating is about a watt. And they give 1000PPM (equivalent of 20K self heating) specification for full scale. I'm sorry, but those accuracy specifications are just not realistic. You cannot just take a number from one datasheet and use that for your specification. In the video Dave measures it out of spec (around 30 minutes). No wonder why.
You made me take a second look at it.
The shunt is isabellenhütte BVS 0.0005Ohm, which has a typical 50ppm/K tempco, but the 0.0005Ohm version has a typical 70ppm tempco. And the spec for the rigol is 50ppm/K.
And they dont include the tempco of the AFE. No way for it to be within that spec.
They also send 40A through that shunt, so self heating is about a watt. And they give 1000PPM (equivalent of 20K self heating) specification for full scale. I'm sorry, but those accuracy specifications are just not realistic. You cannot just take a number from one datasheet and use that for your specification. In the video Dave measures it out of spec (around 30 minutes). No wonder why.
I agree you can't just take numbers from the datasheet, and that could be what they've done here.
But look at the temperature graph for Manganin, the typical response is 17ppm/K, and worst case is 50. So maybe ~24ppm typical, for the 0.0005 ohm version. Its not unusual to use the typical value and not worst case.
Also, I seriously doubt they have done it here, but they could do a full temperature characterization and calibrate it out in software right? But there is no temperature sensor near that shunt that I see.
YOU NORMALLY USE MANGANIN® AS RESISTANCE ALLOY IN THE RESISTORS. THE TEMPERATURE COEFFICIENT OF MANGANIN® IS SPECIFIED WITH < ± 10 PPM/K. WHY DO YOU MOSTLY SPECIFY YOUR SMD RESISTORS < ± 50 PPM/K ON THE DATA SHEETS?
For a two-wire, the TC of a component is comprised of the TC of the resistance material (e. g. Manganin®) and the not completely avoidable influence of the supply line or bonding. For that reason we usually specify the TC of a two-wire with < 50 ppm/K.