0.001 ohm 3W resistor at 30A gets too hot; impact on accuracy and alternatives?

[shapirus]

Let me derail this thread a little bit, especially considering that the OP apparently no longer cares :).

Is the average power dissipation the only parameter that we care about when we choose the size of a current sense resistor?

To be more specific, let's pick a 0.1 Ohm 0603 SMD resistor that will dissipate around 25 mW of power on average, but in very short (tens of microseconds) repetitive pulses with peak current at the end of the pulse reaching, say, 50 A. Will the high current per se cause any damage to it, or the only damaging factor is heat?

[coppice]

Let me derail this thread a little bit, especially considering that the OP apparently no longer cares .

Is the average power dissipation the only parameter that we care about when we choose the size of a current sense resistor?

To be more specific, let's pick a 0.1 Ohm 0603 SMD resistor that will dissipate around 25 mW of power on average, but in very short (tens of microseconds) repetitive pulses with peak current at the end of the pulse reaching, say, 50 A. Will the high current per se cause any damage to it, or the only damaging factor is heat?

Whether its short sharps shocks of heat, or average long term heat its pretty much always heat that is the damaging factor. I commented recently, on another thread, that this is something resistor makers don't specify very well, and someone responded with a data sheet from Yageo, https://www.yageo.com/upload/media/product/productsearch/datasheet/lr/YAGEO%20FMP_datasheet_2023v4.pdf , that does provide proper derating curves for one of their resistor product lines when subjected to this type of thermal shock. I guess the market is responding to most things now being switched mode, and very bursty.

[coromonadalix]

i do think they'll damage to some extend,  you over-spec them   even for a few millisecs ....

[shapirus]

i do think they'll damage to some extend,  you over-spec them   even for a few millisecs ....

I feel intuitively that it might be the case, but what the mechanism would be?

[shapirus]

Whether its short sharps shocks of heat, or average long term heat its pretty much always heat that is the damaging factor. I commented recently, on another thread, that this is something resistor makers don't specify very well, and someone responded with a data sheet from Yageo, https://www.yageo.com/upload/media/product/productsearch/datasheet/lr/YAGEO%20FMP_datasheet_2023v4.pdf , that does provide proper derating curves for one of their resistor product lines when subjected to this type of thermal shock. I guess the market is responding to most things now being switched mode, and very bursty.

Yeah those "Pulse diagrams" are useful: they can be used to extrapolate the values, more or less, to other resistor sizes, I guess.

[magic]

i do think they'll damage to some extend,  you over-spec them   even for a few millisecs ....

I feel intuitively that it might be the case, but what the mechanism would be?

One issue is well known and sometimes characterized in datasheets of semiconductor devices (transistors, diodes). Even if the average temperature of the part stays within ratings, the peaks that occur during load bursts may exceed these limits. Amplitude of those cyclic thermal variations is impossible to predict using only information available in typical resistor datasheets.

Another thing is, I don't know, maybe some mechanical problems due to repetitive thermal expansion. Total speculation, but possible in theory.

[shapirus]

One issue is well known and sometimes characterized in datasheets of semiconductor devices (transistors, diodes). Even if the average temperature of the part stays within ratings, the peaks that occur during load bursts may exceed these limits. Amplitude of those cyclic thermal variations is impossible to predict using only information available in typical resistor datasheets.

Another thing is, I don't know, maybe some mechanical problems due to repetitive thermal expansion. Total speculation, but possible in theory.

This assumes that temparature of the resistor follows the power pulses, which, given the thermal mass of even such small device as a 0603 resistor, doesn't sound very realistic to me: while the power pulses are huge, the momentary temperature values shouldn't be much different from the average value.
On the other hand, resistor's thermal conductivity is not infinite, thus some momentary overheating in very narrow local spots may very well be taking place, leading to the degradation of the conductive (resistive?) material inside the resistor. Aside from thermal processes, I wonder if there's anything on the electromagnetic side of things that may play a role here.

p.s. I can actually test this (well, to be specific, whether the resistor survives and if its resitance changes after a certain duration of such testing). I have everything required almost ready. Will report if/when I have any results .

[magic]

Most of your 0603 resistor is a dummy brick, the actual "resistance" where heat generation occurs is found in a thin layer of metal or metal oxide.

[shapirus]

Most of your 0603 resistor is a dummy brick, the actual "resistance" where heat generation occurs is found in a thin layer of metal or metal oxide.

Yes exactly. And then it's a question of heat capacity of that thin layer, its thermal conductivity and thermal conductivity between it and the rest of the resistor.

Now, I already have some results. My signal generator (JDS2800) is somewhat limited and doesn't allow to adjust frequency and pulse width separately; it is only possible to set frequency and duty cycle, the minimum value for which is 0.1%. This signal generator drives, in the end, a MOSFET switch that turns on when there is an incoming pulse and allows current to flow via whatever (in this case the resistor under test) is connected to a 2-pin terminal block.

Now, the limitation of the signal generator means that the lowest possible average power can be P = 10^-3 * P_p, where P_p is peak power. At 50 A, for example, this means (50A)² * 0.1Ohm * 10^-3 = 0.25W, which is 2.5 times the typical nominal power of an 0603 resistor, so I can't properly test it for what I wanted :).

I still tested it at 50 A, 200 Hz, pulse width ~5 usec, rise time ~1.5 usec. It died in ~2 minutes, failing open. No magic smoke escaped.

I replaced the resistor and continued testing.

1) 15 minutes at 30A pulses (0.09W, again not counting the non-zero rise time, so probably ~15-20% less in reality): all good, no change in resistance. Well, maybe 0.2 mOhm less, if my TR1035 isn't lying, and I have no reason to assume that it does -- but I may have simply placed the test leads differently.

2) 15 minutes at 35A pulses, ~0.12W calculated, probably ~0.1W actual. Or I think even more: 4 usec at 35A + 1 usec rise time. No changes in the observed voltage drop waveform so far, the resistor became warm. I'll let it run until I get bored and then measure its final resistance and call this mode of operation safe enough :).

3) I think I need to build a pulse generator with independent controls for pulse width and frequency.


...update:  having taken a closer look after 35 minutes, I see that the pulsed current has increased to ~36A. TCR in action, I guess.

[magic]

Another potential possibility: electromigration. Happens at sufficiently high current density per conductor cross section, I don't know if resistors are small enough to be meaningfully affected by it. Such damage would accumulate over long term.

[shapirus]

This is interesting. After two hours of being subjected to ~35A pulsed current the resistor's resistance increased from 100 mOhm to 116 mOhm (and it's probably this that made the voltage drop increase over time rather than the effect of TCR).

It's not what I expected. Very interesting. It'd be very interesting to test it at an even higher current, but well below the max nominal average wattage. I guess I'll do it if/when I build some simple pulse generator.

[tszaboo]

Let me derail this thread a little bit, especially considering that the OP apparently no longer cares .

Is the average power dissipation the only parameter that we care about when we choose the size of a current sense resistor?

To be more specific, let's pick a 0.1 Ohm 0603 SMD resistor that will dissipate around 25 mW of power on average, but in very short (tens of microseconds) repetitive pulses with peak current at the end of the pulse reaching, say, 50 A. Will the high current per se cause any damage to it, or the only damaging factor is heat?

You select the resistance to have a meaningful voltage at full scale. For example 100mV is a good goal. You select the size of the resistor based on the dissipation. And you select the type/material of the resistor based on the desired accuracy, for example PPM/K or long term stability. Going below 100mV is possible, but then you exchange heating issues with amplification and thermocouple issues. 100mV is a good compromise.