Software TCR compensation of shunt in DIY electronic load - Page 1
Forum section: electronics / projects

author: RandomUser
author_rank: Contributor
timestamp: September 25, 2024, 06:18:39 pm
content: Hello,almost a year ago I started working on a little electronic load for fun, see schematic attached.The load uses 4x NMOS with a Vishay 0.5R/1W thinfilm resistor (50ppm TCR) as shunt for analog current control.(https://www.vishay.com/docs/28849/ncw0406at-ncw0612at.pdf)I thought to be smart to use that thinfilm because of lowish TCR. But it turns out that the TCR is quite noticable at currents in the range of 1.5A-3A (split across the 4x stages/shunts).The first "power stage" has a temp sensor attached to the shunt to be able to compensate TCR-reated variations of the actual current in software.From a layout perspective only the "GND" side of the shunt is thermally attached to a large copper plane, while the other side is directly connected to the source of the MOSFET.One thought was, that self-heating + a thermal gradient accross the shunt might cause that problem.Unfortunately, the behaviour of the current is not what I expected or know how to handle at the moment.I'm assuming that the self-heating at higher power losses messes with the typ. TCR curve, but maybe someone in the forum can lead me the right way?Attached the measurements for a constant current over temperature of the shunt with thermal sensor connected.I'm glad about any idea to debug or explanation what you think might be going onThanks!EDIT: In the measurements, the axis are not scaled the same (Temperature + Current difference)

author: Kleinstein
author_rank: Super Contributor
timestamp: September 25, 2024, 08:10:59 pm
content: The temperature will be effect from shunt self heating and depending on the layout also from the heat from the FET.The form factor for the shunts is really rather small (1206 size SMD part) so heat can become an issue rather fast. The nominal power is far from practical for any accurate measurement. The norminal power also needs quite some copper area for cooling.Besides the TC there can also be thermal EMF at the shunt, that can set things off, if the thermal layout is asymmetric.Another point, again depending on the mayout could be the ground connection at the shunt. If not careful, one may have some copper trace included in the effecive resistance. Having 4 channels that may want a single ground may not make things easier.A slight simplification reduced sensitivity to the layout could be gained with larger resistors for R6 and R7 (and the corresponding resistors for the other channels). With the rather low value the inverter / sum stage works at low voltage and the exact ground is important. The resistors could get larger (e.g. 50 K range maybe) and as long as they are the same value the gain would not change. However the ground routing gets less critical and mainly the regulator OP-amp ground should go to the shunt.

author: RandomUser
author_rank: Contributor
timestamp: September 26, 2024, 06:22:22 am
content: Hmmm.... thanks for the explanations. I think I messed up the layout on every point you mentionedHere 2x pictures of the1. shunt (4-wire type) for the current measurement (close to the output pad)2. control-loop shunt for the power stage 0 with the temp sensor connectedI still see the chance to calibrate my current measurement close to my DMM and then try to do a digital ctrl loop to improve DC accuracy over temperature?

author: Kleinstein
author_rank: Super Contributor
timestamp: September 26, 2024, 06:51:13 am
content: The control part performace does not look that bad. At least for the smaller currents, it is even better than the measurement.There are 2 separate effects: self heating of the shunt and the general temperature rise, that may well be more from the heat from the FETs. The digital correction may need to parts, one from the temperature and the other from the square of the current. To see which part is more relevant one could do a test with 2 external voltages and the same current (e.g. 2 A). The temperature dependence of the shunt may also not be just linear. They give a linear TC to describe the shunt performance. However especially with better shunt materials the actual curve is far from linear, as they can to a certain degree optimize the linear part to get good performance.Ideally one would really use a shunt with better cooling and also have the heat sink of the MOSFETs more separate from the rest, e.g. with THT parts.