I tried to check what is causing annoying current ripple in Kunkin KP184 DC electronic load.
Reference to the older thread:
https://www.eevblog.com/forum/testgear/fixing-an-manufacturer-issue-of-the-kunkin-kp184-electronic-load/Setup:
Kunkin KP184 ver.04 with C58=10n installed by the manufacturer
Tek P6302+ AM503 current probe. Bandwidth limited to 5MHz.
The oscilloscope is AC coupled.
Linear bench power supply.
Conditions: current set on KP184 is 100mA; voltage applied 5V
Sanity check: noise of the system
Current ripple: as bought
Current ripple: without front panel
Current ripple: without front panel, without fan
Current ripple: without front panel, without fan, without 2nd MOSFET assembly
Now after staring at the PCB and some experimenting I've added a thick ground return path which looks like this
Current ripple: without front panel, without fan, without 2nd MOSFET assembly, with ground return mod
I've also added 1000uF/16V low ESR capacitor in parallel to 470uF cap on the front PCB.
Multiplexed LED display creates current spikes.
I tried to lower them by adding 10uF MLCC but without effect.
Photo of this mod below
Current ripple: with front panel C mod, without fan, without 2nd MOSFET assembly; with ground return mod
Current ripple: with front panel C mod, without fan, with 2nd MOSFET assembly; with ground return mod
Final:
Current ripple: with front panel C mod, with fan, with 2nd MOSFET assembly; with ground return mod
Conclusion:
1. due to 2 layer layout the current return path is not optimal. It can be fixed relatively easily.
2. most of the ripple comes from the fan. The rotor's magnet spins only 1cm away from current sense resistors and induces EMF in it.
It also creates HF content. I tried to add better decoupling but only electrostatic shield make things slightly better however it lowers airflow.
The fan is an integral part of the mechanical assembly.
So only solution that comes to mind is disconnecting the fan and adding another fan as far as possible in the enclosure,
however full current rating probably won't be met due to lower airflow
3. the designers didn't bothered these problems. The load is specified for Imax= 40A and even at 5A these artifacts are barely visible.
Right now I don't see any easy and cheap solution to the fan problem, so I'm staying with ground return mod and C mod applied.