Electronic load testing of PWM'ed H-bridge - Page 1
Forum section: electronics / testgear

author: mindThomas
author_rank: Contributor
timestamp: April 26, 2020, 04:54:51 pm
content: Hi.I am trying to test a low-side current sense setup with an H-bridge circuit.The low-side MOSFET of one side of the H-bridge is turned on, while the other side is PWM pulsed (alternating) at 50% duty cycle.I have tried to connect my KEL103 Electronic load without any luck. It just starts beeping a lot and doesn't show/measure any current.I am afraid that the internal current-control design of the KEL103 is switched/pulsed which is getting confused by the PWM'ed input. If that's the case I must have misunderstood the "Dynamic properties up to 10 KHz" of the KEL103?Any suggestions on how to use an electronic load to test a pulsed H-bridge?Would it be possible to add an inductor in series to smoothen out the pulsed input?Best regardsThomas Jespersen

author: masterx81
author_rank: Frequent Contributor
timestamp: April 26, 2020, 06:29:11 pm
content: Uhm, i've also te kel103, but i've intended the dynamic mode as an unstable signal of a specified frequency (a lot of ripple induced by the device). I've still not used it but that was my first impression.

author: TurboTom
author_rank: Super Contributor
timestamp: April 26, 2020, 07:29:25 pm
content: Most electronic loads, and especially the "bottom of the barrel" ones have considerable capacitance wired across their inputs (several, sometimes tens of microfarads). Moreover, the respnse time of the current control loop usually isn't really fast, also depending on the particular brand / model. It's rather safe to assume the entry level loads expect a "pure", slowly variable DC at its terminals, like a battery or the like will provide. My tests with aRigol DL3021revealed that this load in some modes even won't be able to cope with a (transformed) rectified mains without considerable smoothing. The Maynuo loads will perform somewhat better but I doubt that they will work to load your 10kHz H-Bridge.Depending on what your H-Bridge is designed to supply, you may consider to wire a low-resistance power inductor of a few hundred microhenries in series in order to smooth out the current ripple. At least in constant current mode, the load should then be able to tolerate the signal and maybe this will even more closely resemble the real operating conditions of your bridge. If you need to test polarity reversal of the H-bridge, use a bridge rectifier between the inductor and the input of the load.

author: mindThomas
author_rank: Contributor
timestamp: April 27, 2020, 03:51:42 pm
content: I withdraw my statement from above.After further investigation I can confirm that the KEL103 can indeed be used as an electronic load for PWM'ed signals.When used with PWM signals please be aware that the reported voltage on the KEL103 will be an average and thus affected by the duty cycle and the same applied to the reported current for which I haven't found the actual relationship to the duty cycle.However the set-point current is indeed achieved during the ON-period of the PWM cycle after a short settling period of approximately 0.3 ms.The plot above is from a 200 Hz PWM with 30% duty cycle. I have confirmed that the KEL103 works at least up to 5 kHz PWM (have not tried any higher yet).So this is deemed a success from my side and I can use my electronic load to calibrate and verify the current sense circuit.Note:The reason for the beeping sounds and what I thought was telling me that it could not be used for PWM'ed signals was actually the reverse polarity indicator of the KEL103. The H-bridge was configured to send current in the wrong direction