(Not) another Dummy-Load... - Page 1
Forum section: electronics / projects

author: zli
author_rank: Newbie
timestamp: September 19, 2019, 03:32:31 pm
content: Hello togetherAnother dummy load thread... No, this time something else:) I thought to myself why not try to control a dummy load purely by software? Everybody always says "doesn't work", "too slow" etc... So I searched my pool and soldered the following components to a board:- Teensy 3.5- 2 x external ADC ADS7818 (12 Bit SPI)- OpAmp (only as ADC buffer/amplifier) LT1490- Darlington MJH6284+ various controller/Shunt/Fan-Control/etcEverything was programmed with Arduino. Every 25 us both ADC (voltage/current) are read in by interrupt, the PI controller is calculated and a little bit filtered by software. Thus I reach a Risetime (according to definition 10-90%) at 10 A of 32 us. Noise is 12.6 mV at 10 A (see pictures, measured at the terminals).DUT is a 2S-Lipo 7.2Ah, 90CThe biggest problem that still needs to be solved is the ADC noise. But I think with a SMD, 4-layer PCB that would be possible.Once the noise is eliminated, the next step is a shorter Risetime. Currently I lose about 10 us by the SW filter. With faster ADC, improvements are certainly possible. The final target is a tasktime of 10-15 us. I can make the PI controller faster, but then I have the problem of the overshoot, that needs to be improved.The load can be 13.8 A at maximum 110 W (all monitored by software), fan control is controlled via Teensy-DAC. OLED shows actual current, input voltage, power, temperature and the setpoint of the current. Control is via encoder with built-in switch, 1 x short press output on, 1 x long press output off.GreetingsFranz

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: September 21, 2019, 11:39:22 am
content: Franz,Welcome to the forum.Quote from: zli on September 19, 2019, 03:32:31 pmI have shared one of the most complete studies of a dummy load in this thread:https://www.eevblog.com/forum/projects/dynamic-electronic-load-project/msg288313/#msg288313The thread contains detailed analysis of the control loop in a dummy load controlled by an op-amp.In this type of control loop the circuit action is continuous.When an ADC is included the control loop the system is considered to be 'sampled' or 'discontinuous'.So what is the impact of sampling?Nyquist tells us that the bandwidth is limited to half the sampling frequency. Try and increase the bandwidth beyond this and you get aliasing.In practice the bandwidth that can be achieved is probably between 1/5 and 1/10 of the switching frequency.Loop analysisBecause the control loop is discontinuous the Bode plots have to be approximated.A sampled data has the same properties as a delay. It does not change the amplitude, it changes only the phase. Strictly speaking should not really talk about phase because it is discontinuous.The delay can be modelled like this:The switch and the capacitor form a sample and hold circuit. The statements on the right implement a gain phase meter. I have used a 1us sample period, 1 Msps.The result isBy inspection you can see that average delay is half of the sampling period.Looking at this in the frequency domain, the 1us sampling period results in 18 degrees of phase shift at 100kHz (1/10th of the sampling frequency)At the Nyquist frequency, in this case 500kHz, the phase shift from the sampling, is 90 degrees.LaplaceThe sampling delay can be modelled in the frequency domain using Laplace:The delay has been set to 1/2 the sampling period.The results are:The results are 18 degrees of phase shift at sampling frequency/10 and flat gain.System modelingThis block can be included in the analysis explained in the Dynamic Load thread to show the impact of using an ADC or sampling.Regards,Jay_Diddy_B

author: zli
author_rank: Newbie
timestamp: September 22, 2019, 10:07:33 am
content: Hi Jay_Diddy_B,Thank you for your analysis.I know your threadBut just one question, what is your actual statement?