Franz,

Welcome to the forum.

Hello together

Another 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...

Snip

Greetings

Franz

I 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 analysis**Because 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 is

By 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.

**Laplace**The 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 modeling**This 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