I see that discussion starts to transform into discussion about light bulbs.
Let me explain the situation better.
I am an engineer at Continental Automotive. We are designing and manufacturing a lot of different kind of ECUs including BCMs ( Body Controll Modules which are responsible fo a lot of functions including lights control).
The light bulb is not a trivial load. It may seem like it's nothing easier than turning on and off a light bulb, but if you consider inrush current, temperature and voltage dependency things quickly start to get tricky.
Automotive light bulbs are usually rated for 12V nominal voltage. In case of overvoltage - their lifetime decreases drastically. When the engine is on, the normal voltage on the battery rail is about 13.8-14V (can go up to 18V and all modules must resist for 2 minutes at 25V).
Due to the fact that we are not allowed to overload the light bulbs, we are continuously monitoring the supply voltage and adjust the pwm drive to get the effective voltage down to 12V. This mean that at normal supply voltage we will always get about 85% duty cycle on the bulb.
The majority of the tests are done using original loads ( real light bulbs ) but there is a set of tests which can't be done with them.
The tricky tests are open load detection threshold and overload threshold. This are two software configurable limits individual for every output.
Open load threshold is the current under which we consider the bulb to be interrupted or missing ( it's not zero because you always have some dirt resistance, offsets, noise and so on. Also often one output can controll 2 light bulbs and we must detect that one of them is burned)
Overload is also a software threshold. Basically it's a short circuit protection but it kicks in much soonet than hardware SC protection from drivers.
The ability of precisely measuring low and high currents as well as being able to configure all thresholds are very important for customers.
This is why we must check that during development phases.
The eload should not reproduce the light bulb current profile but must tollerate PWM.
The solution with LC filter is not practical (especially at high supply voltages). In order to have reasonably low ripple current we would have to use inductors over 1H with saturation currents over 5A. Also this solution may cause the ECU to detect open load at turn on of the output because it expects the certain current within a certaion period of time. Inductor will slowdown the current rise time.
You can imagine that we can't adjust the current of the real load to be right at the limits therefore we use an eload for that.
Until now we did these measurements using a separate power supply and a diode to keep the eload in it's linear region but that may trigger another errors in the ECU which slows down the testing process.
The bottleneck is not in the bandwidth of current eloads but in saturation recovery time.
We can't build our own measurement equipment because later we'll get in trouble with our quality mannagers and we'll have to prove that our test jigs are according to standards.
For sure there are shortcuts to evercome these problems ( this is not the first project which we have done ) but I was hoping that there is an eload which can overcome these problems.
Thank you for your answers. Until now the Eload with CC+CV mode looks the most suitable choice but I'll have to investigate that tomorrow more in depth.