I already tried to solve my LTspice problems within another thread here on the forum but I guess my design is bad. So for this reason I decided to start a new thread with some more information about my project. I hope somebody can give me some feedback on how to design this thing.
I'm working for a research group and they asked me to manufacture a "noise generator." It will be used by a university research group. At the university, they have a giant setup with 20 electrical cabinets, each cabinet must represent an industrial building. All the cabinets are connected with a low voltage distribution net. The idea is to generate noise within one cabinet and check how the noise will behave within the distribution net and what it will do to the other users.
They asked me to make a switching device for a high power resistor. The idea is to switch a 20 ampere load at 230VAC within a frequency range from 2-500kHz. So I thought to use a PicoScope to generate the PWM signal. With this signal I want to switch 2 MOSFETs so I can control the load and generate some noise.
I would like to have some galvanic isolation between the PicoScope and the noise generator. My first idea was to use a simple optocoupler but they are all too slow within that frequency range. I tried different and more complex devices like a HCPL3140 and a 6N137.
I first need to simulate the design within LTspice before I can start to design the PCB, and this is where things get difficult. I'm struggling to get the LTspice simulation running.
Also the design should be as simple as possible since we want to manually calculate all the voltages and currents so we can verify the LTspice simulation and make a good scientific report on this project. For this reason I can't get too complex with the galvanic isolation since this would be too hard to calculate (like the 6N137 which has internal logic gates etc).
For this reason I think an optocoupler or something similar is not the way to go. I hope somebody can give me some tips so I can get this thing to work with the least amount of components.
Our supervisor suggested to work with an optical fiber connection to send the PWM signal from the PicoScope to the noise generator, but I think this will be too complex and I have no idea how to start with this.
Attached you can find my current LTspice simulation which doesn't work and always results with the same error.
EDIT:
As suggested below i tried out this schematic.
We want to PWM an AC load, not a DC load.
So i tried to rework the schematic. The result is good in one part of the sinus but not in the other part. It has something to do with how the coils couple but i don't have enough experience about this topic to get this to work. Any suggestions?
Best Answer
Considering your requirements, only, you need (as you say) a pulse generator, that is supplied from 230 Vac, and delivers up to 20 Apk, with a variable frequency from 2...500 kHz. You also mention using galvanically isolated output, which makes sense, since you're supplying from the mains. Since you also seem to favour a half-bridge, then a simple concept would be this:
It makes use of a pulse transformer, which can be replaced by some optocoupler, if you insist, but that would imply additional circuitry to provide power to the isolated side. Or you could use readily available dedicated ICs for this, there are even HV options available.
A1
,R5
, andC4
form an oscillator,A2
provides the inverted pulses, both driving a push-pull stage, driving the power half-bridge. There is no dead-time, no gate resistor for the push-pull or half-bridge, only a bare-bones pulse forming circuit (D1
,R1
,Q1
forM1
, and similar forM2
), which is a very fast driver and, because of that, it will contribute to the noise -- which is what you were going for in the first place. The frequency can be easily tuned throughR5
(make it a potentiometer). If you don't like gates, use comparators, whatever other solution you see fit. As I said, it's a concept.Note that switching 20 A results in some 300*20=6 kW peak power, which is not recommended for a half-bridge -- if you plot the dissipated power on the power transistors, you'll see the horror for yourself. You'll need a full bridge, preferably SiCs (fortunately they're more and more available).
In case you want to fiddle with the schematic, here's the code, save it as
.asc
:Only now I understand what you actually want: a power AC current sink, that is to be connected directly to the mains. The desired current waveform is still a bit fuzzy -- you say noise, but you show PWM, did you actually mean noise (aka continuous bandwidth), or harmonics? If the latter, is the switching frequency meant to be 2 kHz and the harmonics to go up to 500 kHz? It doesn't really make much sense to inject harmonics up to 500 kHz in the mains, but if the latter is true, then a thyristor is the way to go (2 kHz switching frequency). But in the spirit of the new info, here's a reworked version:
I used SPWM, made of
V2
andV3
, but instead of a sine you can choose any other reference. If you need DC, only, then re-use the circuit from the previous schematic. Here, though, I've added some large dead-time (for better viewing), but also to show that the gate drive circuit needs adjusted to account for cross-conduction. Since I only used whatever I saw in LTspice's database, I'll leave you to choose the transistors for whatever requirements you have. I'd still recommend SiCs over MOSFETs, the ones that you see have relatively large Rds for the load current (also Qg & co). I've also increased the value of the inductance for the pulse transformer because of the 50 Hz SPWM, but that's also a matter of requirements; ajust as needed. Here's the code: