Differential Current to Voltage amplifier (high power) - Page 1
Forum section: electronics / beginners

author: pwalther
author_rank: Contributor
timestamp: December 05, 2019, 10:55:33 am
content: I'm an absolute noob when it comes to analog designe. I have a problem coming up with a circuit that converts the output of a DAC to a voltage and amplifies it.Main designe requirements are the following:Output signal that can drive a capacitive load between ground and -48V (slope 48V / uS, 350nF -> 16.8 Ampere) with a 20A output current.As small as possible circuit (phisically) because there is only about 50x50mm of Space and it's a one sided pcb (the other side is aluminium for cooling).The input signal is a differential current (Ip = -In = 0 - 8mA)Available potentials are +5V, -5V, GND and -48VI worked out this circuit to convert the differential current to a single ended voltage:The output stage should probably look something like this:My problem is, that i don't know how to come up with a circuit that connects the two things.Can anyone give me a hint in the correct direction? I probably just miss the terminology to google my way to the solution.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 05, 2019, 11:00:45 am
content: What's the significance of -48V? Why not +/-24 or 30, or +48, or..?What is the equivalent circuit of the load, in particular the stray inductance to it, and any mechanical resonances (I assume this is a piezo stack)?What distortion and bandwidth do you expect?Do you actually need any bandwidth, can it just be driven CW instead?Tim

author: pwalther
author_rank: Contributor
timestamp: December 05, 2019, 11:46:05 am
content: The Load is a Inkjet printhead (a few thousand piezo elements).From my understanding the equivalent circuit is C, R, L in series.From the datasheet:The load presented by the printhead is largely capacitive from about 1000pF to 350nF (depending on the amount of nozzles turned on). The load capacitance has an equivalent series resistance dissipation factor of about 12% for large signal swings.The driver amplifier requires a small signal bandwith of about 1-2 MHz to match the frequency response of the MEMS structure. It should have a very low output impedance.To simplify circuit designe, the driver quiescent output voltage may be biased to a small negative potential (-0.5 to -2.0V).The inductance in the entire current path is about 6 nano Henries.I know, they are very vague in the datasheet ...We have a working driver in the field since a long time with -36V that they designed but we need -48V for a upgraded printhead in about a year so i thought why not look into it in my vacation so i have something to do. We managed to build everything in about 1/8 the size of what they had as reference designe except the firepulse amplifier circuit but i'm sure they overengineerd that aswell so i try to build something smaller that still does the job.I would love to share the designe they gave us but that is probably a legal problem.The max. frequency we output at the DAC is 250 kHz.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 05, 2019, 02:23:21 pm
content: What do you mean by "depending on how many are turned on"? Are there local switches for the nozzles? Why not add a few more switches and turn them into drivers directly? Then the whole thing only needs the 48V supply and signal or logic inputs.How much proportional control does an inkjet have, anyway? I've only ever seen them used in pulsed mode, that I know of. I'm not aware that they can be used for partial output (how can you get 37% of a droplet..?!) but if that's a thing here, I guess that's nice.If you're behind NDA, I understand; would you be open to consultation?Tim

author: pwalther
author_rank: Contributor
timestamp: December 05, 2019, 03:02:18 pm
content: There are 2048 Nozzles that can be turned on / off individually by data in a shift register (inside an asic in the printhead).The Waveform you apply consists of 0 - 3 pulses and an "all on" pulse at the end. The amount of voltage you apply correspondes to the size of the droplet while the slew rate controls the ejection speed.Each pulse is a different size and the up to three droplets will merge in air to form the drop that hits the material you print on. E.g with 3 drop sizes you can have 7 gray levels of the color you print. You need this because of the way the color mixes together and the more controle over the amount of ink the better.Because i do this in my vacation, i can't really afford consultation. If we really need it my boss will designe it, i just figured it was an interesting topic i don't have much knowledge in so i gave it a try. I just found some book about linear integrated circuits that has a great section about Mosfet push-pull amplifiers so i'll try to make it work with that. If it doesn't, at least i learnd something new.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 05, 2019, 04:46:59 pm
content: Hmm, cool physics!Yeah, traditional audio amplifier circuits can be used to that end, with some optimization for the higher bandwidth. Audio only needs 20kHz, but to maintain low distortion, amps often roll off in the 100s kHz. A friend has bragged about hisdrivercircuit delivering 7A to the output transistors in about the time you're looking at (and I forget exactly what that circuit looked like, but I don't think it was very special, just an elaboration on the usual design -- using cascode for the volt amp stage, and Darlingtons for the drivers).These can be built with MOSFETs or BJTs; BJTs are somewhat easier to use, while MOSFETs need more drive voltage. In particular, a complementary source follower (as you suggested initially) needs an input voltage slightly beyond the supply rails in order to saturate. Or else for rail-to-rail inputs, the output saturates some amount -- a bit more than Vgs(th) -- away from either rail. (If you can afford AC coupling, this isn't so hard to deal with -- that is, using a capacitor to store the required additional voltage for a short period of time.)You will need to pay attention to stray inductances. A mere 1cm of wire more than doubles the ESL in your circuit (figure ballpark 1nH/mm of wire or trace length, though it varies with geometry). Wide conductors, many in parallel, and nearby or interleaved grounds, are mandatory. I would hope that the transducer has a wide (stripline?) connection, or many alternating pins?Your spec is blown with only, say,L = V dt / dI(48V) (0.5us) / (16A) = 1.5uHso less than 1/10th of that will be desirable. 150nH isn't hard to achieve, but you can't be careless, and old school construction (like TO-3 devices on separate heatsinks with long wire leads) is definitely out. It certainly won't require RF construction techniques or devices; regular PC mount or SMT (TO-220, 247, D(2)PAK, etc.) will suffice.Tim

author: magic
author_rank: Super Contributor
timestamp: December 05, 2019, 06:39:19 pm
content: How often do you need to produce those swings? There's gonna be some heat.How close to rails is rail to rail?Quote from: pwalther on December 05, 2019, 10:55:33 amAs a minimum, you actually don't need any glue between these circuits. There are opamps capable of 50V/µs, there are opamps capable of 50V supply, there are opamps with rail to rail output, you may find something that will do all of that. It probably won't be cheap.If not, you will have to use a low voltage opamp and some gain stage, presumably a common emitter stage with constant current load. This is known as VAS (voltage amplification stage) in audio-speak if you want search keywords.Are those crazy fast swings required in both directions or only one?

author: RoGeorge
author_rank: Super Contributor
timestamp: December 05, 2019, 06:51:42 pm
content: Search online for service manuals of printers with a similar piezo head, and see what schematic or ICs they use there to control the nozzles.

author: GerryR
author_rank: Frequent Contributor
timestamp: December 05, 2019, 07:55:36 pm
content: I don't hear much about them these days, but you might want to consider using Norton amplifiers for your input stages. They are current driven (input) op-amps and might save some work, at least on the front end.