[Help Needed] switch a AC signal (+/-50V, +/-500mA, <50kHz) with about 500kHz. - Page 1
Forum section: electronics / projects

author: BastelBaus
author_rank: Newbie
timestamp: August 08, 2024, 01:13:03 pm
content: I want to switch a AC signal (+/-50V, +/-500mA, <50kHz) with about 500kHz on and off (<2ohm on resistance would be nice) from a uC/DSP.My thougs so far:- mosfet relais are to slow- using two mosfets anti parallel might work, but I would need quite some circuit (mosfets, +/-30V supply, driver,...)- Analog mux: TMUX8211 was the closest I could findAny suggestions how to do this most effective (small, cheap, easy :-)) ?BR, BastelBaus

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 08, 2024, 05:29:00 pm
content: To what end?This sounds like some sort of power mixer (multiplier or chopper). It may be feasible or it may not. Far more detail is required:- What off resistance?- What bandwidth? What source and load impedance? How much capacitance is tolerable?- What (bi)linearity, THD, IM3, etc.?- What efficiency?- Can it be done in two stages (convert to DC, convert to other AC)? Or synthesized with a power amplifier? A small-signal mixer is a hell of a lot easier to solve (as you've noticed).- You request "small, cheap, easy", but how much really? And what priority compared to the other aspects?Tim

author: BastelBaus
author_rank: Newbie
timestamp: August 08, 2024, 07:03:47 pm
content: Dear T3sl4co1l,thanks for your response. I want to drive an LC circuit in resonance with a controllable resonance frequency. So far I was not really successfull with finding suitable components for a capacitor bank switching but I found this nice very paper (https://eprints.soton.ac.uk/456202/1/hk1g10_thesis_190511.pdf, see also patents at ref  and ) which showed a variable capacitance by a single switch on and off and dedicated timings. It works fine with a simple switch model in ltspice, so I wanted to design a concrete realization.Please see the simulation in the attachment (red the voltage and blue the current accross the switch, green the coil I want to drive in variable resonance between 20 and 40kHz with a series resistance of 1.4 ohm).BR,BastelBaus

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 08, 2024, 07:32:48 pm
content: Ah. Well, they pretty much run through it in the paper, what's missing? MOSFETs will do, just get the timing carefully or you'll waste all the resonant Q in switching losses (note the two capacitors must equalize charge the instant the switch is turned on). You probably want a balanced or bidirectional switch connection.Tim

author: BastelBaus
author_rank: Newbie
timestamp: August 08, 2024, 07:52:32 pm
content: is there any IC doing most of it for me or do I need to have MOSFETS, driver, +/-60V supply all discrete ?BR, BastelBaus

author: BastelBaus
author_rank: Newbie
timestamp: August 08, 2024, 10:07:40 pm
content: So this is my try with MOSFET Source to Source, seams to work when I apply +/-40V switching voltage. Is this really needed? Seams too much, especilly on the -40V side. Is this correct what I am doing ? ANy hints ?BR BastelBaus

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 08, 2024, 10:36:23 pm
content: Putting 60V into an at most 30V rated gate seems rather foolhardy. ...Are you sure this is a project you are prepared to tackle? You may want to focus on fundamentals (like device ratings, characteristics, circuit loops and stray inductance, etc.) first. Or, in the mean time, what's wrong with quasi-statically switching a capacitor in place?Tim

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 09, 2024, 12:22:53 am
content: Quote from: BastelBaus on August 08, 2024, 01:13:03 pmOption 2 is your only real choice.If you really want 500kHz modulation, you need to look for fast isolated gate drivers.Some include power transfer, so you might get this to one part (+ 2 MOSFETS)

author: BastelBaus
author_rank: Newbie
timestamp: August 09, 2024, 06:35:47 am
content: Thanks PCB.WizDo you mean such a componenthttps://www.ti.com/product/UCC21551-Q1? I do not realy find them with +/-60V supply, they seam all onhly positive and up to some 40V.BR,Michael

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 09, 2024, 06:56:44 am
content: Quote from: BastelBaus on August 09, 2024, 06:35:47 amThe whole gate side floats, to >1000V if needed, so there is no +/-60V.It is floating/isolated just like the SSR's, that are too slow.The UCC23313 is a simpler gate driver, still with overkill isolation.You still need a simple DC-DC isolated converter to power things on the floating side.For examples from TI, seehttps://www.ti.com/power-management/dcdc-power-modules/isolated-power-modules/overview.html

author: BastelBaus
author_rank: Newbie
timestamp: August 09, 2024, 07:21:02 am
content: Thank you PCB.Wiz for this very quick responce.But I don't get your idea yet; VCC – VE of UCC23313 is abs max 35V and the voltage over the mosfets I want to switch is +/-50V and the spice simulation guides that I need to drive my mosfets with such a +/- gate voltage. My point is not the isolation to the driving uC but who to get the mosfets controlled right. Maybe my understanding is also not good enough but If you could help it would be nice.BR, BastelBaus

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 09, 2024, 07:30:51 am
content: Quote from: BastelBaus on August 09, 2024, 07:21:02 amThese gate drivers do not switch the signal, they only drive the gates of two back to back MOSFETS.Those gates are floating, thanks to the isolated drivers and isolated power.The MOSFET gates only need ~ 10V swings, whilst their VDS rating defines how many volts they can switch overall.Addit : this appnote covers the types well.https://www.ti.com/lit/po/slvt195/slvt195.pdfIf you can relax the 500kHz switching rate (which is strangely 10x your signal frequency) the TI parts called Isolated Switch Drivers have lower power budgets (50mW) and suit speeds up to about 100kHz.The TPSI3050 for example, specs up to 58.98kHz max at highest transfer power setting, in the data, or if I go to their calculatorhttps://www.ti.com/tool/download/SLVRBI9With no aux load, and fastest setting, that calculator gives 100kHz up to ~ 80nC FET gate charge. (Lower gate loads give more spare AUX mA, tho you do not need that - maybe a LED ? )For your specs, you would target a much lower gate charge FET, with lower Ciss, Coss, Crss

author: BastelBaus
author_rank: Newbie
timestamp: August 09, 2024, 09:09:09 am
content: Thanks PCB.Wiz, We Gemans would say "I stand on the hose" :-)10V on the gate refered to which potential? The si1926dl I used has a V_GS threshold of 2.5V. When I drive my mosfets with +/-10V it does not work (See attachment). The source potential (Vn003) is at -12V.BR, BastelBaus

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 09, 2024, 09:32:12 am
content: Quote from: BastelBaus on August 09, 2024, 09:09:09 amConnect the negative of your gate drive generator V4 to n003, the two source leads.

author: BastelBaus
author_rank: Newbie
timestamp: August 09, 2024, 10:37:13 am
content: Thanks PCB.Wiz, this seams to work, simulation is running (but slow?). I also find the scematics in the datasheet of the TPSI3050 now, unluckily only a PSice model which odes not work in LTspice ...>> If you can relax the 500kHz switching rate (which is strangely 10x your signal frequency)I need to switch within the cycle of the input signal. With this phase I can control the effective capacitance. So I used the 500kHz as ball park figure, might be in this area (a bit higher would be good). In fact it is just the double frequency of the signal with PWM / phase characteristics. So the Bandwidth (rection times) should be higher.BR, BastelBaus