Wish to have 96 separate isolated , high-side NFET gate drives - Page 1
Forum section: electronics / general

author: opampsmoker
author_rank: Regular Contributor
timestamp: September 28, 2020, 07:39:05 pm
content: Please assist…We have 96 back-to-back NFET switches which are to be switched ON/OFF occasionally. We are using 30V NFETs (3x3mm). They will carry 10 Amps when ON.We need 96 separate High side isolated drive voltages (~12V) to switch all of these.What is the cheapest/smallest way to get all these high side isolated drive voltages?Transformer drive chips..We could use 96 Max845’s with 96 Halo TGM-010P3RL transformers with voltage doubler rectifier on each output. However, I reckon the TGM-010P3RL will likely go nil-stocked…..so I am also pondering winding ninety-six 1:1:2 transformers myself on torroids.Flyback converters:However, I am wondering if I could do eight flyback converters, each with 13 separate isolated dsecondaries (one secondary for primary side regulation). I could make it a “triple layer sandwich wind”, with a primary of 4 series layers, with three secondaries in each “primary sandwich”.What do you think is the “lowest component count” way of doing this?MAX845 transformer driverhttps://datasheets.maximintegrated.com/en/ds/MAX845.pdfTGM-010P3RL transformerhttps://www.mouser.co.uk/datasheet/2/706/discrete-max845dc2dc-pcmcia-520302.pdf

author: capt bullshot
author_rank: Super Contributor
timestamp: September 28, 2020, 08:14:58 pm
content: You could use 48 of thesehttps://static5.arrow.com/pdfs/2014/11/20/18/13/55/436/nts_/auto/637an9325.pdfhttps://www.renesas.com/us/en/www/doc/datasheet/hip4081a.pdfThe HIP4081 has a special feature - it can drive the High-Side MOSFETs on for infinite time. One can apply this feature to other high-side drivers by building the charge pump using discrete components. One strong enough oscillator driving 96 charge pumps, each consisting of two capacitors and two diodes ...Or 24 of these (protection included if desired):https://www.analog.com/media/en/technical-documentation/data-sheets/1161fa.pdf

author: Cerebus
author_rank: Super Contributor
timestamp: September 28, 2020, 08:39:54 pm
content: What's stopping you using the usual trick of using a diode and bootstrap capacitor to derive your high-side gate drive voltage, duty cycle?

author: opampsmoker
author_rank: Regular Contributor
timestamp: September 28, 2020, 09:09:32 pm
content: QuoteThese FETs may need to stay ON for long periods, as you know, the bootstrap cap would not get sufficiently refreshed in this useage.Thanks for the photovoltaics, though i believe these suffer degradation in vout and iout over time. Also, we dont need to switch quickly but there is some talk of possibly doiNg say 1kHz PWM of the FETs...As such...We were wondering about the attached push pull driver as a way of doing 4 isolated outputs. PDF and LTspice sim attached.Any improvements on component count possible?

author: opampsmoker
author_rank: Regular Contributor
timestamp: October 12, 2020, 09:59:10 am
content: ...please may i update...?We are seeking an Analog switch (preferably multiple in a chip), which can switch voltages which are outside its Vcc range.......or......the Chip have a wide 0_Vdd range so that voltages up to 40V can be switched.The attached is what we wish to do. We need to turn on a FET that is not referenced to ground. And there are many such FETs, so we want to use an IC full of Analog switchs.We preferably need to switch voltages up to 40V

author: daqq
author_rank: Super Contributor
timestamp: October 12, 2020, 10:40:09 am
content: Would this be an option:https://www.ixysic.com/home/pdfs.nsf/www/FDA217.pdf/$file/FDA217.pdf for the gate driver?As to the high voltage analog switch, what about this one:http://ww1.microchip.com/downloads/en/DeviceDoc/HV2801-HV2901-32-Channel-Low-Charge-Injection-High-Voltage-Analog-Switches-Data-Sheet-20005840A.pdf

author: f4eru
author_rank: Super Contributor
timestamp: October 12, 2020, 12:06:40 pm
content: QuoteClearly the lowest component count is to use integrated protected high side switches, and not discrete FETsThe 2-channel ICs can be (ab)used to provide a back to back configuration, with the driver self powered by the 2 open mosfets's diodes.for example :https://www.st.com/content/ccc/resource/technical/document/datasheet/1a/d2/46/bd/11/38/41/2a/CD00248836.pdf/files/CD00248836.pdf/jcr:content/translations/en.CD00248836.pdf

author: Zero999
author_rank: Super Contributor
timestamp: October 12, 2020, 09:06:24 pm
content: Quote from: opampsmoker on October 12, 2020, 09:59:10 amNo, analogue switches won't work. The voltage switched needs to be within their supply voltage range, so 40V is a deal breaker.Yes they will. Use the HV2801/HV2901suggested by daqq If it was just 20V or so, then an 18V supply and gate driver per channel would work, but 40V makes that infeasible.Why not use P-FETs? They're much easier to control and you only need 10A per channel, which is trivial. As you're not bothered about fast switching, use an NPN transistor and a couple of resistors per channel. Cheap and simple.How about using an oscillator and coupling the signal, via capacitors?EDIT: Although the OP doesn't want to use the circuit on the mains, for those who do, the 100pF capacitors need to have the appropriate safety rating and the circuit needs to be assembled with sufficient creepage and clearances for the voltage used.

author: tom66
author_rank: Super Contributor
timestamp: October 12, 2020, 09:49:28 pm
content: Second the idea of using P-FETs. You can find P-FETs with integral zeners (not the same as ESD diodes) or high side switches for this exact purpose. Unless you are extremely cost sensitive, this will almost certainly be cheaper than engineering some way to use N-channel FETs.

author: f4eru
author_rank: Super Contributor
timestamp: October 13, 2020, 04:14:54 pm
content: Yep. PFETs is a very simple solutionjust a zener and pullup R on the gate, and a ULN2803 to pull this down via a resistor, simple.

author: opampsmoker
author_rank: Regular Contributor
timestamp: October 28, 2020, 07:48:26 am
content: Thanks to all here.Quotedaqq you are a miracle worker....i was searching the entire world for something like the HV2801...I still almost cant believe that a "5v vdd" chip can have its analog switchs switching voltages up to 200V.This is very useful. Thankyou

author: tom66
author_rank: Super Contributor
timestamp: October 28, 2020, 08:09:26 am
content: Quote from: opampsmoker on October 28, 2020, 07:48:26 amThese types of devices (in 96-channel form) used to be used in plasma TVs. They switch to a +150V bias voltage to select the row to be programmed. When the display is 'sustained', each IC had to pass peak currents of around 10-20 amps. Impressive silicon engineering. They usually didn't even need a passive heatsink.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: October 28, 2020, 09:07:25 am
content: How about this device from Silabs :https://www.silabs.com/documents/public/data-sheets/Si8751-2.pdfIt outputs an isolated supply of about 9V at 10uA for driving FET gates. It's significantly faster than photovoltaic optoisolators

author: voltsandjolts
author_rank: Unknown
timestamp: October 28, 2020, 10:15:32 am
content: Quote from: opampsmoker on October 28, 2020, 07:48:26 amIn what way is that chip useful to you?You are switching 10A according to first post. Are you using this as a simple level shifter to drive power PFETS?