Why do bootstrap drive IC's not like negative voltages? - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: January 13, 2023, 07:04:49 am
content: Hi,The attached circuit is NOT OK due to the negative voltage of TS w.r.t GND.Howcome it cant take such a neg voltage?....(LTspice and jPeg scm attache)2ED2182S06https://www.infineon.com/dgdl/Infineon-2ED2182-4-S06F-J-DataSheet-v02_32-EN.pdf?fileId=5546d4626cb27db2016cb8d7368a29e3LTC4440https://www.analog.com/media/en/technical-documentation/data-sheets/4440fb.pdf

author: Faringdon
author_rank: Super Contributor
timestamp: January 13, 2023, 04:15:04 pm
content: Indeed, it is known that in proper useage the bootstrap drivers are used in the low side and the hi side fet they drive is in the hi side...ie thats the standard way to use them. As in the top schem, this modus operandi is unfortunately not available to us.As such, we wonder about the attached configuration instead? Using a digital isolator....one that cares not that its GND1 will fall more than 100V below its GND2.Would you back the attached, may i ask?LTspice and jpeg of alternative modus operandi as attached...The digital isolator in the attached would be eg ISO7810...https://www.ti.com/lit/ds/symlink/iso7810.pdf?ts=1673637471558&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FISO7810

author: xavier60
author_rank: Super Contributor
timestamp: January 14, 2023, 05:07:10 am
content: It would be simpler and more economical to use a part that's directly suited to the application.https://www.st.com/resource/en/datasheet/stgap2s.pdfSome negative spike precautions are still needed if the output channel is to be powered by bootstrapping.Extra: Just realized that bootstrapping will have trouble with some circumstances unless a half-bridge power stage is used.

author: Faringdon
author_rank: Super Contributor
timestamp: January 14, 2023, 03:23:40 pm
content: QuoteThanks, unfortunately we cant use STGAP2S as it cant do 100% duty cycle.

author: xavier60
author_rank: Super Contributor
timestamp: January 14, 2023, 06:34:04 pm
content: Quote from: Faringdon on January 14, 2023, 03:23:40 pmIt will if the driver stage is powered by a floating supply as would most other drivers need. Possibly a LNK626 with small SMPS transformer. I have used hundreds of the LNK626 without a known failure.

author: Faringdon
author_rank: Super Contributor
timestamp: January 14, 2023, 10:03:22 pm
content: Thanks, though STGAP2S is using "magnetic coupling", and AYK, its impossible to do 100% duty cycle with any magnetic core.....it would saturate.

author: xavier60
author_rank: Super Contributor
timestamp: January 14, 2023, 10:14:34 pm
content: Quote from: Faringdon on January 14, 2023, 10:03:22 pmThe magnetics couples modulated HF signals across the isolation zone, not the baseband form of the drive signal.I use the dual channel version of the STGAP2 down to 15HZ in my 3 phase motor drive product.Is this a one-off project? If so, a suitably small SMPS transformer can be salvaged from whitegoods.F&P washer controllers I repair have a nice little transformer with 1.6mH primary and 700mA saturation.

author: Faringdon
author_rank: Super Contributor
timestamp: January 15, 2023, 11:48:53 am
content: Thanks,Actually, the NCD57201https://www.onsemi.com/download/data-sheet/pdf/ncd57201-d.pdf...purportsto be able to handle VS (switching node) pin going +/- 800v either side of chip "proper" ground.I find this hard to beleive, ...if it were capable of that, then it would have wiped all the other bootstrap drivers off the market?Any bootstrap driver IC user will tell you.....negative swing on the "switching node" pin of the bootstrapper is an act of terror to the user....but apparently the NCD57201 just shrugs it off.

author: xavier60
author_rank: Super Contributor
timestamp: January 15, 2023, 12:18:44 pm
content: Quote from: Faringdon on January 15, 2023, 11:48:53 amIt looks suitable and inexpensive.

author: Wolfram
author_rank: Frequent Contributor
timestamp: January 15, 2023, 01:45:22 pm
content: Quote from: Faringdon on January 15, 2023, 11:48:53 amFully galvanically isolated gate drivers have been around for decades. Earlier units using optical isolation, and more modern devices using magnetic or capacitive isolation with a HF carrier. They elegantly solve the problems of damage from Vs undershoot, but they are also a lot more expensive than devices using well-isolation and SOI. Fully isolated gate drivers are pretty common in high power high frequency converters, where the gate driver cost is a small fraction of the system total and dI/dt is high.

author: xavier60
author_rank: Super Contributor
timestamp: January 15, 2023, 02:11:52 pm
content: The cost of galvanically isolated Gate drivers generally isn't much more than conventional types now. The little extra cost is well worth the saving of needing much less critical PCB layout, when both channels are isolated.This is the part I use,https://au.mouser.com/ProductDetail/STMicroelectronics/STGAP2DMTR?qs=W0yvOO0ixfF%252Bp5Pfo9gPjQ%3D%3DIt allows power and control parts of the PCB to be kept separate except for one star point connection at the CS resistor.

author: uer166
author_rank: Super Contributor
timestamp: January 15, 2023, 10:37:43 pm
content: A halfway solution could be to use a digital isolator with enough CMTI, and a normal non isolated single gate driver. It takes more area, but the cost might be lower and there are more dual source options.Normally an isolated gate driver is the best, although I had trouble sourcing the ones I used on a 2kW boost, and had to pay $15 a pop..

author: temperance
author_rank: Super Contributor
timestamp: January 17, 2023, 03:22:14 am
content: This is very easy to do without expensive components.A gate drive optocoupler like a FOD3180. You will need a DC/DC converter to power the opto. But that's easy too. A CM choke which AYK can double as a small transformer driven by some logic gate oscillator buffered with a TC44xy MOSFET driver will do. All that costs nearly nothing and the FOD like drivers can be sources from more manufacturers.About the negative voltage. AYK, there is a diode in the substrate of of the driver IC.

author: Wolfram
author_rank: Frequent Contributor
timestamp: January 17, 2023, 02:07:55 pm
content: Quote from: temperance on January 17, 2023, 03:22:14 amAt 15 V/ns guaranteed CMTI, this part is not suitable for driving most modern MOSFETs, even a lot of IGBTs can exceed that these days. Propagation delay tolerance of +- 90 ns, pulse width distortion of up to 65 ns, and rise/fall times in the same ballpark are also problematic for many control schemes when switching at higher frequencies.Modern magnetic or capacitive isolation parts can improve all of these parameters by an order of magnitude, at a lower cost, and potentially with other features like desaturation detection, overlap protection and dead-time insertion for half-bridge parts, and with UVLO options suited for a wide range of modern switching devices instead of a fixed value. Half-bridge drivers with full isolation can be less than a dollar each in quantity these days.Still, SOI or substrate-isolation parts are cheaper, and they continue being used in lower cost and lower power designs, as they can provide a more cost-effective solution if their limitations are designed around. These days, they are more often than not found as part of integrated switching controllers. You can get an integrated LLC controller with a substrate isolation 600 V half bridge driver for around 50 cents in moderate quantity, likely much less if buying in bulk.For gate drive power, there are a lot of interesting integrated DC/DC options now, from the likes of Mornsun, Recom and Murata. Parts with very low isolation capacitance (3 - 4 pF typically), characterized isolation lifetime under high dV/dt stress, tons of gate drive voltage options and pretty moderate cost. The Mornsun parts are around 2.50 in the 10k quantity bracket.

author: JohnG
author_rank: Frequent Contributor
timestamp: January 17, 2023, 02:15:58 pm
content: ADI also makes fully isolated gate drivers and power supplies, a bit on the pricey side though. Also, Silicon Labs (now Skyworks) makes a series of fully isolated drivers that are quite robust.Onsemi makes a driver that can explicitly tolerate -3.5V on the high and low side drivers (NCP51820?).That being said, countless millions of IR2110s and its progeny have been sold for decades, so it must be possible to make them work.John

author: JPortici
author_rank: Super Contributor
timestamp: January 17, 2023, 04:18:46 pm
content: Quote from: JohnG on January 17, 2023, 02:15:58 pmthis!

author: temperance
author_rank: Super Contributor
timestamp: January 17, 2023, 05:36:38 pm
content: @Wolfram-The FOD3180 is rated upto 15kV/ns and not 15V.-The propagation delay isn't such a problem in this case. The controller is the very old UC38xxI don't know why the op is not looking into things like an IR2110... Something about negative voltage... He should read some app notes for the IR2110 and other gate drivers.

author: Wolfram
author_rank: Frequent Contributor
timestamp: January 17, 2023, 08:27:54 pm
content: Quote from: temperance on January 17, 2023, 05:36:38 pm15 kV/µs, which is 15 V/ns, or 1.5e10 V/s.Modern 650 V Superjunction MOSFETs have rise times of below 10 ns (IPA65R225C7 has a Tr of 6 ns as an example), leading to dV/dt in the 40 - 70 V/ns range with a 400 V DC bus. I've seen latchup and spurious oscillation in Silabs Si823x series drivers (CMTI max = 50 V/ns) with both older generation superjunction devices and SiC MOSFETs, so this is a real problem. After switching to 150+ V/ns fully isolated gate drivers (Silabs Si827x series, some Infineon 2EDBxx), there were no more issues.Even transformer based gate drivers struggle with 50+ V/ns, at which point 10 pF of interwinding capacitance in the GDT will pass half an amp of current into the output stage of the transformer driver, often requiring tradeoffs in GDT leakage inductance or additional protection components. The same concerns apply to DC/DCs used to power high side drivers, many small power transformers and DC/DC bricks can have a lot of pri-sec coupling capacitance. The Recom RD series as an example, has 40 - 110 pF of coupling capacitance, passing 2 - 5 A of transient current if connected to a switching node at 50 V/ns.

author: xavier60
author_rank: Super Contributor
timestamp: January 17, 2023, 08:34:16 pm
content: Faringdon wants to hang the input side of the driver IC from the 175V rail rather than reference it to the negative/ground rail.

author: temperance
author_rank: Super Contributor
timestamp: January 17, 2023, 11:04:07 pm
content: @WolframI was to fast to complain and didn't see the "µs", "ns".The schematic shown switches 175V in about 14ns so the FOD3180 isn't good enough. A FOD3120 would be a better choice with more than twice the required rating.About the DC/DC converter. I suggested a CM inductor because of the low stray capacitance. (not all of them of course)

author: xavier60
author_rank: Super Contributor
timestamp: January 21, 2023, 03:35:56 am
content: Take a look at the SN6505 transformer driver in this post.https://www.eevblog.com/forum/beginners/mosfet-killed-by-back-emf/msg4650082/#msg4650082If an off-the-shelf transformer can't be easily obtain, one could be whipped up.