The 12V level shifter saga

Piotr Wyderski · 2020-02-02T13:44:15+00:00

Hi, I am working on a high-current 4-switch buck-boost converter. The switches are psmnr70-40sshj, which -- with their 202nC gate charge -- makes the 12A gate driver MCP14A1202 a perfect spouse. The problem is with passing the control signals to the upper switches. I want to have true 100% duty cycle operation capability, which excludes AC coupling techniques such as a signal transformer. The high-side switches float at most 15V above the GND. Options: 1. Use a 1.5kV digital isolator. There are units capable of 7ns propagation delay, which makes a total end-to-end delay of about 33ns. Not bad, but I feel it is heavily overengineered. In total that would make 4 gate driver ICs + 2 high-side isolators + 1 2-channel isolator to match the timing between the high- and low-side paths. 7 ICs or 6, is the delay equalizer is based on an RC delay + a Schmitt trigger. 2. Similar to the above, but use a half-bridge driver such as the LM5101 just as a level translator. 6 ICs in total and the propagation delays are equalized out of the box. Disadvantage: the part is not stunningly fast with its own 25ns propagation delay. The fastest HB gate driver I know of, the LMG1210, has 10ns delay. OK, I tested the 5101 variant, works nice up to 2MHz, but has 58ns end-to-end propagation delay... well, meh. 3. A *simple* solution based on discrete components. Total failure in practice, really simple common-base units have ~400ns delay; the best I could make was beyond the pain threshold complexity level and was capable of 78ns, not including the output stage. OK, I have a working solution (2), can go from 25 to 16ns translator delay by replacing the LM5101 with MP18021 or switch to (1) and get 7..10ns, but I have a feeling that something is very wrong, because (3) should be the solution. Why is making a fast level shifter so notoriously difficult? One interesting observation so far is that isolators are so much faster than their non-isolated counterparts. Best regards,...

Feb 02, 2020 45 Replies

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Phil Hobbs 6 years ago

Sort of like the SchmartBoard idea?

Any and all of the above would be very welcome!

Thanks

Phil Hobbs

Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com

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dagmargoodboat 6 years ago

Cute circuit. TI has some boot-strapped gate drivers that essentially integrate that for Si MOSFETs, cap-coupled.

Cheers, James Arthur

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jlarkin 6 years ago

I can send you a few boards with some fets mounted, at least the ones I haven't blown up yet. I'll see what parts are available.

Anyone is welcome to the PADS files and Gerbers, but you'd have to make boards and solder the critters down.

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John Larkin Highland Technology, Inc The cork popped merrily, and Lord Peter rose to his feet. "Bunter", he said, "I give you a toast. The triumph of Instinct over Reason"

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jlarkin 6 years ago

All the commercial gate drivers that I've seen have a lot of prop delay. A home-brew LVDS+GaN can come in at a few ns.

My little Pockels Cell driver (1200v 7 ns pulses, 4 MHz) has Tiny logic driving GaN driving water-cooled SiC. Lotta gain-bandwidth there.

John Larkin Highland Technology, Inc The cork popped merrily, and Lord Peter rose to his feet. "Bunter", he said, "I give you a toast. The triumph of Instinct over Reason"

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Winfield Hill 6 years ago

The TI parts are nice, work to higher voltages, and can drive higher Ciss, but John's LVDS scheme with the 100V, 1A EPC mosfets is much faster!

Thanks, - Win

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jlarkin 6 years ago

Three or six sections of Tiny logic triple buffers can drive the bigger EPC parts. Run them at abs max Vcc or so.

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Then parallel a bunch of those, each with its own gate drivers. Crazy fast and 20 KW peak pulse power *each*

John Larkin Highland Technology, Inc The cork popped merrily, and Lord Peter rose to his feet. "Bunter", he said, "I give you a toast. The triumph of Instinct over Reason"

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