Level shifter runs hot - Page 1
Forum section: electronics / beginners

author: zzattack
author_rank: Regular Contributor
timestamp: July 17, 2023, 01:38:28 pm
content: I'm building a 3.3V to 24V level shifter using parts from the bin, and came up with something that works but gets prohibitively hot.The incoming signal is a 3.3V pulse of 200ns width (at at most 120Hz) that I need to translate to a pulse (of arbitrary duration) of 24V. Given the minimal pulse width I'm "buffering" the signal on a micro and extending it to 10us, so switching speed is no longer a major concern. Initially I tried do to do this without buffering the input pulse, hence the use of the gate driver as opposed to an even simpler NPN - PMOS setup. What I came up with:This works with R1=1k and R2=10k, but both the gate driver and the PMOS get too hot to touch. I'm looking for some guidance on how to size R1/R2, and potentially how to improve the circuit to run not as hot.

author: Andy Watson
author_rank: Super Contributor
timestamp: July 17, 2023, 02:35:47 pm
content: What have you done with the unused gates and inputs?The spare level-shifter should have its input tied to ground.The gate of the unused FET should be tied to its source.And, have you considered de-coupling capacitors ?

author: madires
author_rank: Super Contributor
timestamp: July 17, 2023, 02:36:58 pm
content: IX4426 is a low-side driver and the FDS4935 is a dual p-ch MOSFET which has a maximum V_GS of -20V. And a large gate resistor slows down switching while keeping the MOSFET longer in its resistive mode (problematic if you drive a load).

author: zzattack
author_rank: Regular Contributor
timestamp: July 17, 2023, 03:38:24 pm
content: Quote from: Andy Watson on July 17, 2023, 02:35:47 pmI've actually connected the 2nd output to the gate of the 2nd FET, but the input on the gate driver is left floating, and serves as a 2nd channel which is not always going to be connected. In that case, should I add a (very weak) pull up on the IX4426 input?Quote from: madires on July 17, 2023, 02:36:58 pmSo the max Vgs is exceeded then, making this p-ch unfit for my application?

author: madires
author_rank: Super Contributor
timestamp: July 17, 2023, 06:11:25 pm
content: Quote from: zzattack on July 17, 2023, 03:38:24 pmIt's the other way around, i.e. you need to limit V_GS to a reasonable value (typically 12V) as you won't find MOSFETs with a higher V_GS. Anyhow, you missed the main issue: the driver IC is a low-side driver which means it's meant for n-ch MOSFETs.

author: zzattack
author_rank: Regular Contributor
timestamp: July 20, 2023, 02:32:45 pm
content: @madires: hoping to get a small sanity check. Updated the idea with the MOSFET configuration exchanged for an N-channel and lowering the VCC to the gate driver (and thereby the Vgs) using a 7812, and pull-downs on the gate driver inputs.Looking better?

author: Ian.M
author_rank: Super Contributor
timestamp: July 20, 2023, 03:50:01 pm
content: Since it seems your load is fairly high impedance (as a 10K pullup/pulldown is acceptable), why not drive it direct from the IX4426?The IX4426 datasheet says its output will swing within 25mV of the rails (no dc load), and looking at the thermals, and the max. output resistance, it should be good for about 100mA steady state, far more than you need.

author: madires
author_rank: Super Contributor
timestamp: July 20, 2023, 07:08:47 pm
content: Quote from: zzattack on July 20, 2023, 02:32:45 pmYes! The 7812 needs some caps and a minimum load.