Best MOSFET Type for H-bridge - Page 1
Forum section: electronics / projects

author: i_am_fubar
author_rank: Regular Contributor
timestamp: February 13, 2020, 09:07:39 pm
content: So, there's a lot of MOSFET terminology that I'm a bit wooly on and I don't know how actually relevent it is to application.Trench HEXFETPower TrenchEnhancementDepleation...etcAre there any rule of thumb application information for when to use what type?Example, I'm building a small 5v 5A 3-phase motor driver. And am looking at the FDC8878 MOSFET.It ticks most of the obvious boxed, 18m\$\Omega\$ RdsOn, Sub 50nS turn on/off, low nC for parasitics.But I don't know if the Power Trench ness off it means it's good, bad or just a bit meh.Cheers

author: fcb
author_rank: Super Contributor
timestamp: February 13, 2020, 09:12:30 pm
content: Enhancement/Depletion mode are types of FET's and define how they operate:https://en.wikipedia.org/wiki/Depletion_and_enhancement_modesTrench HEXFET, Power Trench are trademarks of corporations that in-part refer to the construction of the MOSFET:https://en.wikipedia.org/wiki/Power_MOSFET

author: i_am_fubar
author_rank: Regular Contributor
timestamp: February 13, 2020, 09:14:12 pm
content: So enhancment and depletion (in the case of MOSFETs) N and P type. huh, go figure it would be that easy

author: fcb
author_rank: Super Contributor
timestamp: February 13, 2020, 09:52:52 pm
content: Quote from: i_am_fubar on February 13, 2020, 09:14:12 pmNope.It's to do with whether or not the FET is 'on' or 'off' with a zero g-s voltage.

author: Zero999
author_rank: Super Contributor
timestamp: February 13, 2020, 10:01:14 pm
content: Quote from: fcb on February 13, 2020, 09:52:52 pmTo complicate matters it's possible to deplete enhancement mode MOSFETs and enhance depletion mode MOSFETs. Assuming N-channel devices, applying a negative gate voltage to an enhancement mode device can often reduce the leakage current, especially with low gate threshold types and applying a positive voltage to a depletion mode device, can further reduce the on resistance.

author: i_am_fubar
author_rank: Regular Contributor
timestamp: February 13, 2020, 10:23:03 pm
content: Back to the MOSFET drawing board for me

author: Zero999
author_rank: Super Contributor
timestamp: February 13, 2020, 11:03:46 pm
content: Quote from: i_am_fubar on February 13, 2020, 10:23:03 pmBoth depletion and enhancement MOSFETs work the same way. For N-type devices, increasing the gate-source voltage, increases the drain current. This is the same both for depletion and enhancement mode devices. The only difference is, in depletion mode devices, the gate voltage needs to go negative, in order to cut-off the drain current. Just think of a depletion mode device, as a MOSFET with a negative threshold voltage.P-channel devices are the same, just with the signs of both the drain current and gate voltage reversed.

author: Zero999
author_rank: Super Contributor
timestamp: February 14, 2020, 07:59:48 pm
content: A plot of a depletion and enhancement mode devices, biased to act as simple on/off switches. The gate voltages are both swept from -5V to +5V. Note how the turn on curves are similar, except the turn on voltage is negative, with the depletion mode device, yet positive with the enhancement mode device.Full disclosure: I cheated with the depletion MOSFET, which is really a J-FET with a diode in series with the gate. I don't have a depletion mode model and couldn't be bothered finding/making one, but it'll be the same as a depletion mode MOSFET.