FET gate drivers - Page 1
Forum section: electronics / projects

author: LukeW
author_rank: Frequent Contributor
timestamp: November 13, 2013, 10:13:34 am
content: I wanted to have a look at other people's opinions on something.Suppose that a 3.3V signal from a microcontroller is used to switch a high-current load via a low-side N-channel power FET.What's the most economical choice of gate driver IC or circuit to interface between the microcontroller and the gate of the power FET, to provide an appropriately high Vgs to turn it on hard? We will assume that the load operates at 24V, the microcontroller operates at 3.3V and there is a little bit of current available from a spare 10V rail which is probably suitable for the gate driver.What's the most economical way to implement something that will work reliably?Is a dedicated, relatively expensive gate-driver silicon justified, or would something simple such as the attached image, using a common jellybean cheap 2N7002, work effectively?Or is it a simpler, more reliable, or overall cheaper option to choose a logic-level MOSFET with an appropriately small gate threshold to accept a direct drive from the microcontroller? Obviously this will decrease the available choice of devices and may increase cost a little.

author: nctnico
author_rank: Super Contributor
timestamp: November 13, 2013, 11:58:51 am
content: I've used a 74HC544 with several outputs in parallel powered from 6V in a design. The real trick is to choose a MOSFET wisely. If the switching frequency is high then a MOSFET with a low gate capacitance and medium RDSon (0.1 Ohm ball park) could be more efficient. Also try not to use a MOSFET rated for several times the current. It will only increase losses due to higher gate capacitance.

author: owiecc
author_rank: Frequent Contributor
timestamp: November 13, 2013, 04:07:46 pm
content: One way is to add a totem pole driver stage. NPN+PNP plus some resistors.You can use a MOSFET with very low Vgs(th) like AUIRLR3114Z adn drive it directly from your micro. I don't know what is your current level but transistor one should dissipate approx. 1W at 10A at Vgs = 3.3V.You can also use a pFET to switch your load. This way you only need a NPN and two resistors.

author: AndersAnd
author_rank: Frequent Contributor
timestamp: November 13, 2013, 05:23:40 pm
content: Quote from: LukeW on November 13, 2013, 10:13:34 amThat circuit won't work at all.2N7002 is an N-channel MOSFET and won't turn on at all the way you have used it.2N7002 has a 2.5 V max G-S threshold, which means the gate voltage has to be at least 2.5 V higher than the source voltage for it to turn on. But when it's turned on you want 10 V on the source, so you would need at least 12.5 V on the gate for it to be turned on and that's not going to happen from a 3.3 V MCU output.You need a P-channel MOSFET instead if you want to use it in that position get the first MOSFET to turn on at all, but you can't just interface that directly to a 3.3 V MCU either from a 10 V rail to the P-channels source. Then it would always be on regardless if the MCU output is 0 V or 3.3 V, because you are always above (below) the threshold value.

author: gotvolts
author_rank: Contributor
timestamp: November 13, 2013, 05:52:38 pm
content: The question that needs to be ask is what you consider a "high current load."If high current means relatively speaking to a PIC microcontroller circuit and is only a few amps, then the PIC microcontroller can easily drive a logic-level N-Channel FET directly.I'm particular fond of the DMG6968U MOSFET. It can switch up to 6.5A and has a very low RDS and is in a tiny SOT-23 package.So, what would your actual load numbers be that you want to switch?Also, switching frequency is important too. If you are switching low frequency or DC, than driving a FET directly with a PIC may be feasible, but if you need to switch at very high frequencies, you will need a lot of sink/sourcing current to charge / discharge the gate capacitance very quickly.

author: AndersAnd
author_rank: Frequent Contributor
timestamp: November 13, 2013, 06:46:20 pm
content: Quote from: gotvolts on November 13, 2013, 05:52:38 pmHe's switching a 24 V load.Maximum ratings for DMG6968U: Drain-Source VoltageVDSS 20 VSo that's not going to work with DMG6968U.DMG6968U datasheethttp://www.diodes.com/datasheets/ds31738.pdf

author: electros6
author_rank: Regular Contributor
timestamp: November 14, 2013, 01:40:39 pm
content: If you are using low frequency PMW just used a transistor amplifier of used a optocoupler driver to protect the microcontroller and also add 2 or more to the gate driver power supply to put mosfet in full conduction to reduce heat dissipation.

author: LukeW
author_rank: Frequent Contributor
timestamp: November 19, 2013, 10:11:39 am
content: Load current of 12A. No high-frequency switching or PWM, just low-frequency on and off.Quote from: gotvolts on November 13, 2013, 05:52:38 pm