Drive a High-Side N-Channel MOSFET - Page 1
Forum section: electronics / beginners

author: QinX
author_rank: Contributor
timestamp: November 09, 2015, 02:06:50 pm
content: Hello All,I'm building a small computer case that uses 1 or 2 external AC adapters. 1 AC adapter is connected to a Thin ITX motherboard and has a 19VDC input the other AC adapter is 12VDC.I need a small PCB that allows me to switch the 12VDC AC adapter when the Thin ITX motherboard is powered on.I need to use 5VDC from the motherboard because this is the first voltage rail to go high. I need a basic On/Off switch that is based on MOSFETs because they can switch fast. The MOSFET also needs to be on the High-Side because of that is to only place where I can switch off the input of the 12VDC.I think I need a Boost converter or something similar that allows me to apply at least 20VDC to the gate to fully drive the MOSFET but I'm at a loss where to start with that.Do I need to investigate further into Boost converters or is there an even simpler solution?Thank you for your help

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 09, 2015, 04:02:43 pm
content: I don't understand the purpose. Do you need a "wired-OR" function? Do you want to draw power from one preferentially, if both are available?I don't understand why you're talking about switching on/off power when 5V comes up. Seems like that would make a really awful oscillator (transistor comes on --> 5V comes up --> transistor goes off --> 5V goes down --> ...), or do nothing at all (transistor comes on --> 5V comes up --> transistor stays on ... ?!).Tim

author: zapta
author_rank: Super Contributor
timestamp: November 09, 2015, 04:30:55 pm
content: Have you searched DigiKey for gate driver? It returns a bunch of hits. For example, this isolated driverhttp://ww1.microchip.com/downloads/en/DeviceDoc/HT0740%20B060413.pdf

author: QinX
author_rank: Contributor
timestamp: November 09, 2015, 07:05:47 pm
content: Quote from: T3sl4co1l on November 09, 2015, 04:02:43 pmSimply put I have a High-Side N-Channel MOSFET that needs 20-22V on the Gate and I only have 5V available.Quote from: zapta on November 09, 2015, 04:30:55 pmAm I correct in the assumption that if I apply 5V to Vin I get a little over 8.5Vout? That wouldn't be enough or am I missing something?

author: zapta
author_rank: Super Contributor
timestamp: November 09, 2015, 07:58:27 pm
content: You will get 8.5v between gate and source since the output is floating. What Vgs do you need to drive your MOSFET?

author: Kalvin
author_rank: Super Contributor
timestamp: November 09, 2015, 08:02:47 pm
content: P-MOSFET driven by NPN would be more suitable in my opinion. Or just 5V relay.

author: Seekonk
author_rank: Super Contributor
timestamp: November 09, 2015, 09:13:07 pm
content: Just want to mention that those XL6009 boost converters that you can get for about a buck can be driven by lifting pin #5 from ground. Of course this is inverse logic so may want to add a transistor and a pullup. This is a neat little trick that can also be done with the buck converters. It can be PWMed as fast as you want. Makes a cheap little driver circuit for LED or small motor.

author: QinX
author_rank: Contributor
timestamp: November 09, 2015, 09:19:09 pm
content: Quote from: zapta on November 09, 2015, 07:58:27 pmI've got these MOSFETshttp://www.mouser.com/ds/2/196/Infineon-IPD090N03LG-DS-v01_09-en-522767.pdfVgs needs to be 4.5V to be on up to 10V which should get me an RDSon between 7.5mOhm and 13.5mOhm.I need to handle a continuous current up to 25A in rare cases. so with a 13.5mOhm RDSon I want to use 4 in parallel to make sure I don't need a heatink. would that HT0740 be capable of that?Ok, this is new for me but lets see if I understand. in order to get the MOSFET to switch on I need to charge the gate, this MOSFET needs 1600pF max to charge it, I want to use 4 so 6400pF to charge all 4 gates?The HT0740 with a 600pF test capacitance takes 650µs to charge the capacitor. So in order for it to charge the 4 MOSFETs it would take +-7000µs or 7ms to switch all 4 MOSFETs to fully on. That should be more than fast enough for my purposes

author: max_torque
author_rank: Super Contributor
timestamp: November 09, 2015, 09:38:07 pm
content: There are plenty of "smart" high side switches that use N channel Fets with built in gate drivers etc that just need a logic level On/Off signal. They also tend to include automatic protection form short circuits, over temp etc. They are of course, more expensive than a bare Fet, but save a LOT of hassle and messing around.For example:http://www.infineon.com/cms/en/product/power/smart-low-side-and-high-side-switches/high-side-switch/automotive-smart-high-side-switch-profet/BTS50015-1TMA/productType.html?productType=db3a30442239c7bb0122ef87a5fb72f4

author: QinX
author_rank: Contributor
timestamp: November 30, 2015, 02:26:20 pm
content: Can someone confirm for me that the circuit below is correct?Ok, on another forum it is said I don't need a MOSFET driver because I'm not looking to switch at high frequencies, which is correct.I just need a N-Channel MOSFET to switch on a P-Channel MOSFET and I should be good to go.