Double MOSFET action  - Page 1
Forum section: electronics / projects

author: Md Mubdiul Hasan
author_rank: Regular Contributor
timestamp: January 24, 2018, 07:46:21 am
content: Hello There,I am not sure how I can present my problem!Lets try to make it easy.Take a look this image,https://www.eevblog.com/forum/projects/double-mosfet-action/?action=dlattach;attach=389168;image1. I am confused about this problem, when V_USB_MGCN(4.60v) is powered in the circuit with CTRL_PHONE_CHG_ON_OFF ,the V_RING_5V(nearly 4.6v) connects with V_USB_MGCN through the mosfet diode, so the host device goes to charging mode what I dont want.2. For this solution I have change my mind, add 2 mosfet back to back and add a 100k resistor between them.Thinking that the opposite diode action block the system from charging the host mode device.Now the figure looks likehttps://www.eevblog.com/forum/projects/double-mosfet-action/?action=dlattach;attach=389170;imageKindly help me to understand the idea is wrong or not .

author: Zero999
author_rank: Super Contributor
timestamp: January 24, 2018, 11:27:47 am
content: I'm not sure exactly what you're asking. Yes two MOSFETs connected back-to-back is a fairly standard way to ensure it doesn't conduct in the reverse direction, when it's supposed to be off. Other than requiring more components, the main disadvantage is the on resistance is doubled.What does R112 and C39 do? They will slow it down, which I think is a bad idea.

author: Md Mubdiul Hasan
author_rank: Regular Contributor
timestamp: January 25, 2018, 04:24:48 am
content: Dear Sir Hero999,Thank you for your response here.Take a look my feedback on your comment.QuoteAs I said in my post, at first diagram when the V_USB_MGCN ( its a magnetic connector charger port) and CTRL_PHONE_CHG_ON_OFF micro-controller signal pin in action, the power path of both V_USB_MGCN and V_RING_5V (host mode power source) are forced to connect. That makes my device on charging mode, I want to stop it.QuoteCould you kindly explain its drawback? Between the Mosfet(http://pdf.tixer.ru/1031640.pdf), the junction point both positive and negative current path meets together. If its OFF then, dont you think any imbalance in this circuit action? Both source to gate are biased by 100k resistor, when Q18 (second figure) is turned on, dont you see any abnormality on the circuit, where do you think to put components more?QuoteI think these are using a safe switching for Q18, if Q11 turned ON, the collector current need to flow steady.

author: Zero999
author_rank: Super Contributor
timestamp: January 26, 2018, 09:00:31 am
content: The double MOSFET configuration is a tried and tested solution, see the site linked below:http://www.electronic-products-design.com/geek-area/electronics/mosfets/using-mosfets-as-general-switchesThe only problem with your circuit is R112 and C39, which slow it down. The longer it takes for the MOSFET to turn on and off, the greater the power dissipation. If the MOSFET is switched too slowly, there's a risk it could overheat.

author: Md Mubdiul Hasan
author_rank: Regular Contributor
timestamp: January 29, 2018, 01:47:30 am
content: Dear Sir Hero999,Thank you again to respond here.QuoteYes, this link is useful. Most important clue for back to back arrangement of P Channel mosfets is "if the voltage being switched is > Vgs switching threshold of the mosfet used. "I have run a simulation, take a look the attachment,QuoteI did not find any problem using R112 and C39 while simulate them in thermal analysis.One issue in my system would be, I am using CTRL_PHONE_CHG_ON_OFF 3.3v signal. Not sure proper isolation is maintained or not.

author: Zero999
author_rank: Super Contributor
timestamp: January 31, 2018, 11:18:45 am
content: Quote from: Md Mubdiul Hasan on January 29, 2018, 01:47:30 amI'm not sure what you're asking. The DTC304 should work with a 3.3V signal, with no problems. You won't get any isolation, unless you use an opto-coupler.