Author Topic: Double MOSFET action  (Read 1363 times)

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Offline Md Mubdiul HasanTopic starter

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Double MOSFET action
« on: January 24, 2018, 07:46:21 am »
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;image
1. 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 like
https://www.eevblog.com/forum/projects/double-mosfet-action/?action=dlattach;attach=389170;image


Kindly help me to understand the idea is wrong or not .
« Last Edit: January 24, 2018, 08:05:24 am by Md Mubdiul Hasan »
Hasan
 

Online Zero999

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Re: Double MOSFET action
« Reply #1 on: January 24, 2018, 11:27:47 am »
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.
 
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Offline Md Mubdiul HasanTopic starter

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Re: Double MOSFET action
« Reply #2 on: January 25, 2018, 04:24:48 am »
Dear Sir Hero999,

Thank you for your response here.
Take a look my feedback on your comment.


Quote
I'm not sure exactly what you're asking.
As 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.

 
Quote
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.

Could 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?


Quote
What does R112 and C39 do? They will slow it down, which I think is a bad idea.
I think these are using a safe switching for Q18, if Q11 turned ON, the collector current need to flow steady.
 


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Re: Double MOSFET action
« Reply #3 on: January 26, 2018, 09:00:31 am »
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-switches

The 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.
 
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Offline Md Mubdiul HasanTopic starter

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Re: Double MOSFET action
« Reply #4 on: January 29, 2018, 01:47:30 am »
Dear Sir Hero999,

Thank you again to respond here.


Quote
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-switches

Yes, 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, 
Quote
The 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.
I 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.
 
« Last Edit: January 29, 2018, 01:49:19 am by Md Mubdiul Hasan »
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Re: Double MOSFET action
« Reply #5 on: January 31, 2018, 11:18:45 am »
Dear Sir Hero999,

Thank you again to respond here.


Quote
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-switches

Yes, 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, 
Quote
The 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.
I 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.
I'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.
 


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