kevinwinder
New Member
I am using mosfet as load switch application. anybodies can help to calculate if value of resistor R1, R2, RB are correct if AO3401 and MMBT3904 was used?
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Can you help to verify those values by showing the calculation?Looks like you chose a MOSFET that turns on at low voltage. Good job!
The resistors look good to me.
NPN transistor: 100k pull down resistor. Probably not needed. The micro that drives this pin might have all of its pins in 3-state at power up. There might be some noise on this line.Can you help to verify those values by showing the calculation?
This is to turn on off the power to a circuit. I assume it gets turned on at power up, or once/hour. So driving the gate with a 1K resistor is fine. The MOSFET will turn on slow which is good in this case.What is the switching frequency?
NPN transistor: 100k pull down resistor. Probably not needed. The micro that drives this pin might have all of its pins in 3-state at power up. There might be some noise on this line.
NPN transistor: 10k on base. You will have about 1 volt across this resistor. 1.8V-0.65V= 100uA Approx.
NPN transistor: When NPN is just turned on, but the MOSFET's gate capacitor has not charged up yet, there could be 5V across the 1k resistor so about 5mA of current in the collector. 5mA/0.1mA= current gain of 50. That transistor can do that. Even if it could not that is OK for this very short time.
Then the MOSFET's gate has 4.8 volts across it the NPN collector current drops to 5V/470k ohms. I did not check to see what the NPN's collector leakage current is when the base is at 0V and the temperature is high. (50nA??) So what is the collector voltage (gate voltage) with 50nA and 470k ohms? = not much
Do you mean R2=470K is too big?
Have checked with some application notes in internet, some application note for mosfet as a load switch mentioned that the value of R2 should be in ranged 1K to 10K, how to determine the correct value of R1 and R2?
Hello Kevin
You know I have looked and looked at your schematic and felt that something was wrong but couldn't put my finger on it. Then the penny dropped: Q28 is a PMOSFET so the arrow on its schematic symbol should point outwards not inwards. This makes no difference to the circuit function though.
Can you explain the purpose of L20. I asume it's just high frequency filtering.
Data sheets for reference:
AO3401 PMOSFET data sheet: http://www.aosmd.com/pdfs/datasheet/AO3401.pdf
MMBT3904 bipolar NPN transistor data sheet: **broken link removed**
PS: you have put your post inside the closing quote tag so it looks like it's part of ronsimpson's post. Just Lclick on 'edit'. Lclick on 'More Options' and move your text down and outsite the closing 'quote' tag. Lclick 'Save Changes'
The inductor is probably a bead and will have very little energy stored.you have a residual current in the inductor