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Need help with a MOSFET npn type device

graemeian

Oct 2, 2014
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Hi, There are so many different transistor types and I cannot keep up. I need help in a circuit. I am using an FPGA that drives 3v. My need is to switch 3v, 5v, and 30v DC being controlled by the FPGA. I will be using pre configured 5v and 30v power supplies. I need 5v and 30v square waves triggered by the FPGA. My circuit design will be Vo down through a resistor down to NPN device to ground and I will tap the voltage off of the "E" lead of the NPN device. I am looking for an NPN device with a non sensitive gate so I can get clean square waves where the E to C current is all or nothing.
My switching rate will be about 5-10 MHz. Is a MOSFET generally used in this type of application? If not, what is?
Thanks.
 

Arouse1973

Adam
Dec 18, 2013
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I am not sure what you want to do. Can you show us a drawing or something. A npn does not have a gate but an n-channel MOSFET does. If you want to source current to a load you could use a P-channel MOSFET. But you will need a level shift circuit control them. What ars you driving and what current.
Thanks
Adam
 

graemeian

Oct 2, 2014
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P-channel seems to get complicated fast. Please excuse my nomenclature. On the 30V circuit, I would estimate the current to be up to 500ma. I am driving a piezo motor. I will try to upload a pdf. My power load is the hollow black box. In the drawing you see, the 10 ohm shunt is used solely to measure the current by voltage drop across it.
 

Arouse1973

Adam
Dec 18, 2013
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P-channel seems to get complicated fast. Please excuse my nomenclature. On the 30V circuit, I would estimate the current to be up to 500ma. I am driving a piezo motor. I will try to upload a pdf. My power load is the hollow black box. In the drawing you see, the 10 ohm shunt is used solely to measure the current by voltage drop across it.
Why is a p-channel complicated?
 

graemeian

Oct 2, 2014
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Negative voltage. Here is a picture of my diagram.
 

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Alec_t

Jul 7, 2015
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Why a 10Ω shunt? That will drop 5V at 500mA and waste 2.5W of power.
You will need to be careful in your choice of MOSFET to ensure it is switched fully on with a gate voltage as low as 3V.
 

graemeian

Oct 2, 2014
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The shunt was a value I picked as a possible starting point. It will be adjusted to 0 ohms after the power drain is determined. Having the gate voltage so low, is a MOSFET the way to go? The MOSFET has gate capacitance and at high frequencies, it becomes a choke. My FPGA is a Altera Cyclone® V SE 5CSEMA4U23C6N. Does anyone on this forum have the resources to model the gate switching ability? I saw this MOSFET from Rohm RJP020N06T100.
 
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Arouse1973

Adam
Dec 18, 2013
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Negative voltage. Here is a picture of my diagram.
You don't need a negative voltage to operate a p-channel. You connect the source to the positive supply and the drain becomes the output. However P-channels generally have higher RDS ON and switch slower than N-channels, so my first thoughts might not work in your application.
Adam
 

Arouse1973

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Can I ask what are you driving with this voltage? Do you need to source and sink 500 mA?
Thanks
Adam
 

graemeian

Oct 2, 2014
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I am driving a micro motor. It is like a rhombus mixed with a hexagon. The outer part is driven by piezos. I do not know the spects but we were previously able to look at the voltage with an o-scope and saw a 30v square wave from a control board that is no longer available. In some literature, I saw 500ma and I am using it as an upper limit.
 

Arouse1973

Adam
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Why such high frequency? This is the tricky part, 500 mA square wave at 10 MHz is not as simple as it sounds :)
Thanks
Adam
 

graemeian

Oct 2, 2014
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The 5-10 MHz did look large. That is for the 5v and the current is much lower.
For the 5v, I am converting a 3v square wave from the FPGA to 5v for data transmission Frequency at 5-10 MHz. I assume the current is small.
For the 30v, the square wave frequency is 60 kHz and the current could be as high as 500ma.
would MOSFETs be good for both?
 
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