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A "totem pole switch" I got problem with basic electronics understanding.

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LAN_403

lightlinked

Sat Jul 10 2010, 12:05PM

Registered Member #2087 Joined: Tue Apr 21 2009, 08:32AM
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Posts: 115

it looks like the load for the fet is after the source, ie sourcing current. if you want to source current with a fet you would use a p channel device or, for an n channel high side device, have a bootstrap or driver to get the gate voltage high enough. "A N-MOSFET/IGBT needs a significantly positive charge (VGS > Vth) applied to the gate in order to turn on" wikipedia

Bjørn

Sat Jul 10 2010, 12:52PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
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You are not giving the gate 12 V, stick the scope between gate and source and see what the voltage really is. For you circuit to work you need to connect the source to GND so that you get 12 V between source and gate.

cavemen

Sat Jul 10 2010, 06:55PM

Registered Member #2008 Joined: Tue Mar 03 2009, 05:11AM
Location: USA, Frederick, MD
Posts: 118

I guess that the datasheet that tells me that Vgs for this mosfet is 10V in the absolute maximum ratings, is a lie.

What cheap MOSFET can I use that has a low ON voltage and can handle around 9V of power at 25C? It has to be pretty fast.

I see that they tell the gate threshold voltage a lot, but almost never specify what voltage it takes to turn a MOSFET fully on. What would I use to determine it? The graphs?

Is thee a better way to control a powerful load with a 3.3V or 5V chip than 'totem pole switch'?

lightlinked

Sat Jul 10 2010, 09:38PM

Registered Member #2087 Joined: Tue Apr 21 2009, 08:32AM
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How fast with how much power? Here is an app note from vishay

about p channel high side switching. I like the idea of a 2n7000 and p-channel combo. A bunch of 2n7008 are handy to have around in place of NPN BJTs for switching stuff plus you don't need a current limiting resistor like with a bipolar device.

You can use the N-channel mosfet if you put the load before the mosfet. Have the load connected to the drain and the source to ground. Another alternative is a gate driver chip which might be overkill.

in the mosfet data sheet the graph "typical transfer characteristics - gate to source voltage" will give you an idea for how much current you can put through the device at a certain gate voltage, at a low gate voltage it will be in linear operation, like a voltage controlled resistor, untill a certain point where the device becomes saturated.

cavemen

Sat Jul 10 2010, 10:29PM

Registered Member #2008 Joined: Tue Mar 03 2009, 05:11AM
Location: USA, Frederick, MD
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Lightlinked, are you referring to the figure 5 of the publication?
What voltage are those circuits controlled with?
I need 3.3V and 5V to switch my circuit.
I thought of using complementary JFETs but couldn't find a complementary pair.

Figure one cannot be used for switching as it requires more voltage to control than it can output.

I will work with possibly 12V power supply.
Later I will use rectified 120VAC for other projects, but it is not what I am worried about now.

I found logic-level mosfets, but those aren't so common still.

???

I guess the PWM action will be fine on around 500khz. I will do some math that is based on the gate capacity of the mosfet to tell my limits. t = RC

lightlinked wrote ...

How fast with how much power? Here is an app note from vishay

cavemen

Sat Jul 10 2010, 10:40PM

Registered Member #2008 Joined: Tue Mar 03 2009, 05:11AM
Location: USA, Frederick, MD
Posts: 118

How can I find Vgs of the MOSFET in Multisim 11 libraries?
That value somply isn't in eletronic parameters.
How can I import a MOSFET into the Multisim?
There are no NTE semiconductors in it's libraries.

Bjørn

Sun Jul 11 2010, 01:39AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

Simulators generally simulates the smallest possible subset of parameters so it is not unlikly that Vgs it is not there. You seem to make this very complicated. If you don't understand a circuit completely it is not very safe to use a simulator since it will happily simulate complete nonsense without any sign that something is wrong.

Consider this very carefully:
The IRL540 you have in the simulation will turn on nicely at 4-5 V. If it does not then you failed to give it the required voltage. You ignored my advice to measure between Gate and Source to find the voltage you are giving it, you keep measuring between Gate and GND which is completely irrelevant, it is called Vgs for a reason.

Delete everything you did and start again, this time define every requirement and don't change your mind halfway through.

cavemen

Sun Jul 11 2010, 04:09PM

Registered Member #2008 Joined: Tue Mar 03 2009, 05:11AM
Location: USA, Frederick, MD
Posts: 118

groung symbol and spike supression diode were part of the problem

How do I attach the 3.3V signal from the microcontroller to the circuit?

Bjorn, I would gladly define all the conditions and use the right parts, but Multisim doesn't have much in it's databases and I haven't figured out a way to enter components into Multisim.

It appears that in datashees they give common numbers that electronic engineers need, but in Multisim it is all kinds of correction coefficents that change the rational function to fit the specific component. At this point it seems impossible for me to find those numbers.

Things like:
+ (
+ LEVEL= 1
+ VTO= 0.0
+ KP= 2.0e-5
+ GAMMA= 0.0
+ PHI= 0.6
+ LAMBDA= 0.0
+ RS= 0.0
+ RD= 0.0
+ CBD= 0.0
+ CBS= 0.0

I am still not sure how MOSFET works and how it should work.

cavemen

Sun Jul 11 2010, 04:13PM

Registered Member #2008 Joined: Tue Mar 03 2009, 05:11AM
Location: USA, Frederick, MD
Posts: 118

The old Electronic Work Bench has an adventage because it is all "platonic" and places ideal components by default.

Bjørn

Sun Jul 11 2010, 05:17PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

Why don't you measure between Gate and Source and tell us what you get and why you think that voltage is enough to turn the MOSFET on? (This is the third and last time)

You got your Source at 9.61 V and the gate at 12 V then Vgs = 12 - 9.61 = 2.39 V. According to the datasheet that will not turn the MOSFET on properly since it needs a Vgs of 4-5 V.

Since you have not defined your requirements for the circuit we don't really know what you try to do. We can only say that you do it completely wrong. Try swapping positions of the load resistor and the transistor and see if that does what you want.

If that is what you want you can take any 74HCTxx chip and use it to convert your 3.3 V to 5-6 V that will turn on the gate completely. If it is fast enough depends.

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