MOSFETs

I'm thinking of playing with some MOSFETs to try to understand them better. I studied JFETs in college, and I think I have a pretty good handle on them. I'm only interested in digital modes at this point.

Power MOSFETs, on the other hand, are pretty strange in some ways. The gate capacitance issue, for example. What is a good approximation of where you start having to worry about the frequency of the PWM?

The other thing is the 10V on/off signal I keep seeing described for proper operation, with some lesser voltage mentioned for other devices. How do you determine from the data sheet what the control voltage is?

Looking at the IFR510, which is sold by Radio Shack, is a bit confusing. Is the V(GS) (which is +/- 20V) saying it needs 20V to switch fully?

BJ Micro has several, the IFR520 seems pretty similar (same V(DS)).

Assuming capacitance is an issue, I have to use drivers to drive the gates both positive and negitive quickly, such as a push/pull transistor pair?

I also notice all the power MOSFETS are N Depletion types. How common is this?

Thanks.
Due to the input capacitance the yusually have resistors in series with the gate input on amplifiers.
It can also be hard to bias push pull MOSFET amplifiers due to differences in gate turn on voltages and gate temperature/voltage drift.
 

Audioguru

Joined Dec 20, 2007
11,248
The IRF510 Mosfet is very old with poor spec's (RadioShack lousy quality).
Its datasheet lists its max Drain-to-Source on-Resistance when its Vgs is 10V like most Mosfets. A graph shows that increasing the Vgs to 15V makes only a small difference.

Nearly all power Mosfets are enhancement type. Hardly any are depletion type (I have never seen one).
 

Thread Starter

Wendy

Joined Mar 24, 2008
23,415
Yep, Radio Shack has managed to get quite a rep, especially with the older tech set. They are also in a heck of a lot of neighborhoods, and have managed to become a standard for the AAC book experiments. Since I am also writing experiments for this volume I'm using the established standards.

I'm actually playing with IRF520's, they are pretty similar, and much cheaper from the source I bought them from.
 
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Thread Starter

Wendy

Joined Mar 24, 2008
23,415
OK, going through some scenario's here. This is an experiment I'm going to try, it's simple, if I pop the MOSFET no big deal. The toggle switch is shown for conceptual purposes only, moving a wire is the same thing.



I've hear more than one person say a 555 isn't a good driver for a MOSFET. I don't really understand why not, it is pretty heavy duty, and I don't see transients being that big an issue. So why not?



There will be cases that a "soft" output needs to be digitized, so this is my proposed solution for the problem.



The last case is a problem. I haven't seen any pMOSFETs, the nMOSFETs complement, for sale. I need a series pass version similar to what is shown below. It would be ideal for some other projects I have going. Any ideas, especially using a nMOSFET?


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SgtWookie

Joined Jul 17, 2007
22,230
Hi Bill,
Sorry I didn't get back to this earlier. I've been working a lot of hours at a new job, and sadly just don't have as much time for AAC as I used to. :(

You really need a damping resistor between the gate driving source and the gate itself, or the inductance of the wiring and the capacitance of the gate will cause the MOSFET to oscillate on and off, which causes excessive operation in the linear mode. Some resistance helps to damp this oscillation rapidly.

In your first example with the switch, you should have at least a 10 Ohm resistor from the common terminal to the gate of the MOSFET, and a 5k or 10k resistor from the gate to the source, to turn the MOSFET off in case of circuit failure. Switches are certainly not an ideal current source to charge/discharge a MOSFET gate, as the contacts can "bounce" on and off for what to us seems instantaneous, but electronically is an horrifically long amount of time.

A 555 is adequate for driving an N-ch MOSFET's gate at low frequencies, if a suitable resistor is used between pin 3 and the gate. Take another look at the schematic I posted last page for gate charge curves. At higher frequencies, you're better off using gate driver IC's.

As far as the 'soft' driver; it doesn't strike me as a very good solution. Sorry, but I just don't have the time to whip up a simulation for it.

P-ch MOSFETs have largely fallen out of favor, even though you can still get them as replacements. Hole flow is simply not as efficient as electron flow.

Once the high-side driver ICs were developed for N-ch MOSFETs, the P-ch power MOFETs were abandoned in droves. Yes, it's easier to design circuits using more-or-less complementary pairs of p-ch and n-ch MOSFETs, but circuits designed using identical N-ch MOSFETs with high-side drivers made for far more efficient circuits, and a smaller bill of materials.

The driver circuit comparison I posted earlier is admittedly crude, being built from discrete components - however, it demonstrates what kind of improvements could be attained.
 
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