Discussing Advantages of Mosfets for Inverters

Please read the previous post for the continuation...

Hi Ali,

Although capacitors across the windings are not necessarily required, but your values are too small to cause a problem.

 Heatsinks are perfectly alright.

Now the only option left is to connect an ammeter (20Amp) in series with the positive of the battery and check the current consumptions using the original square wave design as well as our PWM design. The difference should instantly surface out.

Let's do this and see whether there's a difference in the consumptions, because current is the key factor that's causing the mosfets to blow.

Regards.

Hi Swagatam, I did what you said and the Square wave circuit drew 2 amps, then I substituted the PWM circuit and it drew 14 amps and blew one of the MOSFETS.

I hope this sheds some light.

Sincerely Ali

Hi Swagatam, I was doing a little reading on Mosfet Basics and I read "

4. The Characteristics of a MOSFET

1) Advantages

1. High input impedance - voltage controlled device - easy to drive.

To maintain the on-state, a base drive current which is 1/5th or 1/10th of collector current is required for the current controlled device (BJT). a voltage controlled MOSFET is a switching device.

2. Unipolar device - majority carrier device - fast switching speed

3. Wide SOA (safe operating area).  It has a wider SOA than the BJT

4. Forward voltage drop with positive temperature coefficient - easy to use in parallel.
So this thing is telling that Mosfets are a Voltage controlled device.
Just  to reiteriate my results from your Current draw test, my Square wave circuit had 2 Amps draw with nothing plugged in to the Inverter. PWM circuit Draw was 14 amps for just a second then it blew one of the Mosfets.
One other Observation , when I power up the Square wave circuit portion of our complete PWM circuit, the voltage at the Clock Output is +6.10V.  Now when I plug the circuit back into the PWM portion of our circuit and POWER UP both circuits, the Voltage at the Transistor Output to the Gate of the Mosfet was + 8.45V. Are these readings shedding any light. Could it be because the voltage is too high at the Gates??
My original Square wave circuit has a voltage of +6.09V at the Clock Output to the Mosfets Gate.
Sincerely Ali

 

 

Advantages of MOSFETs over Transistors in Brief:

Mosfets do not depend on current triggers unlike transistors and therefore get fully triggered even with minutest current at their gates. 

They depend on voltage trigger, which is not an issue because voltage is always available with all inverters control circuit ICs, in contrast to current which is normally not available directly from an IC output. Therefore it becomes possible to drive mosfets directly from IC outputs.

 

Mosfets are positive temperature coefficient devices, meaning they restrict the flow of current proportionately as they get heated up, providing some sort of control from getting overheated and falling into a thermal runaway situation. Transistors are opposite to this and have a negative temperature coefficient, and therefore get damaged instantly when over heated.
Hi Ali,

With the present situation I think we can try out the following things:

Connect a car headlight lamp in series with the positive of the battery. This will stop the mosfets from blowing-off and also its glow will instantly inform you regarding the current consumption of the unit with the two circuits.

Next take a pot of 10K (DUAL) and connect its leads appropriately across the base and the ground of each transistor.

Switch ON the power, adjust the pot, keeping a close watch over the glow of the bulb, try to make its glow as close as possible to that of your square wave design, you may also connect an ammeter in series with the bulb to accurately manage the current while adjusting the pot.

Let's see what happens with the above procedures.

Regards. 

Hi Swagatam, thanks for continued help!! :) 

In your message below everything was clear except one point, when you said " try to make its glow as close as possible to that of your square wave design " not sure what you mean by this ?

 Sincerely Ali

To be Continued

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