Claimed OU circuit of Rosemary Ainslie

[Rosemary Ainslie]

Hi Hoppy.  Bridging the great divide here.  Not too difficult as TK's away - but definitely in need of Dutch courage?  Hopefully Wilby lurks within earshot.

Have been reading through the entire thread.  What a lot of hot air rising.  Don't you guys get tired of talking 'smug'.  It gets awfully repetitive.  But some of it really entertaining.  I'd give 5 stars to TK and Henieck.  But altogether a lot of bombast and not enough of it actually crack out the laughter.

Where has TK gone?  Has he abandoned his argument?  Love to know more.   Truth is I've had such an unadulterated diet of attack that without it I'm in a holiday mood.   

[Hoppy]

Hi Hoppy.  Bridging the great divide here.  Not too difficult as TK's away - but definitely in need of Dutch courage?  Hopefully Wilby lurks within earshot.

Have been reading through the entire thread.  What a lot of hot air rising.  Don't you guys get tired of talking 'smug'.  It gets awfully repetitive.  But some of it really entertaining.  I'd give 5 stars to TK and Henieck.  But altogether a lot of bombast and not enough of it actually crack out the laughter.

Where has TK gone?  Has he abandoned his argument?  Love to know more.   Truth is I've had such an unadulterated diet of attack that without it I'm in a holiday mood.

Hi Rosemary,

If you continue posting as a newbie at this quality, you run the risk of being labelled as a Troll like Wilby 

Hoppy

[MileHigh]

A fun "Experimenter's Corner" project:

Suppose that you have your standard Ainslie circuit setup (without the fly-back diode).

You set one of your scope channels to look at the shunt resistor to ground, which is the standard "Channel B" connection in the white paper.

For the second scope channel, look at the voltage between ground and the MOSFET drain pin.  Note that this is the same node as the "far side" of the coil-resistor, where you normally see the negative spikes.

When you run the setup now the spikes that you see at the MOSFET drain pin after the MOSFET switches off will go POSITIVE.

This is a more realistic view of what's really going on in the circuit because you are looking at the voltage on the second scope channel relative to ground.

The zener protection diode inside the MOSFET has a certain "breakdown voltage" where it will start to conduct electricity.  The reason for this is to protect the metal oxide semiconductor field effect transistor inside the device.

[EDIT: There is no true zener protection diode inside the MOSFET although they use a zener diode icon in the schematic symbol for the MOSFET.  For more information, see later postings below  HOWEVER, the discussion below is still valid but the "zener breakdown voltage" is really the standard MOSFET breakdown voltage.]

If you look carefully at the spikes with this setup you may see some noticeable clipping of the spikes giving the spikes squared-off flat tops.  The squaring off is the giveaway that the zener protection diode is cutting in and conducting electricity.  If you do see the clipping, you should take note of the clipping voltage and compare that with the datasheet of the MOSFET that you are using to see if it checks out.

Note that if the zeneer protection diode is conducting electricity, the battery is discharging.

The second question you should ask yourself if you see the clipping is to wonder if you are possibly going to damage the MOSFET.  The datasheet has some information on this but let's just keep it simple and say that you want to make sure the MOSFET is not running hot by touching it.

The zener protection diode cutting in is not to be confused with the avalanche breakdown failure mode for the MOSFET.

I have a feeling this posting is somewhat academic because I don't recall seeing any spike clipping on any YouTube clips.  Of course YouTube has it's limitations.

The real lesson is to check your maximum spike voltage in your setup against the zener protection diode breakdown voltage for your particular MOSFET so you have an idea if it might be happening or not.  It's a basic heath check for your circuit.

Also visualize that when the MOSFET switches off, the classic "negative back-EMF spike" is just as easily viewed as a massive POSITIVE spike relative to your circuit ground.  That big positive spike represents a potential danger to the short term or long term health of the MOSFET.

MileHigh

[Harvey]

Mile High,

Could you post the datasheet information that shows a zener in the IRFPG50 that is different than the body diode hexfet avalanche path?

According to my understanding this is one in the same and is an integral part of the actual substrate geometry and not a separate internal device as you have stated.

[MileHigh]

Hi Harvey,

I did some searching around and came back to the pleasant sight of your explanation looking like the right one.  It would appear that the standard symbol for a MOSFET shows a "virtual" zener diode that simply is a rough model for what is actually going on with the various effects associated with the substrate geometry.  I am guessing that the zener symbol does double-duty, representing the fact that there is a slow "body diode" that kicks in if you forward bias the source to drain voltage, and it also represents that the MOSFET has a reverse breakdown voltage.

Before doing some searching, I thought that there was a true zener diode that was built into the semiconductor layers with selective doping, etc, or it was even possible a multi-chip module with a true zener diode bonded to the MOSFET transistor itself.

It is worth it to simplify though, especially in the forums, so the icon of a zener diode in the MOSFET schematic symbol helps to visualize.

In my diggings I found this which is interesting:

Beware the MOSFET body-diode !

Those familiar with MOSFETs will know that the fabrication process results in a built-in anti-parallel diode between the source and drain terminals of the device. This is often referred to as the "body-diode." Referring to any MOSFET data sheet will reveal specifications for this intrinsic diode.

At first it appears that the internal body-diodes are a bonus since they provide the desired free-wheel diode function for free. This is often the case in many power electronics applications where they provide the function of the free-wheel diodes with ease. Sadly it is not the case in this application. The MOSFET body-diode is a side effect of the fabrication process and is not a particularly good diode. The same design criteria for good MOSFET characteristics do not produce the best body-diode characteristics. The design of a MOSFET is always a compromise, and it is the characteristics of the body-diode that suffer.

When compared to discrete high speed diodes, the body-diode's reverse recovery time is very long. This means that the diode takes a long time to turn off when the current flowing through it changes direction. As explained previously, this leads to a shoot-through condition when the opposing switch is turned-on. For this reason the body-diodes are clearly not suitable for free-wheel diode duty in this application and should be isolated.

Thanks for the question Harvey.

MileHigh