Why did this zener diode explode?

Thread Starter

fablau

Joined May 14, 2020
167
Both in schematic and a small separate circuit.

When trying to simplify things we often find the problem.

And I'm not going to lie, that schematic is painful to try and follow
Yes, I think I'll do that if I can't find a solution soon. Thanks for your input!
 

Thread Starter

fablau

Joined May 14, 2020
167
Ok guys, after the whole morning of testing and measuring, I think to have understood what happened. For some reason (more about that later below), some of the MOSFETs began degrading, until they broke down and caused a short between the gate and the drain, causing the current to increase to the Zener until it exploded.

So, now that I know that the source of the problem was the MOSFETs breaking and causing the short, I am trying to understand why that happened. My only thought is that since they are connected to an inductive load (electro-magnets), and there are not flywheel diodes in this circuit, I guess the back-current of the inductive load must have damaged the MOSFETs. Please, correct me if you don't see this as correct reasoning.

The fact is, the magnets can be driven in both directions, so a simple flywheel diode won't work. I am thinking to use instead a capacitor or R-C snubbers. What are your thoughts on that?

To make things easier, I created a simple schematic showing the basic circuit I am working on:

Schematic_Zener Diode Discussion_2021-06-13 (1).png


Basically, the power of the magnet is coming from an H-bridge that can switch polarity so that the magnet can either attract or repel. The +32v on the right side (H2) is just used for the MOSFET gates, driven by the Zener diodes (D3 and D4) in order to keep the voltage in the correct range. Please note that the voltage coming from the H-Bridge is the same (32 volts). I forgot to mention that in the schematics. All the rest should be pretty self-explanatory.

How would you suggest protecting the MOSFETs by the possible back-flow from the magnet?

Eager to know your thoughts on all this. Thank you!
 

MrChips

Joined Oct 2, 2009
27,641
That's responsibility of the controller in this design ;) Thanks for the heads-up though!
No.
Somehow I knew you would say that.
Can you guarantee that the controller does not enable both MOSFETs at the same time, especially during power ON and power OFF?
I bet you cannot.

Surely you have heard of Murphy's Law.
 

Thread Starter

fablau

Joined May 14, 2020
167
No.
Somehow I knew you would say that.
Can you guarantee that the controller does not enable both MOSFETs at the same time, especially during power ON and power OFF?
I bet you cannot.

Surely you have heard of Murphy's Law.
Yes, I can because I have programmed it NOT to do that ;)

But I am curious to know, if you like, how would you fix that from the circuit standpoint?
 

MrChips

Joined Oct 2, 2009
27,641
I wish to offer you some advice on circuit schematics.

A circuit diagram is the essential language of electronics.
The purpose of any language is to communicate clearly one's intentions.
Ask yourself if your circuit diagram effectively communicates to another human begin the intent and function of the circuit.
If it does not then it has failed its purpose and needs to be redrawn.

Even your simplified version fails that essential test. A circuit diagram is not about making the correct electrical connections. It is about conveying clearly the function of each element in the circuit.

You show a single GND point. Instead, use as many GND symbols as needed to avoid crossed lines.
The same applies to power rails.

Rather than having crossed lines, use labels.
Keep your lines straight with the minimum number of right-angled bends.
Rotate components to avoid bends in the lines.
Rotate components to make the diagram readable. Pin numbers must be upright.
Inputs come in from the left. Outputs go out to the right.
Put high voltage at the top, low voltage at the bottom.
 

crutschow

Joined Mar 14, 2008
31,091
To make things easier, I created a simple schematic
As MrChips noted, that schematic may be simpler but it doesn't make it much easier.
It's still a hodgepodge.
I assume you write your programs in a logical, well annotated, and relatively easy to understand manner.
You need to do similar thing with your schematics.

Normally such a simple circuit could be understood by many on this forum in a few minutes of study.
As is, I would probably have to redraw it to understand it in a reasonable amount of time.

You have way too many right angle bends and convoluted paths to readily understand the schematic, and related connected components in the circuit are way too far apart (one example, OPT2 and the D3 associated circuitry).
As suggested, signals should go mostly from left to right, and power from plus at top to minus (or ground) at the bottom.

.left to right written is that sentence English an read to trying like rather is schematic Your
 

Thread Starter

fablau

Joined May 14, 2020
167
By adding redundancy.
Have a look at the L298 Datasheet.
You will notice that besides two control inputs there is also an ENABLE input.
Thank you for your note, but I can say, I am doing exactly that. :)

I am controlling the inputs of the H-bridge and the MOSFETs in order to avoid turning them on at the same time.

I also appreciate all your suggestions about writing logical schematics guys, that's something that I must definitively still learn.


Is the gate protected from voltages higher than allowable? Most MOSFETS have a limit of 20V.

Bob
Those mosfets have 40v limit and I can control the input (see the specs I posted earlier)

But please, could we focus on my main question here? How would you suggest protecting the MOSFETs by voltage spikes from the inductive load?
 

MrChips

Joined Oct 2, 2009
27,641
Here is a better way of drawing your circuit.
I have a total of 3 crossed lines which could be reduced to two by moving the H1-2 connector closer to OPT2.
Why is this better?
It is better because it reveals the topology of the circuit and the function of each component in relation to the connected components.
Again, it more clearly communicates what each component is supposed to be doing.

As an example, examine H1-1, OPT1 emitter, R1, and GND.
These are the only components that are connected. This is evident in my drawing.

Compare my circuit diagram with yours.
AAC Magnet Driver.jpg

AAC Magnet Driver - fablau.jpg
 

Thread Starter

fablau

Joined May 14, 2020
167
Wow, that's awesome MrChips, thank you for doing that. Much cleaner indeed, I'll make sure to do it that way the next time.

Any advice on protecting the MOSFETs?

Thank you again very much.
 

Martin_R

Joined Aug 28, 2019
132
Thank you Mr Chips for drawing the circuit in a logical manner! Much appreciated. I'm confused by HS3-1. Is it negative or positive or switching between the two? The problem I'm having is seeing how the PEMOS transistor switches properly, as surely it's source needs to be connected to the +32 volt supply. I can see R3 pulls the gate to +32 and turns the transistor off, and OPT1 provides the voltage from D4 to turn it on, but surely only if the source id connected to +32v.What have I overlooked?
The NEMOS transistor circuit I can see clearly ( now that the circuit has been redrawn properly), though I think R4 value is too high to turn the transistor off quickly, same comment to R3.
 

Martin_R

Joined Aug 28, 2019
132
I wonder how many of (for example) the NEMOSFET transistors could be switched on at the same time by the controller? With several pull down resistors now in parallel the 12v zener voltage collapses and the transistors fail to switch on properly. Same logic applies to the PEMOS transistors. Easy to see now there's a neat and logical drawing.
 

Thread Starter

fablau

Joined May 14, 2020
167
If it is a spike from the coil that is causing it, you can put across the coil: a cap/resistor in series, a MOV, or a TVS.

Have a look into "relay coil suppression for AC circuits"
Thank you for your reply. Yes, I was thinking to use a MOV clamping drain-source for each MOSFET.
 
Top