Driving this MOSFET with PIC

[(*steve*)]

The internet is also full of graphic sex videos. The owner and moderators of this site have reached a consensus not to host information about weapons and other highly dangerous stuff. If it ever came up, we would also probably decide not to produce our own explicit sex videos either. Not all rules are written down.

Without more information about the load (what voltage, is it purely resistive or is it inductive, etc) we can't possibly assist you any more. Depending on the nature of the load we would need to know much more just to be able to give a sensible answer. Is it a flyback transformer? How much current are you switching? If it's a transformer, is it always loaded or could it be unloaded? in essence we need to know possibly more than you can tells us.

 

[TheMaster]

The internet is also full of graphic sex videos. The owner and moderators of this site have reached a consensus not to host information about weapons and other highly dangerous stuff. If it ever came up, we would also probably decide not to produce our own explicit sex videos either. Not all rules are written down.

Without more information about the load (what voltage, is it purely resistive or is it inductive, etc) we can't possibly assist you any more. Depending on the nature of the load we would need to know much more just to be able to give a sensible answer. Is it a flyback transformer? How much current are you switching? If it's a transformer, is it always loaded or could it be unloaded? in essence we need to know possibly more than you can tells us.

The load is purely resistive with no capacitive or inductive properties. It's electric requirements have been predetermined and tested. It operates with the aforementioned transient spikes of about 8kv for a couple of microseconds at 100hz (which according to my research are harmless in the event that it would be exposed to skin)
I can divulge all of the electric properties of the load, but I don't believe that's relevant as that has already been thought out. Really the primary side is all that remains.

This is a flyback converter with no feedback or rectification.
Transformer loaded/unloaded at aprox 50% duty cycle ~100hz. Loaded ~ 100ma
The MOSFET will be exposed to a high voltage, the previous poster stated it can be protected with a TVS diode.

So my only remaining question, innocently enough, is how to supply power to the PIC constantly, if I fully charge the primary. That, in my mind, would create zero voltage on the power rail. I would imagine that there are established ways to solve this.

 

[(*steve*)]

OK, if it's purely resistive then I can't see how you get 8kV spikes unless you're powering it from 8kV.

I can't see how the power rail would fall to zero, especially if it has a resistive load across it. And if it's resistive you can't charge it.

Oh hang on, it's a flyback transformer. that's not resistive!

 

[BobK]

I think he is saying that the load for the HV is purely resistive. Clearly there is either an inductor or transformer to produce the high voltage. If it is a transformer, there should not be 2KV spikes on the input side.

Bob

 

[TheMaster]

OK, if it's purely resistive then I can't see how you get 8kV spikes unless you're powering it from 8kV.

I can't see how the power rail would fall to zero, especially if it has a resistive load across it. And if it's resistive you can't charge it.

Oh hang on, it's a flyback transformer. that's not resistive!

Yes, as Bob said, I was referring to the secondary side load. Why would there not be spikes on the primary as well? From what I understand the switching element on the primary of a flyback converter is exposed to the spikes on the secondary / number of turns (verified in simulator). But if I place a TVS diode I don't believe that will be a problem.

As for my question, I guess it stands to reason that if the voltage across the primary of the transformer falls to zero there would be no way to power a PIC, or anything else for that matter, so the voltage must not fall below that required to power the PIC and to switch the MOSFET.

 

[BobK]

If your transformer is producing 2kV from a 2V input it has a turns ratio of 1:1000. So, given that, what voltage would be required on the secondary to produce 2kV on the primary?

Bob

 

[(*steve*)]

I thought his claim was that a 2kV pulse would appear on the primary?

 

[BobK]

That is his claim, but he is wrong. He is feeding 2V into a flyback transformer.

Bob

 

[TheMaster]

That is his claim, but he is wrong. He is feeding 2V into a flyback transformer.

Bob

That is my claim and I am not wrong. I am not talking about a constant voltage, I am talking about a single pulse. To create a large spike I don't need a high turn ratio (on the contrary). The spike is mirrored on the primary divided in voltage by the turn ratio. And therefor the switching element is exposed to a short lived transient.

 

[BobK]

And, the voltage of the secondary, divided by the primary, is 2V!

If you leave the secondary open, you will get a high voltage pulse, but not if the secondary is driving a load.

Bob

 

[(*steve*)]

Unless he is planning for a 2MV pulse on the secondary!

 

[TheMaster]

And, the voltage of the secondary, divided by the primary, is 2V!

If you leave the secondary open, you will get a high voltage pulse, but not if the secondary is driving a load.

Bob

The voltage spike of the secondary divided by the voltage spike on the primary is the turn ratio.
If you leave it open you will get an extremely high voltage spike. If you place a highly resistive load you will also get a high voltage spike, albeit less so. Where am I wrong? In the OP I stated I'm using a 12V battery not 2V, and I'm not planning for a 2MV pulse, I already stated the order of magnitude. I'm no expert, but this has already been tested in the lab and is successfully powering my product.

Really my final question was how to put maximum current on the primary while still powering the PIC. I think I may have solved this though...

 

[(*steve*)]

What is the turns ratio, and what is the peak voltage you expect to see on the secondary winding?

If the answers are 500 and 12000, the expected peak on the primary is 24V.

 

[TheMaster]

What is the turns ratio, and what is the peak voltage you expect to see on the secondary winding?

If the answers are 500 and 12000, the expected peak on the primary is 24V.

You're assuming I have a high turn ratio. This is a flyback converter, what you just described is not the operation of a flyback converter.

Again, this is already successfully powering my product in the lab (which is fed transient spikes) so the question (at least on my part) does not pertain to this. Really just wondering if there is a good way to charge the primary while still having sufficient voltage on the PIC.

 

[(*steve*)]

Really just wondering if there is a good way to charge the primary while still having sufficient voltage on the PIC.

With any normal circuit this will not be a consideration.

 

[TheMaster]

With any normal circuit this will not be a consideration.

Why not? I understand this circuit may be unorthodox, that's why I haven't been able to just copy an existing one. I want the primary to have maximum current going through it (as much as the battery can supply), so it will be transferred to the secondary upon the opening of the MOSFET. Naturally there is greatest current through it when there is no voltage across it, but then there is also no voltage to power the PIC (which is parallel to the primary). So I guess the only way is to sacrifice some current from the primary, unless there is some method I am not aware of.

 

[BobK]

2MV? That is 1000 times what you said before. And it is unrealistic to think that you will be able to produce that.

Bob

 

[TheMaster]

2MV? That is 1000 times what you said before. And it is unrealistic to think that you will be able to produce that.

Bob

You're not even reading what I'm writing...
"I'm not planning for a 2MV pulse..."

 

[(*steve*)]

Why not? I understand this circuit may be unorthodox,

But since you won't show us the circuit, we can only make assumptions about it. I'm assuming it is wired sensibly.

Also, I think you fail to understand the voltage/current behaviour in an inductor. Or perhaps you fail to understand saturation. Whatever it is, you also fail to be able to give us enough information about your soooper secret circuit for us to do more than make assumptions and apparently useless suggestions.

It's time to close this thread -- it's going nowhere.

 

[davenn]

The load involves a proprietary commercial product which I am not in the position to disclose in full.

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if you cannot discuss a topic because it involves confidentiality etc DONT start a thread involving it
through your own admission you have done this ...


and again read the rules , it is mentioned ......

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we have no idea of your experience ? therefore we cannot know if you can do what you suggest in a safe manner

regards
Dave