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Author Topic: Negative discharge effect  (Read 34289 times)

Dave45

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Re: Negative discharge effect
« Reply #15 on: September 27, 2014, 05:11:53 PM »
Most circuits out there, if you study them are either boost converter's, buck converter's or a variation thereof,
Look at the topology, how their switched, from which rail, how the pulse is hitting the coil, is the output inverted.

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Re: Negative discharge effect
« Reply #15 on: September 27, 2014, 05:11:53 PM »

Offline TinselKoala

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Re: Negative discharge effect
« Reply #16 on: September 27, 2014, 07:54:27 PM »
@ayeaye:a capacacitor is frequency-sensitive. Different frequency means different reactance, different reactance means different amount of charge transfered per period via Miller-Capacitance.
See here: http://www.sengpielaudio.com/calculator-XLC.htm
Capacitive reactance XC = 1 / (2 · π · f · C)
It´s not worth to persue IMHO, exept what Utkin proposes. I have to say however that I could not replicate his results with the negative charged capacitor. Energy in form of negative voltage was always less than existed before in the cap as positive voltage.



Kator01

The voltage on a capacitor in a ringing circuit reverses polarity with every half-cycle of ring.  There is no such thing as "negative" or "positive" voltage as such, because voltage polarity and magnitude are always relative to some reference level. If a ringing cap starts out with a positive peak voltage, the next half cycle will reverse the polarity of the voltage on the cap as it discharges to zero and then _recharges_ to the negative peak voltage, which, sure enough, will be less than the original positive peak because of losses in the circuit. Let it ring long enough without resupplying energy and you get that beautiful exponential decay ringdown, voltage reversing polarity on every half-cycle. So if you arrange your snubbing circuitry properly you can "shut off" the discharging ringing capacitor with either polarity of charge and at whatever level, below the initial charge energy, you like.

Consider two 12volt batteries connected together in series. Measure across the stack. What is the voltage and polarity at the most positive terminal? It is 24 volts (nominally) positive WRT the most negative terminal. Now... what is the voltage and polarity at the "center" terminal where the two batteries are connected together?

SO... I think the voltage on the cap in the device under test will be sensitive to the frequency for at least two reasons: First, the capacitive and inductive reactances depend on the frequency as you note, and also the mosfet is going to be turned on and off at different points in the "ring cycle" by the signal and thus will trap different amounts of energy on the cap, with polarity that could be either way depending on the frequency of drive and its relationship to the resonant frequency of the tank circuit. I think. I haven't done the experiment yet but I may fire up the kit later on today if I have a chance.


Offline TinselKoala

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Re: Negative discharge effect
« Reply #17 on: September 27, 2014, 08:15:45 PM »
The 1n5399 diode is a HV rectifier diode, rated 1 kV and 1.5 A. But it is _slow_, having a typical reverse recovery time of 2 microseconds.

UF4007 is similar in voltage, a little lower in current handling but is quite a bit faster at 75 nanoseconds max.




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Re: Negative discharge effect
« Reply #17 on: September 27, 2014, 08:15:45 PM »
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Offline TinselKoala

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Re: Negative discharge effect
« Reply #18 on: September 27, 2014, 08:22:40 PM »
I see.

But why the effect appeared on different frequency when i repeated the experiment? While everything remained the same, the mosfet, the coil, the diodes, the capacitors. What changed, the mosfet, the coil's core, or the capacitors?
The mosfet or the capacitor or both could be changing. Mosfets can fail progressively especially if they are subjected to lots of avalanching, and the equivalent series resistance of the electrolytic caps can also change if they are subjected to short HV spikes that could exceed their dielectric rating, even as the cap still works as a capacitor.
I doubt if the core of the coil could be changing under your experimental conditions, but the mosfet and caps can definitely change. But I don't think that component changes are needed to account for your results. I'll know more after my scopes warm up.

I just pulled a handful of IRF630s out of an old monitor chassis, by strange coincidence...
 ;)

Offline TinselKoala

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Re: Negative discharge effect
« Reply #19 on: September 27, 2014, 08:37:39 PM »
I will, i had no idea that there is any kind of diode inside.
All ordinary mosfets have a "body diode" that is the result of the manufacturing process. This is often omitted from the mosfet symbol but it is there. It is a reverse-biased Zener diode with anode at Source and cathode at Drain. If your diode-check function on your DMM has enough range you can check this diode in your mosfets yourself.

http://en.wikipedia.org/wiki/Power_MOSFET#Body_diode
http://hephaestusaudio.com/media/2008/11/mosfet-body-diode.pdf

http://www.youtube.com/watch?v=RBJGOOTEwfU

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Re: Negative discharge effect
« Reply #19 on: September 27, 2014, 08:37:39 PM »
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Offline ayeaye

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Re: Negative discharge effect
« Reply #20 on: September 27, 2014, 09:12:38 PM »
Ahh, a Zener diode, but then the backward current through it is also somewhat restricted. Drain-source breakdown voltage? It's 200 V by the datasheet. When its open then i think it should be on its internal drain-source on resistance, 0.4 ohms. But i still didn't understand, is that internal diode active also when the mosfet is closed, because during the back-emf the mosfet is closed.

Yes i ripped all my components from my old CRT monitor. The core is the deflection yoke core, and the wire i also got from the degaussing coil of the monitor.

Offline TinselKoala

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Re: Negative discharge effect
« Reply #21 on: September 27, 2014, 09:20:52 PM »
Ahh, a Zener diode, but then the backward current through it is also somewhat restricted. Drain-source breakdown voltage? It's 200 V by the datasheet. When its open then i think it should be on its internal drain-source on resistance, 0.4 ohms. But i still didn't understand, is that internal diode active also when the mosfet is closed, because during the back-emf the mosfet is closed.

Yes i ripped all my components from my old CRT monitor. The core is the deflection yoke core, and the wire i also got from the degaussing coil of the monitor.

At the end of the video I show the body diode of the mosfet being checked. Select "diode check" function on the DMM and place the Positive probe on the Drain pin of the mosfet. Make sure the mosfet is off by touching the Negative probe from the DMM to the Gate pin. Then put the Negative DMM lead on the Drain pin and the Positive lead on the Source pin. The DMM should read the fwd voltage drop of the body diode, something around 0.45 or 0.5 volt.

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Re: Negative discharge effect
« Reply #21 on: September 27, 2014, 09:20:52 PM »
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Offline ayeaye

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Re: Negative discharge effect
« Reply #22 on: September 27, 2014, 10:39:40 PM »
Ok, i understood now that the internal Zener diode is also active when the mosfet is closed. But we are talking about reverse current. So as i understand, this Zener diode opens when the reverse voltage in the circuit is more than the drain-source breakdown voltage of the mosfet, 200 V in the case of IRF630.

I don't know whether the back-emf is more than 200 V. But it's not only that. As i understand, a Zener diode acts reversely almost the same way as a normal diode acts when there is a forward current. That is, it always has the voltage drop equal to the breakdown voltage, which i understand is 200 V. But this does not explain why the voltage on both the source capacitor and the charged capacitor was almost equal, that is, the absolute value of the voltage was slightly greater on the source capacitor. If this were the reason of the negative charge, then likely the source capacitor had to have much less voltage on it, than the charged capacitor.

Offline TinselKoala

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Re: Negative discharge effect
« Reply #23 on: September 28, 2014, 01:07:06 AM »
Can you confirm the inductance value of the coil?


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Re: Negative discharge effect
« Reply #23 on: September 28, 2014, 01:07:06 AM »
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Offline ayeaye

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Re: Negative discharge effect
« Reply #24 on: September 28, 2014, 10:22:54 AM »
Can you confirm the inductance value of the coil?
The problem is, i have no good tools to measure. I measured it by connecting the coil and a potentiometer in series to a 12 V AC wall adapter, it was really 14.4 V. Then i adjusted the potentiometer so that the voltage on that was half of it, 7.2 V AC. Disconnected the potentiometer, measured its resistance and calculated the inductance by the 2. method there http://www.wikihow.com/Measure-Inductance , when changing the resistance instead of frequency. And with that method i got the inductance 6.6 H.

But then i tried to check that method and measure a known inductance. I had only a small known inductor though, and i don't have so small potentiometer, so i had to use small resistors. I also had to use a 100 ohms resistor to restrict the current of the adapter, to not to burn the adapter. This 100 ohms resistor went so terribly hot that i thought i can use it as a soldering iron. I measured the voltage of the inductor and a combination of resistors in series, and choose resistors so that their voltage was half of that voltage. I got that way 80 mH, but what was written on the inductor was 8,2 mH , that is, 8, what looked like a short vertical line, and 2. I thought that means 8.2 mH but i'm not entirely sure that the thing in between there was a dot, so maybe it was 82 mH.

The core is the core of the deflection yoke of a CRT monitor, and there is 900 turns of a 26 gauge magnet wire on it. I calculated the inductance, and got only 0.16 H. Assuming that the relative permeability is 100, but for such ferrite core it is likely much higher.

So this is what i could get with my primitive tools. As much as i remember,  the potentiometer was 2.2 k when the total voltage was 14.4 V, and there was 7.2 V on the potentiometer. Frequency should be 50 Hz. I measured in the 200 V AC range of my multimeter. So this is how it is, with primitive tools it is not possible to get too good results.

Offline nelsonrochaa

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Re: Negative discharge effect
« Reply #25 on: September 28, 2014, 12:39:46 PM »
The problem is, i have no good tools to measure. I measured it by connecting the coil and a potentiometer in series to a 12 V AC wall adapter, it was really 14.4 V. Then i adjusted the potentiometer so that the voltage on that was half of it, 7.2 V AC. Disconnected the potentiometer, measured its resistance and calculated the inductance by the 2. method there http://www.wikihow.com/Measure-Inductance , when changing the resistance instead of frequency. And with that method i got the inductance 6.6 H.

But then i tried to check that method and measure a known inductance. I had only a small known inductor though, and i don't have so small potentiometer, so i had to use small resistors. I also had to use a 100 ohms resistor to restrict the current of the adapter, to not to burn the adapter. This 100 ohms resistor went so terribly hot that i thought i can use it as a soldering iron. I measured the voltage of the inductor and a combination of resistors in series, and choose resistors so that their voltage was half of that voltage. I got that way 80 mH, but what was written on the inductor was 8,2 mH , that is, 8, what looked like a short vertical line, and 2. I thought that means 8.2 mH but i'm not entirely sure that the thing in between there was a dot, so maybe it was 82 mH.

The core is the core of the deflection yoke of a CRT monitor, and there is 900 turns of a 26 gauge magnet wire on it. I calculated the inductance, and got only 0.16 H. Assuming that the relative permeability is 100, but for such ferrite core it is likely much higher.

So this is what i could get with my primitive tools. As much as i remember,  the potentiometer was 2.2 k when the total voltage was 14.4 V, and there was 7.2 V on the potentiometer. Frequency should be 50 Hz. I measured in the 200 V AC range of my multimeter. So this is how it is, with primitive tools it is not possible to get too good results.

Hi ayeaye,
I make a similar tests and all my mosfet and igbt burn after some time working.
I think that BEMF will fry any mosfet in a common configuration like you illustrate because the gate is not isolated.The diode protection of mosfet in the peak of collapse will not work because the static is so much higher
that diode will let flow the current backward and didn't make the job.
I try the commutation with relay and see what happens. https://www.youtube.com/watch?v=pf_qUlwSZl0
https://www.youtube.com/watch?v=DfxEAQNOjp0.

Good tests

 

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Re: Negative discharge effect
« Reply #25 on: September 28, 2014, 12:39:46 PM »
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Offline ayeaye

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Re: Negative discharge effect
« Reply #26 on: September 28, 2014, 03:15:07 PM »
Yes mosfet has a body diode in parallel to it, which always conducts positive current from source to drain, even when the mosfet is closed. I tested it, connecting positive probe of the multimeter to the source and negative to the drain when the mosfet was closed, in the diode range, and it conducted. This has no importance though for that circuit, because the diode in the circuit is opposite to that diode, and thus when the mosfet is closed, nothing can go through the circuit for that reason. As i said earlier, it is only important for that circuit that the body diode is a Zener diode.

But the real reason why i thought about negative discharge, was because without it, because of the diode, the circuit is disconnected for the forward current in the coil, and this is the only thing that can be caused by switching. And without switched forward current, there cannot be back-emf, and when there is no back emf, then nothing can go to the charged capacitor. So to say in short what i mean, is that negative discharge is the only way how the circuit can be closed (connected) for the forward current. And for the opposite current the main circuit is only closed when the back-emf goes through the mosfet's Zener diode, as we talked.

Offline TinselKoala

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Re: Negative discharge effect
« Reply #27 on: September 28, 2014, 04:13:07 PM »
Circuit elements are not perfect. It seems to me that you are modelling your components in your mind as "perfect". But, for example, the diode you specify is very slow, it has a reverse recovery time of 2 microseconds.
This means it will be ineffective in blocking fast spikes in the reverse biased direction.  Allow me to suggest that you compare the system's performance with different diodes. Try the UF4007 for example and see if your capacitor voltages are the same as when you are using your present diodes. CRT monitors have some fast HV diodes in them, check the internet for the diode data sheets for the part numbers that you find in your scavenging.

The mosfet is not a perfect switch either. For gate voltages near the threshold (usually around 4 volts) the mosfet will be operating in a "linear conductance region" where its on-state resistance is linearly related to the charge on the gate. It is more like a variable resistor than a switch when the gate charge is in this range. So if you have a gate drive that is low voltage and slowly changing, or has only the ability to deliver small currents (filling the gate capacitance slowly) the mosfet is no longer going to be switching cleanly.

Furthermore if there is no way for charge to _leave_ the gate then the mosfet will not turn _off_ cleanly. This is one reason that you sometimes see mosfet gates being driven with AC signals: the reversed (negative) voltage sucks charge out of the gate and turns the mosfet off faster than simply bringing the gate voltage to zero.  Also "pulldown" resistors may be incorporated from gate to source, to allow the charge to leave the gate when the mosfet is supposed to be off.

I speak of course of N-channel mosfets; P-channel are the same but polarities are reversed.

You can make a good inductance meter with an Arduino (or other microprocessor system) and a few other components. You do not need the LCD display, the Arduino can report its data over the serial (usb) line and display on your computer monitor.

http://www.youtube.com/watch?v=S6N8ys8FiA4

http://www.youtube.com/watch?v=SCxypoN8-xc

Offline ayeaye

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Re: Negative discharge effect
« Reply #28 on: September 28, 2014, 05:01:18 PM »
Talking about microcontroller. My microcontroller board is kl25z, and this is a 32 bit arm microcontroller. This means that its logical 1 voltage is 3.2 volts, different from 5 volts in microcontroller boards such as arduino uno. But i tested it, switching the mosfet on and off, and this 3.2 volts switches the mosfet fully on, so that on the resistor in series there is a voltage almost equal to the source voltage.

And yes i controlled the microcontroller, that is generating pulses, through usb, using the minicom terminal emulator.

About pulses, i think that what matters is the exact length of the pulses, not frequency, frequency is important only because when the frequency is higher, the things are changing faster. But i may be wrong about that particular thing.

Offline TinselKoala

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Re: Negative discharge effect
« Reply #29 on: September 28, 2014, 05:54:32 PM »
Talking about microcontroller. My microcontroller board is kl25z, and this is a 32 bit arm microcontroller. This means that its logical 1 voltage is 3.2 volts, different from 5 volts in microcontroller boards such as arduino uno. But i tested it, switching the mosfet on and off, and this 3.2 volts switches the mosfet fully on, so that on the resistor in series there is a voltage almost equal to the source voltage.

And yes i controlled the microcontroller, that is generating pulses, through usb, using the minicom terminal emulator.

About pulses, i think that what matters is the exact length of the pulses, not frequency, frequency is important only because when the frequency is higher, the things are changing faster. But i may be wrong about that particular thing.

No, at 3.2 volts the IRF630  mosfet is not turning fully on, especially if you are sourcing the gate current directly from the microprocessor. The gate _threshold_ voltage of the IRF630 is between 2 and 4 volts but it will not be fully on until the gate voltage is near 8 volts. See the graphs below, taken from the Vishay data sheet.

And no, frequency is always important, since the mosfet has a finite switching time, the reactances vary with frequency, the rise time of the drive pulses probably varies with frequency, etc etc.

You really need an oscilloscope monitoring the mosfet drain voltage to see what you need to see in this experiment.

 

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