What is the purpose of the series diode?

[green]

I do not have another coil with the correct plug available, except one that has the damping resistor built into the coil shell. So I disconnected the 432uH coil and connected it to the scope, and provided external excitation to the coil with a different PI. The resonant frequency was 588.2kHz, which equates to a capacitance of 170pF.

If the total capacitance is 305pF, coil + cable is 170pF (from measurement above), Coss effective is 211pF; then the parasitics would be -76pF. As you can see, there is a discrepancy in the values obtained, as you cannot have negative parasitic capacitance.

At the moment I believe the total capacitance value of 305pF is correct, so the error is most likely in the evaluation of Coss, which varies with applied Vds. The calculation of Coss effective is basically a fudge to determine a fixed capacitance that would give the same charging time as the output capacitance of a MOSFET while Vds is rising from zero to 80% Vds with Vgs = 0V. Perhaps this value is a bit iffy in these circumstances, and needs to be a dynamic value, rather than static.

Thanks for the test. I've tried to measure resonance different ways with the way I suggested giving the highest resonance. Would think the highest resonance would be more correct? Your test gave a resonance of 588.2kHz across the scope vs 602.4kHx with the diode. Does that suggest the diode has less capacitance effect than the scope? Could try putting the scope probe near the test coil not connected.

[Altra]

Interesting thread. I was wondering if you considered the diode's capacitance in your calculations? Which diode did you use in your bench test? For instance a UF4007 has about 27pf and ES3J 45pf.

Two things about diodes, they make good AM detectors and temperature sensors.

[Qiaozhi]

Interesting thread. I was wondering if you considered the diode's capacitance in your calculations? Which diode did you use in your bench test? For instance a UF4007 has about 27pf and ES3J 45pf.

Two things about diodes, they make good AM detectors and temperature sensors.

The presence of the diode's depletion capacitance when reverse-biased is most likely the reason why it is able to block the MOSFET's Coss, since it is effectively in series with Coss. This would also explain why it is not able to provide 100% blockage.
The diode I was using is an MUR460.

[Qiaozhi]

More calculations to think about:

From resonance measurements:
no-diode => 305pF total capacitance
with-diode => 162pF total capacitance

305pF -162pF = 143pF (amount of capacitance blocked by diode)

However, Coss effective = 211pF. So, not all the MOSFET capacitance was blocked.

It is then simple to calculate that the depletion capacitance of the series diode while in reverse-bias is 100pF.
With diode capacitance = 100pF, and Coss effective = 211pF, the total series capacitance becomes 68pF.

MOSFET Coss - series capacitance (with diode) = 211pF - 68pF = 143pF.

So the question now is whether a value for the depletion capacitance of 100pF is reasonable for an MUR460 under these operating conditions?

[green]

More calculations to think about:

From resonance measurements:
no-diode => 305pF total capacitance
with-diode => 162pF total capacitance

305pF -162pF = 143pF (amount of capacitance blocked by diode)

However, Coss effective = 211pF. So, not all the MOSFET capacitance was blocked.

It is then simple to calculate that the depletion capacitance of the series diode while in reverse-bias is 100pF.
With diode capacitance = 100pF, and Coss effective = 211pF, the total series capacitance becomes 68pF.

MOSFET Coss - series capacitance (with diode) = 211pF - 68pF = 143pF.

So the question now is whether a value for the depletion capacitance of 100pF is reasonable for an MUR460 under these operating conditions?

A picture of capacitance vs reverse volts from a spec. sheet. The reverse volts across the diode is over 200volts 10usec after coil turnoff. I'm thinking diode capacitance is less than 7pf, maybe wrong.

[Qiaozhi]

A picture of capacitance vs reverse volts from a spec. sheet. The reverse volts across the diode is over 200volts 10usec after coil turnoff. I'm thinking diode capacitance is less than 7pf, maybe wrong.

In the On-semi datasheet for the MUR460, the diffusion capacitance is show in the graph as 36pF at 50V, and over 100pF at the lowest voltage. The problem here is that both the MOSFET Coss and the diode diffusion capacitance vary non-linearly with applied voltage. Unless we want to solve the full equations, we can only estimate some of these values. At the moment, SPICE simulation agrees more or less with my real world measurements, and my conclusion is as before:

..... inserting the diode doesn't make things worse, but (depending on many factors) there could quite likely be minimal improvement. Practical measurements do however show that the diode "blocks" the MOSFET capacitance, resulting in a higher resonant frequency, and the requirement to increase the value of the damping resistor. In cases where you're trying to sample as early as possible, and "every little helps"; then a series diode can only improve things.

[green]

In the On-semi datasheet for the MUR460, the diffusion capacitance is show in the graph as 36pF at 50V, and over 100pF at the lowest voltage. The problem here is that both the MOSFET Coss and the diode diffusion capacitance vary non-linearly with applied voltage. Unless we want to solve the full equations, we can only estimate some of these values. At the moment, SPICE simulation agrees more or less with my real world measurements, and my conclusion is as before:

..... inserting the diode doesn't make things worse, but (depending on many factors) there could quite likely be minimal improvement. Practical measurements do however show that the diode "blocks" the MOSFET capacitance, resulting in a higher resonant frequency, and the requirement to increase the value of the damping resistor. In cases where you're trying to sample as early as possible, and "every little helps"; then a series diode can only improve things.

The picture was from the On-semi datasheet. If the reverse volts is over 200volts why are you concerned about capacitance at 50v and less? I'm thinking if the resonance with the diode is greater than with the scope connected to the coil the capacitance has to be less than scope probe capacitance, typically less than 15pf with a x10 probe. Just trying to understand what is happening.

I agree the diode doesn't do much with the MPP. I have been using the diode to sample as early as I can with the bench circuit when charting decay curves. I am using an IB coil with my detector and have been using the diode in the Tx circuit. Tried without the diode today. Didn't increase delay time and decay looks a little better, thanks for starting the thread. Think you are looking at graph for the MUR405,410,415 and 420 not the MUR460.

[Qiaozhi]

The picture was from the On-semi datasheet. If the reverse volts is over 200volts why are you concerned about capacitance at 50v and less?

The voltage across the diode is not a constant, but starts at +0.7V just prior to TX-off, and then exponentially increases to over -320V. So it spends about 50% of its time at a much lower voltage than 200V.

I'm thinking if the resonance with the diode is greater than with the scope connected to the coil the capacitance has to be less than scope probe capacitance, typically less than 15pf with a x10 probe. Just trying to understand what is happening.

Same here!

I agree the diode doesn't do much with the MPP. I have been using the diode to sample as early as I can with the bench circuit when charting decay curves. I am using an IB coil with my detector and have been using the diode in the Tx circuit. Tried without the diode today. Didn't increase delay time and decay looks a little better, thanks for starting the thread.

Yes, that's what I'm finding so far in practice. The diode only provides a minimal improvement. Also, Altra made an observation about diodes making good AM detectors, and being affected by temperature. In that case, there may be some negative factors to consider.

Think you are looking at graph for the MUR405,410,415 and 420 not the MUR460.

Haha! ... you're correct. Looking at graphs late at night is not a good idea.

[green]

Interesting test. The coil with cable resonance(94pf, 432uH) should be 789kHz. I've been testing resonance exciting the test coil and cable with an operating PI. Wondering if you could remove the 432uH coil and cable from your test circuit. Connect a different coil and cable to the test circuit. While monitoring amplifier out for resonance bring the 432uH coil and cable near the operating coil to see how close to 789kHz the resonance is using this method.

Thought I would try the procedure with my bench circuit with a 8 inch mono coil shielded top and bottom with graphite paint. Top 5 lines, resonance was measured exciting test coil with the bench circuit(IB coil). Bottom 2 lines test coil was connected to bench circuit(replaced IB TX with 8 inch mono, Rd not connected). x10 probe is labeled 13.5pf, don't know why test calculates 28.4pf. Diode is less than probe and calculates close to the 7pf the MUR460 chart suggests. Coil inductance was calculated from resonance using two different capacitors that two different multi meters read close to the same values. Coil lead, 31 inches twisted pair that measures about .9pf/inch with the multi meter.

[Teleno]

..... inserting the diode doesn't make things worse, but (depending on many factors) there could quite likely be minimal improvement. Practical measurements do however show that the diode "blocks" the MOSFET capacitance, resulting in a higher resonant frequency, and the requirement to increase the value of the damping resistor. In cases where you're trying to sample as early as possible, and "every little helps"; then a series diode can only improve things.

In my recent experience I found out that whether the diode helps or not depends on its reverse recovery time.

A transient in PI is more or less a half cosine with a duration between 300ns-1us. The diode is reverse biased in the second half and it has 150ns -500ns to do the blocking. Its reverse recovery time has to be significantly shorter otherwise it would simply conduct instead of block. Rather than an improvement you get a slight deterioration and power waste because of the voltage drop it introduces when in series with the MOSFET.

There are several possibilities but every one of them implies some kind of trade off. A regular silicon diode (rectifier) is to be avoided at all costs, their recovery times are in the us range. We're left with either fast/ultrafast recovery diodes (25ns -100ns) or schottky diodes (zero recovery because they're based on majority carriers).

The fast/ultrafast recovery diodes have a larger forward voltage (1.3V to 3.6V) which gets larger as the voltage rating gets higher. The drop will be in series with the MOSFET reducing power efficiency significantly.

The schottky diodes have zero recovery time and only their intrinsic capacitance will delay the blocking. Forward voltage is around 1.5V maximum. Higher voltages are hard to come by, usually SiC for tens of amperes with capacitances in the order of 200pF.

I've tried the ultrafast MURS160 (50ns recovery, 10pF) with good results (see the video I posted here: https://www.geotech1.com/forums/show...272#post289272 ). The transient in this case is 700V with a duration of 300ns of which 100ns are actually blocked.
Haven't tried a schottky yet, I'm still shopping for it.

[Qiaozhi]

That's a useful addition to our knowledge on the role of the series diode.
It will be interesting to see the result of using a Schottky diode.

[kt315]

Haven't tried a schottky yet, I'm still shopping for it
--
Schottky diodes are just all low voltage. 50...60 max. unusable.

[bbsailor]

That's a useful addition to our knowledge on the role of the series diode.
It will be interesting to see the result of using a Schottky diode.

Qiaozhi,

If you plan on writing another book, article, or forum thread on Geotech1, please consider doing a trade off analysis between these variables.
1. Coil inductance
2. Resonant frequency
3. Total capacitance as seen by the coil
4. Coax cable effect on capacitance seen by coil
5. Damping resistor value
6. Effect of damping resistor value on the coil discharge slope relative to fully stimulating a variety of target TCs
7. Delay necessary to detect a variety of low TC targets
8. Total power being used to make the TX pulse
9. Coil making techniques needed to work at low delays
10. Now the main point: The trade offs between the above issues to optimize response from various target sizes, shapes, metal type and TC.

If you did this you would synthesize a lot of Geotech1 forum content in one convenient place and help PI detector builders target their own detecting priorities.

Forum members, do you agree?

Thanks

Joseph J. Rogowski

[Teleno]

Haven't tried a schottky yet, I'm still shopping for it
--
Schottky diodes are just all low voltage. 50...60 max. unusable.

Well, there are more than 300 schottkys above 650V on Farnell

https://nl.farnell.com/en-NL/c/semic...-vrrm-max=650v

This one looks like a good candidate, 650V, insignificant 12p capacitance, 20ns switching time and 10uA max leaking current.

http://www.farnell.com/datasheets/2722357.pdf

[sunita.pradhan]

You may try - GB01SLT12-214 also 1.2KV,5p capacitance, less than 10ns , 10uA max leakage...only Power dissipation is 19W vs 64W.