Using low voltage switching fets with zener protection in PI

[6666]

Would anybody know is it practical to use low voltage coil switching fets
in a PI with zener protection , or does the zener just stop the cct from working.

What i mean is this , a typical IRF740 is good for about 650 volts
so does not need protecting
but say you had a fet thats rated at 55 volts, would it work if you put a 55 volt zener across the fet, would that arrangement still switch the PI coil correctly ?

My gut feeling is the answer is no, but any ideas on this ?
thanks
6666

[simonbaker]

Would anybody know is it practical to use low voltage coil switching fets
in a PI with zener protection , or does the zener just stop the cct from working.

What i mean is this , a typical IRF740 is good for about 650 volts
so does not need protecting
but say you had a fet thats rated at 55 volts, would it work if you put a 55 volt zener across the fet, would that arrangement still switch the PI coil correctly ?

My gut feeling is the answer is no, but any ideas on this ?
thanks
6666

I'm not the expert, but I'm pretty sure that would defeat the concept of the PI pulse. You need a super-high flyback voltage to cut off the current fast -- if the zener starts conducting at 55 volts, the current sustains like a normal L/R circuit with low R until the voltage drops back below 55 volts.

-SB

[Davor]

Unless you are happy with the voltage step instead of a short pulse. My simulations say it will be even better. The only shortcoming is that the next step will be in reverse. Some professional MD-s have alternating pulses feature as a standard.

[6666]

Hi
Thanks all
would like to try this cascode cct
if I can get some time.
I wonder if this can be simed ?
The logic level fet is only rated at 30v

[DOOLEY]

hello 6666,

feed the gate on the top mosfet with a 4007 in series ,then protect the gate from exceeding 20v by using a diode from source to gate so the gate will safely "follow" the source voltage when it gets "blown open" by the h.v spike.

[Comp]

???? not sure if I have rad the question right, but surely, thats what they do now???

look at the datasheet for the IRF9640, used in the Surfboard, for instance,.... it is a FET with a parallel high voltage zener????

http://www.datasheetcatalog.org/data...ld/IRF9640.pdf


Cheers, Fred

[simonbaker]

Unless you are happy with the voltage step instead of a short pulse. My simulations say it will be even better. The only shortcoming is that the next step will be in reverse. Some professional MD-s have alternating pulses feature as a standard.

Can you post your LTSpice sim so I can see what you mean? I'm not clear on it, but would like to learn.

-SB

[Davor]

Sure, here it is.
It is not implemented with transistors etc. but as a mere 1V voltage source.
I've left the net names "Coin" and "Rx" for ease of reference, however, it is coil current you wish to check with coin.
I concentrated on tau/delay/zero crossing but don't get alarmed, everything is adjustable.

My reasoning goes like this: even if you load a Tx coil with high resistance, the voltage will jump just to some finite peak value ... which is basically the same as if you loaded it with a diode of certain zener/avalanche voltage, or a real world component with certain avalanche voltage.

Note1: Pulse/zero periods may be adjusted to fit certain requirement
Note2: a voltage pulse can be achieved also with a discharging capacitor
Note3: high resistance in Tx coil is transformed to a target by virtue of impedance transformation, and it is best if kept as low as possible.

[simonbaker]

Sure, here it is.
It is not implemented with transistors etc. but as a mere 1V voltage source.
I've left the net names "Coin" and "Rx" for ease of reference, however, it is coil current you wish to check with coin.
I concentrated on tau/delay/zero crossing but don't get alarmed, everything is adjustable.

My reasoning goes like this: even if you load a Tx coil with high resistance, the voltage will jump just to some finite peak value ... which is basically the same as if you loaded it with a diode of certain zener/avalanche voltage, or a real world component with certain avalanche voltage.

Note1: Pulse/zero periods may be adjusted to fit certain requirement
Note2: a voltage pulse can be achieved also with a discharging capacitor
Note3: high resistance in Tx coil is transformed to a target by virtue of impedance transformation, and it is best if kept as low as possible.

Hi Davor:

I think that a voltage pulse source as you used in your sim is fundamentally different than a typical PI TX coil circuit .

A typical PI TX coil circuit is basically a DC voltage and a switch (MOSFET). When the switch opens, there is ideally infinite resistance in the circuit (a real MOSFET breaks down at some voltage, which is the issue we're talking about).

In contrast, in your simulation, when your pulse goes to zero, it is not like a switch opening, rather there is still very low resistance in the TX coil circuit path (because the voltage source has low internal resistance). So I don't think your sim represents a typical PI circuit, although it is a perfectly good circuit to study for possible use.

Regards,

-SB

[Davor]

You are right, it is not the same as normal PI but a transient circuit nevertheless.

You may think of a system as a contraption made of two (or more) transformers. Tx to target and target to Rx. What you expect to see is some voltage at Rx side, and it relates to voltage at Tx side. In case you stop current in some ideal way, you'll observe infinite voltage pulse at Tx side, and it will surely transform to the Rx side via target as a much higher voltage peak.
However, with infinite resistance at Tx side, you'll stupify the target resistance and lose any chance for discrimination.

How about that?

Or in other words, PI with extremely high Tx resistance makes a superb all-metal detector.

[simonbaker]

You are right, it is not the same as normal PI but a transient circuit nevertheless.

You may think of a system as a contraption made of two (or more) transformers. Tx to target and target to Rx. What you expect to see is some voltage at Rx side, and it relates to voltage at Tx side. In case you stop current in some ideal way, you'll observe infinite voltage pulse at Tx side, and it will surely transform to the Rx side via target as a much higher voltage peak.
However, with infinite resistance at Tx side, you'll stupify the target resistance and lose any chance for discrimination.

How about that?

Or in other words, PI with extremely high Tx resistance makes a superb all-metal detector.

I'm not sure what you're comparing. If you just increase Tx resistance, the current, and magnetic field, gets smaller and smaller and you don't have much of a metal detector.

Are you saying to increase the pulse voltage proportionally (to end up with same Tx current) when you increase Tx resistance so you can compare apples to apples?

-SB

[homefire]

Just food for thought!

What is the Reverse Break Down Voltage of these Zeners?

Are they going to be able to take the few hundred volts Flying back at them?

[Tepco]

Would anybody know is it practical to use low voltage coil switching fets
in a PI with zener protection , or does the zener just stop the cct from working.

What i mean is this , a typical IRF740 is good for about 650 volts
so does not need protecting
but say you had a fet thats rated at 55 volts, would it work if you put a 55 volt zener across the fet, would that arrangement still switch the PI coil correctly ?

My gut feeling is the answer is no, but any ideas on this ?
thanks
6666

Answer is NO. Fist, you don't need any zener. Mosfets break down typ. at 10-15% of rated voltage, acting like high power zener, so you already have one, energy in coil is too low to produce avalanche and destroy Mosfet. If you put real zener of lower voltage rating, it will just dissipate most of the pulse energy instead of FET. Using low volt devices is bad idea, when breakdown happens at lower voltage, coil will need more time to dissipate stored energy, (being bit like "shorted" during breakdown) making achievable sampling time slower.

@Davor: Simulating PI process is not so simple, including breakdown behavior, nonlinear D-S capacitance, coil as distributed, not lumped element etc. Better build and measure instead, sim's can be way off. Alternating pulses are story for itself.

@Comp: Diode is so called "body diode" internal to MOSFET due to it's internal structure, not zener, also wrong polarity to be. "Zener" effect is generated by breakdown behavior.

(I was bit off forum for some time, maybe better to stay that way, i have lot to answer and also ask here but later...)

[Tepco]

Hi
Thanks all
would like to try this cascode cct
if I can get some time.
I wonder if this can be simed ?
The logic level fet is only rated at 30v

If you want to "cascode" something, do it different way. Instead of upper IRF 740 use high voltage bipolar. This is why: typically, IRF 740 is rated for 400pF output capacitance, by far largest in entire circuit, more than coil self capacitance, shield and cable combined. Some HV bipolars, say MJE/MJF18004 are rated for ca.50pF. Just increasing current consumption by 20%, assuming worst case current gain of 10, you will be able to reduce this capacitance 8 times! (spending 3 additional parts and less than 2$)
Also there is 1000+ V breakdown rating benefit.

[Davor]

@Davor: Simulating PI process is not so simple, including breakdown behavior, nonlinear D-S capacitance, coil as distributed, not lumped element etc. Better build and measure instead, sim's can be way off. Alternating pulses are story for itself.

You are absolutely right. My stream of thought went to the extremes of supplying a coil with transient drive: one with pure voltage, and the other with pure current. Neither actually exist, but the conclusions I gathered this way suggest that low impedance drive works better for maintaining phase information and at the very end - discrimination.

As a radio engineer I was more inclined to VLF IB solutions at start, but with only a few modifications to a standard PI it could work as a hybrid and provide the best of both worlds.