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THE AM BULLETIN BOARD => Technical Forum => Topic started by: KK4YY on November 25, 2018, 10:05:27 PM



Title: My first MOSFET design — but, will it work?
Post by: KK4YY on November 25, 2018, 10:05:27 PM
Before I turn a perfectly good MOSFET into a dispiriting wisp of smoke, I thought I'd run my simple as I can make it circuit design past the group.

It's a voltage regulator, I hope, used to drop the voltage on an RF final while maintaining the higher voltage on the modulator. This, to improve the effective voltage ratio of a modulation transformer that has a bit too much step-down. The fact that it may regulate the voltage is secondary in this application, its primary function is to just drop the voltage. Perhaps a simple resistor would work to this end, but what fun would that be? Especially when this circuit also provides a whole string of cool, glowing, regulator tubes! 8)

Okay, I plan to use the same circuit to regulate lower voltages too. Maybe just one 0A2 to get 150V regulated. That's more sensible 'eh? Yeah, yeah, I could use zener diodes too, but I feel bad enough already just using a 'FET. :(

Anyway, with keeping it simple in mind, and despite all those VR tubes, please see the attached.

Comments and suggestions will be graciously received.

Don


Title: Re: My first MOSFET design — but, will it work?
Post by: DMOD on November 25, 2018, 10:42:38 PM
To avoid any parasitic oscillations i would use a 47 ohm resistor between the gate and the top of the 0.1 uf cap.

It looks as if you have a 16.5 ms turn-on delay built in.

Otherwise it should work fine.

Phil


Title: Re: My first MOSFET design — but, will it work?
Post by: VE3ELQ on November 26, 2018, 08:28:55 AM
MOSFETs as source followers make very good simple regulators as you have proposed. Am using them in several of my projects including the station PS for my AM gear which holds to .1 V regulation, not precision but plenty good enough.

Looks good to me with one suggestion.  Ensure there is a bleed path for the gate reference V from gate to source otherwise when the main PS filter caps and output caps bleed down on power off the gate may be left hanging at the reference voltage by the cap which will quickly exceed the gate to source V rating and blow out the gate.  The 2 built in gate zeners are back to back, not sure if that will protect the gate, never used one like that. If not a small 9V zener in series with 100 ohm R from gate to source is all it needs. Does nothing until power off.

73s  Nigel


Title: Re: My first MOSFET design — but, will it work?
Post by: WD5JKO on November 26, 2018, 03:02:58 PM
Don,

   I attach two other FET regulators I have built. These have been operational in a Sherwood S8000 tube amplifier since 2003. These are for comparative reference to illustrate another mans approach.

   The CCS to the zeners really help out. With the CCS feeding the zener string, this helps with line voltage regulation.

   The closed loop +375v regulator uses a 6.2v zener reference diode. This zener with the base-emitter junction of the error amplifier have equal and opposite temperature drift (each about 2.2mv/degre C) such that when thermally bonded together, the regulator is stable as the circuit warms up. High voltage zeners drift UP in voltage quite a bit as they warm UP.

   I like the 51 ohm resistor on the drain. If the supply is switched on when the output is shorted out, the idea is to insure the FET is in saturation such that something up stream dies first (hopefully the fuse). I was always chicken to try this once the power was turned on. Might work there too. But remember the SS regulators like the ones here usually die when the output power is suddenly shorted to ground..

Jim
Wd5JKO


Title: Re: My first MOSFET design — but, will it work?
Post by: PA0NVD on November 26, 2018, 03:46:43 PM
I always put a current limit / short circuit protection by adding a resistor in the source of the series FET and a PNP transistor over that resistor Emitter to the input side, Base via a current limiting resistor to the FET side. The collector can be used to pull up the drive voltage of the series FET when the PNP transistor starts conducting effectively limiting the drive to the FET. The extra series resistor is 0,6/Imax
That circuit did save me a lot of FET's. I made a lot of RF generators for plastic welding and they had all this current limiter.


Title: Re: My first MOSFET design — but, will it work?
Post by: KK4YY on November 26, 2018, 05:29:31 PM
Great replies! I've updated the schematic with the suggestion offered. So far, it's still a nice simple circuit. :)

To avoid any parasitic oscillations i would use a 47 ohm resistor between the gate and the top of the 0.1 uf cap.
Done!

The 2 built in gate zeners are back to back, not sure if that will protect the gate, never used one like that.
Those built-in zeners are one of the reasons I selected that device. I hope they work.
From the datasheet...

"The built-in back-to-back Zener diodes have specifically been designed to enhance not only the device’s ESD capability, but also to make them safely absorb possible voltage transients that may occasionally be applied from gate to source. In this respect the Zener voltage is appropriate to achieve an efficient and cost-effective intervention to protect the device’s integrity. These integrated Zener diodes thus avoid the usage of external components."

I like the 51 ohm resistor on the drain.
Done!

I always put a current limit / short circuit protection by adding a resistor in the source of the series FET...
Hopefully the 51 ohm resistor added in the drain will be enough.

Thanks,
Don


Title: Re: My first MOSFET design — but, will it work?
Post by: DMOD on November 26, 2018, 09:00:39 PM
The 47 ohm resistor needs to be placed as shown below:

I don't think you need a 51 ohm value for the CL resistor, too much Vdrop.

The 16.5 ms time constant will allow for a graceful ramp-up of voltage, limiting current rate.


Phil - AC0OB



Title: Re: My first MOSFET design — but, will it work?
Post by: KK4YY on November 26, 2018, 09:27:00 PM
Phil,

Thanks for the schematic! Looks like I don't need to redraw mine again. ;D

I have LTspice on my PC but I haven't taken the time to learn how to use it. If you already have that drawing as a .asc file can you email it to me? Might be a good time for me to learn. My email is on my profile here.

Thanks,
Don


Title: Re: My first MOSFET design — but, will it work?
Post by: KK4YY on November 27, 2018, 06:05:03 PM
I don't think you need a 51 ohm value for the CL resistor, too much Vdrop.

Pondering this point while at work today, I wondered why. The regulator drops 100V across the FET and dissipates the associated power. The current limiting resistor on the drain drops voltage too. Why couldn't it be made a relatively large value? Let's say the CL drops 50V (half the total drop) at the rated output current. Then, the CL and the FET would equally share the power dissipated. Moreover, the short circuit current is lowered to a safer value. Yes, this would require a heftier power resistor (there's no free lunch) But the FET would run cooler, or need less heat sink, or it could even be downsized.

Would it adversely effect voltage regulation?

Am I missing something? ???

Also, thanks for the .asc file ;)

Don


Title: Re: My first MOSFET design — but, will it work?
Post by: DMOD on November 27, 2018, 06:48:41 PM
Let's look at the math:

With a 51 ohm CL resistor it drops 12.75 volts, 800-12.75 = 787.25V-700V = 87.25Vdrop across FET.

87.25VX0.25A = 21.8W of FET dissipation.

With a 10 ohm CL resistor it drops 2.5 volts, 800-2.5 = 797.5V at the drain of the FET gives a 97.3Vdrop across FET.

97.5VX0.25A = 24.4W of FET dissipation.

The difference is 2.6W of dissipation.

If you are concerned about dissipation use a physically larger MOSFET with greater power dissipation such as the

https://www.onsemi.com/pub/Collateral/FQA11N90_F109-D.PDF


Phil - AC0OB




Title: Re: My first MOSFET design — but, will it work?
Post by: KK4YY on November 27, 2018, 07:58:33 PM
Phil,
I muddled my point. Let me try again. :-[

If the purpose of the CL resistor is to protect the FET from a short circuit (in particular), then a higher value would offer better protection.

   CL          Short
 value       circuit
(Ohms)    current (A)
 —                —
  10               80
  51               15.43
200                 4

So the greater the CL resistance, the lower the maximum short circuit current would be.

And, taken to extreme (given a maximum desired steady-state regulator output current), the CL can drop almost all the voltage (with the FET in saturation) not exceeding the point where the regulator drops out of regulation. Additionally, allowing the regulator to drop out of regulation at a current just above that which is desired, it would offer some measure of protection to the load (a mistuned RFPA, for example).

Making the CL large is at the expense of a larger power rating of the CL (a big expensive power resistor) but does seem to have some advantage for certain applications.

My question: Although it may be impractical in most cases, would this be detrimental to the operation of the circuit as a regulator? ???

Thanks for your patience,
Don


Title: Re: My first MOSFET design — but, will it work?
Post by: DMOD on November 28, 2018, 01:54:36 AM
A simple solution is to place a 500 mA AGC Fast Blow fuse in the Drain circuit if you are concerned about shorts.

Too large a RCL may not allow the cold cathode regulator tubes to "light -off" under certain circumstances. For the RCL, you should also consider the 15 mA or so  current in addition to the forecasted 250 mA.

Any circuit is a compromise so run the numbers and make a choice.


Phil - AC0OB



 


Title: Re: My first MOSFET design — but, will it work?
Post by: WBear2GCR on November 28, 2018, 09:12:57 AM

Another factor is the effective impedance of the regulator, as seen by the load. Usually
a lower impedance is considered to be better. A bit depends on what the load acts like,
but keep in mind that "voltage" alone is not the only thing being regulated.

Another idea here is to add a shunt stage... (more complexity)

Or, alternately, put a "sloppy" regulator in front - aka a "pre-regulator" - such as a capacitor
multiplier set to drop say half the requisite Vdrop, then go to the regulator. The preregulator
voltage will go up and down with the line and load a bit, but the actual regulator will take
care of that, and actually the result will be less hum and ripple... plus you've reduced the
Pd per device.

Alternatively, parallel two devices if the current starts to look high, in any event.



Title: Re: My first MOSFET design — but, will it work?
Post by: KK4YY on November 28, 2018, 06:09:22 PM
My thought process (hold on to your hats):

I, and many others, have, an old boat-anchor transmitter with a common PS and a modulation transformer which has a less-than-optimum turns ratio that limits positive peaks. What to do?

Scratching my head to find a solution, my first thought was to use a resistor, between the PS and the mod xfmr, to drop the voltage going to the RFPA. This would "work" but I was thinking that it would be modulating the resistor along with the RFPA. Then, I though of hanging a big filter capacitor after the resistor to prevent that. Maybe that would work better, but a second filter bank is too big and expensive.

This led me to the MOSFET "regulator". Thinking that it may have a low enough impedance to work better in this application. This, I never stated directly, but Bear figured it out!

Another factor is the effective impedance of the regulator, as seen by the load. Usually
a lower impedance is considered to be better. A bit depends on what the load acts like,
but keep in mind that "voltage" alone is not the only thing being regulated.

That's exactly what I need to know. Can the MOSFET rival the original PS (an LC filter in the DX-100, for example) as far as its output impedance which will be seen by the modulator?

Another idea here is to add a shunt stage...
Maybe that would be better.

I haven't the first clue. Anyone?


Too large a RCL may not allow the cold cathode regulator tubes to "light -off" under certain circumstances.
Phil, I must be on the slippery-slope having chosen to use a MOSFET — I'm ordering a 300pc assortment kit of zeners now. So, it's good-bye to ionized gas and hello to avalanche breakdown. I'll change the dropping resistor network to get a proper current for a zener string (probably closer to 5mA).

Don


Title: Re: My first MOSFET design — but, will it work?
Post by: steve_qix on November 28, 2018, 06:37:40 PM
I think you just need an "amplified zener" and put the circuit between the modulated DC output and the RF amplifier.  Simple and effective.

Sorry for hand drawn circuit - but something like this may do it - and it's a whole lot simpler.

The group will please let me know if I made any mistakes  ;)

(http://www.classeradio.com/amplified_zener.jpg)


Title: Re: My first MOSFET design — but, will it work?
Post by: PA0NVD on November 28, 2018, 07:01:53 PM
simple current limit / short circuit protection


Title: Re: My first MOSFET design — but, will it work?
Post by: PA0NVD on November 28, 2018, 07:36:12 PM
Amplified zener with short circuit protection / current limit and a stabilizing circuit that can use a low voltage zener, also with current limit and short circuit protection.
R1 defines zener current. R2 should lower the collector voltage of the transistor and is equal to R3. Operating they will take a maximum of 2 mA and the voltage drop over R2 and R3 should leave at least 10 Volts Vc-e for each transistor'
The resistor in the base of the PNP transistor is to protect this transistor during short circuit transients. All transistors are just small transistors that can handle the input voltage of the circuit.
All values are approx. and can be adjusted to the closest normal value


Title: Re: My first MOSFET design — but, will it work?
Post by: steve_qix on November 28, 2018, 08:00:48 PM
Has to be a 2 terminal device to be a zener replacement :-).



Title: Re: My first MOSFET design — but, will it work?
Post by: KK4YY on November 28, 2018, 08:43:55 PM
...put the circuit between the modulated DC output and the RF amplifier.
Looks like an elegant solution!
But I don't understand the placement you suggest. There would still be the same voltage on the primary and secondary of the modulation transformer that way, offering no advantage.

I can see using it between the power supply and the "bottom" of the mod transformer secondary (while keeping the full HV on the primary). That would effectively reduce the step-down of the transformer, which is my goal.

Don


Title: Re: My first MOSFET design — but, will it work?
Post by: DMOD on November 29, 2018, 01:39:26 AM
Here is a schematic if you want to use zeners:

The Gate node of a Mosfet Source follower is a high impedance node, whereas the Source node of a Source follower is a Low Impedance node.

I have built these types of regulators for Viking and Valiant modulator screen voltage regulators and they work very well.

Of course here I am using the KISS principle.


Phil - AC0OB


Title: Re: My first MOSFET design — but, will it work?
Post by: MikeKE0ZUinkcmo on November 29, 2018, 09:18:23 AM
Another approach would be to use a sensitive gate SCR and just turn off the FET, hopefully saving the "load" as well as the PS.   At shut down only the very few mils of FET bias current would flow to the load.   The trip is set by selecting the value of the "10" Ohm resistor at upper right.    

Of course the regulator could be scrapped and replaced with the OP's stacked VR tubes, and a couple 100K 1W resistors from the VR tubes to the gate.

(https://photos.smugmug.com/Miscelaneous-/BIGGIES/i-ww3chWw/0/72be08ff/L/SCR%20ShutDown-L.jpg)


Title: Re: My first MOSFET design — but, will it work?
Post by: VE3ELQ on November 29, 2018, 10:11:45 AM
Here is a schematic if you want to use zeners:
Phil - AC0OB

Now we're talking. Best yet IMHO. Dont need the 47 ohm gate R, V gain is unity it cant oscillate. KISS, build this one.

Adding a low value source R and SCR as Mike suggests is exactly what I do in my LDMOS linear amp MOSFET regulator, uses two SUP90140E FETS, 50V with 50A trip out. Different values but very fast trip and works great.

73s Nigel


Title: Re: My first MOSFET design — but, will it work?
Post by: Opcom on November 29, 2018, 12:10:39 PM
I think you just need an "amplified zener" and put the circuit between the modulated DC output and the RF amplifier.  Simple and effective.

Sorry for hand drawn circuit - but something like this may do it - and it's a whole lot simpler.

The group will please let me know if I made any mistakes  ;)

(http://www.classeradio.com/amplified_zener.jpg)


This works very well with a bipolar transistor as well. Very versatile and has the possible advantage of tracking the HV supply as the line or other conditions make it vary.

====

Series resistor was mentioned.. maybe with a large cap across it so the load can 'see' the power supply's filter impedance. But no definite regulation except at a certain load.




Title: Re: My first MOSFET design — but, will it work?
Post by: KK4YY on November 29, 2018, 07:11:09 PM
Wow! Lots of great ideas here. Many thanks!

Phil's "HV Regulator with Zeners" looks very straight forward and would make a good regulator. I'd go with that when I need regulated voltages.

Regulation is not required in my proposed application, and Steve's "Amplified Zener" is a cool two terminal circuit that would provide the Vdrop I need with the fewest parts. When I did a search for "amplified zener" I realized I'd seen the circuit before, used as "B+ reducers", but it didn't come to my mind for this project. Wish I had a good excuse for not recalling it. :-[

I've got 'FETs and zeners on order now, so I'll be anxiously checking my mailbox. My poor DX-100 has been sitting on the desk here, up on her side, for months. I've been modding it on-and-off and any documentation that I did do is scattered around here... somewhere.

I've spent hours watching Jordan Peterson videos recently, but I still keep procrastinating and haven't yet cleaned my room.

Maybe tomorrow... ;)


Don


Title: Re: My first MOSFET design — but, will it work?
Post by: KK4YY on December 01, 2018, 12:59:51 PM
In an earlier post, I suggested using a high value current limiting resistor ahead of a MOSFET 'regulator' used to reduce the HV. My thought was to have the resistor dissipate most of the power while also protecting the 'FET.

Although it may actually do this, upon further reflection, I believe that the 'FET won't isolate the resistor from the load. The load, an RFPA, would still 'see' the resistor as part of the load and modulate it — wasting modulator power.

I, contritely, withdraw the suggestion. :-[


Don


Title: Re: My first MOSFET design — but, will it work?
Post by: WD5JKO on December 01, 2018, 02:43:56 PM


Don,

   I don't know what kind of RFPA you have, but if it a class C plate modulated amplifier, then the plate current is essentially constant during modulation. Even if this was for SSB, the Fet Source will track the gate voltage within a volt or so as the load current varies at a syllabic rate.  In this last instance, the FET drain voltage will vary with load current, but so long as the FET has enough Drain to Source voltage (perhaps > 10v), then all is well.

   You might ponder the situation further.

Jim
Wd5JKO


Title: Re: My first MOSFET design — but, will it work?
Post by: KK4YY on December 01, 2018, 03:26:41 PM
Jim,

With a very high value series resistor, perhaps 200 ohms, ahead of the 'FET I was thinking it would be seen a part of the load when modulated. The 'FET, at best, would be a very low series resistance. So a modulating impedance of 2800 ohms (at the RFPA) would be seen at the modulator as 2800+200=3200 ohms. What's needed is a low impedance source for the modulation to 'work against'. So, it's not really about the 'FET's ability to follow along, but the fact that it doesn't present a low impedance to ground at its source.

I also understand that the current drawn by the RFPA is anything but constant, even without modulation. As I understand it, the class C stage is drawing pulses which when viewed by an analog meter appear constant, showing average current (the pulses are returned to a sine wave by the flywheel effect of the tank).

I struggle to explain some of the ideas in my head. Even when I get things right, I may not write, right. I hope I've got both right this time.

Don


Title: Re: My first MOSFET design — but, will it work?
Post by: MikeKE0ZUinkcmo on December 01, 2018, 11:28:06 PM
So long as the PA current peaks don't cause the FET output to droop, because of excessive voltage drop across the resistor, the modulator wouldn't know, or care, and, there isn't any additional power wasted.  

Further, the average power dissipation of the resistor would remain constant if modulated with a sine wave, and probably won't vary all that much with voice either.


Title: Re: My first MOSFET design — but, will it work?
Post by: MikeKE0ZUinkcmo on December 02, 2018, 12:08:44 AM
Steve, your drawing would appear to work fine so long as the "input to output" differential doesn't exceed 112 VDC.  Once there, it would appear the circuit then simply becomes a pair of Zeners in series.    Or am I missing something?

(http://www.classeradio.com/amplified_zener.jpg)

Mike


Title: Re: My first MOSFET design — but, will it work?
Post by: KK4YY on December 02, 2018, 09:05:59 AM
Assuming the RDS of a MOSFET is quite low, even in its linear region (such as in a series voltage regulator or a zener amplifier), Let's omit the large value series resistor and let the 'FET disipate the power.

With that, let's apply this circuit.

(Put you hands on the radio.)

In the DX-100, if I put the 'FET between the power supply and the modulation transformer secondary, it will lower the voltage to the RFPA by 100 volts while disipating about 25 watts. That 25W would reduce the RFPA input power by 25W, consequentially reducing output power by about 18W (assuming an RFPA efficiency of 72%).

Given that the power supply has been solid-stated, the modulator tubes will have 800V and the RFPA tubes 700V, on their respective plates. This will change the effective voltage ratio of the modulation transformer from 1.69:1 to 1.48:1.

Advantages:
-The modulator now needs to supply 12.5W (half of 25W) less power.
-The modulator doesn't need to be driven as close to clipping to achieve the same percent of modulation, providing lower distortion.
-The percent of modulation is extended further beyond 100%.
-RFPA tube plates dissipate 7w less total power extending tube life.

Disadvantages:
-Total RF output power would be reduced by about 0.7dB.
-An additional 18W of electrical power is turned into heat.

As everyone knows (well almost everyone) it's better to have a fully-modulated, weaker carrier that an under-modulated, stronger carrier.

With components on order, wending their way to my QTH, I hope to conduct experiments, here at the KK4YY SIDL (Silly Idea Development Laboratory).


Don

(Take your hands off the radio.)


Title: Re: My first MOSFET design — but, will it work?
Post by: DMOD on December 02, 2018, 10:27:46 AM
Hark! The He-rald Fi-nals Sing, Glory to, the SI-DL-thing.  ;D



Phil - AC0OB


Title: Re: My first MOSFET design — but, will it work?
Post by: W4EWH on December 03, 2018, 11:12:01 AM
The group will please let me know if I made any mistakes  ;)

Only that you've thrown so many pearls before we swine.   ::)

Bill, W4EWH


Title: Re: My first MOSFET design — but, will it work?
Post by: steve_qix on December 03, 2018, 09:37:15 PM
Steve, your drawing would appear to work fine so long as the "input to output" differential doesn't exceed 112 VDC.  Once there, it would appear the circuit then simply becomes a pair of Zeners in series.    Or am I missing something?

(http://www.classeradio.com/amplified_zener.jpg)

Mike

Hi Mike - Yes, It's designed so that once the voltage drop across the circuit reaches about 106 V or thereabouts, the drop stabilizes (hence the term amplifier zener).

With the bypass capacitor across the circuit as shown in the diagram, it makes a nice constant voltage drop device.


Title: Re: My first MOSFET design — but, will it work?
Post by: KK4YY on December 04, 2018, 07:44:30 PM
Steve,

Well it didn't take long for your drawing to end up in my Google image search results for "amplified zener". :o

https://www.google.com/imgres?imgurl=http%3A%2F%2Fwww.classeradio.com%2Famplified_zener.jpg&imgrefurl=http%3A%2F%2Famfone.net%2FAmforum%2Findex.php%3Ftopic%3D44400.25&docid=addRlHsB5WanJM&tbnid=SCAKaNggmZ0C9M%3A&vet=10ahUKEwjVjtjhtYffAhURnKwKHUWCBRYQMwhCKAUwBQ..i&w=699&h=393&client=firefox-b-1&bih=601&biw=1360&q=%22zener%20amplifier%22&ved=0ahUKEwjVjtjhtYffAhURnKwKHUWCBRYQMwhCKAUwBQ&iact=mrc&uact=8

I received a pack of ten STP7N95K3 MOSFETs today. When the zeners arrive, I'll be busting out the clip leads!
I plan to build an amplified zener, regulator, and sweep tube screen grid driver circuit with them. All to be fitted to my DX-100. It should be interesting.

Thanks for your suggestions here, and on-air encouragement to use MOSFETs.

Don


Title: Re: My first MOSFET design — but, will it work?
Post by: MikeKE0ZUinkcmo on December 05, 2018, 12:54:08 AM
Quote
...Hi Mike - Yes, It's designed so that once the voltage drop across the circuit reaches about 106 V or thereabouts, the drop stabilizes (hence the term amplifier zener).

With the bypass capacitor across the circuit as shown in the diagram, it makes a nice constant voltage drop device....
Understood, but my comment was in reference to the situation where the current set by "R-prot" is exceeded by some significant amount. 

At which point, all excess current, which is undefined but could be substantial, is supplied exclusively through the two Zeners, which probably wouldn't last long. 

Certainly not a "flaw", but I'd opt for some protection so I only need to replace as few parts as possible.   I do enough of that all ready. :)   


Title: Re: My first MOSFET design — but, will it work?
Post by: steve_qix on December 06, 2018, 12:06:44 PM
Quote
...Hi Mike - Yes, It's designed so that once the voltage drop across the circuit reaches about 106 V or thereabouts, the drop stabilizes (hence the term amplifier zener).

With the bypass capacitor across the circuit as shown in the diagram, it makes a nice constant voltage drop device....
Understood, but my comment was in reference to the situation where the current set by "R-prot" is exceeded by some significant amount. 

At which point, all excess current, which is undefined but could be substantial, is supplied exclusively through the two Zeners, which probably wouldn't last long. 

Certainly not a "flaw", but I'd opt for some protection so I only need to replace as few parts as possible.   I do enough of that all ready. :)   

Yes, that could definitely be improved.  And should be in a practical implementation.  Thanks !!


Title: Re: My first MOSFET design — but, will it work?
Post by: KK4YY on December 06, 2018, 05:52:28 PM
Quote
...Hi Mike - Yes, It's designed so that once the voltage drop across the circuit reaches about 106 V or thereabouts, the drop stabilizes (hence the term amplifier zener).

With the bypass capacitor across the circuit as shown in the diagram, it makes a nice constant voltage drop device....
Understood, but my comment was in reference to the situation where the current set by "R-prot" is exceeded by some significant amount. 

At which point, all excess current, which is undefined but could be substantial, is supplied exclusively through the two Zeners, which probably wouldn't last long. 

Certainly not a "flaw", but I'd opt for some protection so I only need to replace as few parts as possible.   I do enough of that all ready. :)   

Yes, that could definitely be improved.  And should be in a practical implementation.  Thanks !!

If "R-prot" were moved to the output of the circuit (just to the right in the drawing), wouldn't the 'FET continue to maintain the ~100V drop and cause no additional current through the zeners?

Don


Title: Re: My first MOSFET design — but, will it work?
Post by: steve_qix on December 07, 2018, 01:00:48 PM
Here's an improved circuit, with protection for the zener.  And real values for the components :-)

(http://www.classeradio.com/amplified_zener_1.jpg)



Title: Re: My first MOSFET design — but, will it work?
Post by: KK4YY on December 07, 2018, 05:12:29 PM
Steve,

Looks good. :)

My build will be using an STP7N95K3. It has back-to-back 30V zener protection built-in (gate to source), but I'll keep D2 (12V) anyway. With 800V on the input, I'll be using a microwave oven HV fuse (5KV rating) at F1. I had already exploded a "3AG" 250V fuse with HVDC some years ago — it wasn't pretty. I'm assuming that the value of C1 isn't too critical and a voltage rating of 200V would be sufficient.

This should be a fun experiment for lowering the B+ on the DX-100 RFPA. I'm still in the process of putting SG driven sweep tube modulators in it with 'FET source followers driving them. Maybe a 'FET voltage regulator will find its way in there too. There's nothing wrong with hybridizing the old girl! ;D

Don


Title: Re: My first MOSFET design — but, will it work?
Post by: steve_qix on December 08, 2018, 05:22:35 PM
Sounds good!  The value of C1 should be such that the capacititive reactance should be low at, say, 30Hz, as compared to the resistance of the load (in this case the RF amplifier and anything else out there such as a screen dropping resistor that comes from the modulated DC.


Title: Re: My first MOSFET design — but, will it work?
Post by: KK4YY on December 08, 2018, 06:59:25 PM
Thanks, Steve.

Let's see if I understand your circuit...

The zener amplifier is acting like a variable resistor, dropping a fixed amount of voltage (100V) regardless of the current drawn through it.

In my proposed application, assuming an RFPA average current of 250mA, it will appear to be a 400 ohm resistor, not only at DC, but also to the audio, which then requires it to be bypassed with a capacitor of low reactance (at the lowest desired modulating frequency) so as not to absorb that audio.

In effect, it's a 400 ohm resistor at DC, but a lower value resistor at audio frequencies (53 ohms at 30Hz). Thus, with 700V at 250mA on the RFPA, the modulating impedance of 2800 ohms is much greater than the 53 ohms, and very little audio power will be lost in the zener amplifier.

Now, if I've got all that right (and assuming the average current of 250mA is drawn by the RFPA), would an equivalent circuit simply be a 400 ohm 25W resistor bypassed with a 100uF capacitor?

As cool as the zener amplifier is, I want to make sure I'm not over-engineering a solution.

Thanks,
Don


Title: Re: My first MOSFET design — but, will it work?
Post by: KK4YY on December 15, 2018, 11:58:56 AM
OR...

I can tap-off the HV before the filter choke on the DX-100 with a diode string and a bank of capacitors and have 1250V. Then, place the Zener Amplifier Voltage Reducer (ZAVR) after that, set at 350V. So, I'll have 900V for the modulator tubes leaving the RFPA at 800V. This would change the modulation transformer voltage ratio from the stock 1.69:1 to an effective 1.5:1 ratio. Tweaking the zener string voltage would allow me to tune the effective ratio for optimum performance. Nice!

Although the ZAVR voltage drop is greater, the amount of wasted power would be decreased, as the average current drawn by the modulator is quite low (for speech) compared the the "steady" current of the RFPA. Cool!

I had speculatively built the 1250 volt "supply" into the rig, so I only need to add the ZAVR. The ZAVR is a two-terminal floating circuit so it needs proper insulation from ground, but is not effected by the high input voltage. Golden!


Don
AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands