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My first MOSFET design but, will it work?




 
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Author Topic: My first MOSFET design but, will it work?  (Read 1487 times)
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WD5JKO
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« Reply #25 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
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KK4YY
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« Reply #26 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
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MikeKE0ZUinkcmo
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« Reply #27 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.
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Mike KE0ZU

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MikeKE0ZUinkcmo
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« Reply #28 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?



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KK4YY
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« Reply #29 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.)
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« Reply #30 on: December 02, 2018, 10:27:46 AM »

Hark! The He-rald Fi-nals Sing, Glory to, the SI-DL-thing.  Grin



Phil - AC0OB
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« Reply #31 on: December 03, 2018, 11:12:01 AM »

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

Only that you've thrown so many pearls before we swine.   Roll Eyes

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« Reply #32 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?



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.
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« Reply #33 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". Shocked

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
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MikeKE0ZUinkcmo
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« Reply #34 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. Smiley   
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« Reply #35 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. Smiley   

Yes, that could definitely be improved.  And should be in a practical implementation.  Thanks !!
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KK4YY
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« Reply #36 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. Smiley   

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
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« Reply #37 on: December 07, 2018, 01:00:48 PM »

Here's an improved circuit, with protection for the zener.  And real values for the components :-)



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« Reply #38 on: December 07, 2018, 05:12:29 PM »

Steve,

Looks good. Smiley

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! Grin

Don
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« Reply #39 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.
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« Reply #40 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
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