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Author Topic: push/pull amp using IRF511  (Read 39314 times)
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WA1GFZ
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« Reply #50 on: October 28, 2011, 09:11:32 PM »

I wouldn't use a choke but I did put a 10k  resistor in series with the diode in my MRI final. I only used 1 voltage regulator for the 2 drivers and 2 fianls. The diodes provide isolation from shorted FETs and a short to ground protects the bias circuit with the series resistors. (1 to 10k not a critical value)
A choke may have a resonance at some weird frequency that could cause instability
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ssbothwell KJ6RSG
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« Reply #51 on: October 29, 2011, 08:25:57 PM »

well. i finished assembling the amp but i'm not sure if its working properly. i was trying to test it with a hp3112a function generator. the most power it can put out is 3.5VRMS. is this not enough power to drive the circuit?

i'm running it off a 13.8V 3A power supply.

heres some photos of the output:
http://i.imgur.com/8eCnC.jpg
input at 3.5VRMS 7MHz with the bias all the way down. i turned up the bias slightly and R9/R10 burned out (1ohm source resistors). all the resistors in my circuit are 1/2W. is that not good enough?

EDIT:
i had been using alligator clips but i just replaced them with bnc connectors and now the waveform just looks like it has nasty crossover distortion: http://i.imgur.com/41mQM.jpg

however, when i turn up the bias, the source resistors start smoking.

EDIT2: with no input signal the circuit is drawing 3.3A. is that normal or is there possibly a short somewhere?

EDIT3: i just looked at my spice model again and realized that there is several watts running through the source resistors. i guess i need to use some sort of power resistors for the source->ground shunts?
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ssbothwell KJ6RSG
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« Reply #52 on: October 30, 2011, 04:41:03 PM »

i did some more tests and now i am even more confused about whats going.

i noticed that the bias voltage would go higher as i raised the resistance on the bias pots. that seems counter intuitive to me. i am using these pots: http://www.bourns.com/data/global/pdfs/3386.pdf

i had the pots connected so that the wipers went to the 78L05 and pin3 going to the 1n914diodes (into the fet gates).
i switched them so that the wipers go to the diodes and pin 1 goes to the 78L05.
i also got rid of the source to ground 1ohm resistors because i only have 1/2W resistors and they burn out in this circuit. the fet Sources are soldered directly to ground.

when i turn on the power supply and have no input signal i see this on the scope: http://i.imgur.com/dCN4K.jpg
this only started happening after i removed the source resistors and switched the pins on the bias pots.

with an input signal and bias turned all the way down: http://i.imgur.com/pqIid.jpg

as i turn up the bias the output voltage gets up to around 7VRMS max and the waveform looks pretty bad. previously it was going up to 20V. (all this is with a 3A 13.8V power supply). sorry i did not get a photo of what this looks like because the the plastic washer on one of the IRF510s melts when i have the bias turned up. this only happens on one of the FETs but i dont want to leave it turned on like this for very long.

adjusting the bias doesnt change the output very much. it stays between 2vrms and 7vrms. the bias pot on the non-melty FET seems to have more impact on the output waveform then the pot on the melty fet.

heres some photos of the circuit board:
http://i.imgur.com/g70D0.jpg
http://i.imgur.com/0G1fb.jpg
http://i.imgur.com/pgevb.jpg

is all this happening because i removed the 1ohm source->ground resistors?
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ssbothwell KJ6RSG
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« Reply #53 on: October 31, 2011, 07:20:27 PM »

i hope i didn't dump too many problems in this thread all at once. i'm guessing whats going on is related to me running the source directly to ground with no 1ohm resistors.

in LTSpice with a 1W input signal, 5V bias, and a 28V power supply there is 22W running through the source->ground resistors the amp produces between 40W and 80W output (depending on the frequency of the input signal).

14MHz: http://i.imgur.com/04GQE.png
7Mhz: http://i.imgur.com/nA5hq.png
3.5Mhz: http://i.imgur.com/f53PK.png
1.8Mhz: http://i.imgur.com/B3F5n.png

does that sound normal and does this mean i need to use massive power resistors from source->ground? will ceramic tube type resistors work?

i dont plan to run the bias that high but i dont want blown resistors to be one twist of the bias pot away.

also, is there any danger i may have damaged the IRF510s by running 13.8V/3A through them at full 5V bias with the source shunted directly to ground?


edit: in my simulation i made a typo no my frequency input. edited post to acccount for correct values.
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DMOD
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« Reply #54 on: November 01, 2011, 02:23:58 PM »

I would recommend putting a SPICE voltage node on one or both sources to see what the voltage across the 1 ohmer's might be.

Then you can calculate real circuit value properties for the 1 ohmers.  

Again, this is why I suggested you do a SPICE voltage sweep (in increments of 0.25 V) for the bias voltage from 1.5 to 5 V and then observe how bias affects output.  

Phil - AC0OB
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ssbothwell KJ6RSG
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« Reply #55 on: November 01, 2011, 09:33:15 PM »

phil i must have missed your earlier comment about setting a voltage sweep. i learned my lesson. one of the FETs was totally blown out. i replaced it with a new one and i put in 5W 1ohm (well actually 10% .87ohm) resistors on the sources.

the amp is behaving much better now but i am still not getting super clean waveforms. actually, above 12MHz or so i get a really nice looking waveform. anything below that and it starts to look distorted.

as an example, here is the waveform at 7MHz (6.3Vrms input into the amp): http://i.imgur.com/S6QUb.jpg
i have the bias adjusted to get maximum output (18Vrms). through bias adjustments alone i cant seem to get the waveform much cleaner then this.

again, my power supply is 13.8V/3A.

do you think i would get a cleaner output if i was able to drive the amp with a higher power input signal?

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WA1GFZ
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« Reply #56 on: November 02, 2011, 10:19:07 AM »

you should only need 4 to 5 volts on the gates. 18 volts you are in saturaation
Sounds like you are pushing the FETs pretty hard. You can go lower with the sourec reisitors but check simulation. 1/2 ohm seemed ok for me. Power rating is an issue I^2R.
Your bias should be set so there is 100 to 300 ma drain current without drive. If you go higher the signal will be cleaner at the expense of a lot of heat.
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ssbothwell KJ6RSG
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« Reply #57 on: November 02, 2011, 01:15:16 PM »

18Vrms is voltage across the dummyload. i'll check the drain current and setup the bias accordingly when i get a chance to run tests later today.

in LTSpice with a 12V power source i get a clean 10W sinewave output with ~4V bias.

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WA1GFZ
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« Reply #58 on: November 02, 2011, 02:02:46 PM »

I hope you are using thermal pads under the FETs not plastic washers. Mylar or Kaptan tape will work in a pinch. You need to transfer heat from the FET to the heat sink plastic won't work.
Source resistors provide negative feedback so help the waverform quality.
If you have a clean signal at 12 MHz and crappy at 7 your transformer inductance is too low.
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ssbothwell KJ6RSG
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« Reply #59 on: November 02, 2011, 07:40:31 PM »

the washers are on the top side of the fet. i'm using to-220 package fets. there is a mica insulator between the FETs and the heatsink.

which transformer are you talking about? the final output transformer (K3 in my screenshots)? do you think an additional turn on both primary and secondary windings would help?
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WA1GFZ
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« Reply #60 on: November 03, 2011, 09:06:16 AM »

The FETs should not run that hot.
Yes add a primary turn or two to get the amp to run at a lower frequency then add a secondary turn to increase output power.
You may need to do both input and output transformers.
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W4AMV
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« Reply #61 on: November 03, 2011, 12:05:15 PM »

The FETs should not run that hot.
Yes add a primary turn or two to get the amp to run at a lower frequency then add a secondary turn to increase output power.
You may need to do both input and output transformers.
I agree. I am using a different FET but idle current is 20-50 mA each device. Pin is 1 W and Pout at nearly 70 W after the LPF. Power gain about 18 dB. Supply V at 24-28 V at 3.5-4 A.
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ssbothwell KJ6RSG
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« Reply #62 on: November 05, 2011, 02:01:37 AM »

I added 2 turns to the primary and one to the secondary of the output transformer  (BN-43-7051) making it 4 windings on the primary and  4 windings on the secondary. this didn't seem to help. in fact now i am getting (with the same signal source and power supply) is 14VRMS output and i get what looks like a little crossover distortion at 12MHz now. i guess i should try adding windings to the input transformer as well? should i also try more windings on the output transformer?
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KD6VXI
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« Reply #63 on: November 05, 2011, 11:43:06 AM »


again, my power supply is 13.8V/3A.

          And also

in LTSpice with a 1W input signal, 5V bias, and a 28V power supply there is 22W running through the source->ground resistors the amp produces between 40W and 80W output (depending on the frequency of the input signal).

Is this perhaps a Davemade or XForce design?

80 watts of output from 50 watts of output seems a bit distorted to me Smiley

40 watts = 80 percent efficiency.  80 watts would be > 100 percent.

Methinks you might be getting into voltage dropping causing your dirty output.

Are you modeling this all with a 3A power supply too, or a pretty much unlimited voltage / current source for B+?  That may show you your distortion your getting is due more to b+ rather than any design type issue.

--Shane
KD6VXI
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ssbothwell KJ6RSG
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« Reply #64 on: November 05, 2011, 05:24:48 PM »

er, sorry for the confusion.

in LTSpice i was testing using a voltage source with no current limit. the 80W output example was with a 28V voltage source. i havent figured out how to setup a current limit, is there a simple way to do that in ltspice?

in real life i am using this power supply: http://amzn.to/uYjqk2

what is 'voltage dropping'?
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WA1GFZ
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« Reply #65 on: November 07, 2011, 09:09:41 AM »

when you set up the voltage source open window and select source impedance. input a value of your source Z. This will make the output voltage drop when current increases. You can't input a current limit but you can monitor current and just stay below maximum.
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ssbothwell KJ6RSG
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« Reply #66 on: November 09, 2011, 02:25:59 AM »

i had a busy past few days but i finally got a chance to get back to this project.

i made a new input transformer so now the input has 14 trifilar turns and the output transformer as 4 turns on primary and 4 turns on secondary. here is what the output looks like with the same ~3.5VRMS input: http://i.imgur.com/U9Sz6.jpg


also, without any input signal i see 3.4A being drawn from the power supply. when i turn on the input signal then the current meter drops to 2.8A.  i any idea why that is happening?


WA1GFZ, is the point of introducing voltage drop into the spice model to see if i can get the spice model to produce the same problems as my real life circuit?

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