Bias setting 50W. PA #qcx #pa


Don, ND6T
 

Another lesson learned! It seems to be most prudent to set the power supply to the highest anticipated voltage when setting the bias adjustment.
I set mine at 13.8 volts when I first set mine but, while doing initial testing, I increased it. When it reached 20 volts both output transistors suddenly and quietly expired. After I replaced them I found that, indeed, the bias adjustment was supply voltage dependent.
Since I was using a current limited supply nothing else was affected. In fact, I spent a couple of hours searching for some other (non-existent) fault since there was no visual evidence of a problem. The output merely dropped from 45 watts to a couple of milliwatts in an instant. 
Great kit! Highly recommend it. I probably won't use the amplifier much but It was too nifty to pass up. Thanks Hans! 73, Don


Ronald Taylor
 

Don, I’m not sure how your issue actully occurred. The bias voltage is taken from a 5 volt regulator that puts out a constant 5 volts to the bias set pot whether your input voltage is 13.8 or 20 volts. So your bias should not change with input voltage. It must have been caused by something else…

Ron

On Mon, Jan 27, 2020 at 7:18 AM Don, ND6T via Groups.Io <nd6t_6=yahoo.com@groups.io> wrote:
Another lesson learned! It seems to be most prudent to set the power supply to the highest anticipated voltage when setting the bias adjustment.
I set mine at 13.8 volts when I first set mine but, while doing initial testing, I increased it. When it reached 20 volts both output transistors suddenly and quietly expired. After I replaced them I found that, indeed, the bias adjustment was supply voltage dependent.
Since I was using a current limited supply nothing else was affected. In fact, I spent a couple of hours searching for some other (non-existent) fault since there was no visual evidence of a problem. The output merely dropped from 45 watts to a couple of milliwatts in an instant. 
Great kit! Highly recommend it. I probably won't use the amplifier much but It was too nifty to pass up. Thanks Hans! 73, Don


George Korper
 

In my case I was greedy and did not take the warning in the manual seriously enough to turn the

Bias anti clockwise from the setting a tad. 

On 20 that tad costs output more than on 40 from what i'm reading. But my lesson was learning to take that warning very, very seriously. 
Again no one will hear the slight difference. Don, I'm happy you noticed this because this is where my troubles started and I melted down before I blew the finals. 
The fact is that even though the bias is at a constant voltage, it is very sensitive. My advice to myself is to spend longer in the practice mode,
as I don't have replacement transistors. They are on order for sure because I see all over the web to keep a few taped to the enclosure (hi hi). 


Don, ND6T
 

Ron, a reasonable person would think that the bias would remain the same over various supply voltages. That's why I didn't bother to test at other than  the 13.8 volts. Replacing those transistors reminded me to always check for my own edification. Yes, the bias supply is rock solid and I have always thought that the source to gate bias was a stable thing even if there was no drain voltage. I was so wrong. It doesn't change much, just enough over that wide range to cause grief. I should have set it at 20 volts. Next time I will check it.
I took my time building it, enjoying the experience, and it worked perfectly the first time. It's very pleasant to have a new kit behave that way. This wasn't a big deal for me as I had plenty of spares, thanks to Hans' choice of commonly available parts.

Try it the next time that you have the amplifier open and with a variable supply. Please let me know if I was wrong. If I wasn't incorrect then perhaps the next version of that spectacular instruction manual migh include it.
73, Don


ajparent1/KB1GMX
 

IF the bias is power supply dependent you have made an error as 
the bias supply is regulated using IC1 a 78L05.  Mine changes very
little from 14 to 20V.  O went and measured mine and its a small
change and not enough to cause failure.

there is a yabut...  Running the amp without a load on the input and
output may result in oscillation and failure.  This would be true if
any of the electronic switching was less than correct as well.

I have seen neither but any error in assembly can lead to the above.

Allison
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Don, ND6T
 

The gate voltage does not change. The conduction point seems to. That is what surprised me.
No, a QCX was attached and transmitting as I slowly advanced the supply voltage. Current was gradually rising. All was well until the current dropped and the power meter did likewise. Quite the surprise.
The new transistors demonstrate the change in conduction point. I had not checked the originals. By "conduction point" I mean the bias setting that just barely increases the static transmit current without input signal. Setting that back to the static current with, say, 10 volts supplied to the amp. Increase the supply 5 or 6 volts and you will see a sudden rise in current without changing the bias control. Yes, I set my bias at a 13.8 volt supply voltage but then increased the supply as an operational test. My error was that I was transmitting continually and not stopping to see if the static current changed while it was still keyed. If I had done that then I would have seen that the static current (around 82 ma.) had increased. Hindsight.

73,
Don


ajparent1/KB1GMX
 

An aside re: IRF510s and so called fragility.

I have four amps that are not QRPL design and all have many hours in
linear service from 10W to 55W and one using parallel device that runs a
very comfortable 220W.    To date and two of those amps are over a decade
old have not blown a device.    The 220W monster (running on 6M) is even
running the poor irf510s at 30V so its in the range of tickling the dragons tail
for explosive failure.  So far 3 contests and its performing well. 

I also have 5 QRPL amps 4 of the 10W over  a year old and running great
and now the 50W.  I see no bad behavior from them.  Should I I'll say what
and why.

Yet I see people that blast them apart at 5W all the time.  So due to poor design,
long leads and other faults some of those are legendary for being unstable
so no real surprise.  Others its a wonder on my part why or how.

Remember in most cases the IRF510 is wired in a circuit where if any reason
they go to full conduction they receive the full wrath of the power supply
(or battery).  Typically that is not milliamps its many amps if not 10s of amps.

Any who the QRPL amp is class C so the bias should be set so there is NO
drain current (less than 10ma per device).  It should not change any amount
(IE from 10 ma to 20ma from 10V to 20V).  

ALSO the devices are called MOSFETs and the gate is indeed static
sensitive.  I have killed a few in winter that way not even installed in
the project. it is possible to partially fail the gate and have it go the
rest of the way [sudden dead device] when running under power.
This also applies to BS170s, 2n7000 and other MOSFETs.

Amplifiers must always have a load, even if there is no RF input.
When testing an amp for the first time I usually put a big dummy load
at the output and a smaller on on the input as then the amp should be
stable.   I bring up the voltage and bias very carefully.   Maybe because
I treat them as if they are expensive and should not be abused.


Allison
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ajparent1/KB1GMX
 

All was well until the current dropped and the power meter did likewise. Quite the surprise.

Don,

How long was key down?  Reason I ask is what you thought you were seeing
was likely not what you thought.

IRF510 like most Hex, Tench, VMOS, and other LDmos FETs  exhibit a bias
point shift not with voltage, but temperature.   That is with a fixed bias and
fixed drain voltage increasing die temperature will see an increasing current,
which will heat the die further and increase the current, rinse later and
repeat to failure..

What that is the Gate threshold decreases with increasing temperature.
The other reason is that with high power out the device also heats, more
power more heat.  So what appears to be bias related and if set right
its very low should not cause that.  However is set high without understanding 
how that can head to failure.

See figure 7 of datasheet page supplies not gate threshold for 175C(very hot)
and 25C (room temp).  Tj means the temperature of the junction (die).

I've mentioned to people in the past (and likely forgotten) running the amp
with bias move s it from fairly efficient class C to less efficeint class AB or
AB1.  What you get for that is linearity (not required for CW) and HEAT. 
That heat is an enemy as the IRF510 has a thermal resistance from the
DIE to the flange that limits its ability to dissipate heat.   That leads to
a calculation where to keep the die under 150C you have to reduce
power for every degree C heating you have to reduce power by .29W
and we start at room temp of 25C.    The other ways to say that is for
every watt of power not put to the load is heating the device and it
increases temperature 2.5 degrees C for every watt.  So when you
run the bias up you get heat, if you key down for long periods you
get heat.  There is even with the generous heatsink there is a
finite limit.

For those the feel the IRF510 is insufficient for a inexpensive amp a
pair of MRF137, or MRFE101 would be far more robust at more than 
60$ for a pair.  Then again they need more supporting parts as well 
so a cheap amp is out of the question.

Allison
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J68HZ
 

You beat me to this and it is the salient point of most in service part failures:

 

I've mentioned to people in the past (and likely forgotten) running the amp
with bias move s it from fairly efficient class C to less efficeint class AB or
AB1.  What you get for that is linearity (not required for CW) and HEAT. 
That heat is an enemy as the IRF510 has a thermal resistance from the
DIE to the flange that limits its ability to dissipate heat.   That leads to
a calculation where to keep the die under 150C you have to reduce
power for every degree C heating you have to reduce power by .29W
and we start at room temp of 25C.    The other ways to say that is for
every watt of power not put to the load is heating the device and it
increases temperature 2.5 degrees C for every watt.  So when you
run the bias up you get heat, if you key down for long periods you
get heat.  There is even with the generous heatsink there is a
finite limit.

Let’s remember that IRF510’s were designed to run in switching power supplies and as motor current switches with a duty cycle of 50% or less at  their listed full ratings.  The thermo-conductivity of the die with adequate heat transfer is what limits this part, and operating it at greater than 50% duty…. in a higher class… say AB2 or greater will cause more heat than design and possible part failure.  While this is mostly a design concern, one can surely get into trouble by cranking the on bias without understanding all of the limiting parameters involved.

 

 

Dr. William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ PJ2/K9HZ

 

Owner - Operator

Big Signal Ranch – K9ZC

Staunton, Illinois

 

Owner – Operator

Villa Grand Piton – J68HZ

Soufriere, St. Lucia W.I.

Rent it: www.VillaGrandPiton.com

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Moderator – North American QRO Group at Groups.IO.

 

email:  bill@...

 

 

From: QRPLabs@groups.io [mailto:QRPLabs@groups.io] On Behalf Of ajparent1/KB1GMX
Sent: Monday, January 27, 2020 1:35 PM
To: QRPLabs@groups.io
Subject: Re: [QRPLabs] Bias setting 50W. PA #qcx #pa

 

All was well until the current dropped and the power meter did likewise. Quite the surprise.

Don,

How long was key down?  Reason I ask is what you thought you were seeing
was likely not what you thought.

IRF510 like most Hex, Tench, VMOS, and other LDmos FETs  exhibit a bias
point shift not with voltage, but temperature.   That is with a fixed bias and
fixed drain voltage increasing die temperature will see an increasing current,
which will heat the die further and increase the current, rinse later and
repeat to failure..

What that is the Gate threshold decreases with increasing temperature.
The other reason is that with high power out the device also heats, more
power more heat.  So what appears to be bias related and if set right
its very low should not cause that.  However is set high without understanding 
how that can head to failure.

See figure 7 of datasheet page supplies not gate threshold for 175C(very hot)
and 25C (room temp).  Tj means the temperature of the junction (die).

I've mentioned to people in the past (and likely forgotten) running the amp
with bias move s it from fairly efficient class C to less efficeint class AB or
AB1.  What you get for that is linearity (not required for CW) and HEAT. 
That heat is an enemy as the IRF510 has a thermal resistance from the
DIE to the flange that limits its ability to dissipate heat.   That leads to
a calculation where to keep the die under 150C you have to reduce
power for every degree C heating you have to reduce power by .29W
and we start at room temp of 25C.    The other ways to say that is for
every watt of power not put to the load is heating the device and it
increases temperature 2.5 degrees C for every watt.  So when you
run the bias up you get heat, if you key down for long periods you
get heat.  There is even with the generous heatsink there is a
finite limit.

For those the feel the IRF510 is insufficient for a inexpensive amp a
pair of MRF137, or MRFE101 would be far more robust at more than 
60$ for a pair.  Then again they need more supporting parts as well 
so a cheap amp is out of the question.

Allison
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Don, ND6T
 

Wasn't heat. It was only keyed about 15 seconds. Immediately upon failure I felt the heat sink and it was still stone cold (the lab was unheated and 10 C.).
I originally thought that there had to be some other cause. Turned out to be the MOSFETs.
Yep!  Dummy load was a 250W thick film mounted on an enormous heat sink and better than 35 dB return loss. Input  during bias setting was BNC  mount with 32 dB RL. Power supply was a well filtered and regulated analog Harrison Labs. A separate 4.5 volt supply applied PTT signal.

Looks like a little bit of DIBL to me. Not something to be expected on a big 'ol power MOSFET. But sure feels like.

The amplifier works perfectly, just exactly like it is supposed to. Up to the point of failure (which is my fault entirely) the original transistors did, too. My point in posting was that it might be prudent to perform the bias setting at the highest value of supply voltage anticipated. Can't hurt.

I would not assume that there are many amplifiers built where such large excursions of supply voltage are encountered when considering bias settings. Most of those are in linear service and not as subject to variations in threshold. This is actually the first time that I have ever blown an IRF510, come to think about it. I've used them in a lot of rigs. An incredible device and dirt cheap. Then again, this is the first time that I've pushed them this hard. My bad.


ajparent1/KB1GMX
 

Don,

It was heat or the device was bad initially from carpet lightning.

Keep in mind you can get a IRF510 hot enough to die without
even warming the heatsink  Simple way is make it dissipate 50W 
(heat it hard and fast its only a 43 W max device) or exceed the
max pulsed drain current of 20 A (blow the bond wires off it).
Usually in those cases you hear a pop and the case ruptures.
Reason the thermal resistance is high enough you can heat it
hard and fast before the heat transfers.

I would verify that nothing else is not quite right.  For example a
intermittent bias pot (or soldering) can easily bounce the bias
to 5V and the device will try to draw maybe 10A (and melt).

I don't doubt they failed but the cause is not conclusive and
heat is still the leading killer. 

The bias setting is per manual just below the threshold. I used a
Tripplite 630 set for 120ma to see that small increase and then
back off from it.

Actually doing the bias setting at 20V is procedure in the manual.

The transistor is not a big old power mosfet at 43W dissipation or
5.6A max current (at 25C).   If you want big ole, check IRF520
at 14A, but the gate capacitance at 670pf makes it tougher to
drive.

Also I've run my 'EBY amp at 26V for years (since '06) and it
can do a mere 55W at 20M with 2W drive. Its heatsink is
not the limiting factor (its larger) its the max current and its
power supply has a current limiter to 4A (that is 26V at 4A
or 104W DC input) and pushed to that limit its 75W
without blowing up.  Gets mighty hot if sustained.   During
testing 90W out (at 30V and 5A) was the smoke test, 
devices got to max die temp well before the heatsink was
warm.   Not bad for two IRF510s in my book.

Testing on my bench a relay driver using a IRF510, shorted coil
meant the moment the gate was driven the mosfet expired
instantly the tab never got warm. Peak current about 14A
at 28V.   Why?  The only limiting fact was the power supply
and leads and the on resistance of the IRF510 is about .6 ohms.
So the bond wires to the die evaporated.  The relay was defective!

FYI I have a MSC 100W 2M repeater amp that has an unregulated
DC supply nominal voltage for the MRF174 is 28V.  However the
Transformer-rectifier-capacitor idles (key up, bias off) at 35V
and at full power the DC droops to 27V.   Not a great design
and the PS is being replaced with a 12v to 28V switcher as it
had too much hum (rough audio) for SSB.

The average nominal 12V radio in the car will see as low as 11.7V
(most stop working right there) and as high as 16V  non -trivial swing.
The spec is + or - 15% centers at 13.8V or a 30% swing in operating
voltage.  

Allison 
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ajparent1/KB1GMX
 

The bit of data sheet figure 4 is the define the limitations of the IRF510
safe operating area.  Safe in this case means no failure if on a infinite
heatsink.

Its current vs voltage and the result is how much heat it can stand.

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