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Author Topic: RD16HHF1 With High Drain Voltage  (Read 15788 times)
TANAKASAN
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Posts: 933




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« on: December 22, 2012, 06:57:01 AM »

Does anyone have experience of using the RD16HHF1 devices with a Vds greater than 13.8 volts? All of the designs I can find assume a 13.8 volt supply but I want to use something a bit higher in the quest for better IMD. Note that the Vds limit in the datasheet is 50v.

Tanakasan
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M0HCN
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Posts: 473




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« Reply #1 on: December 22, 2012, 07:51:43 AM »

Never tried it but note that the miller capacitance will become an increasing pain in the arse at higher drain voltages, still 20V might be Ok if the reduction in SWR tolerance is an acceptable trade off, not sure I would want to go higher than that.

I have wondered about the possibility of isolating the tab and doing source degeneration on those parts, maybe a handful of 1 ohm resistors from the tab to the ground plane? Gain will come down, gate impedance will go up linearity will improve, question is will it be stable? 

Regards, Dan.
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KB1GMX
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Posts: 772




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« Reply #2 on: December 22, 2012, 08:41:06 AM »


>>miller capacitance will become an increasing pain in the arse at higher drain voltages

Not quite true as the Drain to Gate capacitance decreases with voltage.

This is partly why the IRF510 performs way better at Vd of 24-28V ( for a 100V device this
is about right).

Also if the input source impedance is sufficiently low the miller capacitance is not generally
as much an issue.


The biggest issue using any device close to its breakdown voltage is you have reduced SWR
(load variation) immunity.  So running at 30V your tolerence for any reflected power or
loads with high reactance or high resistance will allw the drain voltage to swing higher and
then the device fails due to over voltage on the Drain or Gate.

Its why at nominal 12V (13.8 to 16max) why it is rated for 50V.

If you want to run at higher voltages there are scads of devices for 28, 36 and 50V nominal.
The side effect is they perform at much reduced performance at 12V.

Allison
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M0HCN
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Posts: 473




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« Reply #3 on: December 22, 2012, 09:28:47 AM »

I thought Cdg usually dropped over the first part of the operating region then became more or less stable as Vds increased?

73 Dan.
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KB1GMX
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Posts: 772




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« Reply #4 on: December 22, 2012, 11:25:45 AM »

yes, its' an exponential drop. Usually the first 10V part of the curve has the most change.

Either way if the driving impedance is right operating voltage has no substantial effect.

Most power FETs I've used including the MRF1xx others the Cdg is a number us pay attention
to but Cgs and Cds are far more significant and have to be accommodated in the input and
output circuits.  Far to many ham designs and some commercial leave that to serendipity
or "that's what the other guy did".

The real issue here is running outside ratings for voltage is serious as the device margins
are compromised and the risk of device failure is significantly increased.

There are a few ways to cook a device one is over dissipation (higher operating current)
due to operating into a small than design load the other is over-voltage due to insufficient
load or parasitics from spikes, oscillation or excessive reflected power.   The first is easy
to monitor and avoid. The second and third stresses the voltage breakdown of the device.
The easy way to avoid the later two is adequate breakdown voltage margin and of course
trying to avoid those operating conditions.  The last being often troublesome as you may
have to run into bad load inadvertently (cable or antenna failure) or deliberately during use
of an antenna tuner. 


Allison


 
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G3RZP
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Posts: 4569




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« Reply #5 on: December 22, 2012, 01:11:38 PM »

It's fair to say that  a 'max Vds' is the max that's guaranteed.With batch variations, you  might get one that is safe at 1.5 or even 2 times, or you might get one only safe at 1.1 times.

With receiver IF/detector ICs, we got a batch spread of over 6 dB in sensitivity for a given SINAD. Some customers didn't design to data sheet figures but to the performance of samples......One of them (who had fired me because it was cheaper than laying me off!) had taken no notice of the data sheet and so needed devices from a particular production batch, which needed bringing back from agents all over the world and re-testing.. Marketing made sure they paid ( and paid HEAVILY! - plus an extra for having fired me!) for that...

Sometimes marketing aren't such bad guys.......
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M0HCN
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Posts: 473




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« Reply #6 on: December 22, 2012, 01:22:42 PM »

Ouch!
Karma is a bitch sometimes!

I am guessing someone now owned by Thales group?

Regards, Dan.
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G3RZP
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Posts: 4569




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« Reply #7 on: December 22, 2012, 01:29:46 PM »

The guys who fired me were Burndept. They said everyone laid off would get three months pay. They then told me that they had a four month probationary period and I'd only been there three months....so I got one month's pay.

When the successors to Plessey (who had employed me after Burndept) made me redundant (after 32 years), I got over a year's  pay........at age 64.
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TANAKASAN
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Posts: 933




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« Reply #8 on: December 23, 2012, 01:49:23 AM »

Thanks for all the replies. The idea I had was to use a pair of the devices as drivers to a power amplifier, the P.A. operates at 48V so I was wondering if the RD16HHF1 could be pushed to operate at this voltage as well. My first two experiments failed but I will keep on for now as I have four devices left.

Tanakasan
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M0HCN
Member

Posts: 473




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« Reply #9 on: December 23, 2012, 03:00:49 AM »

No chance mate!

In a push pull stage the drain voltage peaks at twice the supply, due to transformer action in the DC injection bifilar.
I don't see the RD16 surviving 100V somehow.

Regards, Dan.
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