Yes, that's right Don.
With a 9:1 xfmr the device load-line would be such that one will never
achieve the potential power output capability of 4 x VRF2933 devices...it
will flatten / max out at fewer than 1200 Watts with a 48 Volt Vdd supply.
To achieve 1500 to 1800 Watts of output from say 6 of these VRF2933
parallel/push-pull devices one instead needs a more complex to construct
16:1 TLT output xfmr. That is readily doable.
However, the simplification and favoured practicality of the 9:1 output TLT
is one of the reasons why folk tend to deploy multiple solid-state amp
modules at the 1 kW level and combine them to achieve QRO output power.
This is the approach I've taken; two combined 4 x VRF2933 amp modules with a
total saturated output power circa 2400 Watts; and operates very cleanly and
effortlessly at the backed-off circa 1 to 1.2 kW PEP output power level.
Rather than a single big device, I like and prefer the idea of multiple
parallel/push-pull RF power devices to share the load and more evenly
distributed the massive heat into the copper spreader and heatsink system.
Devices tend to prematurely fail when their thermal aspects are not properly
attended to, rather than a failure attributed to load / VSWR mismatch.
Leigh
VK5KLT
-----Original Message-----
From: dsolberg@wi.rr.com [mailto:dsolberg@wi.rr.com]
Sent: Thursday, 12 September 2013 2:11 AM
To: 'Larry'; Leigh Turner; amps@contesting.com
Subject: Re: [Amps] Future of RF AMPS
While the MOSFETs can output 1200 watts with four in parallel/push-pull, the
output transformer ratio will limit the practical output to between 800 and
1KW, using a 9:1 with a 50V power supply. The 9:1 optimal output is at 800
watts. Tokyo HyPower uses this design and they rate it for 1KW. I can also
get 1KW with my homebrew amp with (4) SD2933's, but prefer to run it closer
to 800 watts to improve the IMD.
If you really want the full legal limit, you would be better to have two PA
modules and a combiner. Otherwise the output transformer design is going to
get real complicated. The other option is running higher voltage, which
complicates the selection of a power supply. 50 volt computer server
switching power supplies are low cost and easy to obtain.
I am happy with 800-1KW, but maybe someday I will try the combiner just for
the fun of it.
Don
K9AQ
---- Leigh Turner <invertech@frontierisp.net.au> wrote:
>
> Larry, the RF power MOSFETS to which I refer here are the VRF2933 devices
> from Microsemi. Sometimes one can find surplus and NOS ST Micro SD2933
> FETS on eBay at low prices.
>
> 4 of these rugged 300 Watt rated devices in parallel/push-pull will yield
> a saturated output power of well over 1200 Watts. Six will comfortably
> yield over the US legal limit.
>
> 73
>
> Leigh
> VK5KLT
>
> -----Original Message-----
> From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Larry
> Sent: Wednesday, 11 September 2013 10:48 AM
> To: amps@contesting.com
> Subject: Re: [Amps] Future of RF AMPS
>
> On 10-Sep-13 5:51 PM, Leigh Turner wrote: using a readily available
> multiplicity (6 devices in parallel/push-pull) of cheap 50V MOSFETS
> sourced from the surplus market.
>
> Leigh,
>
> Exactly which cheap MOSFETS from what surplus market are you speaking of?
>
> Larry - W7IUV
>
> _______________________________________________
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> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
>
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