I have followed recent threads that go back and forth about cooling
transformers and use of switch mode technology for HV. I have two practical
examples that I will share that demonstrate that these things can go either
way, there is no hard rule these days, not to mention individual preference and
acceptable design risk. I am not suggesting that I did the right things, but
after lengthy testing and use, the decisions were the right ones. I know that
we hams can be quite stubborn to try new things, and often good old heavy iron
technology is just fine. These are just examples, nothing to force anyone to be
uncomfortable with a choice.
First, I needed several tetrode filament power supplies for 100 kW at 2.8 MHz.
Tubes had 15 Volt AC center tapped filament transformers, with 320 Amps RMS
load. Since they had big filaments, they could not be switched on quickly or
even in step start fashion, instead needing 8 minutes of linear ramped voltage
on turn on and off. Conventional designs would use a big variac with a stepper
motor drive. I disliked anything with moving wiping contacts that must last for
years, as it needs repeated maintenance. I could have found a solid state AC
oscillator to generate a sinewave? I chose a saturable reactor design and
enlisted the late Carl Seivers of SNC in Oshkosh. He told me back in the 1980s
that he made many of them, although there are a lot few companies now making
them. Saturable reactors have a DC flux on one winding (or 2) and AC passing
through the other. They run hot, by design. Insulation is rated for it, so
there is no reason for concern. The SR is essentially in seri
es with
the 480 VAC power line to the filament transformer. I was mounting this in a
large chassis, along a small linear ramping supply for DC bias, a logic card
and some relays and breakers. I HAD to have a fan to remove some of the heat
buildup, that would otherwise have cooked my circuits nearby. The fan I chose
was a very reliable Pabst, and it has an additional thermal cutout mounted on
the SR that will detect a dead fan and prevent power until it is fixed. At the
time I was nervous about having to add a fan to a unit to help cool the effects
of big iron, and then the clap trap logic to protect in case the fan died. Its
been in service in two power amplifiers since 1998 now, without any problems or
failures besides replacing the mechanical running time meters when they give
up. Had it used variacs, I wouldn't be saying the same thing.
In more recent work, I had a filament power supply that needed 20 VDC at 1000
Amps, for a bigger tetrode at VHF. Note i said DC, not AC here. This would have
been 3 phase primary, and needs rectification. The SR got too complicated,
followed with a big rectifier and filter. Carl was no longer around at SNC. I
talked to several companies who make big iron, and there was a lot of risk due
to their experience (lack) plus the cost of the copper and iron. Everyone
wanted to make an SCR controlled power supply. We chose one, ran it for a year,
and it was a POS. I won't name the company, but reliability was low, not so
much the power conversion but the controls were very susceptable to noise, RFI,
transients when the HV capacitor vault crowbarred, etc. I spent a lot of time
cussing it when it would trip off with no warning, and slam off my expensive
tube's filament. The answer was staring in my face, but I didn't want to use it
due to the old fears about switchers. True that in the
1970s
and 80s, they were just moving towards MOSFETs and later IGBTs to improve over
old BJT circuits. I did select a nice switcher (not so easy at this power,
needed to parallel two commercial units) and it ramps up beautifully, never
faults *yet* and has run for half a year. And it is really efficient.
I use switcher supplies for screen, control grid and filament, now, and am very
satisfied with COTs designs.
I would still use iron if I could, in places with simplicity prevails, more
immune to RFI and transients in a multi-megawatt RF system, but cost, size,
cooling are important factors to realize for new designs too. I took the risk
of using switchers there, and was pleasantly surprised with how well it worked
out.
73
John
K5PRO
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