For the past 10 years, more power supplies are being built that use high
frequency switching techniques (not to be confused with what hams call high
frequency!). We used to buy 6 KW 2.45 GHz band microwave generators for
dielectric heating, plasma sources, from a NY manufacturer that took a fork
lift to move, with conventional 3 phase 208 VAC line supplies. Then one
company built a unit that provided 6 kW that I could carry in my arms. The
8000 V 1A power supply for the maggie was all on a circuit board with a
bunch of ferrite! And the rectifiers were water cooled, the thing was
controlled over a fiber optic link for feedback regulation. I took spectrum
analyzer measurments of the magnetron RF output, and the normal 360,720 Hz
ripple sidebands were gone, to be replaced by weak 60-70 KHz sidebands.
This was my first experience with 'HF switching' power supplies for HV DC.
Many of the US HV power supply companies (Glassman, Universal Voltronics,
Spellman, EMI) have made capacitor chargers that are switched mode. These
are low current, high voltage switchers that are typically connected to big
capacitor banks for one bang shots or low rep rate pulsed loads. That is
not new technology. What is new is applying switching supplies to provide
high average power. Like in radio transmitters, and industrial RF
processes.The military has been using inverters for HV for years, to reduce
weight and volume for aircraft, for instance.
In the past several years, some high-end broadcast radio transmitter
manufacturers have built solid state high level modulators, for the
plate(s) of the final RF tube(s). In some of them, a number of nearly
identical HF power supply modules are connected in series, and turned on
and off by binary weighting, sort of a giant power A/D converter driven by
the analog information (audio). Similar modules are now being used to
provide high potential for IOT's, the inductive output tubes, for UHF TV
transmission. They operate at lower HV than klystrons (like 30 KV DC) and
need to be crowbar protected in case of a tube arc. The idea is to use a HF
switching power supply to provide the HT (high tension, as the UK calls
it). If a stack of modules are in series, then the output can be controlled
(regulated) by varying the output of one module, or turning on and off a
string of them in a binary weighted fashion. These are now being rated for
higher DC currents (Amperes) so that large capacitors for stored energy are
no longer needed. And crowbar circuits can be eliminated. We call these
low-stored energy power supplies. They now offer ramp up, fast shut off in
an arc (bandwidth of the controls at least audio rates), and lower noise
than previous switching technology. Instead of free running switchers with
flyback transformers, some are using quasi-resonant networks with H bridges
of Mosfets or IGBT's as drivers.
Off the shelf capacitor charging power supply manufacturers are now
offering 'stiff' CW rated switching power supplies for HV DC. Electronic
Measurements (EMI) ALE series is now up to 50 kV at 50,000 Watt output.
Spellman now has a 72 KW CW magnetron power supply. It weighs 460 lb, in a
single rack.
The old iron-clad mains power supplies are slowing being replaced in
numerous applications. It's not cheap,for high power yet. We priced a 18
KV, 2 Ampere DC supply recently, and to use a low stored energy
resonant topology, it would cost roughly 3-4 times what a conventional
mains operated SCR-regulator power supply cost -- went with convention, as
couldn't afford the technologically advanced approach. It's only a matter
of time before the non-recurring engineering charge for these things is
down, and they will be the baseline products, with mains supplies costing
more for the extra mass involved!
John
K5PRO
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