3-22
Similarly, the output of A3Q1 is denoted
+28RLY; this source is regulated to +15V dc
(VRLY) by A4VR3 for use by the output selec-
tion relays. The +28U supply is also used on the
A4 Power PWB as a bulk source for the +12V
regulator VR1, which supplies the gate driver U3
and the Isolated Power supply U2, Q10.
Half of bridge A6BR2 also serves as a center-
tapped full wave rectifier to source the +14U
supply, which is filtered by A4C15. The +14U
supply is delivered via A6W6 to the A3 PWB, for
the +5V regulator A8VR1, and to the A2
Display PWB to source all of the LEDs. This sup-
ply is also used on the A4 PWB by the +5V reg-
ulator VR2, which supplies reg5Vdc to
the A2 PWB via A6W6.
3.7.3 RF Power Supply
Refer to Figure 5.7a for schematic details. This
circuitry comprises a 100 KHz boost mode
switching regulator to generate reg72V
dc (RFSUP) for use by the RF Power Amplifier.
When the unit is not activated, this supply is shut
down, and RFSUP drops to about 50V dc unreg-
ulated.
Power FET Q1, inductor L1, rectifier CR1, and
capacitors C1 and C2 serve as the 100 KHz
power switching components which boost the
unfiltered, rectified 40 V input to +72V dc.
When Q1 is turned off, as during deactivation,
RFSUP becomes an unreg50V dc
source, passing through CR1 and filtered by C2.
Unlike offline switching power supplies, this cir-
cuitry is isolated from the mains by A6T1 and
thus can be worked upon without use of an exter-
nal isolation transformer.
RFSUP feeds Q3, which by use of 18V zener
D1, provides reg17V dc to the RF
Supply controller, U1. Overvoltage protection is
provided by Q4, which will kill the +17 supply if
RFSUP ever rises above about +95 V. LED CR2
serves as a rough indicator of the voltage on
RFSUP, and glows noticeably brighter during
activation when Q1 is switching and RFSUP is
up to +72V. It also serves to indicate when C2
has bled off below about 20V after the mains
switch is turned off.
IC U1 is a special type of switchmode control IC
known as a Power Factor Controller (PFC). This
device operates to force the low frequency current
draw from the 40V source to retain the same
sinusoidal waveshape as the +40U input source
voltage. Switching and regulation continues over
the entire mains frequency cycle, so the current
drawn from the 40V secondary of A6T1 appears
the same as if were supplying a simple resistance.
Thus the AC input power factor is unity, and the
AC power drain is equal to the DC output power
drawn from RFSUP, plus a few watts of switch-
ing loss. Ordinary capacitively filtered DC sup-
plies draw 180% or more AC volt-amps from
their source as a function of DC power output.
The PFC control used by this ESU requires a
much smaller and lighter power transformer than
would otherwise be required, while retaining
excellent mains isolation, conducted noise sup-
pression, surge tolerance and power factor.
U1 operates much like an ordinary current mode
device in that it turns Q1 on at the beginning of
every 100 KHz clock cycle and turns it off when
the instantaneous current through L1 and Q1
rises to a predetermined level. That current is
measured as the negative voltage VRS across the
0.05 ohm equivalent resistance of R1 and R2.
The turnoff threshold for VRS is determined on
each switching cycle by the DC output voltage
RFSUP, the RMS value of the AC input voltage
and the instantaneous value of the AC input volt-
age. A sample of RFSUP is divided down by R8
and R9 to +7.5V at pin 11 VSN. An internal
error amplifier adds in loop compensation via
feedback network R10, C18 from its output at
pin 7, VAO. The RMS value of +40U, moni-
tored on pin 8 VRMS, is found by lowpass filter-
ing and dividing down by R15, 16 and C22.
Finally, the low frequency value of the input cur-
rent is monitored by pin 6 IAC via R17,18. The
instantaneous turnoff value of VRS is derived as:
VRS(off) = K x [(7.5V - VSN) x IAC] /VRMS2
where K is determined by other circuit constants.
RFSUP does contain mains-frequency ripple.
This is because the power delivered by the switch-
ing section is a maximum at the peak of the AC
input voltage, where its input current is also at its
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