Power Regulator Circuit Board - Crown FM600 User Manual

4.9 Power Regulator Circuit Board

The power regulator boards are the shorter of three boards mounted under the chassis
toward the front of the unit. The board has the isolating diode for the battery input, the
switch-mode voltage regulator for the RF power amplifier, and circuitry for PA supply current
metering.
Illustration 6–10 and accompanying schematic complement this discussion.
Diode D4, in series with the battery input, together with the AC-supply diode bridge, pro-
vides diode OR-ing of the AC and DC supplies.
U1 and U2 form a switching regulator running at about 35 kHz. U1 is used as a pulse-width
modulator; U2 is a high-side driver for MOSFET switch Q1. Power for the two IC's comes
from the 20–volt supply voltage for the RF driver (available when the Carrier switch is on).
The voltage is controlled at 16 volts by zener diode DZ1. Bootstrap voltage provided by D2
and C9 allows the gate voltage of Q1 to swing about 16 volts above the source when Q1 is
turned on. Current through the FET is sensed by R12A and R12B. If the voltage from pin 5
to 6 of U2 exceeds 0.23 volts on a current fault, drive to Q1 is turned off. This happens on a
cycle-by-cycle basis. The speed of the turnoff is set by C5.
U3 and Q2 are used in a circuit to convert the current that flows through metering shunt,
R19, into a current source at the collector of Q3. Forty milli-volts is developed across R19
for each amp of supply current (.04 ohms x 1 amp). Q3 is biased by U3 to produce the
same voltage across R16. The collector current of Q3 is the same (minus base current) as
that flowing through R22 resulting in 40 microamperes per amp of shunt current. R5 on the
metering board converts Q3 collector current to 0.1 volt per amp of shunt current (.04 ma X
2.49 k). (See section 5.4.)
4-12 FM600 User's Manual