Pfc Boost Stage; Bridge Isolation Stage - Crown CE-4000 Service Manual

CE 4000 Service Manual
overcurrent condition. The circuit of U24-A and U24-B
will latch if either of these conditions is detected.
U11-B will hold the node low if U11-A detects a
low line voltage condition (about 60VAC) such as
a brownout.

3.3.5 PFC Boost Stage

After the relay closes and the soft-start pins are
opened, the PFC controller, U2, starts to run. The
GDRV pin (20) on U2 controls gate drivers U9 and
U26 which drive the gates of Q1 and Q2 in a boost
mode confi guration. Several inputs to the controller
affect its operation. The controller uses feedback from
the "V BULK" node, compensated by R24, C314,
and C14 and fed into U2-13 and U2-14 to maintain
approximately 400VDC at this node. The feedback
loop is quite slow, in the "10's of Hz" range, unlike
that of a power amplifi er, and so depends on the
large energy reserve capacitors to supply impulsive
power but overall, the voltage is regulated to 400VDC
regardless of the line voltage or the load current. Note
that 400VDC is higher than the highest expected
peak line voltage so the PFC stage is always boosting
to a known voltage. The PFC controller also looks at
the incoming "RECT AC+" voltage and makes sure
that the current the boost stage draws is in phase
and shaped like the voltage. This makes the amplifi er
look like a resistor to the AC mains, and allows it to
pass certain regulatory requirements. Pin U2-3 looks
at the incoming line voltage to make sure it is within
safe limits and if not, shuts down the FETs. R36 sets
the current limit of the PFC stage to approximately
47Amps peak. Using all of these inputs, U2 causes
the boost stage to take small amounts of current from
the line 62,500 times each second.
Theory of Operation 3-6

3.3.6 Bridge Isolation Stage

The 400VDC is then "bucked" down to the ground
referenced rails (+Vcc and –Vcc) needed by the
audio amplifi er. Controller U4, through gate drivers
U17, U31, U29, and U32 and transformers T2 and
T3, turns Q4 and Q7 on at the same time, then turns
Q4 and Q7 off and Q5 and Q6 on at the same time,
causing T1 to convert voltage and current to its
secondary. This process is repeated so that any two
of the four FETs are on about 85% of the time. U4
runs at 125kHz (the 250kHz sync signal is divided
down within the IC). Diodes D3-D6 and capacitors
C7-C8 rectify the secondary AC to DC. Transformer
T1 provides isolation between primary (mains) and
secondary (ground referenced) power for safety.
Controller U4 is called a "Phase-Shifted Bridge
Controller" because as its soft-start pin is released
and U4-19 climbs in voltage (U4-19 is a current
source), the controller slowly allows the power
transfer to ramp up by phase-shifting the power FETs
properly. This stage is referred to as a "buck" stage
(as mentioned previously), an "isolation" stage for its
transformer action, and a "bridge" stage since the
four FETs form a full bridge topology.
The AC primary current travelling through the
transformer is also passed through a small current
transformer in order to sense the current. This
AC signal is passed through diodes D9-D10 and
D12-D13 and cap C26 to convert it to DC. If the
bridge current is high and remains high for several
seconds (such as when the amplifi er is producing
high voltage, high current sine waves), the average
power limiter will tell the audio amplifi er to invoke the
compressor and slowly reduce the power output. The
DC signal from C26 is also compared to a reference
in the Over Current Detect Circuit and will latch both
U2 and U4 off if currents above 50-55 Amps are
detected in the bridge primary. The power to the
amplifi er must be cycled to reset this latch.
130485-1 Rev. B
©2002 Crown Audio, Inc.