QSC K.2 Series Service Manual page 13

Power supply
The amplifier module's switch mode power supply is a regulated, fixed frequency, active clamp flyback. It is similar to a typ-
ical flyback converter except that when the main switch is off another switch turns on and recovers the transformer energy
into a resonant tank. A typical flyback power supply has good cross-regulation and can operate over a wide input voltage
range, but the power dissipation of a passive clamp limits the power level a flyback can reasonably output; adding the active
clamp allows a flyback converter to scale to much higher power levels. The power supply accepts universal AC voltages, the
incoming mains voltage is bridge rectified to DC and stored in a bank of 2 primary capacitors in series. This rectified voltage
varies directly with mains voltage.
The main and clamp FETs, operating at approximately188 kHz, couple energy to the secondary diodes through the trans-
former. The regulated ±77V secondary voltage is stored in a pair of secondary capacitors, one per rail. Regulation is taken
across the entire +144V secondary voltage, relying on proper circuit operation to keep the rails balanced. Adjusting the duty
cycle of the PWM signal driving the power supply FETs keeps the power supply's output voltage constant regardless of
changes to input voltage or load.
C132 C55
PRI_HI
0.022UF 0.022UF
BR1 10 PCNT 10 PCNT
500V 500V
KBU1005
- +
C130 C31
C112
1000UF
0.022UF 0.022UF
20 PCNT
10 PCNT 10 PCNT
200V
500V 500V
PRI_MID
C113
1000UF
20 PCNT
200V
Figure 4.3.3 - Active flyback converter circuit used in the K.2 series SMPS
Auxiliary voltages of ±15V, ± 5V, and +12VLOW are created with separate transformer windings from the main secondary
rails, but all windings are tied together at ground. Under load, the amplifiers will pull energy from the main rails and the
power supply will increase the duty cycle to compensate and keep the main rails at ±77V; this means the housekeeping
voltages increase under load. The auxiliary voltages are used to power numerous circuits across the amplifier module, like
the op-amps, DSP, microcontroller, and amplifier drivers.
Figure 4.3.4 - SMPS circuit, PCB
TD-001517-01
K.2 Series Service Manual
321MW EACH
R144 R152
D51
10K 100K
5 PCNT 1 PCNT
RB751S40T1
1W 1/8W
R67
R145
AUX_OUT
R172
10K
5 PCNT 100K
1 PCNT
1W 1 PCNT
1/10W
1/8W
R146
10K
C114
5 PCNT
1W 0.022UF
10 PCNT
500V
R147
10K
R188
5 PCNT
1W 100K
1 PCNT
R105
1/8W
10K
5 PCNT
R189
1W
100K
1 PCNT
R129
1/8W
10K
5 PCNT
1W
PRI_LO
VIN LINE_UV
CURRENT SENSE
56K FOR PASSIVE
T1
(RC) CLAMP MODE
R123
56.2K
1 PCNT
1W
C137 D40
C128
RB751S40T1
1UF
1 4
10 PCNT
25V
22.6
1UF
T3
10 PCNT
D35
T3 R164
25V
RB751S40T1
2.00K
1 PCNT
3 6
1/10W
D27
R121
1N4148
1 PCNT
R157
QM_DR
10
1 PCNT
1/16W
R68
243
6
1 PCNT
1/10W
C49
SEC 2
120PF
5 PCNT
200V
7
C50
120PF
5 PCNT
SEC 3 T2 200V
R69
243
8
1 PCNT
1/10W
Figure 4.3.5 - Auxiliary voltages on the secondary
EE42 3C90
22AWG AUX
100X40AWG LITZ PRI
100X40AWG LITZ SEC
C116
3 9
0.68UF
5 PCNT
250V
PRI 2 SEC 1
XF-MID 2 10
Q13
STP11NM60FD
PRI 1 SEC 5
T2
T2
Q18
1 11
MMBTA56
DNI
R52
10.0K
1 PCNT
1/10W
PSU-VSW
10
Q14
1/16W
STP45N65M5
C124
R150
10.0K
R165
0.0022UF
1 PCNT
20 PCNT
1/8W 250VAC
100
1 PCNT
1W
C52
R166
100
220PF
10 PCNT 1 PCNT
250VAC 1W
+15V CURRENT:
MC33078 = 5.5MA
TP1
L78M05 = 6.8MA
+15V
+5VA CURRENT = 95.15MA
F2
D31 TOTAL = 107.45MA D62
ES1D MBR230
5A
24V
C91 C72 C94
1000UF 0.1UF 1000UF
20 PCNT 10 PCNT 20 PCNT C15
25V 25V 25V
0.1UF
10 PCNT
25V
C4
DNI
C82
A1 0.1UF
10 PCNT
C92 C74 22UF
10 PCNT 25V
1000UF 0.1UF
R73 DNI
25V
20 PCNT 10 PCNT C83
25V 25V
22UF
0
10 PCNT
DNI
NA
F1 25V
D32
NA D12
ES1D -15V CURRENT: MBR230 -5VA CURRENT:
5A
24V MC33078 = 5.5MA 2X IRS2092 = 11MA * 2 = 22MA
-15V
TP2
L79L05 = 6.25MA TOTAL = 22MA (187MW)
-5VA CURRENT = 22MA NOMINAL -15V RAIL IS AT -13.5V
TOTAL = 33.75MA WITH ZERO COPPER
JUNCTION AT 25C=33
+15V
A1
R56 R156 Q16
2.00K 2.00K IRLML2502
1 PCNT 1 PCNT
C95
1/10W 1/10W D29
SMD110PL
1.0UF
R71 R70
10PCT
2.00K 2.00K
100.0VDC
1 PCNT 1 PCNT
D36 1/10W 1/10W C125
D20 D37
SMD110PL SMD110PL SMD110PL
1.0UF
10PCT
100.0VDC
AUX OUTS
DNI DNI
C126 C121
100.0PF 100.0PF
5 PCNT 5 PCNT
500.0 VDC 500.0 VDC
DNI DNI
R161 R7
100 100
1 PCNT 1 PCNT
1W 1W
R162 R45
330 330
1 PCNT 1 PCNT
2W 2W
C127 C122
100.0PF 100.0PF
5 PCNT 5 PCNT
500.0 VDC 500.0 VDC
+3.3VD CURRENT = 10.21MA + 270UA = 10.48MA
VAC MONITOR = 657UA
TOTAL = 95.15MA (809MW)
NOMINAL +15V RAIL IS AT +13.5V
ON 10MM SQ PAD, THETAJA = ~55C/W
IN 50C AMBIENT, JUNCTION IS 95C
+5VA
TP11
D61
1N4148
U7
L78M05
1 3
IN OUT
GND
C19
10UF
20 PCNT
6.3V
C6
LM79L05AC
10UF
20 PCNT
GND
7
VIN4
6.3V
6
D15
VIN3
3 1
VIN2 VOUT SMD110PL
2
VIN1
D1 TP12
U4
1N4148
-5VA
THETAJA = 180C/W
+12VLOW CURRENT:
HF + LF = 89MA (MEASURED)
HF DEADTIME = 948UA
2X LO LEDS = 500UA * 2 = 1MA
TOTAL = 91MA
+12VLOW
OUT
C56 C86 R99
1000UF 0.1UF 10K
20 PCNT 10 PCNT
1 PCNT
25V 25V
1/16W
VSS
13