Power Supply Mosfets And Repair - QSC K.2 Series Service Manual

8.4 Power supply MOSFETs and repair

Power supply failure is often related to shorted power MOSFETs Q13 and Q14. Both MOSFETs often fail together, but just
one can fail in some instances. Unfortunately most power supply MOSFET failures cause catastrophic damage to PCB traces
or cascading damage to other components down/up stream. Check all components in the schematic below before deciding
to perform a component-level repair.
Symptoms:
• no power
• The PSU attempts to start but does not succeed.
Failure verification:
• Q13 or Q14 shorted between any two pins of gate, source, or drain.
(verifiable from the top of the PCB, with the clamp off - Figure 8.4.1)
Repair notes:
• First consider replacing the AMP/PSU board with a new one. Repairing
the power supply at a component level is not recommended due to
the complexity and small components in the circuit.
• Use the highlighted schematic in Figure 8.4.2 as a component check guide. Replace components in red and check com-
ponents in blue.
• Q13 and Q14 don't always fail in pairs, but always replace both when one has failed.
• A failure of Q14 could cause direct damage to the PWM controller at U15. If this occurs, replace the board with a new
one instead of repairing.
• Check gate drive components (resistors and diodes) tied to gate signals QM-DR and AUX_OUT.
• If the LEDs or LCD screen blink but the module won't start, suspect a failure to the power supply controller, U15.
• AC current draw at idle should be 0.1—0.3A at 120Vac and 0.1—0.2A at 230Vac. If the idle current is not in this range,
there is a problem.
• MOSFET clamping:
Take good care during the reassembly process when replacing the power MOSFETs. Apply a fresh
and thin layer of thermal grease on the MOSFET body. The torque specification on the 2 clamp screws is very critical to
ensure good heat transfer between the electrical component and heatsink. Sufficient clamp force is required to achieve
the best result. Refer to the installation steps in "6.2 AMP/PSU board".
TD-001517-01
K.2 Series Service Manual
CURRENT SENSE
T1
D51
RB751S40T1
C128
R67
AUX_OUT 1 4
22.6
1UF
T3
1 PCNT 10 PCNT
T3
1/10W 25V
RB751S40T1
3 6
PRI_LO
Figure 8.4.2 - Some components to check if the MOSFETs fail.
Figure 8.4.1 - Measuring PSU MOSFETs from top
56K FOR PASSIVE
(RC) CLAMP MODE
R123 C116
0.68UF
56.2K
5 PCNT
1 PCNT
250V
1W
C137 D40
RB751S40T1
1UF
10 PCNT
Q18
25V
D35 MMBTA56
R164
DNI
R52
2.00K
1 PCNT 10.0K
1/10W 1 PCNT
1/10W
D27
R121
1N4148
10
1 PCNT
1/16W
R157
QM_DR
10
R150
1 PCNT
10.0K
1/16W
1 PCNT
1/8W
3
PRI 2
XF-MID 2
Q13
STP11NM60FD
PRI 1
T2
1
PSU-VSW
Q14
STP45N65M5
C124
0.0022UF
20 PCNT
250VAC
C52
220PF
10 PCNT
250VAC
41