QSC PLX Series

T r o ubleshooting
PLX 1202 l PLX 1602 l PLX 2402 l PLX 3002
Power Supply - -
Replacing Blown IGBTs.
In order to improve EMI performance, reduce cost, and increase current capacity, the PLX IGBTs
are driven by an active, direct coupled integrated circuit, rather than a gate drive transformer. IGBT or
driver failure should be rare (when correctly assembled) but when the IGBT’s blow, it usually damages the
following parts:
CHECKLIST AFTER BLOWN IGBT’S
Q96, Q97, (IGBT’s generally fail in pairs)
D78, D79, R358, R359, gate drive coupling comonents, check after removing blown IGBT’s.
Ul8, lR2110 high-side gate driver, Fault current when low-side IGBT shorts to upper rail. Such currents
also typically damage the gate coupling parts noted above.
Ul9, 3525 controller, Blows from currents shorted thru Ul8, or possibly by overvoltage on the supply rail
SOMETIMES Ul4, 556, powered from 5V output of 3525, which may fail high when 3525 fails.
RARELY Ul3, which has fairly high supply voltage ratings.
PROBABLE CAUSES OF MASSIVE IGBT FAULTS
SHORTS IN CONTROL CIRCUIT.
The parts operate well within their ratings and should hold up well in the field. The usual cause of failure is
when both IGBT’s turn on at once, shorting Pri-Hi to Pri-Lo. This occurs when something causes the drive
signal to one part to remain on when the other part is supposed to turn on. Shorts from solder or debris are
one obvious cause.
SHORTS IN THE LOAD.
Although there is peak current shutdown, shorts in the power amplifier transistors or secondary-side supply
components can cause currents to increase too quickly to prevent damage.
OVERVOLTAGE ON THE BIAS SUPPLY.
If the TOP-210 bias supply fails to operate, no harm occurs, the unit simply does not operate.
However, open circuit (missing part) in several key components can cause the Bias supply voltage to be
much too high, This blows the 2110 and thus the IGBT’s.
QUICK TEST OF BIAS SUPPLY.
Ramp the AC voltage up slowly to 25% of regular voltage (30V for 12OV unit). If the bias supply is working
normally, the green “power” LED should come on between 30 and 35Vz with its usual, steady “half-brighr
start-up level. If the LED comes on at 2OV, or not until 5OV, or blinks, DO NOT RAISE VOLTAGE PAST 60V
until you have measured the bias voltage. The switching will not start until you reach 9OV, so you can save
the IGBT’s from blowing.
Confirm that bias voltage at Cl38 is 18-19V. Open or missing D63, 64, 65, 66, 67 or R349 will break the
feedback to U16 and cause overvoltage.
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