Crown XTi 1000, Service Manual

Page: 26 / 167

The energy is magnetically coupled to the secondary of the transformer and hence C43-
C48.
Energy is also transferred back to C196 through D26. If the voltage at C196 is
maintained at a point greater then the lower bound for the Vkstart sense comparator,
U16B, then the controller circuitry becomes sustained by the converter itself and the kick
start cycle is terminated. If for any reason the converter does not sustain the voltage on
C196, the kick start cycle will repeat indefinitely approximately every 500-800mS.
Power up
Now that the converter is sustaining the control circuitry, the amplifier has rails of either
half of the nominal or nearly the full amount depending on the input source. If the input
voltage is 120V, the rails are nearly half of the nominal value. If the input is 240V the
rails are now sitting nearly at the full operating voltage.
The microprocessor, U1, samples the across capacitor C182, on the RAIL node. The
voltage on this node is reflective of voltage present on the secondary side of the
transformer, T1. It should be noted that the voltage here not only accounts for converter
sag due to the converter throughput impedance, but also AC line sag due to significant
AC line impedance.
The relay K1 is now commanded to close by U1 at which time the voltage at RAIL is
measured. If it is below 1.4V, the microprocessor commands K1 to open followed by the
closing of K2. The closing of K2 creates a half-wave doubler out of D24 and the bulk
capacitors C57-C60. By doubling the voltage on the primary side, the secondaries are
also doubled and the relay K1 is re-closed finishing the power up sequence.
If the voltage at RAIL is above 1.4V, no more processing is required and the power up
sequence is finished.
Normal Operation
The IGBTs, Q28 and Q31, switch at approximately 42% duty cycle each at a frequency
of 125-130kHz. At idle, circulation current is provided through the magnetization of the
core within T1. This current provides a soft switching action to the IGBTs by
charging/discharging the parasitic capacitance of the devices during dead time, thereby
reducing low power losses. At the HB_OUT node the voltage will have a trapezoidal
appearance.
At higher power levels, another form of soft switching is achieved due to the resonant
tank, C68 and L3. The resonant frequency of the tank circuit is greater than that of the
switching frequency thus the current through the switches is commutated to the opposing
anti-parallel diode thus allowing resonant turn-off of the devices.
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