Xti Power Supply Operation; Brief Summary Of Operation; Initial Start Up; Kick Start - Crown XTi series Service Manual

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XTi Power Supply Operation

Rev 1 8/9/06 EMB
Note: Any component references will be to the –8 revision level but should be backward
compatible except where additions have been made.
1K: 139087, 2K: 139086, 4K: 138349

Brief summary of operation

The power supplies within all of the XTi models are very similar. The topology
employed is a half-bridge series resonant converter. The use of this topology allows for
soft switching of the main semiconductors Q28 and Q31 reducing heat both at idle and
throughout a good portion of the operating power range.

Initial start up

Upon closing the switch S7, current is drawn from the AC source through the PTC RT6,
rectified by D24, and stored in the bulk storage capacitors C57-C60. The control
circuitry is powered directly from this stored charge through a series of resistors R370-
R373. The resistors immediately begin charging capacitor C196 which will provide a
kick-start to the power supply control circuitry.
Comparator U16 plays two rolls in the power up sequence. Firstly the voltage on C196
must be greater then 26-volts or so before the output of U16A is allowed to enter a high
impedance state. Secondly, the AC voltage must be at least 95-100Vrms before the
output of U16B is allowed to enter a high impedance state. Both of these comparator
circuits have hysteresis around the trigger points such that chatter does not result. U16A
allows a high state from about 26V all the way down to 12V. U16B allows a high state
of a few volts from the 95-100V trigger point.
Once in a high stage, the resistors R374 and R378 will pull the voltage across D31 up to
16V allowing the regulator made up of Q39-Q41 to power the controller, U14, and the
gate driver, U18.

Kick start

Once voltage is present on U14, switching begins. The clock frequency is determined by
C69 and R367 and oscillates at approximately 250-260kHz. 180-degree phased clock
signals are provided to U18 for generation of the low and high side gate drive waveforms
on pin 7 and 5, respectively. The transistors Q26-Q30 serve to increase the current drive
to the IGBTs Q28 and Q31.
Switching commences for the time allotted by the discharge of C196, or approximately
100mS. During this time period the IGBTs transfer energy from the primary bulk storage
(C57-C60) and the AC line through the resonant tank (C68 & L3) to the transformer, T1.