Error Amplifier; Error Signal Translator; Tl494 Control Ic; Gate Driver - Crown XTi series Service Manual

找资料学技术，故障求助

Error Amplifier

U23A is an error amplifier is of the inverting type. The demands and feedback
signal both feed a virtual ground at the inverting input. Both local feedback around the
error amplifier and global feedback from the amplifier rail are employed. A single diode
(D73) provides an asymmetric voltage clamp that properly bounds the error amplifier.

Error signal translator

U23D is a matching stage that translates the error amplifier output voltage to the
proper range for the TL494 control IC.

TL494 control IC

U22 is an old and inexpensive switch mode power supply (SMPS) control IC, that
is also used in the main power supply and 3.3V supply. It contains two internal error
amplifier op amps that are to slow to be of use, so they are disabled. The error signal is
driven directly into the feedback pin (#3). An internal sawtooth oscillator runs at
approximately 500 kHz. This frequency is set by C71 and R76. R75 creates a small
positive offset in the sawtooth to extend the maximum duty cycle beyond the published
capability of the IC, while schottky D33 shunts R75 to keep the falling edge of the
sawtooth steep. The internal comparator compares the error signal and the sawtooth
creating the pulse width modulated output (PWM). Note that the signal at pin #3 is
backwards of most of most control ICs. 0V is 100% duty cycle, and 4V is 0% duty cycle.
This makes the modulation method leading edge. Therefore the switching transistors will
always turn off during the falling edge portion of the sawtooth waveform.
PWM level translator
This circuit translates the PWM output of the TL494 control IC at ground level, to the
gate drive IC which is referenced to the negative rail. This circuit is very similar to the
circuit used to translate the BCA modulated output down to the -VCX rail on the CTS
BCA and Itech designs. Q55 is a PNP grounded base stage (cascode), which accepts
current from the emitters of the output stage transistors in the TL494 control IC. The
combination of resistors R77 and R734 sets the translator current. R734 serves a special
purpose to help turn off Q55, by removing stored charge from its emitter to base junction.
Q46 is another grounded base stage (cascode) that is referenced to the negative rails gate
drive supply. It receives the current signal from Q55, and then the current signal is
changed into a voltage signal across R99. Q46 is never turned on. It serves to cancel out
the parasitic capacitance present in Q55.

Gate driver

The purpose of the gate driver is to receive and buffer the logic level signal from
the PWM level translator to a higher level signal that is suitable to drive a power
MOSFET. Each channel has its own gate driver IC (U26). The gate driver has a low and
a high side that operate completely independently. The high side is connected to the
trackers for the positive rail, and the low side for the negative rail. The gatedrive IC and
the negative gate drive buffer (Q43, Q44) are powered from an auxiliary winding off of
the main power transformer that is regulated by linear pass regulator (R1, D1, Q1, Q2).
The high side gate drive supply via a bootstrap supply (D32) and a current source (Q140).
Zener diodes D77 and D79 protect the gate drive IC from over voltage.
Power Stage
Q47, D35, L4, C84/C49 form a buck switching cell. This is area of the circuit
that processes the power. The gate drive signal turns the MOSFET on and off at
请到http://bbs.bjjdwx.com 海量维修资料免费下载