Theory of Operation 4
Detailed Circuit Description
Op amp U606B senses the
High Voltage Amplifier Feedback |
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Feedback for the High Voltage Amplifier is provided by R606, R607, and R608 from the output signal MVOUT. This means that the high voltage signal transformers are driven open loop by the High Voltage Amplifier. Feedback for amplitude leveling is provided on the High Voltage Sense Assembly (A6).
The turns ratio of the hf transformer is less than the other transformers. Resistor R608 compensates for the difference in turns ratio so the overall gain of the High Voltage Amplifier, plus the signal transformers, is
Signal Transformers |
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Three transformers cover the four frequency ranges as shown in Table
Table
FREQUENCY
RANGE
40 Hz to 120 Hz
120 Hz to 3.4 kHz
3.4kHz to 30 kHz 30 kHz to 100 kHz
TRANSFORMER
LF (Series)
LF (Parallel)
MF
H F
TURNS RATIO
1:5.5
1:5.5
1:5.5
1:4.5
HIGH VOLTAGE
AMPLIFIER
OUTPUT(MVOUT)
40 to 200V
40 to 200V
40 to 200V
49 to 167V
HIGH VOLTAGE TRANSFORMER OUTPUT
220 to 1100V
220 to 1100V
220 to 1100V
220 to 750V
Four control lines: HFPD, MFPD, LFPD, and VLFPD, control the frequency ranges. The control lines are generated on the High Voltage Sense assembly. These lines control relays K601 to K604, which direct the High Voltage Amplifier output to the primary of the appropriate transformer and switch in feedback resistor R608 to compensate for the hf transformer turns ratio. Relay K604 switches the primary windings of the lf transformer into a series connected configuration for 40 to 120 Hz operation, and into a parallel connected configuration for 120 Hz to 3.5 kHz operation.
Relays on the Interconnect assembly (A1) switch the secondary transformer windings. These Interconnect relays are driven by the same control lines as the High Voltage Amplifier assembly. These lines originate from latch/driver U157 on the High Voltage Sense assembly (A6).
Temperature Monitoring |
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The temperature monitoring circuit estimates the junction temperature of the power MOSFETs by measuring the temperature of one of the high voltage heat sinks and adding a calculated temperature rise from heat sink to junction, based on the current in the +400V supply.
The heat sink temperature is sensed with a