Fluke 5725A, Instruction Manual, Page: 87 / 196

Theory of Operation
Detailed Circuit Description
4
4-29
Op amp U606B senses the -400V supply and forces the autobias to the minimum bias
condition when the
±
400V supplies are turned off. Transistor Q606 forces the minimum
bias condition when the window comparator is activated.
High Voltage Amplifier Feedback 4-51.
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 -100 independent of frequency range. Resistor
R608 is switched in by K601 only in standby mode and in hf operation.
Signal Transformers 4-52.
Three transformers cover the four frequency ranges as shown in Table 4-2. The lf
transformer is a C-core. The mf and hf transformers are toroids. All three signal
transformers are located in the transformer box on the left side of the instrument when
viewed from the front.
Table 4-2. Signal Transformer Usage
FREQUENCY
RANGE
TRANSFORMER TURNS RATIO
HIGH VOLTAGE
AMPLIFIER
OUTPUT(MVOUT)
HIGH VOLTAGE
TRANSFORMER
OUTPUT
40 Hz to 120 Hz LF (Series) 1:5.5 40 to 200V 220 to 1100V
120 Hz to 3.4 kHz LF (Parallel) 1:5.5 40 to 200V 220 to 1100V
3.4 kHz to 30 kHz MF 1:5.5 40 to 200V 220 to 1100V
30 kHz to 100 kHz H F 1:4.5 49 to 167V 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 4-53.
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 stud-mounted thermistor mounted into the P-
channel heat sink #2 via connector J603. Its negative temperature coefficient is linearized