Fluke 5725A High Voltage Amplifier Feedback, Signal Transformers, Temperature Monitoring

Theory of Operation 4

Detailed Circuit Description

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.

4-51. High Voltage Amplifier Feedback

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.

4-52. Signal Transformers

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

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).

4-53. Temperature Monitoring

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

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