
5725A
Instruction Manual
4-28
compensate for the capacitance seen at the gates of the power MOSFETs on the #1 heat
sink assemblies. Their sources establish the voltage on the gates of the cascode FETs in
the midstage, Q654 and Q662.
The power MOSFETs are in the TO3 package and are mounted to the heat sinks using a
heat conductive pad (electrically conductive as well) to minimize thermal resistance to
the heat sink. This means that the heat sinks can have
±
400V on them during normal
operation. Do not open the 5725A cover without referring to the access procedures and
warnings in Section 6.
Lower voltage supplies of about
±
50V are available on the board to replace the
±
400V
supplies during test and troubleshooting. How to power up in the reduced-voltage
troubleshooting mode is described in Section 7.
The four heat sink assemblies are attached to the High Voltage Amplifier assembly (A3)
via connectors P661-664. A thermistor is stud mounted to the P-channel heat sink
assembly #2 and attaches to the High Voltage Amplifier Assembly through J603. The
temperature monitoring circuit, described further on, uses this thermistor.
Autobias Current Source
4-49.
FET Q653 is a current mirror that provides a dc current to bias the midstage. FET Q654
is a cascode stage to share the voltage and power. Since the loop forces the dc output of
the amplifier back to zero if the midstage current is increased, the effect of increasing the
midstage current is to increase the voltage drop across R674. Midstage current is
increased until the gates of the output devices (connected to either end of R674) are
driven just hard enough to establish the desired idle current in the output stage.
The current that Q653 mirrors comes from Q657, which is driven by BIAS from the
Autobias Sense Circuit. FET Q657 stands off the high voltage to the +400V supply and
makes the autobias circuit less sensitive to its ripple.
Autobias Sense Circuit
4-50.
The autobias sense circuit senses the +400V IM from the Power Supply assembly (A4).
400V IM is a voltage developed across a R313 in the low side of the +400V
supply, which provides a voltage proportional to the current in the 400V supply. If the
High Voltage Amplifier were operating Class B, the +400V IM waveform would be half
sines for a sinusoidal output current waveform.
The autobias sense circuit forces the average current in the +400V supply to be greater
than the average of a half sine to produce Class A-B operation. This results in Class A
operation for small output currents and for high output currents, a +400V IM waveform
that looks like a half sine at the peak but is more rounded at the base. The autobias sense
circuit does this by measuring the average of +400V IM and comparing it to the peak of
+400V IM / pi (the average that would have been obtained for Class B operation), plus
an offset.
Op amp U606A and C621 are configured as a peak detector. Resistors R647 and R648
divide the peak by pi. Resistor R643 and the 15V supply provide the offset: (15V * 10K)
/ (4.3M * 0.39) = 90 mA.
These signals are summed into integrator U601B. The integrator is an error amplifier that
drives the autobias current source circuitry, via BIAS, to achieve the desired relationship
between the peak and average values of the current delivered by the +400V supply.
Summary of Contents for 5725A
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Page 78: ...5725A Instruction Manual 4 20 aq20f eps Figure 4 3 Current Amplifier Assembly Block Diagram...
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Page 132: ...5725A Instruction Manual 8 6 aq24f tif Figure 8 1 Final Assembly...
Page 133: ...List of Replaceable Parts 8 8 7 aq25f tif Figure 8 1 Final Assembly cont...
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Page 135: ...List of Replaceable Parts 8 8 9 aq27f tif Figure 8 1 Final Assembly cont...
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Page 137: ...List of Replaceable Parts 8 8 11 aq29f tif Figure 8 1 Final Assembly cont...
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Page 139: ...List of Replaceable Parts 8 8 13 aq31f tif Figure 8 1 Final Assembly cont...
Page 141: ...List of Replaceable Parts 8 8 15 5725A 1611 aq36f eps Figure 8 2 A1 Interconnect PCA...
Page 145: ...List of Replaceable Parts 8 8 19 5725A 1650 aq37f eps Figure 8 3 A2 Current Amplifier PCA...
Page 149: ...List of Replaceable Parts 8 8 23 5725A 1661 aq38f eps Figure 8 4 A3 High Voltage Amplifier PCA...
Page 153: ...List of Replaceable Parts 8 8 27 5725A 1631 aq39f eps Figure 8 5 A4 Power Supply PCA...
Page 155: ...List of Replaceable Parts 8 8 29 5725A 1650 aq40f eps Figure 8 6 A5 Digital PCA...
Page 159: ...List of Replaceable Parts 8 8 33 5725A 1601 aq41f eps Figure 8 7 A6 High Voltage Sense PCA...
Page 161: ...List of Replaceable Parts 8 8 35 aq32f tif Figure 8 8 A12 Transformer Enclosure Assembly...
Page 162: ...5725A Instruction Manual 8 36 aq33f tif Figure 8 8 A12 Transformer Enclosure Assembly cont...
Page 163: ...List of Replaceable Parts 8 8 37 aq34f tif Figure 8 8 A12 Transformer Enclosure Assembly cont...
Page 164: ...5725A Instruction Manual 8 38 aq35f tif Figure 8 8 A12 Transformer Enclosure Assembly cont...
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Page 169: ...Schematic Diagrams 9 3 9 Figure 9 1 A1 Interconnect PCA 5725A 1611 aq48f eps...
Page 170: ...5725A Instruction Manual 9 4 Figure 9 1 A1 Interconnect PCA cont...
Page 171: ...Schematic Diagrams 9 5 9 Figure 9 1 A1 Interconnect PCA cont...
Page 172: ...5725A Instruction Manual 9 6 Figure 9 2 A2 Current Amplifier PCA 5725A 1650 aq52f eps...
Page 173: ...Schematic Diagrams 9 7 9 Figure 9 2 A2 Current Amplifier PCA cont...
Page 174: ...5725A Instruction Manual 9 8 Figure 9 2 A2 Current Amplifier PCA cont...
Page 175: ...Schematic Diagrams 9 9 9 Figure 9 2 A2 Current Amplifier PCA cont...
Page 176: ...5725A Instruction Manual 9 10 Figure 9 3 A3 High Voltage Amplifier PCA 5725A 1661 aq56f eps...
Page 177: ...Schematic Diagrams 9 11 9 Figure 9 3 A3 High Voltage Amplifier PCA cont...
Page 178: ...5725A Instruction Manual 9 12 Figure 9 3 A3 High Voltage Amplifier PCA cont...
Page 179: ...Schematic Diagrams 9 13 9 Figure 9 4 A4 Power Supply PCA 5725A 1631 aq59f eps...
Page 180: ...5725A Instruction Manual 9 14 Figure 9 4 A4 Power Supply PCA cont...
Page 181: ...Schematic Diagrams 9 15 9 Figure 9 4 A4 Power Supply PCA cont...
Page 183: ...Schematic Diagrams 9 17 9 Figure 9 5 A5 Digital PCA cont...
Page 185: ...Schematic Diagrams 9 19 9 Figure 9 6 A6 High Voltage Sense PCA cont...
Page 186: ...5725A Instruction Manual 9 20 Figure 9 6 A6 High Voltage Sense PCA cont...
Page 187: ...Schematic Diagrams 9 21 9 Figure 9 6 A6 High Voltage Sense PCA cont...
Page 188: ...5725A Instruction Manual 9 22 Figure 9 6 A6 High Voltage Sense PCA cont...
Page 189: ...Schematic Diagrams 9 9 23 aq45f eps Figure 9 7 A12 Transformer Enclosure Assembly...
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