System Overview
2-55
Filament Driver Board
The filament current sense circuitry uses hall current sensor to provide analog output proportional to the actual
filament transformer current.
The FDB is mounted in the Mainframe E-box. It is responsible for several functions as follows:
- Provide PWM output for driving Filament Transformers in High Voltage Tank
- Switch large or small filament focus
- Stop PWM generation for a while delay when switching large or small filament
- Over current protection
- Provide the over current alarm signal to MCB
- Respond to alarm signal from MCB and stop PWM generation
- Filament transformer current closed loop control
The structure of FDB is shown as follows.
SG3525
Hall current
sensor
Over current
protect
Delay
Circuit
40-110kV
SM
TANK
PWM
Driver
Filament
current
regulator
RMS Current
Filament
Transformer
Current
+24V
24V_GND
FCSSW_FDB
OCALM_FDB
FILDISA_FDB
MAC_FDB
Invertor
Filament
Select
Circuit
PS1
MCB
The FDB consists of 9 function blocks: Inverter, Overcurrent protect, PWM control, Filament select circuit, Hall
current detect, Filament transformer current control, L/S focus switch, 10V reference, and Interlock.
Inverter
The filament drive pulses PWMA and PWMB drive power MOSFET Q3 and Q4 in a switching stage. When
either input is high, one of the two MOSFET’s gates is positive and the other negative. Power to the switching
stage is constant +24V, needed 1.4A power input, which is provided from external switching power PS1.
Overcurrent protection
The transformer primary current is monitored across a 0.255Ω, comprising R4 and R5 in parallel, in the DC
bus. The sample voltage inputs to Q1 base. The transformer primary current for the large focus is less than
1.4A in normal condition, which yields less than 357mV across the resistor pair. When the transformer
primary current exceeds 2.75A, the voltage will exceeds 0.7V, Q1 will drive the base of Q2 on, then Q2
collector will be grounded, OCALM_FDB will go high, OC_ALM_S will go low to drive U3(1/4) on to pull SMAC
to -15V, SG3525 output will shut down until S1 is pressed or the system restarts. Inductor L1 limits the peak
switching current through transitions and reduces high frequency radiated energy from the output stage.
PWM control
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