TM-9-1200-215-34&P Technical Manual for M109 Automatic Fire Control System (AFCS) Download
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TM 9–1200–215–34&P 6–6 The PWM is controlled by two voltage comparators. The first voltage comparator monitors the vehicle voltage level. If this level is below 20 volts the PWM is enabled and the output of the PCU is supported by the voltage doubler. The second voltage comparator monitors the voltage drop across a current sense resistor that is connected between the source of the MOSFETs and vehicle return. If the voltage drop across the sense resistor exceeds a predetermined level the PWM will be powered down. This only occurs if the system is demanding too much power to be safely delivered from the PCU or a short has occurred in the PCU. This is a pulse by pulse current limiting technique and will only power down the PWM as long as the overcurrent situation continues. 2 = OUT OUTPUT FILTER A B CR1 CR2 TRANSFORMER 2 2V – + + – V R IN IN FET BANK A FET BANK B DRIVE IN TO SYSTEM = OUT IN CURRENT SENSE IN PWM CONTROLLER V I I I I Figure 6–3. Voltage Doubler Block Diagram The output voltage of the doubler, due to the resistance of the interconnections and losses in the transformer and diodes will not exactly be double the input voltage but will support the AFCS. (7) Back–up battery charger. The back–up batteries are charged by the PCU. The back–up battery charger in the PCU is a DC to DC converter. The back–up battery charger block diagram is shown in figure 6–4. Figure 6–4. Back–up Battery Charger Block Diagram CR1 CR2 + L R +28V C FROM VEHICLE V – IN RTN INPUT FILTER GATE DRIVE CURRENT SENSE PWM CONTROLLER TEMPERA TURE SENSE OUTPUT VOLAGE SENSE TO BACKUP BATTERY ASSEMBLY OUT TO THERMISTOR IN BACKUP BATTERY ASSEMBLY FETS V OUT + – INPUT VOLTAGE SENSE I