QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 891 HIGH CURRENT CAPACITIVE CHARGING CIRCUIT LT3750EMS DESCRIPTION Demonstration circuit 891 is a High Current Capaci- there is no need to connect components to the high tive Charging Circuit featuring the LT3750EMS. It is a voltage output. The charging current is set by an DC/DC flyback converter used to charge large capaci- external sense resistor and is monitored on a cycle- tors to high voltages. The circuit implemented on by-cycle basis. The LT3750 is available in a space the board is a simple example of how to charge a saving MSOP-10 package. 400uF capacitor to 300V in approximately one sec- ond, depending upon the line and load conditions. WARNING LETHAL VOLTAGES ARE PRESENT ON An output capacitor discharge circuit is included on THE DC891 CIRCUIT BOARD. DO NOT OPERATE the board for the convenience of the user. This dis- THE ASSEMBLY UNLESS YOU ARE TRAINED TO charge circuit can be disabled by the user or perma- HANDLE HIGH VOLTAGE CIRCUITS. nently enabled at the expense of a minor degradation in the charging profile. The LT3750 is a current-mode flyback controller op- Design files for this circuit board are available. timized for charging large value capacitors to a pre- Call the LTC factory. determined target voltage. This target voltage is set , LTC and LT are registered trademarks of Linear Technology Corporation. by the turns ratio of the flyback transformer and just two resistors in a simple, low voltage network, so Table 1. Performance Summary (T = 25C) A PARAMETER CONDITION VALUE Input Voltage Range Vout = 0 to 320V 6-20VDC Output Target Voltage Vin = 6 to 20VDC 300 + 5%VDC Average Input Charging Current Vin = 0 to 20V, Cload = 400uF 2.2A QUICK START PROCEDURE WARNING LETHAL VOLTAGES ARE PRESENT ON 1. With the power source turned off, connect the in- THE DC891 CIRCUIT BOARD. DO NOT OPERATE put power supply to the board through the VIN THE ASSEMBLY UNLESS YOU ARE TRAINED TO (E1) and GND (E2) terminals. HANDLE HIGH VOLTAGE CIRCUITS. 2. Connect the load to the VOUT (E3) and GND (E5) terminals. Demonstration circuit 891 is easy to set up to evalu- 3. Set the CHARGE jumper at JP1 to the STOP posi- ate the performance of the LT3750. For proper tion. Alternatively, the user may connect a 0-24V equipment setup, follow the procedure below, refer- signal source to the three pin JP1 header. Refer to ring to Figure 1. the data sheet and schematic for proper signal lev- els. 1 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 891 HIGH CURRENT CAPACITIVE CHARGING CIRCUIT 4. Position the OUTPUT BLEEDER jumper at JP2 to 6. Change the CHARGE jumper at JP2 to the GO posi- the NORMAL OPERATION position for initial test- tion. If a signal source is used, apply an active ing. The user can also leave the jumper set to transition. DISCHARGE, but there will be a minor degradation 7. The output should start charging towards 300V. to the operation of the charge time. 5. Turn on the input power source to at least 6V, but below 20V. OPERATION WARNING LETHAL VOLTAGES ARE PRESENT ON and GND. The data sheet defines a signal profile that THE DC891 CIRCUIT BOARD. DO NOT OPERATE must be used to properly activate the CHARGE input, THE ASSEMBLY UNLESS YOU ARE TRAINED TO so it is recommended that the user employ a suitable HANDLE HIGH VOLTAGE CIRCUITS. signal source to control the charge cycle. The DONE bit is pulled up to VIN through a high im- pedance 100K resistor. This may not be compatible Demonstration circuit 891, featuring the LT3750, is a with all electrical systems. DC/DC Flyback converter used to charge large valued capacitors to a predetermined voltage. The charging DC891 is equipped with a safety circuit that drains the current is set by the current sense resistor R10, while output capacitor of charge when the controller is not the target voltage is determined by the turns ratio of delivering energy to the load. When JP2 is set to the transformer T1. For a complete description of the NORMAL OPERATION, this safety circuit is disabled operation of the LT3750 integrated circuit, please re- and the output capacitor will bleed down from its own fer to the data sheet. leakage current. If JP2 is set to DISCHARGE, the output capacitor will be discharge through the com- The DC891 demo board is equipped with two physical bined parallel resistance of R2, R3 and R4. interfaces to control the charge cycle. One is a sim- ple solder terminal at location E6. The other is 2mm Due to the high charging currents made possible by header at location JP1. For the convenience of the the LT3750, it is possible to simply engage the safety user, a high impedance 100K pull up resistor is con- circuit all the time by leaving JP2 in the DISCHARGE nected between the CHARGE pin and the VIN power position. This will result in only a minor degradation input. A jumper has been installed between CHARGE in the charging profile. 2