BAP-1950 Three Phase SCR Driver
The BAP-1950 is a versatile three-phase bridge SCR firing board with advanced features and functions. The BAP-1950 is the ideal firing circuit for large industrial power supplies, motor controllers and generator controllers. It can be used to phase control AC mains; soft-start high power systems; produce variable, unregulated DC in it's open loop configuration; and with the closed loop feedback option, produce regulated DC output with voltage control and current limiting. The BAP-1950 is insensitive to phase sequence and mains voltage distortion. It features high gate voltage isolation, soft and instant start/stop functions, phase loss inhibit and can be used at mains frequencies between 30 and 90Hz without adjustment. Its form factor and size are industry standard and does not require auxiliary boards to implement all of its functions and options.
SIGNAL CONDITIONING OF INPUT REFERENCE
Variable control of SCRs requires varying the SCR turn-on phase angle in order to vary the amount of input voltage that is conducted to the output. The BAP-1950 phase locks to the input source in order to create the reference signals required to control the conduction angle of SCRs in any topology. References can be derived from the cathodes of the three SCRs on J2, which are connected to the input source; or from an auxiliary inputs supplied at connector J5. In either case, the signals used to create the references are filtered to remove unwanted harmonics that could affect the accuracy of the controlled delay angle .
The three-phase input phase rotation is sensed and the SCR gating signals are automatically adjusted to account for either ABC or ACB phase rotation. This eliminates incorrect phase rotation of the three-phase input source. However, if auxiliary phase reference input at connector J5 is used, the three inputs at J5 must be properly connected to be consistent with the phase sequence of the input phases connected to J1 and J2. Refer to the connection section on page 5 for a more detailed explanation.
SCR GATING PHASE LOCKED TO THE UTILITY INPUT
In order for the delay angles to control the conduction angle of the SCRs, the delay angle is phase locked and phase shifted from the utility input by the amount determined by the delay angle control. The BAP-1950 employs phase locked loop circuitry to maintain the SCR gating signals phase relationship with the three-phase input. An additional control loop implements the delay angle to remain constant as the input frequency varies from 30Hz to 90Hz.
Delay Angle Control
The magnitude of the delay angle determines the point on the input waveform that each SCR will be switched on. This controls the output voltage of a Converter (AC in, DC out) or an AC Controller (AC in, phase-controlled AC out). The BAP-1950 accepts either a voltage or current input signal that allows user control of the delay angle. The default scaling for the Delay Angle Control input is:
To provide a controlled and orderly start up sequence, the delay angle control input provided by the user cannot be instantly applied to the SCRs at turn-on. At start up, the BAP-1950 forces the delay angle to maximum value (minimum conduction) and only ramps the delay angle after the SCR control signals are phase locked to the input references and no errors are present. At that point, the delay angle will ramp down from maximum value to programmed value in approximately 500mS. While in operation, the SCR gate firing can be turned off using either the soft stop function (shorting J3-12 to J3-11) or the fast turn off feature (open contact closure between J3-4 and J3-6). When the soft stop is activated, the delay angle ramps to its maximum value in approximately 50mS. If fast turn off condition is activated, the BAP-1950 forces all SCR gate signals to turn off within 20µS.
The delay angle control logic is implemented in an FPGA (Field Programmable Gate Array). This programmability allows factory modifications to suit customer specific requirements.
DC Gate Drive
The BAP-1950 implements DC gate drives, rather than picket fence drives, which offer improved performance in circuits with discontinuous load currents. If an SCR loses its holding current when being driven with a picket fence, the SCR will turn off and may not turn on again until it is turned on with the higher current leading edge pulse of the next turn on transition. The DC drive keeps current flowing into the gate so that the SCR will continue to be commanded on for the entire time that the SCR can be in conduction.
The current waveform sourced to each SCR gate is an initial 2 Amp peak pulse (rising at a rate of approximately 1A/µS) approximately 10µS wide, followed by 500mA of DC current for the remainder of the turn-on signal. The open circuit voltage applied to the gate is 24 volts, which enables the BAP-1950 to drive large area devices under high dI/dt conditions.
Options for Powering the Board
There are several options for applying power to the BAP-1950. Transformer T2 can be driven by 120 VAC or 240 VAC via connector J4. There are jumpers between T2 and J4 that configure the board to accept either of these inputs. The transformer can also be powered directly from the three-phase source that the SCRs are controlling. By removing J4 and installing jumpers JP5 and JP6, the voltage at the cathodes of the phase A and phase B load-to-line SCRs is connected to the primary of T2. In this condition the turns ratio of transformer T2 is determined by the magnitude of the input source.
Fault Detection and Shut Down Sequence
Under normal operation, the delay angle is controlled directly by the delay angle control voltage supplied by the user at J3-10, or by the error amplifier when configured as a DC power supply. It can also be turned off fast by removing the contact closure between J3-4 and J3-6 (this will illuminate the INHIBIT LED) or ramped down slowly by shorting J3-12 to J3-11 (this will illuminate the DISABLE LED).
If one or all of the input phases are lost or the input frequency changes too fast for the circuit to remain locked, an out of lock condition is detected. In such a case, the PHASE LOSS LED is illuminated and a fast turn off is initiated which inhibits all gate signals within 20µS. When lock is restored, the unit will ramp up to the programmed delay angle.
If the temperature sensing circuit is used, the OVERTEMP LED will be illuminated and the gate signals are inhibited 20µS after the over temperature threshold is exceeded. The gate signals will ramp up to the programmed value after the heatsink temperature drops 8°F (4.4°C). This value of thermal hysteresis can be modified to suit the customer’s requirements.
Figure 1: Mechanical Drawing of BAP-1950
Gate Drive Connectors J1 and J2
Two Mate-N-Lok™ type connectors allow for a convenient interface with the SCR gates. The mates for these connectors are supplied with the board along with keying plugs to eliminate the possibility of inadvertently swapping J1 and J2. Only one mating connector is included if the board is to be used in a semi converter application.
Each connector has three pairs of wires to drive three SCRs. J1 is configured to drive the three upper SCRs in a converter topology or the three SCRs with the anodes connected to the utility in an AC controller topology. J2 is configured to drive the three lower SCRs in a converter topology or the three SCRs with the cathodes connected to the utility in an AC controller topology. For this reason, the phase reference signals are obtained from the J2 connector. Therefore, if an AC controller or a semi converter is being controlled by the BAP-1950 without using J2, J5 must be used to obtain the reference signals.
Control Signal Connector J3
The control signal connector is the BAP-1950 interface to the system controller. The table below describes the pin functions of J3 when the BAP-1950 is being used with an external controller to vary the delay angle.
When the BAP-1950 is configured as a DC power supply, J3-10 no longer controls the delay angle. One jumper on the board, OPN LP, is removed which isolates J3-10 from the circuit and another jumper M, S or I is installed connecting the regulation loop’s error amplifier to the delay angle control input. The jumper is installed if the board is operating independently. The M jumper is installed if the board is operating in the master mode, controlling another slave board. The S jumper is installed if the board is operating in the slave mode, being controlled by a master board. The rest of the pins on J3 perform the same function in DC power supply mode. An explanation of the feedback connectors and how they should be wired is included in the DC power supply option section on the following page.
Power Supply Connection Options J4
The necessary power supplies to run the logic on the board and drive the SCR gates are generated through T2. There are several options for applying input power to T2:
Phase Reference Options J5
The default method of deriving references is to sense the cathodes of the three SCRs on J2 that are connected to the input voltage. This is a convenient point to obtain the utility inputs, which are then attenuated and filtered so they can be phase locked to the delayed gate commands. The magnitude of the utility input must be known when the board is ordered so that the correct components are inserted into the interface circuitry on the board.
Phase references may also be obtained by using auxiliary connector, J5. J5 is a Mate-N-Lok™ series connector that may be used if the circuit topology does not allow the input voltage to be sensed via the SCR cathodes normally available on J2. It is important to connect J5-1 to the input phase that is controlled by J1 pins 1 & 2, J5-3 to the input phase that is controlled by J1 pins 4 & 5, and J5-5 to the input phase controlled by J1 pins 7 & 8 (see Figure 3).
High Voltage Feedback J6
If the closed loop option is ordered, the DC output of the power supply is brought back to the board via J6. An isolation amplifier attenuates the high voltage and isolates it from the output so that the feedback can be referenced to the signal ground.
Current Limit Control J8
The value of current at which the power supply will foldback can be adjusted with the on-board pot or J8 can be used to connect an external pot (a 10K pot should be used in this application).
Remote Voltage Control J10
The output voltage of the power supply can be controlled remotely with an external pot (the minimum pot used in this application should be a 1K) or a 0 to 5V signal. This connector can be replaced with an on board 10K pot to control the power supply output voltage. The 5V reference at J10-1 has a limited source capability of 10 mA. Therefore, it should not be used for any circuitry other than the pot.
Current Feedback J11
The BAP-1950 provides a connectorized interface to an open loop current transducer for current feedback to be used for an inner current loop and/or current limiting. The inner current loop enhances system performance by improving stability and allowing the user to set or vary a current limit. The 4 pin header on the board interfaces directly with the HAS and HAX open loop hall effect sensors from LEM, providing an inexpensive means for obtaining accurate current feedback.
The LEM current transducer can be placed on either side of the load. All diagrams show the current transducer on the positive load side, with the appropriate the current flow (arrow). If the current transducer is required on the negative side of the load, to eliminate floating the current transducer in high voltage supplies, for example, the current flow is reversed (arrow points away from the load).
Temperature Sense J12
A temperature sensor can be used to interface with the board via J12. The temperature sensor can be mounted on a heatsink to prevent the SCRs from operating at a temperature beyond their ratings. A threshold can be set on the board, so that when the temperature is exceeded, the BAP-1950 will inhibit SCR gating and illuminate the OVERTEMP LED. The temperature sensor used is an LM-35A mounted near the hottest point of the heatsink. Selecting this option includes an LM-35 assembly that is to be mounted on the heatsink.
The following will provide the user with an explanation of how the BAP-1950 controls SCRs in several common applications, basic DC converter, regulated DC converter and AC Controller. These are the most common circuit topologies found in industry. Included is a functional description of the circuitry and instructions for connecting the BAP-1950 in a system.
DC Power Supply – Full Controll Converter
The basic design of an SCR three-phase bridge power supply is shown in Figure 2, below. This is called a full converter, converting 3-phase AC into DC. Connections to the BAP-1950 board are shown. The SCRs are connected to the BAP-1950 with connectors J1 and J1. The phase angle delay is controlled via J3. The customer control signals used to control the phase angle delay for firing the SCRs is accessed via J3. See the J3 connector connections on pages 4 and 5.
When the BAP-1950 is configured as a basic AC to DC converter, see Figure 2, the phase angle delay is determined by the voltage (0V to 5V) on J3-10. The jumper on the board, OPN LP is shorted, and the M (Master), S (Slave) jumpers are opened. The I (Independent) jumper is shorted, since the board is operating independently. The other connections on J3 are used as needed. See J3 connector description on pages 4 and 5.
DC Power Supply - Half Control Converter
The default means of deriving references is to sense the cathodes of the three SCRs on J2 that are connected to the input voltage. However, in a half-control configuration as shown in Figure 3, rectifiers replace the SCRs and no reference is available.
Phase references may be obtained by using auxiliary connector, J5. J5 is a Mate-N-LokTM series connector that may be used if the circuit topology does not allow the input voltage to be sensed via the SCR cathodes normally available on J2. It is important to connect J5-1 to the input phase that is controlled by J1 pins 1 & 2, J5-3 to the input phase that is controlled by J1 pins 4 & 5, and J5-5 to the input phase controlled by J1 pins 7 & 8 (see Figure 3 below).
Regulated DC Power Supply
Designing a regulated DC power supply requires changing 2 jumpers and installing the isolation amplifier circuitry on the board. This allows the design of a DC voltage regulation loop with an adjustable current limit as well as an inner current loop for stability. Information must be provided to APS when ordering, regarding the system’s output filter and required step response so that error amplifier compensation components can be selected. Computer simulations of the system’s dynamic performance can be performed at APS to determine the compensation components that will optimize system response and ensure stability. With this information, one can use the BAP-1950 board in the circuit configurations shown in Figures 4 and 5.
Figure 4 is the most common circuit used for regulated power supplies. The BAP-1950 board has the optional isolation amplifier and compensating components installed. In addition, a Hall effect current transducer is employed for current feedback to the BAP-1950. In order to optimize the performance of the system, the user must inform APS of the filter (capacitor and inductor) values. This will allow us to install the proper values of compensating components to insure proper operation, stability and response.
Figure 5 is another common circuit employed for regulated DC power supplies. This topology is used when the output voltage is higher than 600V to 1kV rms. The advantage of this topology is that the mains voltage is usually below 600V rms and SCRs with blocking voltages below 2kV can be used. It is usually easier to put rectifiers in series strings than it is to put SCRs in series. In addition, the current rating of the input mains SCRs is usually lower then they would be in the transformer secondary circuit. Depending on the specifics, the SCR and rectifier total costs may be lower with the circuit topology in Figure 5 than the circuit topology in Figure 4.
Higher than Six Pulse Operation
In order to set up the BAP-1950 as a DC power supply, OPN LP must be removed and an M, S, or I jumper must be installed. This will render the delay angle control at J3-10 inactive. The delay angle will now be controlled by the output of the error amplifier in the voltage regulation loop. The on board pot, external pot (connected to J10), or a remote voltage will command an output voltage which will drive the error amplifier to the required delay angle. The Soft Start/Stop and Fast turn off functions controlled by J3 remain active when the board is configured as a DC power supply.
When the I jumper is installed, the BAP-1950 will independently control a three phase SCR bridge, regulate the DC voltage and limit the current. If 12 pulse, 18 pulse, or up to 36 pulse regulation is to be used, one board will be the master, requiring the installation of the M and I jumpers. Any slave boards will require the S jumper. The output of the voltage error amplifier of the master board will be exported out to J3-12 and will control the conduction angle on the master board and the conduction angle on any slave boards (the slave boards will import the error amplifier signal on J3-12). A DC power supply with a 12-pulse rectifier using two BAP-1950s in a master/slave configuration is illustrated in Figure 6.
Figure 3: Sensing input voltage with auxiliary connector J5 to obtain phase reference, with half-control topology
Figure 6: Example of 12-pulse operation using two BAP-1950 boards, one configured as a Master the other as a Slave.