BAP-3012 Twelve Pulse SCR Driver

The BAP-3012 is the latest generation of twelve pulse gate firing board for SCR control.  Standard BAP-3012 features provide robust and reliable operation driving parallel or series connected SCR bridges.

BAP-3012 standard features include: 

• instant inhibit
• phase loss inhibit
• soft start & soft stop functions
• controls AC main voltages up to 600 VAC 
• on-board diagnostic LED indicators
• on-board current limit control
• active balancing of parallel bridge currents to +/-2%

The BAP-3012 provides complete SCR firing control for industrial/commercial power supplies of any power level, delivering hard firing gate drive power to control larger SCRs from 16mm to 100mm. 

Without the BAP-3012, slight mismatches (.5%) in transformer secondary voltages may result in current imbalances between paralleled bridges as high as 20%.

The BAP-3012 directly interfaces with low cost hall effect sensors or an optional shunt interface to monitor and actively force the output currents of each bridge to be matched within +/- 2%.  This eliminates expensive alternatives used to balance bridge output currents, such as interphase or custom transformers.  With a minimal series inductance inserted in each bridge, the BAP-3012 will adjust the delay angle of each bridge independently to actively force the currents to match.

Functional Description

SIGNAL CONDITIONING OF INPUT REFERENCE

SCR GATING PHASE LOCKED TO THE UTILITY INPUT

In order for the delay angles to properly control the SCRs conduction angle, the delay angle must be phase locked and phase shifted from the utility input by an amount determined by the delay angle control. The BAP-3012 employs phase locked loop circuitry to keep the SCR gating signals in phase with the three-phase input. An additional control loop has been added that will force 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 an 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-3012 accepts voltage or current control input that allows control of the delay angle.  The default scaling for the Delay Angle Control input is:

0V corresponds to maximum delay angle (minimum conduction angle) or zero output
and
5V corresponds to minimum delay angle (maximum conduction angle) or maximum output.

The control input can be modified to accept a current input or a different scaling of the input voltage. 

In order to provide a controlled and orderly start up sequence, the delay angle commanded by the user is not instantly applied to the SCRs at turn-on.  At start up, the delay angle is forced to the maximum value.  When the SCR control signals are phase locked to the input references, with no errors present, the delay angle will ramp down from the maximum value to the 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 the contact closure between J3-4 and J3-6).  When the soft stop is used, the delay angle will ramp up to its maximum value in approximately 50mS.  If the board is forced into a fast turn off condition, all SCR gate signals will be turned off within 20µS.

Logic Implementation

All of the logic required to perform the delay angle control is contained on a single FPGA (Field Programmable Gate Array).  Since it is programmable, it can be modified to adapt to customer needs in certain applications.

DC Gate Drive

The BAP-3012 provides DC gate drive outputs rather than picket fence drive outputs, which offers improved performance in circuits with discontinuous load currents.  If an SCR loses holding current when being driven by 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 BAP-3012 DC drive maintains continuous current flow into the gate to insure the SCR continues to be turned on for the entire SCR conduction time .

The BAP-3012 output drive current waveform applied to each SCR gate provides 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-3012 to drive large area devices under high dI/dt conditions.

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: Example 12-Pulse Connection Diagram