David L. Huffman

The BLS supply is a linear supply powered from a 20Amp 208VAC 3 Phase circuit. It provides about 400 Watts of DC power per BLS Electronics Crate. The supply is capable of delivering 707 Watts per side.

  • Control Power Board
  • There is a circuit on the Control Power Board that prevents the application of high power to the transformer at start-up. If all conditions are good for turn-on the Control board will connect the main transformer to the line first through a set of power resistors to soft start the core and then directly using solid state relays. This is necessary to prevent the breaker from tripping due to high inrush currents. The breaker is magnetic and sensitive to peak currents.

  • Cooling
  • The chassis is completely closed and therefore requires an efficient method of cooling. Water cooling is used on all heat producing elements inside the chassis. The output modules, that have the pass transistors and current shunts, are cooled with a commercial heatsink from Wakefield. The rectifiers, main transformer and solid state relays are mounted to a cold plate that parallels the output module water circuit. Quick disconnects on the end of short hoses make the water connection to the rack manifolds off the back of the supply.

  • Display Board
  • display.jpg (150K)

    The Display board provides user information about the condition of the supply. The top half is divided into Left and Right with LEDs to indicate wheither the supply is On/Off and the fault condition of each output. These LEDs are Green/Red type so they are lit for all conditions. When a fault occurs this indicates either a voltage, current or temperature condition. The particular fault must be read back through the computer connection. The bottom half has indicators for the common portion of the supply. The input AC phases and DC control voltages are displayed along with the External Interlock signal and supply over temperature LED.

  • Logic Board
  • The logic board handles all control of the supply. It sums all fault conditions and muliplexes the digital readback. To readback the large number of digital bits the signals are multiplexed. A 5 bit control word is generated in the Rack Monitor chassis and used by the power supply to encode the digital status bits. A top, middle or bottom Module Select plug is connected to the logic board depending on the supplies position in the rack. This module plug prevents the supply from operating if it is missing and it programs the multiplexer for the correct bit transmission.

    The logic board latches faults so diagnostics can be determined.

    There is a circuit on the Loagic Board that does the following. Normally when a fault occurs the supply turns the outputs off. What actually happens is the reference is shorted out (zero volts) and the supply regulates to zero volts. If for some reason the regulator breaks and the output cannot regulate low a timer is started and after the timeout if the output is still high the main power to the transformer (high power circuit) is disabled.

  • Fault Board
  • The fault board compares the voltage, current and temperature of each output to a preset reference. If the set trip point is exceeded a logic signal is passed to the Logic board. These signals are momentary and the Logic board captures and holds them until a reset is issued.

    The voltage outputs are also checked against a minimum value for an undervoltage fault. This level is disabled when the supply is off or during the turn-on period so no false trips are generated.

    Since the Fault board is where the transition between analog and digital is performed it is also the card that has the analog readback for the Rack Monitor.

  • Output Module
  • Output modules are basically the pass transistor of the regulator. A commercial water cooled heat sink has transistor and emitter resistors for six regulated outputs. There are three temperature sensor transducers, one for each pair of regulators, that provide feedback of the transistor temperature. If water flow is low or absent the transistor temperature will quickly rise about the trip set point and the supply will turn off. There is also a thermodisk switch mounted on the heat sink that provide backup protection should the primary thermo protection fail.

  • Raw Supply
  • The main power supply uses a double Faraday shielded three phase transformer to provide the raw DC voltages for the regulators.

    The raw portion of the supply has four separate transformer windings that are rectified using fullwave Schottky diode bridges to provide the following voltages and currents. The two 18V rectifiers are operated off the same transformer winding


    Although each half share the same raw dc they are allowed to operate independently of the other.

  • Regulator Board
  • A linear regulated supply uses an amplifier to create a difference signal between a voltage reference and the output of the supply. If the output voltage is larger than the reference the amplified error tells the pass transistor to turn off somemore. If the output is lower than the reference the transistor is driven on more.

    The regulator board has a turn-on circuit that controls the regulation of the output. If the supply is in the OFF position the reference for the regulator is clamped at zero volts. When the supply is switched ON the clamp is removed in a slow linear ramp. The regulator (output) follows this ramp up to the reference level. At this point the outputs are fully ON and the supply regulates against the reference.

    Outputs are paired (plus and minus) and these pairs a ramped together. If needed the ramp rate of each output can be controlled but not there sequencing.

    There are shunt resistors for each output on the output modules used to measure the current. The Regulator board has instrumentation amplifiers and gain amplifiers for the monitoring a protection circuits

  • Turn-On
  • The AC is controlled first by a switch/breaker on the front panel. No power is present in the supply pass the breaker when it is off. If the breaker is switched On control power will energize all the electronics inside the chassis. The supply will indicate faults and require a reset before any output power can be enabled. The front panel has three toggle and two push button switches that control the action of the supply. The supplies are two sided each side powering a BLS electronics crate. The middle toggle switch is used for Local/Remote control. In the Local position the left and right toggle switch will turn the outputs on and off. Each half works independant of the other. If an one of the voltage rails, each side has six voltages, faults all outputs for that side will be inhibited. This prevents electronics from being partially powered.