Preamp Power Supply Principles of Operation

Run II electronics required new power supplies. One major consideration was to keep changes to the existing system to a minimum. Existing water and AC power would be sufficient to operate the new system, however other changes would be needed.

Power requirements have gone from 520 Watts to 1810 Watts delivered and yet the volume the supply can occupy remains the same. In order to meet this new power level a switching type supply was utilized.

The presence of a magnetic field further complicated supply construction. Although the field was not considerably high, 200-300 gauss, its volume is large. The supply has a volume 10-1/2 x 10-1/2 x 18 makes shielding difficult. The supplies are built around a commercial Vicor MegaPAC supply that will operate from the 20Amp 208VAC source. It was found that these supplies fail to operate in a field of more than 175 gauss when orientated in the vertical direction. To reduce the field adequate shielding must be provided by the chassis. A two-layer shield was devised to lower the magnetic field to an excitable level. The outer shield is named the steel box and is welded together. The second shield is directly around the Vicor supply and is made from high-saturation magnetic material. It has two halves that fit together.

Since the supply will be enclosed in a steel chassis there is a need for cooling. Water will be used on heatsinks inside the box. The Vicor modules were modified for direct cooled which greatly decreases their operating temperature. Additional cooling is provided for the current sensing shunts.

The supply provides three separate voltages to power the Preamp hybrid circuits. This power is distributed to each column of 24 motherboards. Each power rail will be interfaced so the voltage and current can be monitored via a computer connection. In addition any voltage or current exceeding a set level for more than 8 milliseconds will signal a shutdown to all outputs of the supply. This is done to prevent the application of partial power to the hybrids. The unit has fuses on the primary 208VAC input and a solid-state 3-phase relay to control applying of the main AC power to the Vicor supply. There is a local On/Off, Reset and Local/Remote switch on the front panel. There are LED indicators for local information.

The Vicor supplies have built-in analog temperature sensing. This signal is 2.5V/25C and has a range of 0-100C. There are digital signals for Phase Fault, Over Temperature Warning and AC Power OK that are monitored.

The Phase Fault signal is on active high, it will drop low if the input reaches the over current level of 30 Amps due to a missing phase or severe line imbalance.

The AC Power OK signal, active high, will drop low about 3ms before the output regulation is lost. An AC Power Fail signal is the compliment of the AC Power OK.

The Over Temperature Warning signal, normally high, drops low somewhere between 65-76C. The recovery point is 1C below the actual trip point. Actual over temperature shutdown occurs when the inlet temperature exceeds 70-81C. Recovery is 10C below the actual trip level.

There are two conditions that will turn the supply off in the event of a fault condition. The first condition will be any time the output current exceeds a preset level for more than 8 milliseconds. This is what might be considered a typical fault condition. A second level overload condition will be when the fault exceeds the trip level for 100 milliseconds. For this case the main AC power to the supply will be removed. The design intent here is that this is a more serious fault and more direct measures need to be taken. This might be caused by a direct failure of a module which fails to respond to the inhibit signal. A third and final level of protection will be inline fuses on each output from the supply. This will ensure that current in the wiring will be at a safe level. The fusing will be rated above the normal operating levels and should not open under a typical fault condition.

All faults will be latched to allow analysis of the condition that occurred.

Logic control is implemented with a programmable logic gate array. This provides the best flexibility and reduced circuit board size for the assembly.

Control power (+5V and +/- 15V) is a separate supply that will be ON whenever the AC is applied to the box.

Additional temperature sensors are used as an indicator of water flow and will need to be below a preset limit during normal operation.

There is a thermo-switch mounted on the water manifold that provides a failsafe interlock should the water flow stop. This switch is in series with the solid-state relay removing the power when water flow is interrupted and the supply becomes hot.

There is a Hall probe circuit that provides information about the magnetic field inside the steel shielding box. The sensor is operated from 10V provided by a zener diode. The output signal of the sensor is at 5V with zero field present and goes above and below that point depending on the magnetic field direction. An OPAmp circuit removes the offset and adds a gain of 4 to the signal.

Analog circuitry: Reference the Monitor board schematic.

The analog section of the Monitor board provides two general things. First it measures the voltage and current on each output and feeds these signals, conditioned, back to the Rack Monitor. Second it compares these signals to trip points that provide the logic level for fault detection. These logic levels are passed to the logic control discussed below. The current is measured as a voltage drop on a shunt. The shunt is chosen to have a low voltage drop (less than 250mV). The shunt is on the high side of the output that can be +-15V. Therefore an OPAMP with very high common mode rejection is used to translate the signal with respect to ground. The INA117 amplifier can handle common mode ranges as high as 200V when operated on +-15V rails. All channels on the circuit board are the same. The Personality modules change the way a channel is used. A positive output uses a blue Personality module and a negative output uses a green module. The module set the trip level for voltage and current trips and it reverses the signal from the shunt so all measurements are positive when supplying current.

LOGIC Control: Reference the logic circuit with regard to this explanation.

The digital signals from the analog comparators are latched. Signals from the voltage, current, phase-fault and temperature warning (18 signals) are time latched. The External Interlock signal, however, is latched anytime the signal goes false.

The time latch is done with an external one-shot that sets a delay time, 30ms typically, that must be exceeded before the fault is captured and the supply is inhibited. All 18 inputs signals are ord together and triggers a one-shot. The same signal is the input to a flip-flop that is clocked by the falling edge of the one-shot. If the signal is still faulted after the delay the latch gate is opened and the fault is captured and sent to the supply to inhibit the outputs. All outputs are disabled when a fault occurs. There are two logical elements needed beside the usual AND, OR, NOR and NOT etc. One is a transition detector. Whenever a signal changes logic level this element outputs a single pulse the width of the clock. A 20MHz clock is used with the logic. The second element is a transient-up circuit. It outputs a clock pulse anytime the input transition is low to high but does nothing when the level goes from high to low.

The rest of the logic is simple and will not be explained except to say that there is one line, VICORENAB, that is the inhibit signal for the power supply. Other output signals provide status.