High Current MOSFET Polarity Switch

The synchrotron community utilizes high magnetic field manipulators called Undulators to generate high intensity X-rays from intense electron beams. Rows of precisely positioned magnets create a sinusoidal magnetic field that causes the electron beam to undulate, which produces the desired X-rays. The intense magnetic field, nearly 1 Tesla, is further shaped by electro-magnetic correction coils mounted to the undulator, in order to direct and influence the shape of the X-ray beam. These correction coils typically carry high DC currents, up to 50 amps, but at low voltages.

The direction of the current in the correction coil must be reversible to allow the greatest influence on the magnetic field. Power supplies with reversible polarity are not commonly available. The high currents coupled with the need to reverse current direction (polarity) leads to a design problem. Most contactors that can carry such currents are usually large, expensive, and electrically noisy (to actuate) because they are designed to interrupt high voltages as well. The correction coils are typically driven by low DC voltages between 10-24 volts. Servo amplifiers have been used in the past to reverse the current flow in correction coils but while these can control current precisely, the voltage is quite variable which can affect the total power delivered to the coil. Custom power supplies have been designed (by Agilent) with reversible outputs but these are expensive and require PC communications.

For these reasons a High Current MOSFET Polarity Switch was conceived by ADC for use in this application. The switch consists of 4 MOSFET devices arranged in an "H Bridge" fashion. This H bridge can be switched so that current is directed in either a forward or reverse direction to a load. The MOSFETs that were chosen have very high current carrying capacity (160 amps) and very low on-state resistance (6 milli-Ohms). However, these are mounted on large heat sinks and do require ample cooling air to reduce the heat that will be generated at the higher currents.

The switch requires a separate 24 v DC supply for control logic that resides on the card. Dual inputs for direction and enable are provided at 5v TTL logic levels as well as 24 v DC logic levels. The gates of the low side MOSFETs are controlled by simple transistors but the high side gates use a unique high side gate control made by Maxim (MAX 1614EUA). The Switch can reverse the polarity on power supplies from 10-24 volts DC up to 80 amps. On card logic prevents both sides of the H bridge from turning on at the same time.

The card is designed to fit on a European standard 3U x 160 mm format and employs a DIN 41612, 32 pin, Type D connector. This card will fit in a 3U VME style card rack consuming 8H in width but will also fit in the Phoenix Contact SKBI 32/D single card holder. A separate terminal block is provided for stand-alone mounting and flying lead connection. Special Power and Load terminal stud bolts are provided with M8 threads. Optional 24 VDC fans can be mounted to each MOSFET heat sink for cooling and wired to the 24 VDC supply. For very high currents, two MOSFETS are mounted back to back on each heat sink to cut the on-state resistance in half.

LEDs are provided to indicate 24 VDC power, on board 8v Power, forward and reverse command direction and forward and reverse conducting direction. A toggle switch is provided for local shut down and BNC is provided for easy connection to a DVM for voltage monitoring.