2.6. Principle of Operation
Magna-Power Electronics MagnaLOAD DC electronic loads are offered in three variations: the ARx Series, the WRx Series, and the ALx Series. The ARx Series and WRx Series is comprised of a bank of MOSFETs and a series connected array of power resistors configured in a binary format. The ALx Series, which utilizes a more conventional electronic load circuit topology, is comprised simply as a bank of MOSFETs. Both models use three MOSFETs arranged in modular subassemblies called linear modules. These modules contain the necessary drive, feedback, and protection circuitry.
Figure 42 shows a simplified version of the ARx Series and WRx Series, which includes two basic series connected elements: resistor R1 and current source Q1. The current source consists of MOSFET Q1, shunt resistor R2, and operational amplifier U1. Load current, IL, is set with voltage V1 applied to the positive input of operational amplifier U1. V1 drives the gate of Q1 high until the voltage across the shunt resistor, VR2, equals that of V1. The load current, IL, will be equal to V1/R2.
Assuming the power across the shunt resistor is insignificant, the power dissipated in load resistor R1 is IL x VR1 and the power dissipated in MOSFET Q1 is IL x VQ1. The resistors can be operated at higher temperatures than the MOSFETs, simplifying cooling requirements of the passive elements. Keeping VQ1 small and VR1 large lowers system costs in comparison with purely semiconductor electronic loads. Adjusting the value of resistor R1 is accomplished with a binary switching matrix.
Keeping the resistor switching increments small and over a wide range maintains the smallest voltages across the linear modules and over the widest operating range. The ARx Series and WRx Series has 31 resistor switching states to accomplish this goal. In parallel with the resistors is a shorting switch which provide a very low impedance to bypass the resistor matrix. With the shorting switch enabled, the ARx Series and WRx Series operates like a conventional electronic load and is capable of providing a near zero impedance. The power limit in this mode is that of the linear modules.
With the linear modules fully conducting, the ARx Series and WRx Series can operate solely with the resistor matrix providing 31 resistor states. This mode, called rheostat mode, has no feedback controls for automatic adjustment. Rheostat mode eliminates any possible instabilities between the source and load and is basically a programmable power resistor.
The linear modules have protection circuitry to protect itself from abusive conditions. Each linear module is protected against over current and over voltage conditions. These fault conditions are independent of normal feedback operation. Over current conditions causes shutdown of the system. Over voltage conditions operate in two states. If the voltage applied to the load exceeds it rating, the linear module first acts as a power Zener diode to clamp any transient voltage such as that produced by Ldi/dt. After short instant in time, the ARx series shuts down like over current protection.
The ALx Series uses the same linear modules as the ARx Series. The ALx series has no resistor matrix and no shorting switch. The ALx Series uses 12 linear modules for 2,500 Watt operation, while the ARx Series and WRx Series uses 4 linear modules for 6,750 Watt and 12,500 Watt operation, respectively. The ALx Series has a wider constant power range since there is no restriction with application of the resistor matrix.
The control system has multiple digital signal processors (DSPs), a minimum of 4 are used with others depending on external communications. The combination gives both the ARx and ALx Series powerful control in regulating current, voltage, power, resistance, and rheostat mode. Both the ARx Series and ALx Series use the same control scheme. Like all Magna-Power products, all control circuitry is referenced to earth ground eliminating ground loops between communications equipment.