MagnaLOAD Operating Profiles and Ranges

Magna-Power’s MagnaLOAD DC electronic loads utilize two different energy dissipation topologies: linear and active resistance.

In the linear topology, used in the air cooled ALx Series and consistent with most electronic loads available on the market, MOSFETs driven into the linear operating region are used as the primary heat dissipation device. Using MOSFETs to dump energy is a simple, known approach, which enables operation at full power over a very wide voltage range. The downside of the topology, however, is the relatively high cost of linear electronic loads, driven by the high dollar per wattage pricing and the large number of active devices necessary for sizable power applications.

In the patented (US Patent 9,429,629) active resistance topology, used in the air cooled ARx Series and water cooled WRx Series, a switched matrix of passive resistors provides the primary power dissipation, which is then series coupled with a small relative number of active devices. New programmed set points or variations in the DC input bus results in resistors being switched in and out of the circuit, with simultaneous digital control of the active devices to make the transition seamless. By substituting passive devices for active devices as the primary heat dissipation element, active resistance electronic loads are offered at a significantly lower price than traditional electronic loads, enabling economical application of electronic loads for high-power requirements. As passive resistors have a fixed impedance, the voltage range in which full power can be achieved is more limited than linear technology loads. An electronic load’s operating profile is an important consideration to make when selecting a product.

MagnaLOAD Operating Profiles

MagnaLOAD models are specified by the series name and three numbers, in order: maximum power, maximum voltage, and maximum current. For example, model ARx13.5-1000-28 is rated for 13.5 kW maximum power, 1000 Vdc maximum voltage, and 28 Adc maximum current. With the MagnaLOADs wide maximum full power operating range, it may not be immediately clear from the model over what range those ratings can be achieved. The full power operating range can be determined easily by:

• Dividing the model’s maximum power rating by the maximum voltage rating to determine what the current will be at the product’s maximum voltage.
• Dividing the model’s maximum power rating by the maximum current rating to determine what voltage will be at the product’s maximum current.

Between these two voltage and current points, the full power operating range is determined, for example, for model ALx2.5-200-600:

2.5 kW rated full power operation for model ALx2.5-200-600 is achieved from 482 Vdc and 56 Adc to 1000 Vdc and 27 Adc. Shown on a plot, below, with both power and current versus voltage:

As another example, MagnaLOAD DC electronic load model ARx27-1000-56:

27 kW rated full power operation for model ARx27-1000-56 is achieved from 482 Vdc and 56 Adc to 1000 Vdc and 27 Adc. Shown on a plot, below, with both power and current versus voltage:

As a simple rule of thumb for active resistance ARx Series and WRx Series MagnaLOADs, full power can be achieved from 48% maximum voltage to 100% maximum voltage. For the ALx Series, with its much wider full power operating range, it’s best to use the calculation from Eq. 1-2 above.

Of course, MagnaLOADs can also operate outside of the Figure 1 and Figure 2 operating profiles, at reduced power; this is when it’s necessary to review the respective product’s specified operating profile in its datasheet or user manual. Given the wide range of models and power levels, Magna-Power normalizes these profiles about the product’s full scale ratings for a respective product series. To derive a model’s operating profile, multiply the profile’s normalized values by the ratings from the model number. Figure 3 shows the normalized ARx Series high power range operating profile and Figure 4 shows the ARx Series high power range operating profile applied to model ARx40.5-100-840.

Low Power Operating Range

Active resistance MagnaLOADs, the ARx Series and WRx Series, have a second operating range, low power, which can be selected from the front panel or computer command. This operating range allows these products series to achieve maximum rated current at low voltages by bypassing the passive resistors and using the MOSFETs only for power dissipation. While the MagnaLOAD can only dissipate approximately 20% of its rated power in this low power range, it is useful for low voltage applications that cannot otherwise be addressed by the high power operating profile.

Figure 5 shows the normalized ARx Series low power operating profile and Figure 6 shows the ARx Series low power operating profile applied to model ARx40.5-100-840.

MagnaLOAD Operating Profiles and Ranges Applied to Example Applications

Among the many applications for MagnaLOAD DC electronic loads, several were selected to best demonstrate the product's operating profile and ranges.

12 Vdc and 48 Vdc Automotive Testing

Magna-Power was approached by an automotive manufacturer to use the MagnaLOAD in testing conventional 12 Vdc automotive applications, but also for testing 48 Vdc plug-in hybrid electrical systems. The MagnaLOAD would be used for battery discharge testing for 12 Vdc lead-acid and 48 Vdc lithium-ion batteries, but also for loading down the 12 Vdc alternator and also simulating the battery energy recovery on the 48 Vdc bus of a hybrid vehicle. The worst-case discharge specifications provided were as follows:

• 350 Adc maximum discharge current at 12 Vdc from the lead-acid batteries to simulate the starter motor cranking
• 400 Adc maximum dissipation at 48 Vdc from the lithium-ion batteries during acceleration

The highest power requirement for this application, 48 Vdc at 400 Adc for the lithium-ion indicates that this requirement is in the power range of the ARx Series (6.75 kW to 40.5 kW):

Based on this 19.2 kW power requirement and Magna-Power's product offering, the closest fit ARx Series model would be in the range of 20.25 kW and above. Now it's important to ensure that the operating points fall within the product's operating range.

Referring to the red line in Figure 3, the ARx Series MagnaLOAD DC electronic loads can achieve full power in high power operating range from 48% to 100% of maximum rated output voltage. From Magna-Power's offering, the only voltage range that can achieve full power at 48 Vdc would be the MagnaLOADs rated for 100 Vdc maximum voltage, as 48 Vdc is equilvalent to 48% of a 100 Vdc MagnaLOAD's rated voltage. 48 Vdc would therefore fall within the full power operating range of a 100 Vdc MagnaLOAD.

Referring to the dark blue line in Figure 3, at 12 Vdc the current capability for the 100 Vdc ARx Series MagnaLOADs is only 22.5% of maximum current rating in the high power range—this is where the low power range comes in.

For low voltage applications, configuring the MagnaLOAD for low power range will provide the operating profile from Figure 5. In this range, the MagnaLOAD can achieve near its rated current capability at low voltages. Referring again to Figure 5, at 12 Vdc—equivalent to 12% of a 100 Vdc MagnaLOAD's rated voltage—it can achieve 95% of its rated current.

Considering the closest power rating over 19.2 kW, model ARx20.25-100-420, we evaluate the operating conditions:

Equation 6 shows that at 48 Vdc, MagnaLOAD model ARx20.25-100-420 will be able to achieve 421.8 Adc in high power range, satisfying the requirements for the lithium-ion battery testing.

Equation 7 shows that though MagnaLOAD model ARx20.25-100-420 is limited to 95% of its rated current at 12 Vdc in low power range, 399 Adc provides enough current to satisfy the requirement of the lead-acid battery testing.

Therefore, model ARx20.25-100-420 satisfies both requirements, through the use of high power range (Figure 3) and low power range (Figure 5).

500 Vdc Electric Vehicle Testing with Expansion Capability

Magna-Power was approached by an electric vehicle (EV) manufacturer for 500 Vdc DC bus testing for their electric vehicle. The MagnaLOAD would be used in place of their battery pack to dissipate reverse energy. In addition, the MagnaLOAD would also provide loading for the vehicle on-board battery chargers.

The on-board chargers drove the power requirement, which was 12 kW. The user preferred an air-cooled solution, which was a fit for the ARx Series (air cooled, 6.75 kW to 40.5 kW). The user first selected the ARx13.5-500-56, with maximum ratings of 13.5 kW, 500 Vdc, and 56 Adc. This selection seemed like the most logical fit, as it would handily satisfy their the full power demand providing up to 27 Adc at the 500 Vdc voltage requirement.

Though the user's selected model would satisfy the present demand, the future generation of their vehicle would push the DC bus voltage to 800 Vdc; the ARx13.5-500-56 would not be able to satisfy testing demands for this new generation. As a result, despite the immediate need for only 500 Vdc, the customer selected the ARx13.5-1000-28. Referring again to Figure 3, this new model would still be able to provide 27 Adc at the 500 Vdc voltage (50% of the maximum rated voltage) requirement, but could now go up to 13.5 Adc at 1000 Vdc, allowing the product to be used for the generation to come.

12 Vdc ASIC Mining Power Supply Testing

Cryptocurrency mining power supplies provide high-power +12V buses for power hungry ASIC miners. During production validation, these mining power supplies are cycled and burnt-in using DC electronic loads. In this particular application, five power supplies are tested simultaneously in a custom power supply tester. The power supply is rated for 1200 watts, capable of up to 100 Adc on its +12V rail. Therefore, the maximum power requirement for five miner power supplies under burn-in was 6 kW.

For a 6 kW requirement, the ARx Series initially seems like the best fit, with models ranging from 6.75 kW to 40.5 kW. Given that the requirement is at 12 Vdc, however, in low power mode the ARx Series would be limited to less than 25% of its rated power (Figure 5), which would require the product to be vastly oversized to accommodate the 12 Vdc, 6 kW requirement. Instead, looking at the operating profile for the ALx Series, it is capable of full power over an extremely broad range, making it ideally suited for high power, low voltage applications. Referring to the the Figure 1 excerpt from the MagnaLOAD model ALx2.5-200-600's operating profile, it is capable of achieving its full power rating at 12 Vdc.

While the maximum power rating for an ALx2.5-200-600 is only 2.5 kW, multiple units can be digitally master-slaved in parallel using the product's included MagnaLINK™ interface. With MagnaLINK™ master-slaving, there is no sacrifice in performance and the units perform measurement aggregation to truly act as a single product. Master-slaving three ALx2.5-200-600 units will allow power capability up to 7.5 kW and current capability up to 1800 Adc. While operating at 12 Vdc, the master-slave system had more than enough capability for the ASIC mining power supply tester's 6 kW, 600 Adc requirement.