How susceptible is circuitry to noise on your DC rail lines? How can you test the maximum power point tracking performance of an inverter? How can you use instrumentation to meet the dynamic test requirements for electric vehicles? These are just some examples of the challenges that you face when developing test routines to verify the performance of your products.
The solution requires a combination of a DC power supply or a load and a signal generator to modulate the DC power device. One option is to combine the two instruments. You could use a bias tee to connect an AC signal generator and a DC instrument together. That is a simple connection, but the problem is you will be hard-pressed to find a commercially available bias tee that can pass more than 700 W with a maximum voltage of 100 V and a maximum current of 7 A. If you are testing a solar panel inverter, an electric vehicle battery and vehicle circuits, or any circuit operating at higher than 100 VDC, then a bias tee with 100 V and 7 A capacity is completely inadequate. Another alternative is a wide band power amplifier that that has a bandwidth down to DC. Most power amplifiers do not amplify frequencies down to DC. They are large and expensive. Neither of these options solve the challenge of combining a low power AC signal with a high-power DC level.
Figure 1. A bias Tee combines a signal generator output with a DC power supply output, but is power limited
Built-in function generator for AC + DC signal generation
The best solution is to do what EA Elektro-Automatik does which is build the function generator into a power supply or a load. Then you do not have to worry about protecting a low-power signal generator from a source of high power. In fact, there is no low power hardware to protect as the EA function generator is entirely firmware.
The function generator can generate sine waves, triangular waves, rectangular waves, ramps, as well as custom waveforms. You can use these waveforms to:
- Test a circuit’s susceptibility to power line noise immunity by adding a 60 (50) Hz signal (and harmonics) onto a DC voltage representing the DC rail lines
- Test a circuit’s response to varying voltage rail ramp-up rates
- Determine the range of rail voltages over which a circuit will remain powered
- Simulate voltage spikes to test a circuit’s level of protection from voltage transients
- Test a circuit’s susceptibility to a kHz custom noise signal representing the output of a switching power supply.
These are just some examples of how an arbitrary waveform generator built into a DC power supply can conveniently enable a test engineer to more thoroughly test a circuit or a product. More extensive testing improves product reliability.
EA’s built-in function generator has even more capability. The function generator has the capability to create I-V curves to simulate devices such as solar cells, batteries, and fuel cells.
Simulating a solar panel or a solar panel array for inverter testing
Let’s simulate a solar panel. We need to create an I-V curve for the solar panel. First let’s review how a solar cell works so we can understand what the function generator will define for the solar panel, a set of connected solar cells. A solar cell model consists of a current source which represents the solar light creating current, a p-n junction represented by a diode, and resistances in series with and in parallel with the p-n junction. Photons impinge upon electrons in the p-n junction and provide sufficient energy to enable the electronics to transition to the conduction band. The electric field that builds up turns on the diode, and current flows to the output of the solar cell. Figure 2 shows a simple model for a solar cell.
Figure 2. Circuit model for a solar cell
The series and shunt resistances represent the losses in the solar cell. The series resistance represents the resistance of the solar cell semiconductor material and the resistance of its metal contacts. The shunt resistance represents the insulation resistance defined by the leakage current through the p-n junction. Having a lower series resistance and a higher shunt resistance increases the efficiency of the solar cell.
Based on this model, the solar cell has an I-V characteristic as shown by the red curve in Figure 3. The p-n junction exhibits an inverse diode characteristic. The key parameters that define the curve are the solar cell’s short circuit current, ISC, its open circuit voltage, VOC, and the point where the solar cell’s power output is a maximum, the maximum power point, MPP. The ISC and the VOC are the maximum current and voltage that the solar cell can generate. VMP and IMP, as shown by the blue curve in Figure 3, represent the I-V parameters of the maximum power output that the solar cell can generate. Operating the solar cell at its MPP ensures maximum performance from the solar cell and is the target operating point.
Figure 3. I-V characteristic of a solar cell (red curve) and the power output (blue curve)
The built-in function generator of the EA Elektro-Automatik instruments, such as the PSB bidirectional DC power supplies, makes it easy to create the solar cell I-V curve. The function generator requires four parameters: the open circuit voltage, the short circuit current, and the maximum power point current and voltage. Figure 4 shows the setup screen which will create the I-V curve.
Figure 4. Function generator setup screen for solar cell simulation.
Test engineers can use the simulated solar cell to test solar cell inverters and how well they can track the maximum power point of the solar cell or solar panel. The function generator enables testing of an inverter’s efficiency in compliance with standard EN 50530 Overall efficiency of grid connected phototovoltaic inverters. The EN50530 test mode allows determination of inverter response to changes in the maximum power point. In addition, the test mode allows inputs to change irradiance on the solar panel and changes in its surface temperature. Both parameters affect solar cell output.
Using a test rack of as many as 64 PSB 30 kW supplies, a test engineer can simulate a 1.92 MW solar farm. This allows complete testing of solar inverters for power distribution applications.
Simulating a battery for testing a battery-operated system and its charger
A second example of an application for the function generator’s XY generator is the simulation of a battery. Figure 5 illustrates a simple model for a battery. The model uses an ideal voltage source representing the battery’s open circuit voltage and an internal resistance representing the electrochemical and the electronic resistances in the battery. As the load current increases, the output voltage of the battery declines due to the increase in the voltage drop across the internal resistance. A test engineer can determine how a load, such as an electric vehicle motor, responds to the drop in battery voltage when the motor increases its current draw.
Figure 5. Simplified model of a battery using an ideal voltage source and an internal resistance
Using the function generator in the ELR electronic load, the ELR load can simulate a battery for charger testing. The load can determine how well a charger can supply a high current output for fast charging a battery. Also the simulated battery load can test the chargers ability to safely trickle charge the battery when 100% charge has been achieved.
Benefits of including a function generator in a power supply and a load
The built-in function generator in the EA power supplies and loads gives the test engineer flexibility to more thoroughly test products under a wide range of conditions. The built-in generator eliminates the complexity of combining a low-power signal generator with a high-power instrument and avoids potential safety concerns. With all the capability that the function generator adds, does it make sense to consider a new power supply without a function generator?