Measurement accuracy is a fundamental specification for any test system. Accuracy specifications are a combination of all devices in the system and the overall layout of the system, including wiring selection and configuration. Determining test system accuracy must include the accuracy of all programmable DC power supplies in the test system. So, what defines the accuracy of a programmable dc power supplies?
programmable power supply dc Accuracy Specifications and Terminology
Display Accuracy : This specification is usually the primary contributor to the accuracy of the device. This parameter is essentially the error budget for all components in the output and feedback paths of the power supply. Display accuracy is expressed as a percentage of the full-scale voltage or a percentage of the output voltage setting.
Load Regulation : Load regulation defines the drop in output voltage as more current is required to maintain a fixed voltage value as the load resistance decreases. Higher current output increases the voltage drop on the output path of the power supply circuit. Load regulation is expressed as a function of the full-scale voltage or the programmed output voltage.
Stability : The drift over a specified time interval determines stability. That time interval could be an eight-hour work shift. Manufacturers may define stability as a percentage of the full-scale voltage. Not all manufacturers define this parameter.
Noise : Variations in the output come from noise generated in the electronic components. Manufacturers define noise as ripple or just noise. Noise is the random variation in voltage in all electronic circuits. Ripple is the periodic variation in the output voltage due to imperfect rectification of the AC input voltage. Specifications can include rms (root mean square), peak.
Importance of Remote Sensing
Measurement accuracy alone does not determine how well a programmable DC power supply will perform in a test system. Another important consideration is how well the power supply maintains the required voltage tolerance at the load. The voltage applied to the load is affected by the output setting of the power supply and the wiring to the load. The best way to ensure that the voltage at the load is the desired voltage is to use remote sensing, which compensates for the voltage drop caused by the resistance of the test leads.
Using only the two output terminals of the power supply to connect to the load is called local sensing, and the voltage at the load is:
VLoad = VSupplyOutput – 2·VLead
= VSupplyOutput – 2·Iload·RLead
No matter how good the accuracy of the power supply is, the voltage applied to the load will not have the same accuracy. Power supply accuracy is defined at its output terminals, but engineers are primarily concerned with the accuracy of the voltage at the load. If the load current, ILoad, is large, this will reduce the voltage at the load, according to the formula, VLead = Iload·RLead, and the resulting error can be large.
As shown in Figure 1, remote sensing uses two sense terminals to measure the voltage at the load and feed this voltage back to the power supply control circuit. The load voltage sensing circuit has a high input impedance, so the current drawn from the load is negligible. The control/power amplifier circuit adjusts the VSupplyOutput to keep the voltage at the load at the programmed output voltage. The actual VSupply Output = the programmed output voltage + the voltage drop in the test leads (2* Iload·RLead).
Figure 1. Using remote sensing to ensure that the voltage at the load is the programmed output voltage
Wiring Methods
While using remote sensing can compensate for the voltage drop in the test leads and the resulting voltage drop at the load, there is a limit to how much voltage drop in the test leads the power supply can maintain its accuracy. For the PSI 10200-70 and EA-PSI 10000 series, the total voltage drop in the leads must be less than 5% of the rated output voltage.
If the output voltage of the power supply is 24 VDC, the maximum voltage drop in the test leads must not exceed 1.2 V. Therefore, we need to keep the test leads as short as possible and select wire sizes that both meet safety standards and minimize the voltage drop in the leads. Following these recommendations will ensure that the power supply can accurately deliver power to the load.
Reducing circuit noise and improving stability are more challenging tasks, and test engineers need to try different wiring configurations based on the capacitive and inductive characteristics of the test leads and the load.
For example, for resistive loads, separate the power leads and sense leads. Use twisted pairs for the sense leads to minimize the loop area so that external magnetic fields (such as motors) cannot sense voltage in the wire leads. See Figure 2.
Remember that the voltage sensed by the magnetic field is V = ∫B·dA
Due to the high input impedance of the load voltage sensing circuit, a small amount of external electrical noise on the sense input can produce an error voltage. Shielded twisted pair wiring can be used to eliminate the effects of external electrical interference sources.
Figure 2. One of the wiring options for remote sense leads is to use shielded twisted pair wiring
If the load has a more complex input impedance, other wiring schemes may be required to eliminate oscillations that start in the sense circuit and propagate to the output of the power supply.
Recall that test leads like coaxial cable are a distributed RLC network. Although we are discussing a programmable high voltage dc power supply, the source-load circuit is a complex impedance network. Step changes in the programmed voltage and external noise can cause oscillations on the DC power supply line.
Placing each sense line next to its corresponding power line can eliminate possible oscillations. In some cases, adding capacitors to the load can help eliminate oscillations.
It is best to try different configurations of remote sensing to ensure that the source-load circuit is the most stable.
Device Warm-up
When powered on, all devices require a warm-up time to allow the electronic equipment to reach thermal equilibrium. The stability and accuracy of the device are specified after the warm-up. For example, most EA power supplies typically require a warm-up of 30 minutes to achieve the specified accuracy.
Don't overlook this detail! Most electronic engineers know that they need to allow their equipment to warm up, but often forget this detail. If conditions do not allow the power supply to warm up, its initial output may not meet the specified specifications.
Summary
The specifications of the programmable dc power supply and the wiring configuration of the power supply in the circuit will affect the accuracy of the load voltage in the power-load circuit. With these tips, electronic engineers can get the best accuracy from the power supply. IDEALPLUSING can help users select DC power supplies and achieve the best connection to the load.
