Power Supplies
While often viewed merely as ancillary equipment to provide power to a Device Under Test (DUT), precision DC power supplies are critical metrological instruments in their own right. The stability, accuracy, and noise characteristics of the power supply directly impact the performance and measurement integrity of the entire test system.
Topologies: Linear vs. Switching
DC power supplies generally utilize one of two primary architectural topologies, each with distinct advantages and trade-offs.
- Linear Power Supplies: These utilize a large mains transformer to step down the AC voltage, followed by a rectifier and a linear regulator (functioning essentially as a variable resistor) to maintain a constant output voltage.
Advantages: Extremely low output noise (ripple), excellent regulation, and fast transient response.
Disadvantages: Heavy, bulky, and highly inefficient (excess power is dissipated as heat). They are the preferred choice for sensitive analog circuits, RF applications, and precision metrology where noise must be minimized. - Switch-Mode Power Supplies (SMPS): These directly rectify the incoming AC line, chop it at a high frequency (tens to hundreds of kilohertz), pass it through a small high-frequency transformer, and then rectify it again.
Advantages: Highly efficient, compact, and lightweight.
Disadvantages: Inherently generate high-frequency switching noise and electromagnetic interference (EMI), and typically have a slower transient response than linear supplies. They are common in general-purpose testing and powering digital logic.
Key Performance Specifications
The ability of the power supply to maintain its output voltage (or current, in constant-current mode) despite changes in the load resistance. It is usually specified as a percentage of the output or as a fixed voltage value (e.g., < 0.01% + 2 mV).
The ability of the power supply to maintain its output voltage despite fluctuations in the input AC line voltage.
Periodic and Random Deviation (PARD) represents the unwanted AC components superimposed on the DC output. It is typically specified in terms of RMS (Root Mean Square) voltage and peak-to-peak voltage over a specific bandwidth (e.g., 20 MHz).
The time required for the output voltage to recover and settle within a specified tolerance band following a sudden, significant change in load current.
Advanced Features
- Remote Sensing (4-Wire Operation): When drawing significant current, the resistance of the test leads causes a voltage drop ($V = IR$), meaning the voltage at the DUT is lower than the voltage at the power supply terminals. Remote sensing uses two additional, high-impedance sense leads connected directly at the DUT to measure the actual voltage delivered, allowing the supply to automatically compensate for the voltage drop in the power leads.
- Constant Voltage (CV) / Constant Current (CC) Modes: Precision supplies can operate in either mode. In CV mode, the supply acts as a voltage source, providing whatever current the load demands up to a user-defined limit. If the load attempts to draw more current than the limit, the supply automatically crosses over into CC mode, acting as a current source and dropping the voltage as necessary to maintain the limited current.
Calibration of Power Supplies
Calibrating a precision DC power supply ensures that its programmable settings and internal readback meters are accurate.
The process generally involves using a highly accurate digital multimeter (DMM) and precision current shunts.
- Voltage Calibration: The output voltage is programmed to various points across its range and measured directly with the DMM. The readback display of the power supply is also compared to the DMM reading.
- Current Calibration: A precision current shunt (a resistor of a highly stable, known value) is placed in series with the output. The supply is programmed to deliver specific currents, and the DMM measures the voltage drop across the shunt. The actual current is then calculated using Ohm's law ($I = V/R$).