R&S®Essentials | DC Power supplies fundamentals
Author: Anja Fenske, Product Manager R&S Essentials Power supplies
When choosing a DC power supply, one of the first questions to answer is what type of power supply fits your application best. There are three most common types of DC power supplies. The different designs allow for highly constant voltage, low ripple and noise or higher efficiency.
Linear regulated power supplies provide highly constant output voltage, low ripple and noise and fast regulation, even with high line and load transients. They produce significantly less electromagnetic interference than switchedmode power supplies. A conventional mains transformer isolates the power-line from the secondary circuits (output stages). It is followed by a rectifier that supplies the unregulated voltage to a series actuator. Capacitors at the input and output of the regulator circuit serve as buffers and decrease the ripple.
A high precision reference voltage controls the analog output amplifier. This amplifier is generally fast and allows very short recovery times for load changes.
R&S Essentials linear power supplies: R&S®NGA100, R&S®NGL200, R&S®NGM200, R&S®NGU201/401.
SMPS, e.g. R&S®NGP800, have much higher efficiency than linear regulated power supplies. In a first step, the line voltage is rectified. Due to the high input voltage, buffer capacitors with a small capacitance can be used. In a second step, the DC voltage to be converted is chopped at a high frequency.
This takes place in the switching transistor and requires only comparatively tiny and light ferrite chokes or transformers with low losses. The switching transistor is switched fully on and off, hence switching losses are low. The output voltage is usually regulated by changing the duty cycle of the switching transistor. A rectifier and low-pass filter improve the output quality.
If current flows into the positive voltage terminal, the power supply acts as an electronic load. It is sinking power instead of sourcing power. Instruments that function both as a source and sink can simulate batteries or loads; they are called two-quadrant (or four-quadrant) power supplies. Rohde & Schwarz offer two- and four-quadrant architecture. The instruments automatically switch from source to sink mode. When the externally applied voltage exceeds the set nominal voltage, current flows into the power supply, which is indicated by a negative current reading.
The architecture of power supplies can be fully defined using a Cartesian coordinate system. The four quadrants show all combinations of positive and negative voltage and current. The figure below illustrates a coordinate system with voltage on the vertical and current on the horizontal axis.
As mentioned above, standard power supplies typically generate voltage of positive polarity only (i.e. they work in the first quadrant), for example from 0 V to 20 V. If a power supply can provide either positive or negative voltage at its output terminals without having to switch the external wiring, it is referred to as a bipolar power supply and will work in quadrants 1 and 3, providing voltages from –20 V to +20 V, for example. Such instruments can be used, among other things, to test the characteristic behavior of semiconductors for bipolar voltages across the 0 V point.
Power supplies that can operate in quadrants 1 and 3 typically also offer sink functionality for positive and negative voltages and currents. They can operate in all four quadrants and are referred to as ソース・メジャー・ユニット（SMU）. In the first and third quadrant, current flows out of the voltage terminal; the instrument is sourcing power. In the second and fourth quadrant, current flows into the voltage terminal; the instrument is sinking power.
Most of the Rohde&Schwarz power supplies offer the same voltage range on all channels. This means it does not matter which channel you choose for a specific application. Each channel can be regarded as a separate power supply.
Advanced, complex electronic circuitry is very sensitive to voltage variations on the supply lines. To minimize interference when powering devices under test (DUTs), power supplies must provide extremely stable output voltages and currents. Ideally, an output is free from voltage variations. In practice, there are two types of variation that can possibly affect the circuit or device: periodic variations (ripple) and random variations (noise), also referred to as periodic and random deviations (PaRD). Linear power supplies exhibit significantly lower high-frequency ripple compared to switched-mode power supplies.
Specialty power supplies as well as some basic power supplies such as the R&S®NGA100 employ linear voltage regulation for minimal residual ripple and noise.
The linear design of the output stages makes it possible to supply low-interference voltage to sensitive designs such as complex semiconductors. Low ripple and noise values are also perfect for developing power amplifiers and MMICs.
The outputs of specialty power supplies can be configured in various ways. For example, parameters such as the output impedance, a switch-on delay and different trigger modes can be set. Power supplies should have an output impedance as low as possible to avoid loading effects on the DUT. However, there are applications that require simulating batteries in a controlled manner, or simulating the increase in internal impedance as the battery discharges. The R&S®NGL200, R&S®NGM200 and R&S®NGM200 power supplies support these applications with adjustable output impedance.
