Supplies Low-Noise Switching Power Supplies

For test and measurement applications, power supplies must operate with very little interference so as not to distort the measurement signals. If such devices are operated on the mains, linear regulators are still the first choice, but they cause high power losses. However, it is also possible to optimize switching regulators to generate supply voltages with extremely low noise over a wide frequency band.

The wide availability of many, sometimes very sensitive sensors for many measurement tasks leads to a wide spread of electronic sensors – even to self-configuring sensor networks. In addition, the analog circuitry has also evolved, so that today extremely sensitive measuring instruments can be built that offer extreme resolutions. All of this, however, requires power supplies like the one presented below, which is mains powered. Some of the secondary-side techniques can also be applied to battery power, for example, as a battery is usually low-impedance and low-noise, but not noise-free (Y type capacitors).

High demands are placed on measurement technology because the interference signals of the power supply must never falsify the measuring signals. In the following, therefore, a distinction is made between three types of interference signals:

  • Power disturbances: Pulses of any frequency that are generated by non-resistive loads and may disturb other equipment.
  • Power supply faults: Interference generated by the power supply itself.
  • Load disturbances: Noise signals caused by the load to be supplied on the output voltage itself.

Regardless of the noise generation mechanism, it is necessary to measure them where they are most disturbing – directly at the load or output of the power supply. For the measurement, an electronic load is suitable, which is at the same time able to tap the alternating voltage components at the output broadband and make available for further measurements, for example for a spectrum analyzer.

For the measurements, the power was supplied via a Line Impedance Stabilizer Network (LISN) to achieve defined impedance values ​​at the input and to attenuate noise from the mains. Using a spectrum analyzer, the noise at the output can be examined much more accurately than with a simple measurement of the voltage ripple.

The coupling of an interference voltage from the mains input into the measurement signal (output voltage) is described by the power supply rejection ratio (PSRR) – a parameter known from operational amplifier data sheets. This parameter describes, for example, that a noise voltage of 10 mV at the operating voltage and a PSRR of -70 dB leads to a noise signal at the output of U = 10 mV × (-70 dB) = -107.8 dBu, which is quite good at first sounds. In an analog-to-digital converter (ADC), whose signal-to-noise ratio (SNR) is not otherwise degraded, this corresponds to a resolution of 18 bits. In a high-precision measuring system with 24-bit resolution, 6 bits would be of no use. Here, at most, a noise voltage of 145 μV should still be tolerated on the supply voltage (Y type capacitors).