Low Noise Power Supply
Why a Low Noise Power Supply is the Right Choice for Your Design
The power supply design considerations for sensitive equipment, whether for medical equipment, high-end audio systems, or RF transceivers, focus strongly on reducing noise.
Taking into account both output noise and output ripple can be an arduous engineering task.
Instead, choosing a low noise power supply can greatly simplify the design process.
The Challenges Presented by Noise in the Design of Sensitive Equipment
Critical applications require a stable dc output that is achievable only with a low noise power supply. Most standard ac-dc power supplies have considerable frequency bleed through, regardless of the quality of the switching chip.
This periodic ripple combines with the more random noise spikes that come from the device itself or from the influence of external sources.
The control chip itself isn't the sole determinant of the input-referred ripple. The entire circuit must be considered when governing ripple through line regulation within any design. This can be a complex problem for many designs.
A low noise power supply is the only reliable ready-made option. The power-supply rejection ratio also plays a major role, attenuating any deviations at the input significantly.
One method for improving the line regulation is by controlling the gain for the control circuit. With a higher gain for the control loop, the error at the output can be reduced. This is one method that can counteract ripple when applied correctly.
Increasing the size of the input capacitors can have a similar effect. There is still the matter of random noise, including thermal, flicker, and shot noise. Handling these presents a more formidable design challenge.
This challenge is greatly simplified through the use of a low noise power supply.
Filtering Is Not an Effective Solution to Noise for All Applications – Some Require a Low Noise Power Supply
The same principles that allow for the filtering of noise from signals can be applied to the filtering of noise from power supplies. Increasing the output capacitance can reduce noise when viable.
A major design consideration here is that both equivalent series resistance and equivalent series inductance must be taken into account. These design complexities can be eliminated with a low noise power supply.
A capacitor with both low equivalent series resistance and inductance can effectively provide noise filtering. However, power-supply circuits often rely on equivalent series resistance for their feedback error signal.
The level of reduction required to filter noise for sensitive equipment can destabilize the power supply. This problem isn't present when using a low noise power supply.
Output noise can also be reduced through the addition of a series inductor and a filter capacitor. The inductor introduces negligible loss to the circuit and impedes high frequencies. In combination with the additional filter capacitor, this can effectively filter noise. Of course, this presents further design problems.
This type of circuit's natural resonant frequency can destabilize the power supply and reacts poorly to transient load changes. A low noise power supply provides the same reduced noise without added instability.
Further filtering methods involve obscure components and additional complexity, making them highly unattractive design options. All filtering considerations can be avoided by instead choosing an ultra low noise power supply. The soft switching topology allows for a low noise power supply with minimal current leakage.
A Low Noise Power Supply Solves Multiple Design Problems
When choosing a low noise power supply, there are multiple factors by which to gauge performance. Ideally, a power supply will have low ripple noise, low conducted emissions, low radiated emission, and low leakage.
Conducted emissions from devices are controlled closely in the same way that radiated emissions are. Beyond regulatory requirements, sensitive equipment requires further improvements in conducted emissions.
Within a hospital or laboratory environment, the conducted emissions from equipment can have adverse effects on other equipment on the facility's grid. The low-frequency noise produced by conducted emissions can be avoided with a low noise power supply.
Traditional electric filters are not suitable for the reduction of conducted emissions; power supplies require dedicated filters. Incorporating these elements into a design can be a difficult process.
Interference from radiated emissions is of particular importance for sensitive instruments. Reducing radiated emissions requires a significant number of design principles.
The proximity of components within a device, resonance values, and more must be considered during the design process. A low noise power supply ensures minimal radiated emissions, protecting sensitive equipment and ensuring proper operation.
Common mode noise is a complex problem to solve, with voltage occurring across the power supply and the reference ground. This noise can produce radiated emissions that will interfere with sensitive equipment.
Solutions to common-mode noise such as decreasing loop area and shortening cable length aren't always viable to implement. Choosing a low noise power supply reduces the impact of common mode noise at the source, eliminating the need for countermeasures.
The traditional methods to reduce noise, like coil-based filters, introduce increased leakage current. These methods make the power supply less safe by increasing the chance of electric shock.
A designated low noise power supply eliminates the need to filter noise, meaning that additional components are not present within the circuit to increase leakage.
Soft Switching Provides Superior Performance in Low Noise Power Supply
A low noise power supply produces superior results in current leakage, radiated emissions, conducted emissions, and common mode noise by incorporating soft switching technology. Traditional ac-dc switching generates noise during the switching process.
The raising and lowering edges of the waveform generate noise at the zero-cross point. This conventional type of inefficient switch is called "Hard Switching."
The soft switching of a low noise power supply incorporates a sinusoidal waveform to drastically reduce noise by softening the raising and lowering edges. Minimum current and voltage cross the 0 level concurrently in soft switching, unlike the difference in crossing with hard switching. This method increases efficiency while reducing noise generation.
The noise reduction produced by soft-switching produces more reliable low noise power supplies for critical applications. A variety of medical imaging systems require minimal noise that a low noise power supply provides much more effectively than other methods.
Furthermore, inspection systems for circuit component production and other delicate electronic applications benefit from reliable noise reduction.
Soft switching allows for a reduction in ripple, conducted emission, radiated emission, and common-mode noise. These are precisely the qualities required in a power supply for sensitive equipment in medicine and other fields.
Choosing a low noise power supply with soft switching eliminates the compounding design problems of reducing noise through traditional methods.