Bench power supplies: the evolution of laboratory power

4 mins read

While digitally oriented switch mode technology is the norm in most areas of the power supply market, pure linear regulation still has its place – particularly for users requiring the lowest output noise, the best transient response and the most benign stability characteristics when driving complex loads. But linear units are larger and heavier for a given output power and generate more heat.

Traditionally, bench power supplies (PSUs) – also known as laboratory power supplies, despite widespread use elsewhere – have offered a fixed maximum voltage and maximum current for each power rating. In this type of unit, the maximum power output decreases in direct proportion to decreasing output voltage. The disadvantage is that users need to specify their voltage/current requirements exactly to match each application, with the result that manufacturers had to offer many models within each power range. Over the past two decades, however, improved switch mode techniques have seen the introduction of PSUs with a semi constant power characteristic. This enables higher current to be provided at lower output voltages, thus providing a degree of flexibility to meet varying applications. This technique, although simple in principle, is difficult to apply to a laboratory PSU, which must be able to operate down to zero voltage and zero power output and to perform correctly under all load conditions. Typically, a linear final regulator is an integral part of the design in order to achieve the required minimum power performance as well as ensuring low noise and good dynamic response. Early units that used this approach, known as PowerFlex, were limited to a maximum current capability of twice that available at the maximum voltage. Later products increased the range to 3:1 and beyond, but difficulties remained in retaining the required performance characteristics over very wide voltage and current ranges. However, dsp based control techniques have become available which can maintain stability over a wider parametric range. Power supplies using this technique can achieve an output current to voltage ratio in excess of 6:1. These techniques can also offer faster step load response and better power efficiency. It has also become possible to achieve very low noise and good dynamic recovery performance without the need for a linear final regulator. Although cost and absolute efficiency considerations still favour fixed maximum voltage and current type PSUs, the flexibility offered by the semi constant approach is proving increasingly popular with users whose requirements extend beyond a single application. Analogue or digital controls? A recent survey among users of bench power supplies produced by Aim-TTi found that, while most users understood the benefits of digital control, many saw it as unnecessarily complicated for adjusting the basic parameters of voltage and current. Traditional analogue controls were seen as simpler, quicker and better fitted for the job. To solve this dilemma, a hybrid system has been developed which combines true analogue control knobs with internal digital circuitry. For the user, the PSU operates in a manner identical to a traditional analogue controlled unit. However, if the user wants the benefits of digital control, they are available at the press of a button in the form of functions known as 'S-lock' and 'V-span'. The S-lock function enables voltage and current settings to be locked, transferring control of voltage and current from the analogue controls to internal digital circuitry. This not only offers security, but also stability: each setting is controlled by a high resolution instrumentation quality d/a converter. The V-span function allows the user to redefine the end stop values to create a specific voltage range. When working with any particular piece of equipment, engineers often require a voltage source variable over only a narrow range. V-Span enables the 300° rotation of the voltage control to cover whatever voltage range the user requires. Consider, for example, an engineer working on a circuit that will operate from four NiMh cells. In this situation, V-Span can be used to set a maximum voltage of 5.8V (to prevent over voltage damage) and a minimum voltage of 3.6V (to ensure the circuit does not reset). The result is a PSU which provides high resolution analogue control over the exact voltage range the user requires: an ideal solution for those requiring a linear regulated precision bench power supply with conventional analogue controls. Another important feature is the use of on/off switches for the main outputs. This enables voltage and current settings to be viewed before the load is connected and for multiple outputs to be controlled individually. Equally important on multiple output supplies is the ability to switch all outputs on and off simultaneously: critical where lockups and device damage can occur if some rails are not applied correctly. Remote sensing Most engineers are aware of the need to make remote sense connections between a PSU and the device under test (DUT) in order to achieve good regulation. However, they are often less aware of the practical effects of omitting this and the benefits to be gained from correct use of remote sense. This lack of awareness is partly created by the published specifications for some laboratory PSUs which, despite their lack of remote sense facilities, boast regulation and voltage accuracy figures which will not be achieved in practical situations. For example, load regulation is commonly quoted as around 0.01%. Consider a DUT operating at 5V, drawing 3A, and connected via a 1m length of 24/0.2 wire. Typically 24/0.2 wire has a resistance of 26m?/m, so the pair of connection leads will total more than 50m?. This results in a full current load regulation of 3%, a far cry from the 0.01% of the PSU itself. Of greater concern is the error between the voltage indicated on the PSU and the voltage being applied to the DUT. In the above example, the error between the two would be 0.15V – for a PSU setting of 5V, the DUT voltage is only 4.85V. Before the advent of the digitally metered PSU, the engineer would probably have set the power supply up by measuring the DUT voltage with a separate voltmeter. With the PSU now featuring a high accuracy meter it, they are likely to accept that reading as correct. With remote sense connected, both problems are removed. Regulation is improve from 3% to the 0.01% of the PSU itself and the voltage at the DUT equals that shown on the PSU's voltmeter. A dc power supply operating in constant voltage mode uses a control loop to compare a 'sensed' voltage with a reference voltage. In modern high quality laboratory PSUs, the voltage to be sensed can be either at the output terminals of the power supply (local sense) or at the device to which it is connected (remote sense). Using local sense does not compensate for the voltage drop caused by the resistance of the connecting leads, resulting in poor regulation. With remote sense connected, the control loop monitors the voltage at the DUT and maintains it at a constant level, allowing the voltage at the output terminals to rise to compensate for the drop in the connecting leads. This results in near perfect regulation at the DUT. Mark Edward is sales director for Aim-TTI.