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Advanced Test and Measurement
Instruments and Power Supplies

Power Supply Technologies

REGULATION

Linear Regulation

A linear power supply typically uses a large transformer to drop voltage from an AC line to a much lower AC voltage, and then uses a series of rectifier circuitry and filtering processes to produce a very clean DC voltage.

Linear regulation refers to using circuitry (e.g. transistors etc.) in their linear operating region to behave as a variable resistor to regulate the output voltage (as opposed to using it as a switch - see Switched Mode regulation below). Pure linear regulation still provides the lowest output noise and best transient response. The main disadvantage is lower efficiency as any excess voltage gets dissipated as heat. This leads to larger components, more weight and higher heat output with the consequent requirement for more heatsinking and/or fan cooling. Pure linear power supplies also typically have a fixed input voltage requirement and are configured for the mains supply of the region in which they are sold, although some are user configurable.
 

Switched Mode Regulation

This refers to using power transistors as high speed switches to vary the duty cycle of a waveform in order to vary the output. This is a considerable over-simplification, but will do for this point. Since the transistor is either fully on or fully off, it will dissipate much less heat and the high switching frequency allows smaller passive components to be used. The result is a smaller, lighter and more efficient power supply, usually with the ability to be used over a wide range of input voltages ("universal" input). The main down-side is increased electrical noise due to the switching transients.
 

Mixed-Mode Regulation

For higher power levels, Aim-TTi have developed a technology that uses switch-mode pre-regulation and linear final regulation. This technique combines exceptional efficiency with noise levels that are close to those of pure linear supplies.
 

RANGING

Fixed range outputs

Most simple laboratory power supplies offer a fixed maximum voltage and maximum current capability. So, for example, a 30V/3A PSU can supply up to 90W, but only at the combined maximum voltage and current settings. Its power capability falls in direct proportion to the output voltage so that when being used at 12V, for example, the current limit remains at 3A and the maximum power available has reduced to 36W. Many such power supplies are available as a range of different models with the same power capability but with different maximum voltage and current combinations. For example a range of 100W power supplies might have 5V/20A, 10V/10A, 20V/5A, 100V/1A variants to suit different requirements. For a single application of known parameters, this may be the most cost effective route.
 

Multi-Range Outputs

The output channels of some Aim-TTi power supplies can be configured to give a higher maximum voltage or a higher maximum current at the same power to suit different applications so extending their flexibility. For example, the MX100T outputs can be 0-16V at up to 6A or 0-35V at up to 3A. This does mean that the maximum voltage (here 35V) and the maximum current (here 6A) are not available simultaneously as they are in different ranges and care must be taken when interpreting the specifications. For more general use, the ability to trade voltage for current by choosing different ranges greatly increases the flexibility of the device.
 

PowerFlex Autoranging

The TTi PowerFlex system uses a modified form of mixed-mode regulation to provide an auto-ranging or semi constant power characteristic so that the current capability rises as the voltage falls. The same model can provide full power over a wider range of voltages thereby greatly increasing its flexibility. For example, the CPX400 can provide up to 60V or up to 20A with a power limit of 420W. It should be noted that a little caution is required when interpreting the specifications of these and other multi-range power supplies as the stated maximum voltage and maximum current are not available simultaneously and maximum power available is still limited by the peak current at low voltages. Although producing slightly higher noise levels than standard mixed-mode regulation, performance is still excellent. Both the CPX and QPX series use PowerFlex with linear post regulation to provide wider output voltage and current capabilities with low noise.
 

IN PRACTICE

Multi-range linear regulated models

The Aim-TTi QL Series is a range of high precision single output and triple output power supplies using all linear regulation. Linear regulation offers the lowest output noise and the best transient response (recovery time from a sudden current step). It also offers the most benign stability characteristics when driving complex load conditions.  The QL Series uses twin secondary winding on the mains transformer that can be switched between series and parallel wiring using relays.  This provides two unregulated voltages, one of which is approximately half the other, but with potentially double the current capability. Because of the inevitable losses in the rectification and output regulation stages, the efficiency of the lower voltage, higher current range is not as good as the higher voltage range.  Nevertheless, the two ranges greatly increase the usefulness of the power supply.  The QL564T, for example, offers ranges of 56V/2A and 25V/4A from each of its main outputs.  Easy series or parallel wiring of the outputs is facilitated by an isolated tracking mode which results in a true linear power supply capable of outputting up to 112 volts or up to 8 amps.

 

Multi-range mixed-mode regulated models

Aim-TTi were innovators in the development of high performance laboratory power supplies using switch mode pre-regulation for efficiency followed by linear final regulation to minimize noise and improve transient response. The MX Series is a range of triple and quadruple output power supplies incorporating extensive range switching to provide unrivalled levels of flexibility in the choice of voltages and currents. Key to that choice is the ability, not just to change the voltage and current capability of an output, but also to be able to combine the power capability of two outputs to offer further combinations.
 
By increasing the designed operating range of the linear output stage of the MX models. Aim TTi can offer a wider range of maximum voltage and current. With the addition of the internal combining of the outputs the max levels can again be doubled. Thus a single output of an MX100T (315W triple) or MX100Q (420W quad) can be switched between 16V/6A, 35V/3A and 35V/6A.  Other outputs provide choices of 35V/3A, 70V/1.5A or 70V/3A. For higher voltage or current requirements the MX180T (378W triple) has an output switchable between seven ranges from 15V/20A at the high current end through to 120V/3A at the high voltage end.
 
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