Posted on 17 July 2019

The Correct Power Supply

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The choice has become easier

To find a suitable power supply for your own application: developers and project engineers can tell many a tale about this. Despite the fact that there is an enormous choice on the market, the one that fits the bill exactly is never amongst them. Compromises regarding dimensions, quality of output voltages, performance and price have to be made.

By Dipl. Ing. (FH) Oliver Kistner, Principal Engineer Power Electronics, Schroff GmbH, Straubenhardt, Germany.


Split decisions between linear regulators and switch mode regulators

Especially in Measurement and Control Technology, where a high quality of output voltage with minimal interference is of great importance, linear regulators do fulfil the demands, as they separate the input voltages via a transformer, stabilize them with a linear regulator and filter them extremely effectively. However, they are large and heavy, with a low efficiency. Switch mode regulators work very efficientlly, have small dimensions and a low weight. This is achieved by switching the input voltages at high frequencies (50 to 500 kHz) and then isolating them galvanically with a small transformer. Through the following regulation and filtration with the aid of a pulse width modulation regulator the required output voltage is obtained. The multiplevoltage changes do however create high frequency interferences, which can be found as ripples and spikes on the output voltages. The classic switch parts of a network have cost many developers much time, as the anti-interference measures for the voltages are very costly and time consuming.

Product photograph of slimpower

Interference-low input and output voltage

With ‘slimpower’ Schroff has created a 19” power supply range that combines the positive properties of switch mode regulators as well as linear regulators (table 1). During the design and development of the power supplies, input and output voltages with the least possible interferences were considered, to lower the typical interferences of a switch regulator. The use of SMD performance components and linear technology reacts positively on the interferences due to the low parasitic effect. Ripples and spikes lie below the tolerance of 10 mV for low emission units. Simultaneously, both the incoming interferences and HF emissions have been considerably reduced. The levels are at least 15 dB below the tolerance levels set by EN 55022 Class B.

Comparison 19” linear regulator – 19” switched mode regulator

Small dimension, large performance

Comparison of residual ripple and spikes between the switch regulator – slimpower

During development further emphasis was put on small dimensions and high output performances. The units are arranged as a standard Euroboard, 3 U and a depth of 160 mm (Euroboard format) and a width of only 3 HP (15 mm) with a standard H15 connector. For the transformer instead of the common copper coils, etched copper channels, which have been placed on a board retainer by multi-layer technology, are used. This multi-layer construction is a planar transformer and fits exactly between two ferrite cores and saves a lot of space.

Transmission interference curve slimpower

As the real primary switch regulator a highly integrated circuit is used for these power supplies and this element combines a complete pulse width modulator including the power transistor. Cooling takes place via a SMD-cooling unit, which is thermally linked with flexible heat conducting material to the housing of the power supply. Despite the small dimensions, output performance of 5 V at 30 W, 12 V/15 V/ 24 V at 42 W in a temperature range between 0 to 50 °C and an efficiency of 80 % is obtained, while no fan cooling is needed. Depending on the application, the power supplies can be operated with or without redundancy diode. During redundant operation the current share operation guarantees an even distribution of power at the parallel switched ports. The V/I starting point for the parallel operation is configured ‘hard’ or ‘softly’ and allows the user to run high capacity loads such as batteries with the unit. This is a considerable advantage for portable measuring instruments, for instance. The indication of the output voltage is also configured as ‘high active’ and ‘low active’ signalling. The secondary power limitation, over voltage protection (OVP) with automatic restart and the over temperature protection improve the function and safety of the power supplies. The MTBF values of these power supplies lie at more than 400.000 hours at 40 °C ambient. With a range of 85 to 254 VAC (120 to 360 VDC) worldwide use is possible.



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