Cutter Electronics

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Everyone knows this situation: a regulated voltage supply is required. In itself, nothing difficult. Not much board space is available - no problem. The input voltage supply is unstabilized and can vary over a wide range - so what? The load is also not exactly steady - so tell me something new! BUT: what if everything must be accommodated in a small case without ventilation and no heat-sinking can be used? Or if the regulator losses must be kept low because the unit is battery powered and you can't afford to waste precious battery life as heat? Then it already looks more difficult using the standard off-the-shelf linear regulator solutions….

The remedy is the new switch-mode converter range from RECOM that unites both the specifications and the simple application of the ubiquitous 78xx linear regulators with an extremely high efficiency of up to 97% to give a minuscule heat generation that avoids all of the physical and mechanical problems associated with heat sinks and waste heat transportation.

Just like standard linear regulators, the R-78 series has a drop-out voltage of 1.5V and an input voltage range that extends up to 34V: an input range of up to 7:1! All of the R-78xx-0.5 series deliver an output current of up to 500mA and R-78xx-1.0 types with up to 1A output current are in preparation The output voltages are available off the shelf in all of the usual values of 1,8V, 2,5V, 3,3V, 5V, 9V and 12V as standard. Non-standard values can very simply be set in the production by special request.

However, these are not the only advantages of this new converter. Another attention-grabber is the design: the R-78xx-0.5 is accommodated in a SIP3-plastic housing with the measurements of only 11.5 x 7.5 x 10.2mm, therefore approximating the footprint of a 78 series linear-regulator in TO220 casing without a heat-sink but with a clearance gap to the other components on the board. The pin-out is identical: Pin 1 is the input voltage, Pin 2 the common and Pin 3 the output. An external trim of the output voltage by means of a fourth pin will be available as an option in the coming months, as well as a 79xx series for negative outputs as a counterpart to the analogue 79xx regulators and implementations for both types in SMD designs.

The RECOM R-78xx-0.5 is continuous short circuit protected and incorporates a thermal shutdown protection function that switches off the converter if the internal temperature exceeds 160°C. The short circuit protection circuit restricts the input current with a shorted output to typically 25mA and so helps to avoid further damage to the supply circuitry during fault conditions. The operating temperature range is from -40°C to +70°C with full load, and extends from +70°C to +85°C with a derating to 80% of maximum load.

The outstanding efficiency of up to 97% comes mainly through the use of a switch-mode design using the Buck principle. The 30 years of experience of RECOM in the design and development of DC/DC converters has enabled the high specification of this non-isolated step down converter, in particular the reduction of internal losses to only a few percent. This switch-mode design inserts itself seamlessly into the recently introduced RECOM INNOLINE as a supplement to the non-isolated point-of-load converters with similar high efficiencies and consequently expands the range for lower power and miniaturised designs. It presents itself as ideal supplement for distributed power supplies, where together with the isolating DC/DC converters from the RECOM POWERLINE and the R-5xxx, R-6xxx and R-7xxx series from the RECOM INNOLINE, allow supply voltages to be down-converted as close to the load as possible. Consequently the entire distributed power supply chain can be supported by RECOM products.

A frequent and normally justifiable point of criticism concerning switch mode designs over linear designs is the noise generated on the outputs as well as the disturbances reflected back into the input supply. The RECOM R-78xx-0.5 series has a relatively high operating frequency of around 300kHz which is easy to filter out internally and so generates very low inherent noise, as the first EMC-Tests confirmed. The switch-mode converter requires no external components and has typical values of ripple and noise of 30-50mVpp, which can be even further reduced with external filters. RECOM recommends that designers who need very low ripple and noise levels should insert low pass filters: a simple first order LC-low pass output filter with a corner frequency of approximately 10% of the operating frequency brings the ripple and noise levels down to 5mVpp or below Also the reflected noise (disturbances that a switch-mode design generates on its inputs due to its switching frequency) are inherently very low with the R-78xx-0.5 and can be if wished further reduced with external filtering of the input supply. The compromises that a designer must make in order to insert a switch mode regulator are therefore not great, as in most cases the standard application with no external components is perfectly acceptable.

Of course, this switched mode converter won't replace completely the analogue linear regulator that is used the world over. For one thing, the price is a deciding factor. A linear regulator can be bought even in small quantities for a few tens of cents while any switched-mode solution will always be many times more expensive. But the total costs needs to be examined: input and output capacitors are recommended for analogue linear regulators, but these components are already integrated into the standard switching regulator design and while the switching regulator needs no heat sink, the linear regulator requires one in most cases which both uses up valuable board space and costs money and assembly time.

However, this is not all: presuppose that the circuit shall be placed in a small, non-metallic, airtight case - then a heat sink rapidly loses its effectiveness. Even a large heat sink will become useless if air convection and air circulation are blocked. The only help here is to solve the problem at the root cause. If the regulator is highly efficient with low internal losses, then almost no heat is generated that needs to be taken care of. Here, the switching regulator comes to the fore because it generates so little waste heat in the first place.

If one now compares the entire costs of a power supply solution - i.e. switched mode regulator vs. linear regulator + heat sink + assembly + maybe forced cooling with fans, as well as the further expenditure for external components, then clearly the cost relationship is not so unbalanced. Possible problems in design, production and assembly of heat-pipe or chilled circuits and the question of whether it makes any sense at all for sub-15W supplies are left to the interested reader and will not be discussed further here.

If all of the above points mentioned above in this article are considered, even if only some of the aspects apply, then maybe the alternative of using a switching voltage regulator makes more sense than at first meets the eye.

Application Note for Creating Negative Output

New Extended Package offers extended input voltage to 34Vin on the 1 Amp version and offers an 'L' package option for right angle mounting to lower the package profile

New Datasheet on 1 Amp version

New High input voltage now available offering 30-72Vin DC in outputs from 3.3V to 24VDC and is also available in the new 'L' right angle package

New Datasheet available on the High Input Voltage Version

New High Current version is now available offering input range of 4.75 to 18V and outputs to 1.5 Amp at 1.5Vout to 6.5Vout

New Datasheet on 1.5Amp Version

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