DC/DC converter - setting up supply systems

There is a wide range of potential applications for DC/DC converters. As their name implies, they convert voltages to match each other. They increase the voltage on long supply lines, thereby balancing out voltage drops caused by cable losses.

The converters use galvanic isolation to separate electrical circuits and protect sensitive loads by decoupling them.

That is why primary-switched power supply units from the Quint product range have an intermediate internal circuit used as a filter.

This makes it possible to separate, for example, grounded and ungrounded electrical circuits. Another advantage is that it protects critical loads against disruptive voltage fluctuations.

If, for example, a motor turn-on requires a lot of power for its start-up torque, this causes a brief voltage drop. The same occurs when loads with a high incoming capacitance are connected.

When these types of temporary interruptions occur, troubleshooting often turns out to be difficult and time intensive. In view of this, DC/DC converters provide an attractive option when sensitive appliances have to be supplied with a stable 24 VDC.

The fuse protection on the secondary side makes a significant contribution to load availability. In this context, three different cases can be distinguished:

• No fuse: if cable cross-sections are designed to match the output current’s maximum effective value, then it is not necessary to have a fuse on the secondary side to protect load circuits and lines.
• Reduced cable cross-sections: if there is a reduced cable cross-section, then the affected load feed lines must be secured with a suitable line fuse. The fuse is dimensioned according to the cross-section of the feed line.
• Selectivity: if a load that has short-circuited has to be turned off quickly to secure continued interruption-free operation of other loads, this requires selective deactivation. Devices that would satisfy this are in the Clipline range.

For selective deactivation, standard commercial circuit breakers are the most economical solution. They can trip electromagnetically or thermally via a bimetallic strip. To trip within a few milliseconds, however, the integrated electromagnet requires a significantly higher current than the circuit breakers rated current.

This is why selective fuse breaking technology from the Quint product range is now also available for DC/DC converters. The dynamic power reserve reliably trips standard circuit breakers within a few milliseconds.

Additionally, the devices supply six times the rated current for 12 ms. Faulty current paths are selectively turned off, the error is localised and important system components remain operational.

The selective fuse breaking impulse on the primary side is comparable with the impulse on the secondary side. This makes it possible to deactivate SFB technology in devices from the product range.

This is necessary when the upstream source cannot provide the requisite energy if there is a short circuit. A case like this can occur, for example, when input is being supplied with a battery with low capacity or a power supply without SFB technology.

If the SFB impulse has been overridden, then the converter will not retrieve electricity from the feed grid. When SFB technology is activated, the converter at the output delivers up to six times the rated current for 12 ms.

This means that the feed source has to provide a comparatively high current. The SFB switch is on the upper side of the DC/DC converter and can be activated using a screwdriver. The SFB technology is always activated in the standard setting.

A project planning matrix is useful in designing the secondary side. It describes the maximum wire lengths based on the device power class, cable cross-section and the circuit breaker.

On the primary side, cable cross-sections should be sized as large as possible and cable lengths should be planned as short as possible so as to keep the line impedance as low as possible.

The continuous monitoring of output voltage and current enables DC/DC converters to be comprehensively diagnosed. Preventative function monitoring visualises critical operation states before errors appear in the system. Voltage is controlled on the input side.

Active switching outputs and zero potential relays are used for remote monitoring. With the static power boost power reserve, devices constantly have 1.25x rated current. This even enables loads with a high start-up current to start reliably.

The Quint range includes five DC/DC converters with component widths between 32 and 82 mm and rated currents between five and 20 A. In power boost mode, output currents between 6.25 and 25 A are available; with SFB technology, those values increase to between 30 and 120 A.

The nominal input of 24 VDC is split into start and operation. A Quint converter requires at least 18 VDC to start up, while the input voltage can drop to 14 DC during ongoing operation.

The device will only switch off beneath this threshold. The upper threshold is 32 VDC during both the start-up and operation.

All DC/DC converters support a broad temperature range of -25 to 70°C, which is why they are suitable for applications that require high resistance to cold and heat. Shock resistance up to 30 g pursuant to IEC 60068-2-27 and resistance to vibrations up to 2.3 g pursuant to IEC 60068-2-6 ensure trouble-free operation, even under highly demanding mechanical conditions.