Alessandro Giuseppe Antonio Anastasio Volta and André-Marie Ampère never imagined it, but today our highly technical world can no longer function without voltage and electricity. Pretty much everything would collapse without an electrical power supply. From simple consumer devices to LED lighting, computer networks, a medical device that helps us stay healthy, satellites or the Hubble telescope that we use to explore distant worlds. And then there are the navis and the electric vehicles. Nowadays, ships would find it difficult to reach their port of destination without satnavs, and aeroplanes would need an additional navigator. And what about “e-mobility”? Electric vehicles have to be safe and now they are even supposed to drive autonomously! The car is being rethought. There is no longer a steering wheel. And we sit in the back seat. Three on-board computers, countless sensors, endless software, artificial intelligence! And when everything works together and is sufficiently developed, we will rely on the technology, pull down the blinds and arrive relaxed. Fine, still a bit of a future, but it already exists for vehicles for the disabled.

Reliability

The big fat traction battery powers the engine. And all the other consumers, such as the on-board computers with their AI programmes, are also powered from the traction battery, but they need lower voltages, such as 12V, 18V, 24V, and so on. And the actual microcontrollers and their peripherals work with 5V, 3.3V or 1.8V and these are again derived from the 12V. And now we have already arrived at the first hard requirement for DC/DC converters. They have to work just as safely and reliably as the entire computer technology. And unfortunately, for physical law reasons, they also get warm. And whenever something gets warm (possibly hot), there is ageing. At low temperature increases this is insignificant, but the hotter the components get, the more ageing processes set in. In the case of electrolytic capacitors, even disproportionately. With the existing component technology, only good heat dissipation can help. Fans are usually out of the question, so the only way to dissipate heat is via heat-conducting material, such as a base plate made of aluminium, which transfers the heat directly to the enclosure. And then there is a second “trick”: instead of using one large DC/DC converter, several small converters are distributed over a large area (distributed power architecture). This also has the advantage that the voltages are generated where they are needed. From a 12V rail, an internal supply bus so to speak, 3.3V are generated directly at the microcontroller, and the analogue circuit for processing the sensor signals receives its own 12V/5V converter in its immediate vicinity. This architecture is also known as PoL (Point of Load). In addition to distributed heat, it also has EMC advantages.

Size and EMC

Based on what has been said so far, we can already see that the importance of small, highly efficient DC/DC converters is increasing and determining the architecture of a device.

In addition to the requirements already mentioned, the converters must have the smallest possible dimensions, provide a highly constant and accurate output voltage that remains within defined limits even when the load jumps. They must have a wide input voltage range and also be very cost-effective. In most cases, the requirements can practically only be realised with switching regulators, because a series regulator would generate too much waste heat. However, switching regulators require EMC-compliant filtering at the output and sometimes also at the input. When selecting a converter, it is therefore important to consider which standards it already fulfils and which additional filters are required.

Converters commonly available on the market range in power from a few hundred milliwatts to several hundred watts. As different as the power ratings are, so are the common designs. These range from SMD, SIL or DIL for the power range up to approx. 10W, to the inch sizes for powers up to approx. 40W, to the brick converters for high power. A so-called full brick measures 117mm x 61mm.

An example

To give an example of a switching voltage regulator, we deliberately choose a low-power converter, because high efficiency and high power density are particularly difficult to achieve with small converters. It is the RPX-4.0, a miniature step-down converter in a thermally optimised QFN package with an extremely high power density.

It weighs only 2g, making it particularly suitable for weight-constrained applications such as flying drones, acceleration circuits and portable devices. At 5V output voltage, the power density is 180W/cm³!

The wide input voltage range covers all common battery voltages. Therefore, it can be used in practically all battery-powered products.

The useful CTRL input can be used to put the RPX-4.0 into sleep mode or to switch a high output current with a low-power control signal. It can also be used very easily for an emergency stop function. It is unusual for such a small device, the RPX.4.0 can supply up to 4 amps – sufficient for stepper motor drivers, high starting current applications or laser power supplies.

The RPX-4.0 is designed for 3.8 to 36V input voltage and delivers – adjustable with two resistors – 1 to 7V output voltage with 4A. With its compact dimensions of 5mm x 5.5mm and a height of only 4.1mm, the step-down converter module sets new standards in power density. The RPX-4.0 is fully protected against undervoltage, short circuit, overcurrent and overheating.

Efficiency

The high efficiency of the RPX-4.0 allows operation at full power up to 65°C and with power reduction up to 90°C depending on the variant and mounting arrangement. The efficiency curve rises for low output powers, i.e. the converter can also be used advantageously for medium and low output powers. This is achieved, among other things, by intelligent control of the switching frequency and an integrated and shielded storage choke, which additionally ensures low EMI. The design follows RECOM’s ‘3D Power Packaging®’ technology for high power density and uses a flip-chip on leadframe construction. It comes with a 3-year RECOM warranty. An RPX-4.0-EVM-1 evaluation board is also available to allow customers to test all product features and optimise filtering to meet target system requirements.

Lossless power source

If LEDs are fed from a voltage source, they require a series resistor for current limitation and operating point adjustment. Significant power loss is generated in it. It can be avoided if a current source is used to power the LED. With a voltage regulator and an operational amplifier, a simple circuit can be realised that has the current source characteristic with respect to the LED:

Size comparison

With RECOM’s 3DPP^® technology, the RPX-4.0 has a significantly higher power density than conventional converters, as shown in these comparative images, all at the same scale. Despite its incredibly small size, the excellent internal thermal management design still allows full load operation without forced cooling.