Project examples

High-Power DC/DC Converter

Development of a 28 kW DC/DC converter with an efficiency of more than 98%

In reply to a customer inquiry, we developed a demonstrator of a 28 kW DC/DC converter within only 6 months in 2017:

Technical data:

The focus of the customer was not on high efficiency but on a construction which was as inexpensive as possible. However, in this performance category mechanical construction and cooling are a cost factor which should not be neglected. It might therefore make sense to realise the converter itself as small and as light as possible and with a high efficiency, but on the other hand somewhat more expensive. The money invested here can then be saved again with the mechanical construction, cooling and energy costs.

For this reason an approach was used with which all power semiconductors are soldered directly onto PCBs as SMD components.
On the basis of previous experience, a platform consisting of single phases with a rated out­put of 400 W was developed. Twelve phases per PCB enable outputs of up to 4.8 kW per PCB and power module (19″ size: 3 RU/14 HP). This corresponds to a power density of about
4 kW per litre.

The 12 phases are operated in an interleaved manner, meaning that the PWM cycles are controlled with a time delay so as to cancel the ripple cur­rent as efficiently as possible. The advantage of the relatively small phases is that they can be constructed in a very compact manner and therefore with low magnetic leakage. This enables very steep switching edges and thus high switching frequencies. With high switching frequencies relatively small inductive com­ponents can be used, which are available at low cost in large quantities. Due to the 12-phase interleaving the base clock is, from the point of view of EMC, multiplied by a factor of 12 and its amplitude is considerably reduced. This means that the first significant emission per power module is, as regards EMC, 2.4 MHz. EMC filters can thus, despite the high output, be designed to be very small and inexpensive.

50% of the rated load could already be realised in laboratory operation without a heat sink and air flow. For cooling at full load, a heat sink was screwed directly onto the back of the PCB and fastened with a heat conduction pad. Cooling takes place exclusively exclusively through PCB vias, which is completely sufficient in view of the low power loss per transistor. For this reason special expensive PCB technology is unnecessary.

A small FPGA drives the interleaved 12 phases and reads the analogue value of the discretely constructed current controller with an ADC.

Due to the parallel connection of six 4.8 kW modules in one 3 RU 19″ subrack the peak out­put of 28.8 kW is attained: the efficiency is still above 97%. An only half-full second 19″ subrack houses the higher-level control module, which performs voltage control, monitoring and the interfaces with the outside world via an STM32 microcontroller. The energy self-supply and several other peripheral components are also located inside this subrack.

To visualise the performance data for the specific customer application a Windows 10 tablet was used, whose LabVIEW front end is supplied with data through the DC/DC microcontroller via a USB connection.

For possible series development there exists among other things the following optimization potential:

• Single phases with higher power output (1..2 kW) so as to decrease the costs of single-phase drive (gate driver, FPGA, monitoring . . .)
• Fully digital control for more flexible parameterization
• Optimisation of switching frequency -> trade-off between power density and efficiency
• Optimisation of installation space by downsizing the overdimensioned cooler and realising the whole of the device in only one 19″ subrack.

The use of this demonstrator successfully showed the advantages which a high-channel multiphase  DC/DC converter can offer at high outputs.

Fuel Cell

DC/DC converter for fuel cell vehicle

Development and production of a customized full functional 21-kW DC/DC converter in only 8 months.

Starting point

Our customer needed a DC/DC converter for his former prototype to charge a battery with a fuel cell. First, he scanned the current market – unfortunately the required component did not yet exist and therefore needed to be developed. Essential specifications: Functionality, robustness, time-to-market. Less important: Size, weight, suitability for series production.

Concept

Based on standard components, Michael Rübig developed an initial solution. The limited IGBT modules available on the market were purchased immediately to prevent delivery delays. A first circuit design was simulated using SPICE circuit simulation technology.

A housing for the converter

In order to install it in a car, the DC/DC converter was accommodated in a die-cast aluminium housing. Kaiser Ingenieurbüro, our partner in this development, designed a matching base plate with a water cooling system. Based on our standard microcontroller platform, the circuit diagram was generated. Two CPLDs were designed to support the controller in time-critical operations. Herbert Spinner implemented our customer’s control concept. An emulator for power electronics was developed to enable the software engineer to immediately begin creating the software while the 21-kW version was being worked on.

Test phase

Due to the limited amount of AC electrical power available at our premises, the initial commissioning with up to 12 kW proved unspectacular. During tests at the customer’s test facilities, some limits began to appear, requiring a derating at a higher temperature. All other requirements were satisfied up to 21 kW.

Product

The customer was provided with two identical converters. Since neither of the converters failed during the development phase, the IGBTs, which had been purchased as reserves, had never been needed.

Evil little diode?

Automotive electronics with EMC problems

Electromagnetic suppression of an automotive component. Time and manpower were scarce resources for our customer.

Starting point

Our customer had developed an automotive component that had failed in EMC tests. Locating the error was not a simple matter. A lack of manpower had hampered more intense troubleshooting. Preis Ing. performed the EMC examination.

Initial tests

We analyzed the behaviour of the electronic components in a variety of working environments and located a source of disturbance on the circuit board. A silicon diode in a switching regulator was exhibiting critical behaviour.

Finding a solution

The silicon diode was replaced by a Schottky diode to get closer to the threshold values. Nevertheless, at high operating temperatures, a leakage current appeared, leading to a thermal runaway. We therefore continued to search for a solution.

Further research and comparisons of Schottky diodes were conducted – the EMC limit was not achievable by merely replacing the diode.

The error shows up

The behaviour of the deficient component was increasingly examined in order to understand the principle of the disturbance effect.

Finallythe solution became clear: The layout of the circuit board was the source of the problem. The conductor tracks needed to be rearranged. Unfortunately, the customer was unable to implement the changes within a short amount of time.

A tangle of wires

However, we were able to help: We soon developed a proof of concept layout with manually wired connections on top of the PCB board. Using an optimized ground connection the seemingly tangled wires began to work more efficiently and did not exceed the threshold values.

The solution

In light of this new knowledge, we recommended a new layout to the customer. Some weeks later, we learned, that the EMC problem had been solved with new board generation.

Project management under pressure

Intensive support of an automotive supplier while developing his products towards the series production stage

Starting point

An automotive supplier contacted us, wanting to develop his product to the large-scale production stage to coincide with the manufacturing start-up of a new vehicle model. The customer needed project management support as well as technical support for solving hardware issues.

Finding a solution

Karl-Heinrich Preis began working full-time as a project manager on-site. Based on his background knowledge, the additional manpower and the wealth of know-how available at Preis Ing. as well as his experience in negotiations with OEMs, he has the ability to solve problems in a very effective and efficient manner. As an external employee, it is often easier to think outside the box and tackle the unsolved problems.

Solution and implementation

Weaknesses were soon located and discussed. With the additional manpower provided by Preis Ing., the customer’s team obtained the support it needed. Design changes to the circuit diagram were implemented, EMC examinations performed and solutions developed.
Furthermore, Preis Ing. handled the contact with automobile manufacturers and suppliers.
Finally, the customer was able to commence large-scale production successfully and on schedule!