Standard-Stromversorgungslösungen und Anwendungen für AC/DC- und DC/DC-Leistungswandler

The step-down (buck) converter is able to convert a higher DC input voltage at its input and transform that into a stabilized, but lower, DC voltage at its output. See Figure 1.



Buck converters have decided advantages in that their losses are quite low, and will be able to reach efficiencies higher than 97%. Their switching frequencies will be up in the many hundreds of kHz; this will lead to a very good power density architecture via the use of smaller inductors along with a fast transient response capability. When the FET switch is disabled, during its switching cycle, the output level will drop to zero. This enables the no-load power consumption to achieve a very low level. All of these advantages make the buck regulator quite an attractive replacement for a linear voltage regulator in quite a few applications.

There is a disadvantage with the buck converter, due to the fact that the Pulse Width Modulation (PWM) regulator feedback circuit will need to have a minimum output ripple to reach proper regulation. This is because the regulation is usually cycle-by-cycle. In addition, the output ripple will be dependent on the duty cycle, which will be a maximum at 50%. In this case, it will not possible to achieve ripple and noise down to μV levels, which can be achieved by linear regulators which are non-switching.

In the event that a designer requires a cleaner power supply, a linear regulator can be placed after a buck regulator, in order to achieve the best performance from both topologies. This is made possible since the linear regulator Power Supply Rejection Ratio (PSRR) will greatly reduce ripple/noise at its output.

A step-up, or boost converter power design, will convert a lower input voltage into a stable and solid, higher voltage at the output. Figure 2.



The advantage of using a boost converter architecture is that the output voltage is able to be varied with the mark-space ratio (this is the ratio between the high time and the low time). This is actually the duty cycle. For example, a 50% duty cycle is when the voltage high time is equal to the voltage low time of the Pulse width modulator (PWM) signal in order to be equal to, or ‘boosted’ above, the input voltage VIN.

This functionality enables the use of the Boost Converter to be quite useful, for example, in boosting a low battery voltage output to, a more useful, higher voltage level. However, a boost ratio that is more than two or three times the input, will make feedback stability much more difficult to maintain at a solid level. In addition, since the input current pulses will increase proportionally, as compared to the boost gain, a converter which triples its input voltage will draw three times its input current. This level of pulsed input current can even lead to increased EMI, as well as, voltage drop levels at the battery input leads.

One other disadvantage, of the boost converter topology, is that its output cannot be switched off, without the addition of a second switch, placed in series with the input. This is because just disabling the PWM controller alone will not disconnect the load from the input.

Power designers must be aware that, with the Boost Converter, they should not allow its input voltage to rise higher than its output voltage. In such a case, the PWM controller could then keep S1, in Figure 3, open continuously. In this case the input and output would be connected directly from L1 and D1 without regulation. Highly damaging currents may flow that can fast damage the converter along with its load. If designers are not able to avoid this condition, a replacement topology, which permits both buck and boost operation, will be necessary.

The buck-boost converter, aka inverting ‘flyback’ converter, is able to convert an input voltage to a regulated, negative output voltage which is able to go higher or lower, than the absolute value level of that input voltage. The image in Figure 3 shows a simplified schematic of the Buck/Boost converter.



The buck/boost converter has a nice feature: the input voltage may be higher or lower than the regulated output voltage. Having such a capability, can prove useful for any applications that may require a stabilized voltage output from a battery which can have a terminal voltage between 9V, when discharged, to a fully charged 14V. Buck/Boost converters can prove quite useful in stabilizing the outputs of photovoltaic cells. Solar cells are able to deliver quite high voltage and current when in bright sunlight, but will revert to low voltage and low current when the sun is blocked or its light is diminished on a cloudy day. As the voltage/current relationship changes in such an example, a buck/boost converter can be quite useful for maximum power point tracking (MPPT) since the input/output voltage ratio can be continuously adjusted.

The greatest disadvantage of the buck/boost converter is its inverted output voltage. Here, if used in conjunction with a battery, the output voltage inversion is irrelevant, due to the battery supply which can be left floating and the -VOUT may then be connected to ground, in order to give a positive-going output voltage. Another disadvantage here is that the switch S1 is without a ground connection. This means that a level translator will be needed in the PWM output circuit; this can quickly add cost and complexity to the design.

DC/DC converters have a great many applications in the electronics industry. In this section we will discuss the primary application areas for DC/DC converters.

In general, the main requirements for DC/DC power supplies to be used in automation applications are the importance of isolation. This would ensure that the power design avoid any interference with other equipment. The DC/DC should also be designed into a system, taking care to avoid any ground loops or potential differences that may disturb the operation of the automatic control system.

One good example, of isolated DC/DC converters in automation, would be to help break up ground loops. This will allow separation of parts of a circuit, sensitive to noise, from the source of that such noise. A regulated and isolated DC/DC converter can help minimize electrical noise using that isolation. The choice of an isolated and regulated DC/DC converter, for voltage conversion, will enable prevention of electrical noise isolation, as well as immunity to line surges and dropout/dips.

Internet of Things (IoT) and the Industrial Internet of Things (IIoT)

Basically, the IoT is consumer-oriented while the IIoT, a subset of the IoT, is industrial-oriented.

The IoT and the IIoT are systems of inter-related connected objects, connected on the Internet, which makes it possible to collect, share and transfer data over wireless networks with virtually no human intervention. These two incredible systems feature: distributed intelligence, countless interconnected sensors and actuators along with decentralized control.

The IIoT is made up of devices which collect large amounts of data vs. IoT devices which will generate a relatively lower volume of data. An IIoT example is a single turbine compressor blade that can generate greater than 500 Gb of data daily.

Both the IoT and the IIoT are not only about connected devices, but also regarding the information those devices can collect. Useful insights can be gained from such information.

Rugged, Isolated DC/DC converters are typically needed to power IIoT sensors such as those monitoring the condition of industrial machines. Large power surges may occur with the starting and stopping of heavy machinery; Isolated DC/DC converters with high isolation, in the order of 3 to 4 kV, are necessary here especially in protecting sensors.

The IoT and the IIoT both create smart spaces, environments/objects via the use of sensors which can communicate to gain intelligence.

Some IoT examples can be in a business office setting in which area lighting will have smart capabilities to adjust to ambient light levels, and even to the number of people present. Within the IoT, various machines are able to monitor their own functionality, or in homes which adapt to daily routines. The IoT will automatically save energy while providing human comfort.

The IoT also needs power designers that will choose cost effective and good power density DC/DC power supply modules. Applications in this space include Smart Offices, that are filled with many intelligent sensor nodes. Energy harvesting is another excellent application which uses DC/DC converters.

Power supplies for the IoT and the IIoT will need to be very efficient, at low and full load levels. DC/DC power supplies, that are designed to handle fast transient and dynamic load currents, will be best suited for these kinds of environments. Such power supplies have to have physically compact features, be reliable and, need to be cost-effective. These DC/DC power supplies will be as ubiquitous as are the sensors, processors, radios and actuators which they are designed to power.

See this video entitled “What is the ‘Internet of Things.”

Industrial High Power DC/DC Converters

In industrial applications, with fork-lift trucks and other materials handling equipment, these applications use traction batteries rated from 320V to 600V. There is a series of on-board power supplies which can optionally generate 24V or 48V from the high voltage battery with a rating 4kW with high efficiency. 19“ rack product versions can be baseplate- or liquid-cooled.

Electric Vehicle charging stations

In this present world of ever-decreasing availability of fossil fuels, electric vehicles (EVs) have greatly grown in their usefulness as a solution to reduce fossil fuel dependence which will help sustain our natural resources.

One of the prime features in EV development is their growth of battery capacity, lower charging time, greater lifespan and improved power density.

Electric Vehicles are vastly increasing in numbers on our roads. These vehicles (cars, buses, trucks, etc.) will need more charging stations, which are fast being deployed along major roads and highways as well as in local areas.

EV charging stations power levels are fast reaching several kW of power. DC/DC converters with increased isolation, with high insulation strength, are a critical part of EV charging stations.

The types of DC/DC converters which can be typically designed into chargers for electric vehicles.

The EV charging designs typically employ isolated DC/DC converters with power isolation. Converters exist that are cost effective and can be in a SIP package with 1kVDC isolation. The isolation will help to reduces EMI.

This type of DC/DC can be soldered into a PC card in the same way as any integrated circuit (IC). These converters have a 5VDC input and a regulated 5VDC output voltage which is short-circuit-proof. Their power is at 1W. These devices have a temperature range of -40oC to +85oC. The application for this DC/DC converter in EV charging is for regulated power in the network interface area of the EV charger. This part enables communication between the EV devices and the Internet.

A key area application, for industrial high power DC/DC converters, is in railway applications. This sector has applications for railway rolling stock, on-board and trackside application, industrial applications, high voltage battery-powered applications, and distributed power supply architectures.

DC/DC converters are typically used in railway environments for converting DC battery voltages to a lower voltage for use in a plethora of control and energy circuitry. Railway rolling stock designs have a DC power distribution system that employs batteries which will be deployed to maintain electrical power in case of generator failures.

DC/DC converters, for railway use, have to be constructed in accordance to EN 50155 to ensure that harsh environmental conditions do not affect their proper operation. Potential harsh conditions of heat, frost, vibration, and mechanical impact may cause serious damage to electronic assemblies. DC/DC converters used in railway technology are exposed to extremely tough conditions and must work properly and reliably for their entire service life.

Engineers are seeking DC/DC converters that can withstand these potentially catastrophic conditions, as well as being certified for railway applications.

1.2kW, 3.5kW, and 4kW DC/DC converters have flexible input/output voltage levels and have railway application approvals. This series is also suitable for general industrial use, with five DC input ranges available covering 16.8V to 170V and three DC output variants, nominal 24V, 48V and 110V.

The power density of a DC/DC converter is a measure of the amount of output power divided by the volume of the DC/DC converter. This is measured in ‘watts of output power per cubic centimeter’. A high value provides an advantage to the designer, allowing more power for a given volume.

Power in small packages: 3D power packaging for low power DC/DC converters

Low power, non-isolated DC/DC switching regulators are cost-effective solutions which maintain increasing demands for better performance with improved power density. Their packages need to be small so that they can compete with discrete designs. Non-isolated DC/DC designs are challenged to be highly efficient with a small sized design. The use of faster switching techniques, such as in wide bandgap designs, will help reduce size while maintaining a good efficiency.

RECOM achieves high power density with an over-molded ‘flip-chip-on-lead frame’ construction. EMI is reduced due to smaller switching current loops in the design.

See the YouTube video to increase power density with 3D Power Packaging: ‘Big power in small packages’

High power density DC/DC Converters for Industrial and Electro-Mobility Applications

The RP and RPA series are board-mounted DC/DC converters with a power rating of 30W to 240W and a high power density up to 4.5W/cm³. This is one of the highest power densities currently available on the market for this class of DC/DC power.

One of the main reasons these power devices reach such an excellent power density is by the use of planar transformers in their design which reduces the overall package size without compromising efficiency or output power. This construction method supports a fully automated production process which leads to high reliability and excellent cost-effectiveness.

These two series are best for space-constrained industrial, test-and-measurement, transport, railway and other demanding applications that require a 4:1 input voltage range and even an excellent 10:1 input voltage range.

Isolated DC/DC converters can have these advantages:

1. Isolating the grounding between input and output means that the grounding scheme of the DC source can be different from the load on the output
2. Designers will be able to “map” a large range of different levels of DC voltage on the input relative to the output
3. If they have very low capacitance on their output, they will more readily and safely allow multiple, isolated DC/DC converters to be placed in parallel on the same DC bus

Non-isolated DC/DC converters have this advantage:

1. Their DC input and output are connected to the same potential. These types of DC/DC converters are Buck, Boost or Buck-Boost.

A bidirectional DC/DC converter is a relatively new architecture in many new applications, including automotive, server and renewable-energy systems. Low-voltage bidirectional DC/DC converters are typically non-isolated.

Three key applications for Bi-directional DC/DC converters are automotive, server and renewable-energy systems.

The transformer in Figure 4 has two primary windings of 115V which are shown connected in parallel or series via the input voltage selector switch. The two series-wired 6V secondary windings lead to a nominal 12VAC output which becomes rectified by the bridge rectifier BR and then DC-smoothed by the output capacitor, C. This gives a typical output voltage of about 14VDC. The full-bridge rectifier uses a four-diode typical configuration.

There are many key applications for AC/DC converters and in this section, the primary application areas for AC/DC converters will be discussed.

When electronic devices do not come into direct contact with the patient and are only handled by trained operators they are classified in category Means of Operator Protection (MOOP), which means they typically only need to meet 60950-1 and 62368-1 ITE standards for Laboratory Environment Compliance.

Medical electronic designs use a Means of Patient Protection (MOPP), an electrical safety standard set forward by standards organizations across the globe; these are the American National Standards Institute (ANSI), Canadian Standards Association, and European Commission in IEC60601-1. MOPP safety standards set basic the safety requirements for medical electrical equipment.

Medical applications for AC/DC power supplies with 2 x Means of Patient Protection (MOPP) safety approval for mains-powered medical applications. The highest level of safety protection comes from using 2 x MOPP. Note that some power supply manufacturers typically highlight power supplies as meeting medical approvals whether the unit has 1 MOPP or 2 MOPP.

These compact medical grade power supplies have a universal AC input voltage range, 4kVAC isolation, low standby power consumption, active PFC (> 0.95) and do not require a minimum load.

See this podcast for more details

Power density is a measure of power output per unit volume. This is important in a power supply, especially in a space-restricted area.

Efficiency is a prime issue in power electronics. Improving efficiency in power supplies will improve power density. One sure way to increase power density is to reduce component sizes. Designers should choose the smallest possible capacitors, inductors, transformers and heat sinks that will meet design needs.

Applications that require the highest efficiency, (in the order of 99%) and high power density (73 W/in3), are high-end servers and telecom. AC/DC converters are used in such an applications.

Modern industry demands decreasing size, footprint, with solid power density numbers than AC/DC predecessors. Modern advances in switching controllers, topologies and components enable AC/DC power supplies to have 2X the power density than in previously designed converters. New designs must improve upon safety, reliability, efficiency, and performance in order to be competitive.

Designers must know how much input AC input voltage swing their selected AC/DC converter will be handling in their design. In most industry in the Western hemisphere, the output from the AC mains range from a nominal 100 VAC up to 277 VAC for use around the globe.

It is critical that the AC/DC converter function efficiently over the entire load range from full load down to light load, and even to no-load conditions.

Most AC/DC converters are internationally safety certified to UL/IEC/EN standards, with Certification Body (CB) Reports. Saving energy is critical to customers and many applications switch automatically into standby in order to reduce power consumption.

In the Test & Measurement sector, devices can span from desktop products to server rack installations. Such systems need AC input voltages which can span a nominal 90VAC to 277VAC, and often times higher for some industrial applications.

Low power, PCB mounted AC/DC converters, range from 3W to 20W are frequently designed into these systems. If higher power is needed, such as from 40W to 550W, designers may choose chassis-mounted options.

Operating without a fan environment is often challenging in a Test & Measurement environment. All selected AC/DC converter solutions for this environment must be able to provide useful power at high temperatures without the benefit of forced air.

230W and 550W products are available with the addition of baseplate cooling.

Safety certifications for different end-application environments are available according to IEC/EN 61010, IEC/EN 62368, IEC/EN 60601, EN 60335 requirements.

Good AC/DC converters will need to meet ElectroMagnetic Compatibility (EMC) standards without the use of any added components. All AC/DC devices will need to have low mains leakage current that is critical in Test & Measurement applications, and in particular in medical environments. Applications that require sensitive measurements, will need low output noise AC/DC power supplies which is a significant benefit to designers in many Test & Measurement systems.

Counterfeit power supply components can wreak havoc in the design arena. These devices can malfunction or not even function at all. If they do function, they can still be a danger to injure people or even cause a fire. These will severely hurt a supplier’s reputation.

It is highly recommended that buyers purchase known manufacturer products from their global distributors.

Savvy designers and purchasing people know that acquiring electronic components and power supplies, from trusted distributors, and manufacturers will be the best route to ensure a fully functional, reliable, and safe end design.

(1) Chapter 1 An Introduction to Buck, Boost, and Buck/Boost Converters, RECOM DC/DC Book of Knowledge
(2) Chapter 2 Linear AC/DC Power Supplies, RECOM AC/DC Book of Knowledge