Challenges in the design of higher voltage DC/DC switching Regulators

Due to the market demand for SMD power modules, RECOM has created the RPM family of compact, low profile SMD switching regulators. The RPM concept delivers fully functional power supply building blocks with the switching regulator controller IC, power inductor, voltage setting resistors and input and output capacitors included in one stand-alone part, all in a standardized size low profile metal case and with a DOSA-compatible pinout. This family of five different low voltage products (RPMxx-1.0, RPMxx-2.0, RPMxx-3.0, RPMxx-6.0, and RBB10-2.0) has been further extended with three more higher voltage series to handle increased input and output voltages (RPMBxx-2.0, RPMBxx-3.0, and RPMHxx-0.5), without deviating from the same footprint or case dimensions, permitting an easy upgrade from existing designs.

However, the new RPMH and RPMB product series are not simply modifications of the existing RPM product series. Although physically they look identical, they are internally quite different. This white paper explores some of the design and component choice decisions required to cope with higher input voltages up to 36VDC or 65VDC respectively and higher outputs up to 24VDC and 3A load current.

Switching regulators have long been the workhorse of efficient conversion of DC power rails to lower or higher voltages, either directly for the load or as part of a distributed power architecture. The first designs from the 1950s used vacuum tubes and showed dramatic improvements in conversion efficiency compared with the alternative ‘linear regulator’ approach and also opened up the possibility of boosting DC voltages, only practical previously with unwieldy mechanical ‘vibrators’. It was only in the 1970s that the first switched-mode power supply IC controller appeared, the Silicon General SG1524, using ‘voltage mode’ control.

The success of this device opened the floodgates to alternatives using different control and conversion techniques. Today, there are many alternative topologies and control methodologies; from synchronous vs asynchronous, minimum on time vs minimum off time, averaged vs cycle-by-cycle current control or fixed frequency vs variable frequency designs with pulse-skipping for light loads, to name just a few examples. Although each topology has its own advantages and disadvantages, the selection and specification of the main power stage components; the switching transistor, the inductor and the output capacitor are still often key to achieving the optimum performance and the lowest possible losses under all operating conditions.

A measure of the development of the switching regulator is its conversion efficiency - over the years, the figures have been steadily climbing from around 70-80% to 97% and higher in the latest designs. Higher efficiency allows a higher power density, measured in watts/volume, representing how much power can be delivered from a given size without the converter overheating. Much of this improvement in power density is due to increasing integration; the low RDS(ON) switching transistor(s) are commonly integrated alongside the controller die in the same package and increasingly the inductor is also on-board. Peripheral functions such as fault monitoring, current sharing, synchronisation and sequencing have also been increasingly swept into the controller IC design. In addition, the switching frequencies are moving upwards from a few hundred kilohertz up to 1MHz or higher. Increasing the switching frequency allows smaller components to be used, but at the cost of increased power dissipation and higher EMI levels. The challenge is to find the optimum balance between physical size, thermal performance and EMC filter costs.

The RPMBxx-2.0 and RPMBxx-3.0 are higher voltage versions of the RPMxx-2.0 and RPMxx-3.0 respectively. The continuous input voltage can go up to 36 VDC and short excursions up to 38 VDC are permissible without causing permanent damage to the switching regulator. This input voltage range allows popular 24 VDC industrial bus power voltages to be used, accommodating any over-voltages and surges up to 150% - a condition that can sometimes occur in heavy industrial installations with long cable runs. This input voltage range also allows an unregulated 24V battery back-up system to be used to keep the system running during a power outage.

A higher input voltage range also permits a higher output voltage. The RPM series are limited to a nominal 3.3V or 5V output, adjustable from 0.9V up to 6V, which is a practical use for nominal 12V supply rails. However, with an input voltage range of up to 36V, the RPMB series also offers 12V or 15V outputs, adjustable from 9V up to 24V, as well as 3.3V or 5V outputs, adjustable from 1V up to 9V. This makes the RPMB series much more versatile. For example, the 3.3V version can be used with 5V, 12V, 24V or 36V supplies, making it a universal input power module.

So, for a 24V input and 5V output, the duty cycle would be around 21%. If the output voltage is increased to 15V, the duty cycle increases to about 64%. The worst case conditions are at 36V input and 3.3V output (smallest duty cycle of around 10%) and at 16V input and 15V output (highest duty cycle of around 94%).

The inductor value is determined by the following, more complex relationship:


This equation tells us two things:

1. The greater the input-to-output difference, the larger the inductor needed.
2. The higher the switching frequency, 𝑓, the smaller the inductor needed.

So for a fixed operating frequency and maximum load current and the same maximum input voltage of 36V, a larger inductance value will be needed for the lower output voltages than for the higher output voltages.

For the RPMBxx-3.0, the difference in the inductor value for the 3.3V output and for the 15V output is a factor of 7 and it makes sense to use two different inductor values; one for the lower output voltages from 1V up to 9V and another value for the higher output voltages from 9V up to 24V.

There is a growing trend towards the use of 48 VDC as an industrial bus supply voltage as there are several factors that make a 48V supply more attractive than the standard 24V industrial voltage supply:

1. It is more efficient. The power losses in a wire are proportional to the square of the current flowing through it, so by increasing the supply voltage from 24V to 48V, four times more equipment can be powered using the same supply cable. This allows upgrades to be made to the existing plant without having to re-wire the power supply cabling.
2. It is more secure. The push for mild-hybrid electric vehicles (for example, city cars with a downsized fuel-burning engine that use an additional electric motor to improve the acceleration and to allow short journeys to be purely battery powered) has meant that the cost for a Lithium-Ion 48V battery pack has fallen sharply. These maintenance-free 48V Li-Ion batteries are smaller, lighter and now often cheaper than an equivalent lead-acid battery-backed system.
3. It is safe. Operating industrial machinery on a 48 VDC supply means that the system is still classified as Safe Extra Low Voltage (SELV), meaning that additional insulation and safeguards against electrical shock are not required. This can substantially reduce the installation costs in heavy industrial applications which require more power than could be supplied by a 24V supply compared to the alternative of using mains powered equipment.

The output voltage of an automotive-grade 48V Lithium-Ion battery pack is specified by the LV148 industry norm:


It can be seen from the LV148 norm that any buck regulator powered from a 48V Li-Ion battery pack needs an input voltage range of ...