Isolated Power Modules for Wide Voltage Ranges

Challenges for an electronic design engineer for industry control in terms of supply voltage from the 24V bus, with the use of isolated DC/DC converter modules.

 

Introduction

Every industry control circuit needs a supply voltage. But as the industry control circuit is only a small part in a bigger electrical environment, a lot of specification parameters have to be considered to choose the proper supply device. During the design of a DC/DC converter, the ever-present questions from the designer are:

• What is the input voltage range?
• What is the output voltage (range)?
• How much power do I need?

The following overview provides a brief summary of the essential facts that explain the story behind key parameters.

 

What kind of isolated power module do we need?

The following applications are typical for an industrial environment such as bottling plants, rolling mills, conveyor belts and printing presses:

• Interface/Bus Isolation – RS232, RS485, CAN, Interbus, Profibus
• Isolation of Digital circuits
• Sourcing isolated amplifier, analog-to-digital converter
• Measurement and data acquisition

All these applications have one thing in common, which is the input supply voltage is isolated from the bus voltage. But why should you galvanically isolate a supply from a bus or switching components in general? Galvanic isolation prevents faults that can propagate from the supply voltage into the bus and disturb its operation. The following schematic (figure 1) shows a typical application for an isolated power module. It shows the set-up for isolated RS485 communication with the essential functional units.

 

Figure 1: Typical application for an isolated power module.

 

The following functional units are necessary to set up isolated communication:

The Micro Controller Unit (MCU) provides the data for the RS485 transceiver and receives data from it. The signal isolation unit implements galvanic isolation of the signals using optocouplers. Galvanic isolation of the grounds between the signal isolation unit and the transceiver unit is achieved with a power isolation unit - a DC/DC converter power module.

 

Wide Voltage Range, Enhanced Application Area

In terms of input voltage range, 8V to 42V has been set as the typical industry range for decades. This voltage range has been selected for two reasons. First, it is inspired from existing relevant standards such as IEC61131-2 for programmable logic controllers (PLC). Second, field experience with electrical supply and installation conditions confirmed and agreed with this voltage range. It should be noted that the most commonly used rail voltages of 12V and 24V can be covered with this classical voltage range.

 

Figure 2: Industrial voltage range versus converter types.

 

Typically, industry uses 2:1 and 4:1 isolated converters to cover the wide input voltage range of 8V to 42V as shown in figure 1. As an explanation to the terminology, the first number of a 2:1 or 4:1 converter indicates the multiplication factor. And this value is multiplied by a minimum input voltage value to get the maximum voltage range value. This means for a 2:1 converter with a minimum input voltage value of 4.5V, the input voltage ranges only from 4.5V to 9V. If another voltage range is needed, a different type of module needs to be chosen. But none of the commonly available 2:1 and 4:1 modules cover the industrial voltage range in total.

The 5:1 SIP-8 module from Würth Elektronik cover with one type the complete industrial range of 8V to 42V. If we also take into account the adjustable output voltage (3.3V to 6V) of the SIP-8 module the added value is even more obvious. It can address common applications like CAN or RS485 for isolated converter with 1W need a supply voltage of 3.3V or 5V.

To supply an application with 3.3V and 5V, two types of power modules are needed instead of one with adjustable output voltage like the SIP-8. The 5:1 SIP-8 with wide input/output voltage range – reduces the type of different converters as well as the number of designs that have to be conceived, configured, tested, EMI conformity proven, built and logistically handled.

 

Wide voltage range – Input Voltage limits Basic considerations

For further considerations, it is helpful to have a common understanding of the structure of an industrial application, such as which voltages are present and why.

Industrial applications are known for long connecting lines between the separated parts of the application. Due to the spatial extensions the length of these connecting lines can be in the range of tens of meters.

Figure 3 shows the basic structure of an industry plant. Nowadays, the electrical supply is realized through cabinets with switched mode power supplies or transformer power supplies. Transformer power supplies are still common for higher power application supply. The separated parts of the applications are supplied through a dc bus. On site, every separated electrical load is connected via a sub distribution with 24V. It is easier to generate the 24V in a centralized cabinet and distribute them through a dc bus than to distribute the hazardous 230Vac / 400Vac. This also reduces the number of separated power supplies.

 

Figure 3: Basic structure industrial plant

 

Based on that structure there are three main influencing phenomena to the dc bus voltage:

• The voltage from the electrical supply
• Disturbances to the dc-bus from running cables that are placed in parallel
• Voltage drops due to current flow

In order to explain the lower voltage limit the voltage drop due to current will be considered:

 

Minimum input voltage – Lower limit

Usually the cable cross-sections for the DC-Bus is selected based on experience, rough estimation or with the use of tables. It should be noted that the commonly used design constraint for cable sizing is to avoid overheating. This means the voltage drop of the connecting line is mostly overlooked and therefore not considered. This voltage drop in turn means a difference in the voltage levels between output of the electrical supply (Vout) and the input of the application (+VIN).

For a better clarification a numerical example calculation is shown with real values that could be found in an industrial plant:

Based on the cable cross sectional area, A, cable length, l, and the specific resistance the electrical resistance, R, can be calculated as 1.376Ω using equation (1). If we consider a 100W supply, a rated current of 4A flows through the 24V DC bus. Based on equation (2) we will get a voltage drop across the connecting lines. That means, at the supply input of the application, e.g. PLC, the nominal 24V cannot be provided as it is only 24V-5.5V=18.5V. If we take a closer look at the PLC standard IEC 61131-2, the input voltage range for the supply voltage is defined to 19.2V to 30V. With a supply voltage of 18.5V, the undervoltage shut down of the PLC will be tripped and it will stop its operation.

The lower operating voltage limit of 8V of the SIP-8 enables a placement in an application far away from the supply cabinet.

In addition, an undervoltage detection circuit can be installed to protect against input voltage drop below 9V, in a typical 9V application.

 

Maximum Input Voltage – Upper limit

For the derivation of the maximum input voltage, the industrial plant (figure 3) has to be decomposed to its functional units which are the electrical supply, the DC-bus and the electrical loads. The electrical supply itself, e.g. transformer power supply without post regulation, is going to be supplied with 3x380Vac -15% / +20% whereby it is possible that the dc bus voltage differs from the nominal 24V. Furthermore voltage fluctuations of the ac input voltage due to relieved ac motors that are connected in parallel change the output voltage of a transformer power supply.

As already mentioned, the supply and the loads are connected through a dc bus, with some 10 meters cable connections. These cables can act as an antenna and receive disturbances from neighboring pulse loads such as frequency converters. These disturbances can then be distributed to the entire dc bus and every connected application. In addition, the input side physical connection of the different applications itself through the dc bus could lead to interactions. These include e.g. voltage spikes due to inductive induced switching transients and back feeding overvoltage from dc motors. As a basis for the explanation of the maximum input voltage value two parameters are relevant:

First, the value of the maximum output voltage of the electrical power supply that is technically possible. Second, the maximum peak value of an input protection element for a nominal 24V application. Every switched mode power supply or transformer power supply has one or more output electrolytic capacitors for stabilization and filtering the output voltage, see figure 3. These capacitors have a voltage rating of 35V for a 24V nominal output voltage. The IEC 60384-4, chapter 4.14 defines peak voltages and their frequency for the lifetime of an electrolytic capacitor that can be applied without visible damages of the capacitor or a capacitance change of less than 15%. The bearable peak voltage is set to the 1.15 times of the rated voltage. This leads to 40.25V for a 35V capacitor. To protect the input of the application against transient over voltages, Transient Voltage Suppressor Diodes (TVS) are commonly used. The diode conducts if the breakdown voltage VBR is reached and the energy of the impulse is bypassed through the diode and thus protects the load. No destructive voltage greater than the clamping voltage VClamp of the TVS can be present.

To protect a 24V application against transients, following basic guideline is a suitable reference point: The diode starts conducting at the maximum reverse voltage (VRMW) and the current is negligible with only few µA. Therefore, the nominal operating voltage of the load and its tolerances has to be above VRMW. For a nominal 24V rail, a TVS diode from Würth Elektronik with 26V VRMW is a common value. When the transient voltage reaches VBR the diode conducts and a current of 1mA flows. Due to the technology of a TVS diode, the breakdown voltage has a tolerance between a minimum and maximum value. Therefore a precise tripping point cannot be defined. For our 26V VRMW example we have the region between 28.9V and 31.9V. The diode is able to clamp the maximum voltage VClamp while conducting the maximum allowable current of IPeak. For a TVS diode with a 26V reverse voltage VRMW the clamping voltage VClamp is typically 42.1V. If you compare TVS diodes from various suppliers the characteristic values are all nearly in the same range. The TVS diode protects a DC/DC power module in the 24V system against overshoots above the absolute maximum ratings VINMAX. In general the higher this value is specified the easier it is to design the right TVS diode and the input filter. That means that it is more difficult to find the right diode if the nominal operating input voltage is close to the maximum input voltage VINMAX of the module. In conclusion, a specification of the maximum operating input voltage VIN of the SIP-8 isolated power module, 42V is a proper value to withstand the 40.25V and 42.1V transients as shown above.

 

Wide voltage range – Output voltage limits

3.3V and 5V are the common IC supply voltages in industrial control applications like:

• Interface/Bus Isolation – RS232, RS485, CAN, Interbus, Profibus.
• Isolation of digital circuits.
• Sourcing isolated amplifier, analog-to-digital converter.
• Measurement and data acquisition.

Common isolated power modules on the market provide a fixed output voltage. The SIP-8 isolated dc/dc module provides an adjustable voltage range because in some cases it is helpful to set the output voltage a little bit higher than the nominal operating voltage value of the load so that it is more robust against, e.g. voltage dips. Therefore the bulk capacitor of the load can also be reduced in its capacitance value because the undershoot at the module output is less.

 

Power Boost – “Power more than you expect”

In an industrial plant with its variety of applications, a lot of interaction between the supply, the loads and interference takes place. Many parameters are hard to calculate and hence can change during the implementation

One important point is the power that is needed to supply a load. Therefore, it is useful to have some kind of flexibility without changing the design.

 

Figure 4: Power boost abilities

 

A power boost feature is the ability of a power module to provide more than the nominal output power. There are two possible kinds of Power Boosts: static and dynamic. The static power boost provides a durable extra power. The dynamic Power Boost even provides a multiple of the nominal power at a limited time. It needs periodic cool down cycles. During the power boost event, the maximum ambient temperature rating is lower. This is related to the increased power dissipation of the power module.

Thanks to the ability to deliver more than the nominal power, the following additional positive aspects enhance the applications of the power module:

• Unforecasted increases in load demands are supported – see figure 4 [1].
• Monotonic charging of capacitive loads is provided without voltage dips – see figure 4 [2].
• Backup power for momentarily higher energy demands of the application.
• Tripping input fuses of downstream applications in case of an overload (ensures higher current for safe tripping) – see fuse tripping characteristic see figure 4 [3].

To fulfill all of these requirements the VISM 17791063215 SIP-8 from the “Fusion” series was developed.

 

Figure 5: SIP-8

 

This new MagI³C module can work with 9V / 12V / 24V and 36V-bus voltages with its ultra wide input voltage range from 8V to 42V. The module is a functionally isolated DC/DC converter that includes the PWM control IC, power stage, transformer, input and output capacitors. The precisely regulated output voltage can be adjusted from 3.3V to 6.0V. The output is continuously short circuit protected. The 1W power module can offer triple of the rated power capability with Power Boost. Therefore, applications with a peak power consumption of up to 3W can be supplied. An ON/OFF Pin turns the module into a remote-controlled power source. By virtue of its unique features, the module is suitable for applications like supplying interfaces, microcontrollers, industrial control and test & measurement equipment.

 

About the Author

Timur Uludag works as a Product Manager at MagI3C Power Modules, München und Umgebung, Deutschland. He is particularly skilled at product management, electronics and project management. He earned his Engineer's Degree in Mechatronics at the Regensburg University of Applied Sciences in Regensburg, Germany. He studied accompanying training in medical technology and additional study in international competence also at the same university.

 

This article originally appeared in the Bodo’s Power Systems magazine.