A supply voltage is required by every industry control circuit. However, because the industry control circuit is only a small part of a larger electrical environment, many specification parameters must be considered when selecting the appropriate supply device.
The designer’s constant questions during the design of a DC/DC converter are:
- What is the voltage range for the input?
- What is the voltage range of the output?
- What amount of power do I require?
The overview that follows provides a brief summary of the key facts that explain the story behind key parameters.
What kind of isolated power module do we need?
Bottling plants, rolling mills, conveyor belts, and printing presses are examples of typical industrial applications such as:
- Bus Isolation/Interface – RS232, RS485, CAN, Interbus, Profibus
- Digital circuit isolation
- Obtaining an isolated amplifier and an analog-to-digital converter
- Data collection and measurement
The input supply voltage is isolated from the bus voltage in all of these applications. But why should you galvanically isolate a power supply from a bus or from switching components in general?
A galvanic isolation prevents faults from propagating into the bus and disrupting its operation. A typical application for an isolated power module is depicted in the schematic figure below. It depicts the configuration for isolated RS485 communication with the functional units that are required.
A typical use for an isolated power module.
To set up isolated communication, the following functional units must be present:
The RS485 transceiver receives data from the Micro Controller Unit (MCU), which sends it data. Using optocouplers, the signal isolation unit performs galvanic isolation of the signals. A power isolation unit, a DC/DC converter power module, is used to achieve galvanic isolation of the grounds between the signal isolation unit and the transceiver unit.
Broad Voltage Range, Expanded Application Area
For decades, the typical industry input voltage range has been set at 8V to 42V. This voltage range was chosen for two reasons.
For starters, it is based on existing relevant standards, such as IEC61131-2 for programmable logic controllers (PLC). Second, on-the-job experience with electrical supply and installation conditions confirmed and agreed on this voltage range.
It should be noted that this classical voltage range can cover the most commonly used rail voltages of 12V and 24V.
The range of industrial voltages versus the different types of converters.
As illustrated in figure above, industry typically employs 2:1 and 4:1 isolated converters to cover the wide input voltage range of 8V to 42V. To clarify the terminology, the first number of a 2:1 or 4:1 converter represents the multiplication factor. The maximum voltage range value is obtained by multiplying this value by a minimum input voltage value.
This means that the input voltage range for a 2:1 converter with a minimum input voltage of 4.5V is only 4.5V to 9V. If a different voltage range is required, a different type of module must be selected. However, none of the commonly available 2:1 and 4:1 modules cover the entire industrial voltage range.
Würth Elektronik’s 5:1 SIP-8 module covers the entire industrial voltage range of 8V to 42V in a single type. The SIP-8 module’s adjustable output voltage (3.3V to 6V) adds even more value. It can address common applications such as CAN or RS485 for isolated converters rated at 1W and requiring a 3.3V or 5V supply voltage.
Instead of a single power module with adjustable output voltage, such as the SIP-8, two types of power modules are required to supply 3.3V and 5V to an application. The 5:1 SIP-8 with wide input/output voltage range reduces the number of designs that must be designed, configured, tested, EMI conformity proven, built, and logistically handled.
Basic Considerations for a Wide Voltage Range – Input Voltage Limits
Long connecting lines between the separated parts of the application are common in industrial applications. The length of these connecting lines can be in the tens of meters range due to spatial extensions.
The diagram below depicts the fundamental structure of a manufacturing plant. Nowadays, electrical power is provided by cabinets equipped with switched mode power supplies or transformer power supplies.
Industrial plant’s basic structure.
Transformer power supplies are still used for high-power applications. A dc bus connects the various parts of the applications. On-site, each separate electrical load is connected to a 24V sub-distribution. It is simpler to generate 24V in a centralized cabinet and distribute it via a dc bus than it is to distribute the hazardous 230Vac / 400Vac.
There are three major influencing phenomena to the dc bus voltage based on that structure:
- The voltage from the power supply
- Disruptions to the dc-bus caused by parallel-running cables
- Voltage falls as a result of current flow.
The voltage drop caused by current will be considered to explain the lower voltage limit:
Lower Limit – Minimum Input Voltage
Typically, the cable cross-sections for the DC-Bus are chosen based on experience, rough estimation, or the use of tables.
It should be noted that the most common design constraint for cable sizing is to avoid overheating. As a result, the voltage drop of the connecting line is frequently overlooked and thus ignored. This voltage drop results in a difference in voltage levels between the electrical supply output (Vout) and the application input (+VIN).
A numerical example calculation with real-world values from an industrial plant is shown for clarity:
Using equations, the electrical resistance, R, can be calculated as 1.376Ω based on the cable cross sectional area, A, cable length, l, and specific resistance (1). In the case of a 100W supply, a rated current of 4A flows through the 24V DC bus. We’ll get a voltage drop across the connecting lines based on equation (2).
That is, the nominal 24V cannot be supplied at the application’s supply input, for example, PLC, because it is only 24V-5.5V=18.5V.
When we look at the PLC standard IEC 61131-2, we can see that the input voltage range for the supply voltage is defined as 19.2V to 30V. With a supply voltage of 18.5V, the PLC’s undervoltage shutdown will be tripped, causing it to stop operating. The SIP-8’s lower operating voltage limit of 8V allows it to be placed in an application far from the supply cabinet.
Upper Limit – Maximum Input Voltage
To calculate the maximum input voltage, the industrial plant (figure 3) must be divided into functional units, which are the electrical supply, the DC-bus, and the electrical loads.
The electrical supply itself, for example, a transformer power supply without post regulation, will be supplied with 3x380Vac -15% / +20%, which means that the dc bus voltage may differ from the nominal 24V.
As previously stated, the supply and loads are linked via a dc bus with 10 meter cable connections. These cables can function as an antenna, picking up on disturbances from nearby pulse loads such as frequency converters. These disturbances can then be distributed to the entire dc bus and every application that is connected.
Furthermore, the physical connection of the different applications on the input side via the dc bus may result in interactions. Voltage spikes caused by inductively induced switching transients and back feeding overvoltage from dc motors are examples of these.
Two parameters are relevant as a foundation for explaining the maximum input voltage value:
To begin, the maximum output voltage of the electrical power supply that is technically possible. Second, for a nominal 24V application, the maximum peak value of an input protection element Every switched mode or transformer power supply has one or more output electrolytic capacitors for voltage stabilization and filtering.
For a nominal output voltage of 24V, these capacitors have a voltage rating of 35V. Peak voltages and frequency for the lifetime of an electrolytic capacitor that can be applied without visible damage or a capacitance change of less than 15% are defined in IEC 60384-4, chapter 4.14. The maximum allowable peak voltage is set to 1.15 times the rated voltage. For a 35V capacitor, this yields 40.25V. To guard against transient overvoltages at the application’s input,
TVS (Transient Voltage Suppressor Diodes) are widely used. If the breakdown voltage VBR is reached, the diode conducts, and the energy of the impulse is bypassed through the diode, protecting the load. There can be no destructive voltage present that is greater than the clamping voltage VClamp of the TVS.
The following basic guideline is a suitable reference point for protecting a 24V application against transients: The diode begins conducting at the maximum reverse voltage (VRMW), and the current is negligible with only a few A. As a result, the load’s nominal operating voltage and tolerances must be greater than VRMW.
A TVS diode from Würth Elektronik with a VRMW of 26V is a common value for a nominal 24V rail. The diode conducts and a current of 1mA flows when the transient voltage reaches VBR. The breakdown voltage of a TVS diode has a tolerance between a minimum and maximum value due to its technology. As a result, a precise tripping point cannot be determined. For our 26V VRMW example, we have a voltage range of 28.9V to 31.9V.
When comparing TVS diodes from different suppliers, the characteristic values are nearly all in the same range. The TVS diode in the 24V system protects a DC/DC power module from overshoots above the absolute maximum ratings of VINMAX.
In general, the higher this value, the easier it is to design the correct TVS diode and input filter. That is, if the nominal operating input voltage is close to the maximum input voltage VINMAX of the module, it will be more difficult to find the correct diode. Finally, the maximum operating input voltage VIN of the SIP-8 isolated power module, 42V, is a suitable value to withstand the 40.25V and 42.1V transients shown above.
Wide voltage range – Output voltage Limits
The most common IC supply voltages in industrial control applications are 3.3V and 5V.
- Isolation of interfaces and buses such as RS232, RS485, CAN, Interbus, and Profibus.
- Digital circuit isolation
- Obtaining an isolated amplifier and an analog-to-digital converter.
- Data collection and measurement
The majority of isolated gate drivers on the market have a fixed output voltage. The SIP-8 isolated dc/dc module has an adjustable voltage range because it can be useful in some cases to set the output voltage slightly higher than the nominal operating voltage value of the load to make it more robust against, for example, voltage dips. As a result, the capacitance value of the load’s bulk capacitor can be reduced because the undershoot at the module output is less.