Application context and voltage disturbance scenarios

It is often assumed that supply voltages in battery-powered systems fluctuate only between the voltage levels of a charged and discharged battery. This is true only for simple and small systems; however, even a car's 12V grid drops to 9 volts and lower during engine cranking, and can rise to over 60 volts if the battery is disconnected while the engine is running (load dump)
In more complex systems like trains or DC driven robotic applications with many dynamic loads, connectors, and long cables, significant voltage fluctuations and drops occur.

Voltage tolerance ranges per EN 50155 (railway standard)

As shown in figure 1 the railway standard EN 50155 specifies operating voltage ranges from 0.6 x Un up to 1.4 x Un. In addition, battery system voltages in trains depend on many factors like size of the train and region.

Figure 1: EN50155 specifies nominal voltages and tolerances for railway applications

Short circuits in the DC transmission lines can cause the supply voltage to drop to zero until a fuse or circuit breaker reacts. For many railway systems EN50155 requires continuous operation during supply voltage interruptions of 10ms (S2) or 20ms (S3), see figure 2.

Figure 2: Supply voltage interruptions specified by EN 50155, Un = nominal supply voltage, see figure 1

Supply interruptions and continuity requirements

Other applications using DC grids or conductor rails can also experience supply interruptions and as in train applications hold-up solutions are required to ensure continuous operation.

Hold-up capacitor sizing and limitations of the conventional approach

Energy stored in a capacitor is E = ½ C x U² and to calculate the necessary capacitor following formula can be used:

C = capacitor in Farads, Pout = load in Watts, t = required hold-up time in seconds
η = converter efficiency, Vnom = supply voltage, VUVLO = UVLO threshold voltage.

Therefore, many customers opt for the simplest solution: adding a larger capacitor and a diode to the input of the DC/DC converter. The diode prevents energy stored in the capacitor from flowing back into the system during a supply interruption (figure 3).

Figure 3: Hold-up circuit with larger input capacitor and diode blocking reverse energy flow

Let’s calculate an example with the following data:
100W load, 88% converter efficiency, 10ms hold-up time, 24V nominal voltage and the UVLO threshold is 12V:

Important note:
This calculation does not include tolerances and degrading with age of electrolytic capacitors. Please consider these factors when selecting the capacitors.

When designing a power supply able to work over the complete train voltage range (14.4V – 154V) this capacitor must have a rated voltage of 200V. Size of a typical 680µF/200V capacitor is 22mm(Ø) x 55mm and taking a typical tolerance of -20% into the equation, eleven capacitors connected in parallel are needed.

Adding around 6000µF or more to the input of a 100W converter significantly exceeds the 150 - 300µF typically recommended for stable operation and fast transient protection. It leads to another significant drawback of this solution. These large capacitors directly connected to the input lines create higher and longer inrush currents. Without an additional active inrush current limiting circuit, it can overload connectors and switches and trip circuit breakers (figure 4).

Figure 4: Adding larger hold-up capacitors at the input of a DC/DC converter increases the inrush current

Patented Bus Pin hold-up architecture: reduced size without inrush current penalty

P-DUKE designed a patented solution enabling customers to backup supply voltage drops of over 20ms without increasing the inrush current and with significantly smaller capacitors. It consists of DC/DC modules with very wide input voltage ranges and a charging circuit integrated into the modules for separate hold-up capacitors connected to the Bus pin (figure 5).

Figure 5: Basic schematic of P-DUKE’s patented backup circuit

Operating principle and key design advantages

Let’s look into the details and advantages of this circuit:
The ultra-wide input voltage ranges of these modules cover all possible train voltages from 14.4V up to 154V and one single module can be used for all different applications.

The Bus pin provides a fixed voltage of 21.4V to charge the external hold-up capacitor. In case of supply voltage interruptions, the energy stored in this capacitor is fed via D2 to the input of the DC/DC converter and keeps it working. Making the same calculation as in the example before and considering a 0.7V voltage drop across D2, the required capacitor value would be

The formula can be simplified by introducing a coefficient k (ranging from 1.3 to 1.5) to compensate for the D2 voltage drop and electrolytic capacitor tolerance.

It is larger than in our first calculation, but the Bus pin voltage is limited to 21.4V and a capacitor with 25V voltage rating can be used for all different input voltages. Size of a typical 6800µF/25V capacitor is just 16mm(Ø) x 40mm, and considering a -20% tolerance, only two pieces are needed. An over 90% volume reduction compared to the first example, which used 11 capacitors rated at 200V. Since only 2 capacitors are needed, this also reduces material and production costs.

A further cost reduction is achieved as one single solution covers the complete train voltage range (figure 6).

Figure 6: One single solution covers all train voltages

This patented circuit offers another significant advantage in addition to its substantial size and cost savings for the required hold-up capacitors. Whatever value is added, it does not increase the inrush current as these capacitors are not connected to the input lines. Instead, they are charged by a current-limited circuit inside the module (figure 7).

Figure 7: Adding hold-up capacitors to the Bus pin of the P-DUKE converters does not increase the inrush current

P-DUKE’s RCD10U-K, RCD20U-K, RED40U-K and the QAEXXU-K families offer this Bus pin and in the QAE family diode D2 is already integrated into the modules.

More details of this patented circuit, comparison graphs and maximum capacitor values can be found in P-DUKE’s application note.

To make capacitor selection easier for customers, this document also contains simplified Chold-up calculation formulas. They already include tolerances plus some safety margins to ensure that the required hold-up times are achieved under all circumstances.

Figure 8: simplified calculation for hold-up capacitors of different product families

Application examples

Let’s look at two examples where customers achieved significant space and cost savings by using this patented solution from P-DUKE:

Example 1: Railway Ethernet switch

At the beginning the customer was looking for the simplest possible solution to supply various railway Ethernet switches requiring 24V with power levels of 40, 60, and 100 W. Input voltages range from 14.4 up to 154V and all units have to meet S3 requirements, which means a minimum 20ms hold-up time. Due to limited available space, large hold-up capacitors and the necessary inrush current limiting circuit could not be used at the input of the converters.

By selecting P-DUKE’s QAE40-72S24U-K, QAE60-72S24U-K, QAE100-72S24U-K, a solution with minimum space requirements for the hold-up capacitors was found. The other big advantage was that all modules have the same ¼ Brick size and therefore one single PCB layout designed for all capacitor variants could be used. Depending on the required power, the appropriate module and capacitors are mounted on the PCB, see figure 9.

Figure 9: final Ethernet switch solution covering 3 different power requirements

Example 2: Railway microcontroller power supply

Due to the significant space and cost saving the customer also redesigned the power solutions for their microcontroller boards requiring 5V/20W. P-DUKE’s RCD20-72S05U-K also provides the Bus pin to charge the hold-up capacitors but does not have enough space inside to also integrate D2 (figure10).

Figure 10: Microcontroller supply using the RCD20-72S05U-K

If another supply voltage is needed for a CPU, control or display board, the customer can just change the module to get single or dual output voltages up to 24V.

Example 3: Smart-factory robot control system

As mentioned before, not only train applications face longer supply voltage drops. In this example a customer designing control systems for robots in smart factories contacted P-DUKE with the following requirements:

Input voltage: 24Vdc (18 – 36) or 48Vdc (36 – 72V) with up to 10ms interruptions
Output: 12V/60W
Size was a critical factor for this design

The customer initially planned to use two converters from the RED60 series: the RED60-24S12W for 24V input voltage and the RED60-48S12W for 48V input voltage. They also wanted to add large capacitors at the input of the modules.

P-DUKE suggested a solution using modules from the QAE60-XXXXU-K series with wide input ranges and the Bus pin option. This module family is available with two nominal input voltages: 36Vnom (9 – 75V) and 72Vnom (14 – 160). With an input voltage from 9 – 75V, the 36Vnom module does cover the full supply voltage range in this application. This module's advantage is an operating voltage down to 9V (8.1V UVLO threshold is in the calculation). Compared to the 14V minimum voltage of the module with 72Vnom input, it provides more margin for the capacitor calculation.

also reflected by a smaller multiplication factor in the Chold-up calculation table

Following table shows the capacitor values for the various options and as expected the smallest capacitor and overall solution size is achieved by using the QAE60-36S12U.

Figure 11: Comparison of hold-up capacitors using various DC/DC models.

Similar to the previous example, a solution covering both voltage ranges was realized using one module and one capacitor (figure 12). In addition to the size reduction, significant savings were achieved in production and logistics.

Figure 12: Robot Controller solution covering 24V and 48V supply voltages and providing 10ms hold-up time

Conclusion

P-DUKE’s patented circuit allows customers to achieve long hold-up times of over 20ms without increasing inrush currents, tripping circuit breakers or overloading cables and connectors. 25V-rated capacitors and one DC/DC converter module with ultra-wide input range can be used to cover all nominal input voltages in train, industrial or telecom applications. This article only covers the basics and three example applications. Explore the best solution for your application—contact P-DUKE’s application engineering team today for customized technical support >>Contact Us