The majority of industrial grade converters have either 2:1 or 4:1 input voltage range. For example, a 2:1 converter with a nominal 24V input can operate from 18-36Vdc (the voltage range of a 24V lead-acid battery including the battery charger open circuit voltage), while a 4:1 converter can operate from 9-36V (the voltage range of either a 12V or a 24V lead-acid battery).
There are very few isolated converters that offer more than 4:1 input voltage range. This is because of two reasons; firstly, the input current at the minimum input voltage will be at least four times higher than the input current at the highest input voltage and it is difficult to design an input stage that can handle both the high input voltage at one end of the range and the high input current at the other end, and secondly, isolated converters typically use a PWM (Pulse Width Modulation) controller to regulate the output voltage independently from the input voltage or output load and the modulation only has a 4:1 effective range, defined by the ratio between minimum on-time and maximum off-time.
The reason why there is a minimum on-time is the reaction time of the power stage. If the pulse width is reduced too much, then the power switching transistor simply cannot react quickly enough. Moreover, any jitter in this very narrow pulse will be amplified by the power stage and cause the output voltage to be less well regulated (if a 1ms wide pulse has 100µs jitter, the output voltage will vary by 10%). On the other end of the scale at minimum input voltage, the driving transistor cannot be kept on all of the time so has to switch off, also with a minimum off-time. This pulse off-time has to be long enough to allow the transformer to reset and is usually much longer than the minimum on-time. These two limitations on the pulse widths mean that the PWM controller cannot operate over much more than a 4:1 input voltage range and still regulate properly.
Fig 1: Principle of Operation for a DC/DC converter with PWM control (source: DC/DC Book of Knowledge, RECOM)
The standard industrial DC/DC converter input voltage ranges were defined by the telecoms industry (12V, 24V or 48VDC battery-based supplies). Railway applications also use higher battery voltages of 72V, 96V or 110V. The most common mainline railway bus voltage is 110Vdc, but light railways and trams often use lower battery voltages. The most recent edition of the EN50155 standard for Electronic Equipment used on Rolling Stock also includes 28V and 36V options:
Fig. 2: DC Input voltage range, including transients, according to EN50155:2017
We can immediately see a problem with the input voltage range of the EN50155 standard and the traditional DC/DC input voltage specifications. Traditionally, three DC/DC converters (nominal 24V, 48V and 110V) would cover all of the standardized railway supply voltages including transients, but the addition of a 28V battery voltage option disrupts this pattern. The nominal supply voltage of a 28V system is within the input range of a 24V DC/DC converter, but the maximum supply voltage is too high. The 28V maximum output voltage is within the range of a 48V DC/DC converter, but the minimum is too low. Neither fit!
This quandary has two possible solutions; add a nominal 36V input DC/DC converter so that four different designs are needed to cover all of the input voltage options or design an 11:1 input range converter that would cover all eventualities in one converter.
At this point, you may be thinking, “Hold on! First of all we hear that 4:1 is the limit, and now an 11:1 solution is proposed!” Well there is no way around the 4:1 input range problem, but you can combine two converters into one design: a low input voltage pre-converter boost stage followed by a high input voltage isolated 4:1 converter. The advantage of the boost converter topology is that when the input voltage is higher than the set output voltage, then the power transistor is off all of the time and the input voltage is passed straight through minus volt drop across the diode (Fig 3.) Another advantage is that the boost converter stage does not need to be isolated, so there is no transformer reset time to be considered and the input voltage range can be much higher than 4:1. Therefore the input range is not limited to 8:1 (4:1 followed by 4:1), but can be up to 12:1 (8:1 followed by 4:1) as shown in Figure 4.
Fig. 3: Boost converter topology. The output follows the input if Vin>Vout.
Fig. 4: 12:1 input range DC/DC converter. The non-isolated boost stage is followed by a conventional 4:1 input isolated DC/DC converter.
RECOM offers four different all-in-one ultra-wide range DC/DC converter solutions based on this principle with either 10:1 or 12:1 input voltage ranges:
Each converter series can operate with 24V, 28V, 36V, 48V, 72V, 96V or 110Vdc nominal supply voltages including under and overvoltage transients as defined by the EN50155 standard. They deliver isolated 12V, 15V, 25V or 48V outputs. The 40W and 60W versions also have 5V outputs.
The converters have a consistent efficiency over the entire input voltage range, so only one product is needed for all possible railway supply voltages, simplifying the logistics, documentation and technical support overheads of the final installation. It also means that the same power supply can be used for mainline, light rail or tram applications reducing the development and production costs.
Steve Roberts, RECOM