A DC-to-DC converter’s input-range ratio specification provides a single number to characterize the span of acceptable input voltages. This concept is generally applicable to all DC-to-DC converters. However, for this article, we focus on small PCB-mounted converters such as the one shown in Figure 1.
Key Takeaways
- Calculate input-range ratio as V_in_max divided by V_in_min. For example, 75/9 = 8.33 approximated as 8:1.
- Novice trap: Do not use input-range ratio as a quality indicator.
- Novice trap: Be careful as both 9–72 V and 15–120 VDC converters have an 8:1 range.
- Novice trap: The ratio describes input-voltage characteristics only; it does not indicate output voltage, power rating, efficiency, or regulation quality.
- A narrow ratio is appropriate when an upstream well-regulated source is used.
- A wide-ratio converter can reduce logistics overhead. You can design one PCB to accommodate many voltages such as those encountered in battery or solar applications.
Author: Aaron Dahlen | MSEE | Senior Applications Engineer, DigiKey
Last update: 30 Jul 2026
Figure 1 Representative PCB-mounted MEAN WELL DC-to-DC converter.
Comparison of DC-to-DC Converters
Let’s explore two converters to better understand the specification. The MEAN WELL MDS02M-03N and SPAN02W8-03 make a good comparison as shown in Table 1. MEAN WELL rates both converters for 2 W with a 3.3 VDC output voltage.
| Parameter | MDS02M-03N | SPAN02W8-03 |
|---|---|---|
| Minimum input voltage | 10.8 VDC | 9 VDC |
| Maximum input voltage | 13.2 VDC | 75 VDC |
| Nominal input | 12 VDC | Wide |
| Input-range ratio | 1.22:1 | 8.33:1 |
Table 1: Calculated input-range ratio for the MEAN WELL converters.
Why Input-Range Ratio Matters
Like most design optimization questions, the answer depends on your requirements. In this case, the dominant parameter is clearly the anticipated range of input voltages.
From Table 1, we see that the power supplies are very different. The MDS02 converter has a ratio of approximately 1.22 while the SPAN02 converter has a ratio of 8.33 (nominally classified as 8:1).
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The MDS02 lives in a relatively controlled environment. In fact, the range is so narrow that we can assume an upstream regulated 12 VDC source. Consider that the float voltage for the typical lead-acid battery is about 13.8 VDC. This is higher than the MDS02 input specification even though both converter and battery are described as having a nominal voltage of 12 VDC.
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By comparison, the SPAN02 operates in a changing environment. I would describe it as a 9 to 75 VDC universal power supply within its stated range. This is the same idea as describing an 85 to 264 VAC supply is considered universal. It implies that one converter may be used for many applications.
Be sure to carefully compare the specifications. System efficiency may be different depending on the actual input voltage. Remember, for a given power, input current increases with decreasing voltage. This requires careful analysis as efficiency is no longer a single fixed value derived from the nominal input voltage. It is now a surface that varies with both input voltage as well as output current.
Note that the higher input-range ratio can increase cost. In this specific example, at the time of this writing, the MDS02 has a DigiKey cost of $5.99 USD in quantities of 10, while the SPAN02 has a cost of $8.32.
Tech Tip: As an academic exercise, we can compute the range ratio of a voltage source. For example, a flooded 12 VDC lead-acid battery has a range ratio of about 1.5. This is calculated assuming a 15.8 VDC equalization charge relative to the severe 10.5 VDC deep discharge voltage.
Looking back to the MDS02, we see that it shares the same 12 VDC nominal voltage with the battery. However, the converter’s ratio is less than the battery. Consequently, it is not intended for direct interface with a lead-acid battery.
Contrast this with the SPAN02, which easily accommodates the battery based on the ratio and absolute input voltage ranges captured in Table 1.
Applications with Wide-Ranging Input Voltages
A DC-to-DC converter with an 8:1 input-range ratio is useful when the specific input voltage is unknown yet bounded by the operational limits of the converter. This has the potential to reduce inventory as a single assembly could be used in multiple applications. A few examples include:
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Connection to external battery: Assume the converter is powered directly from a battery. In this case, the nominal acceptable battery range is 12 to 48 VDC (one 12 VDC battery as compared to a series string of four). This assumes a lead-acid battery with headroom for charge (15.8 VDC equalization) and discharge (10.5 VDC) voltages. Adjust these numbers to accommodate your chosen battery chemistry.
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Solar source: Once again, the converter will operate over a wide range of voltages. Watch out for nominal-vs-actual voltage distinction as a lightly loaded solar panel on a cold yet sunny day can have a high voltage.
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Field-configurable assembly: A small DC-to-DC converter can be used alongside a larger converter. Consider a stepper motor drive. You set the DC input voltage in the field according to the needs of the chosen motor. The smaller PCB-mounted wide input range converter is then used to power the logic.
Parting Thoughts
As a gentle reminder, don’t get too excited about taking that last sip of energy out of a battery or capacitor. The physics are working against us.
- Deep discharge of a battery leads to reduced life.
- Stored energy in a capacitor increases as the square of voltage. There simply isn’t that much energy left when the voltage drops.
About This Author
Aaron Dahlen, LCDR USCG (Ret.), is a Senior Applications Engineer at DigiKey in Thief River Falls. His background in electronics and industrial automation was shaped by a 27-year military career as both technician and engineer, followed by over a decade of teaching.
Dahlen holds an MSEE from Minnesota State University, Mankato. He has taught in an ABET-accredited electrical engineering program, served as coordinator of an electronic engineering technology program, and instructed military technicians in component-level repair.
Today, he has returned to his home in northern Minnesota, completing a decades-long journey that began with a search for capacitors. Read his story here.