Multiplying Your Power: When and How to Parallel AC/DC Power Supplies

There are two primary motivations for combining multiple power supplies in a system: redundancy or increased power capacity. 

Redundancy is a powerful tool to add to your system when you have an always-on requirement, but there are important considerations to ensure your system can handle a supply failure. You need to know how much redundancy is required and that your supplies are properly protected.

When designing or upgrading an electronic system, the gut reaction to a demand for more power is usually straightforward: buy a bigger power supply. If your system outgrows its current 600W requirement, logic suggests you should just hunt down a single 1200W supply, drop it in, and call it a day. However, in industrial automation, telecommunications, and high-reliability electronics, bigger isn't always better. Sometimes, deploying multiple power supplies in a parallel configuration is the smarter, more reliable, and more cost-effective engineering choice.

Paralleling power supplies allows multiple units to connect to a single, shared load. But doing this successfully involves far more than just twisting output wires together. Let's break down exactly why you might want to parallel your AC/DC power supplies, some possible pitfalls, and the best practices to follow.

How Do I Design a Redundant Power System?

Redundancy is critical when your system simply cannot afford to shut down. In mission-critical configurations, such as medical equipment, data servers, or even manufacturing lines, a power failure could mean catastrophic data loss or expensive downtime.

Here are a couple design elements that are critical to consider:

1. How much redundancy is required? 

• Terminology such as N+1, N+2, and N+M help define your level of redundancy.

• N refers to the number of power supplies needed to meet your power requirement. 

• M or +1/+2 refers to the number of redundant supplies in the system. This dictates how many power supplies can fail before the system as a whole fails.

• Example: If you have a 3000W system where you are using 1000W power supplies and want an N+2 level of redundancy, you would use five 1000W power supplies (N = 3 and M = 2.) Then even if two units fail, your system still has 3000W available!

2. Are your power supplies protected from each other when a failure occurs? 

• If a power supply in your system fails into a short condition on the output, without the proper protection, this could pull your entire system down.

• To combat this scenario, OR-ing diodes or OR-ing MOSFETs are generally recommended. Both of these solutions will stop current from flowing back into the power supplies, preventing one of them from becoming a sink. See figure 1 below.

Figure 1. OR-ing Diodes / MOSFETS connect from +Vout to +Load

How and Why to Parallel AC/DC Power Supplies for Increased Power Capacity

When design engineers we’re working with need more power from their system, our solution is often to simply recommend a higher power supply. This is especially true when working with smaller power supplies, as many smaller supplies are not designed to be used in parallel. Most power supplies with current sharing are > 600W. There are a number of situations however where using two or more power supplies in parallel is preferable, or even required. Here are some examples.

7 Real World Scenarios Where Connecting Power Supplies in Parallel is Practical

• Situation 1: There is no higher power supply available. If your design already pushes the upper limits of available power tech and you are using one of the largest single units on the market, paralleling two units is your only path forward.

• Situation 2: Strict form factor limitations. High-power units are notoriously bulky. If you are constrained by a slimline enclosure or rigid length, width, or height requirements, a large unit might not fit. Paralleling two or three smaller, low-profile units allows you to orient them creatively to fit tight spaces.

• Situation 3: Low-voltage availability at high wattage. This is an incredibly common roadblock. High-power AC/DC supplies are typically optimized for higher output voltages such as 24V, 48V, or even 380V! Finding an industrial-grade, single 3000W supply that outputs low voltages like 3.3V, 5V, or 12V can be incredibly difficult. Paralleling two readily available 1500W, 12V supplies solves the problem immediately.

• Situation 4: Input voltage restrictions. As single power supplies scale up in wattage, their input requirements change. Many high-power units require high-line voltage (180Vac+) or even three-phase power. If your application only has access to standard low-line input (like 115Vac), paralleling multiple lower-power units that accept low-line input is the only way to hit your target capacity.

• Situation 5: Supply chain availability. The more specialized and high-power a component is, the more fragile its supply chain tends to be. Lower-power units are mass-produced commodities. Sticking with a highly common power unit keeps your manufacturing pipeline moving because you know replacements will always be in stock.

• Situation 6: Upgrading an existing system. If you are boosting the power of a product that has been in the field for years, it may be safer to keep the power supply you already trust. You already know its longevity, reliability, and electromagnetic compatibility (EMC) profile. Adding an identical second unit in parallel avoids the costly engineering headaches of validating a completely new, massive power brick.

• Situation 7: Dispersing thermal hot spots. Concentrating massive power generation into one large block creates a singular, intense hot spot inside your enclosure. By splitting the load across two separate supplies spaced a distance apart, you distribute the thermal load and mitigate the risk of overheating nearby sensitive components.

How to Parallel Power Supplies: Three Core Methods

1. Active Current Sharing 

If you want to achieve the most accurate load balancing with the best efficiency, active current sharing is a feature you want from your power supplies. This requires a signal connection between the parallel supplies for current sensing / communication. Many high quality larger power supplies have this feature, including various Cotek products that include parallel function. Figure 2 below shows some of the signal connections you would make when paralleling and using remote sense.

Figure 2. Current sharing connections with remote sense on Cotek AEK-3000.

2. Droop Current Sharing

This type of current sharing does not require communication. Instead, the power supplies are engineered to automatically lower (or "droop") their output voltage slightly as the load current increases. When one supply starts pulling more than its share of the load, its voltage drops slightly, naturally forcing the second power supply to step up and take over the excess current. This method generally requires toggling a switch or jumper to tell the supply to enter droop mode.

3. Brute Force Current Sharing

This is what happens when two power supplies that are not designed for paralleling are put in parallel. With the right considerations, this can work successfully in a system. External resistors can even be added to cause artificial droop. But it carries a higher risk of failure where one unit goes into overcurrent, taking most of the load. 

No matter what type of current sharing you are using, these are some best practices to employ when putting two or more power supplies in parallel. 

Critical Considerations When Connecting Power Supplies in Parallel

1. Match the voltages precisely before connecting outputs together.

This should be done before connecting to the load or the other supplies. Whichever power supply has the highest voltage will take the majority of the load just based on Ohm’s law, so this is vitally important. Target a maximum of 0.1% difference.

2. Ensure identical cable lengths and gauges.

From the positive of each power supply to the positive of the load should measure the same between each supply. Same for the negatives. This is sometimes referred to as the star connection method. Any difference between lengths and wire gauge will result in different voltage drops (back to Ohm’s law.)

3. Derate the total output power of the system.

To avoid a situation where one power supply goes into overload, you want to stay away from the maximum power of the supplies combined. A derating to 90% of total capacity is a good rule of thumb. 

• For example, if you have 2 x 1000W power supplies connected in parallel, it is recommended your system power draw should not exceed:

2 x 1000W x 0.90 = 1800W

4. Use OR-ing diodes or OR-ing FETS.

While this is most important when designing for redundancy, it is generally good practice, especially when using brute force paralleling. If one power supply output comes on before the others, you don’t want that power back-feeding into the other supplies.

Conclusion

While there are different reasons and different methods of paralleling multiple power supplies in a system, there are similar considerations that should be taken. When you need more power, sometimes bigger is better, but sometimes more is merrier. The best solution for your system will depend on your precise requirements and design limitations.  TRC carries a variety of power supplies with parallel functions. 

👉 Explore Parallel Function Power Supplies By Cotek at TRC Electronics

TRC Electronics has engineers that can help you determine the best product and the best way to achieve either redundancy or higher power.

Give us a call at 📞 1-888-612-9514 and talk to a power specialist who can help you make powerful decisions. 

How to Parallel Power Supplies FAQs:

Q: How do we make two power supplies share the load equally?

A: The best way to do this is by using power supplies that have active current sharing and being sure to connect that current sharing pin. Droop parallel function can also work and may be simulated with external resistors. No matter the method, it’s important to adjust voltages as close as possible before connecting to the load, and to use the same length and gauge wire to match voltage drop.

Q: If I have two of the same 12V power supply, can I just connect the positives together and the negatives together to get more current?

A: This is known as brute force paralleling; if you take the precautions of matching voltage setpoints and cabling, it can potentially work. But you must be wary of unbalanced supplies and derate your system to avoid one supply going into overload.

Q: Which is better, active current sharing or droop current sharing?

A: Active current sharing is more accurate and allows for more precise voltage control than droop current sharing. If voltage variation doesn’t matter, droop current sharing can be less expensive to implement and could be considered more robust.

Q: Should we choose ORing diodes or ORing FETs, and what specs are important?

A: OR-ing FETS require external control circuitry if being added externally to the power supply, but they offer better efficiency, especially in high current systems. OR-ing diodes are generally easier to implement, but suffer greater power loss when current is higher. When spec-ing diodes you need to make sure the current limit and reverse voltage are high enough. Two diodes on the same die with the lowest voltage drop possible is also ideal. If working with power supplies like the Cotek supplies that already have OR-ing FETS or diodes, it doesn’t matter since the efficiency and specs are already built into the system. 

References:

TRC Electronics, “Cotek AEK-3000 ORingFET Series Datasheet,” TRC Electronics, [Online]. Available: https://files.trcelectronics.com/datasheets/AEK3000LVORING.pdf

TRC Electronics, “Cotek Power Supplies with Parallel Function,” TRC Electronics, [Online]. Available: https://www.trcelectronics.com/collections/cotek-parallel-current-sharing-blog

Cotek, “AE-1500-LV ORingFET Announcement,” Cotek, [Online]. Available: https://www.cotek.com.tw/ae-1500-lv-oringfet.