How Pulsiv OSMIUM technology eliminates inrush current

This article originally appeared in the June'24 magazine issue of Electronic Specifier Design – see ES's Magazine Archives for more featured publications.

In this article, Darrel Kingham, CEO, Pulsiv explores the cause of inrush current and traditional methods of managing it – and explains how Pulsiv OSMIUM technology completely eliminates the problem.  

Electrical installations must manage inrush current, for example, using circuit breakers  

What is inrush current?

As the name suggests, simply put, inrush current is the surge of current that enters an electrical device when power is first applied. Although it only occurs for milliseconds, inrush current can typically be 40-80 times higher than the steady-state operating current for which a system may have been originally designed. This is particularly challenging where multiple units are connected together and powered-up simultaneously (for example, with LED lights, servers, EV chargers, alarms, or air conditioners), or in scenarios where future expansion is possible. If ignored, the consequences can be catastrophic. Inrush current can lead to blown fuses, damaged circuit breakers, welded relay contacts – and major service interruptions.  

Figure 1. What is inrush current?  

What causes inrush current?

All legacy AC to DC power supply designs contain energy storage components (capacitors, inductors, and transformers) that naturally draw significant amounts of energy during the start-up phase. Traditional power electronic circuits provide a low resistance path that allow large currents to flow. Think of it like trying to push a car: to begin with, a huge amount of energy is needed to move it; but less is required once there’s some momentum. Steps must be taken to avoid the potential consequences in systems exhibiting this behaviour.  

How is inrush current managed today?

Various methods are available, but all come with their own compromises. Some involve the use of specialist components in the power supply design (ohmic resistor, NTC thermistor, PTC thermistor or a soft start circuit), which introduce cost, complexity, and energy waste. In cases where the power supply design does not actively manage inrush current, system installers must upgrade circuit breakers – or limit the number of LED drivers, power supplies or battery chargers that are connected to a single wiring circuit (adding time and cost).  

Figure 2. How is inrush current managed?

How has Pulsiv eliminated inrush current?

In a conventional power supply design, a series inductor is used to achieve power factor correction (PFC). Pulsiv has developed and patented a completely different method by charging a capacitor in parallel. This approach enables greater control and prevents a large current from flowing into the circuit. Measured using accepted techniques, Pulsiv OSMIUM designs have been proven to completely eliminate inrush current.  

Figure 3. Pulsiv OSMIUM design circuit schematic  

Kingham explains: “Inrush current has always been a problem in certain applications due to the behaviour of legacy power electronics. The obvious safety and reliability implications mean that it must be addressed somewhere in the system design or installation process. By completely eliminating inrush current, Pulsiv OSMIUM technology reduces cost, complexity, and risk in any application where it currently needs managing.”  

Pulsiv OSMIUM technology provides manufacturers with a cost-effective method of designing always-efficient power supplies to reduce overall energy consumption while delivering a range of other benefits. It offers engineers complete flexibility to adapt and scale for virtually any application from just 20W to several kW, with evaluation boards and reference design document packages available online to simplify the design process.  

What are the benefits of no inrush current?

It goes without saying that a circuit without inrush current delivers significant advantages:  

Design phase  

• Overall system design is simplified  
• Reduction in component count, PCB space and assembly time  
• A scalable solution for different applications  

End applications  

• Reduce manufacturing costs  
• Improve reliability  
• Extend product lifetime  

Installations  

• System installation is simplified  
• Reduction of cabling  
• Installation time/cost reduced  

Figure 4. Pulsiv OSMIUM eliminates inrush current

Fire alarm system example

In a typical fire detection system used for large commercial buildings, there will be several notification devices on a 24Vdc circuit that may be several kilometres long. Due to strict regulations, it is not possible to increase voltage or add equipment to address voltage drops caused by inrush current. Output lines from the fire panel are protected, so the combined effect of inrush from multiple devices can cause fuses to blow and lead to complete system failure. Alarm sounder manufacturers do not always document inrush current data for their devices, so the problem is often discovered – and must be resolved – during installation. Using a power supply based on a Pulsiv OSMIUM design will eliminate inrush current, reducing application cost, complexity, and risk.