Is it Safe to Connect the Output of a Non-Isolated DC/DC Converter as an Input of Another?
The title Is it safe to connect the output of a non-isolated DC/DC converter as an input of another one 60V down to 24V and then from 24V to 19V raises important questions about the safe and efficient use of DC/DC converters. Here we explore the pitfalls and provide solutions to ensure stability and safety in your designs.
Understanding the Challenge
Paul Lee's expertise on the matter is well-founded, and as he suggests, the most practical solution is to isolate each DC/DC converter in AC terms. This means inserting an inductor in series and/or a capacitor in parallel between each stage. The capacitors, positioned as close to the input of each stage as possible, provide a practical solution, while the inductors should generally be within the range of 100uH. Capacitors, rated at 60% of the maximum voltage, size from 10 to 1000uF, ensuring that their voltages are appropriately rated.
Stability Concerns in DC/DC Converters
The inherent instability in switched mode DC/DC converters arises from their negative input impedance for incremental changes in input voltage. These converters work to maintain a constant output voltage under given load conditions. Since efficiency tends to remain constant, constant power out translates to slightly higher constant power in. In situations where the input voltage increases, the current must decrease, effectively creating a negative resistance.
When the first converter sees this as a load, it may need to raise its output voltage to compensate. This leads to an oscillation—initially the first converter lowers its voltage, leading to an increase in current, followed by a decrease in voltage and a decrease in current. If this oscillation is not controlled, it can lead to instability in the system.
Fixing Instability
To mitigate this oscillation, a large capacitor at the intermediate voltage can be introduced to overwhelm the effect. However, large capacitances can also introduce instability, so depending on the DC/DC converters being used, there may not be an ideal solution.
Another issue to consider is the presence of input noise filters with series inductance in some high-performance DC/DC converters. This can also contribute to the preceding DC/DC converter becoming unstable. If a linear type DC/DC is used instead of a switched mode converter, the problem is avoided.
Designing for Efficiency
For large step-down ratios, DC/DC converters can be inefficient. In such cases, it might be more beneficial to use an isolated type, even if the input and output grounds are the same. The transformer will handle the required conversion ratio, ensuring a more efficient and stable design.
For those who need to find a solution involving only switched mode converters, transformers like those sold at Fry's and Digikey that run on 72V maximum and output 24V provide an alternative. However, the efficiency and stability should be carefully evaluated when implementing such a solution.
The key takeaway is that while it is technically possible to connect the outputs of non-isolated DC/DC converters as inputs to another, it is crucial to consider the stability and efficiency implications. Proper isolation and careful component selection are essential to avoid potential failures and ensure optimized performance.