The world of power supply design is complex, with numerous factors to consider when creating efficient and safe systems. One crucial aspect is galvanic isolation, particularly in switched-mode power supplies (SMPS). This blog series explains the importance of isolation in DC-to-DC converters, exploring what it is, why it's necessary, and the consequences of neglecting it. SMPS can be separated into two categories: non-isolated and isolated. Board designers and users must be able to recognize these differences. Techniques are elaborated to materialize various isolation architectures allowing optimum power transfer, voltage ranges, robustness, integration, and, of course, cost.
Why Isolation?
Isolation must be understood here as electrical isolation. Two portions of a circuit are electrically separated when there is no solid conductive link between them. Such isolation is also called galvanic isolation. A good example of that characteristic is the electrical isolator between two sections of a high voltage cable. Today’s electronic AC-to-DC and DC-to-DC controllers deal with high voltages approaching kilovolt and current up to several hundreds of amperes.
Figure 1: Isolator
In a circuit, such quantities appear on circuit nodes and branches. To avoid accidental contact with them by humans or by surrounding devices is vital. Serious damage or injury up to death can occur. Isolation is therefore often linked with safety and protection.
Figure 2: Isolation Purpose: Safety and Protection
Damage and Risks of Non-isolation
First, is the injury caused to a person who accidentally touches a circuit containing high voltage potential nodes. Depending on which part of the body is in contact, the skin condition (dry, humid), the incident duration, the path offered for the current (more particularly if the heart is in the track) the injury can be more or less urgent. On average, a human body can be modelized as a resistance of a few hundred Ohms to several tens of kilohms.
From medical views, it is generally admitted that on average 10 mA can cause involuntary muscle contractions, 30 mA can lead to respiratory arrest, and currents of 100 mA or more can cause ventricular fibrillation or cardiac arrest, which can be fatal.
High voltages/currents are also dangerous for surrounding and connected equipment; especially when the various components are not rated correctly. Damage can range from simple performance degradation, temporary or permanent, to full destruction.
Figure 3: Overvoltage/Overcurrent Damages on Equipment and Components
What Is Isolation in SMPS?
Isolation and safety are concerns for many electronic domains. The focus here is on power supplies since SMPS are dealing with high voltages and currents. More particularly one can consider a DC-to-DC converter as two portions: a high power part (usually connected to the input sources) and a lower power part, usually connected to the output side. We consider power (and not simply voltage or current).
Figure 4: High/Low Voltage Parts in Any Step-Down
Note that linear and low-dropout (LDO) regulators are not part of the discussion because they do not work with high power values. It’s also important to note that when properly designed and robust components are used, there should not be danger for the low power section that is in contact with sensible electronics (for example, data converters, sensor interfaces, human hands/fingers) BUT, user inattentions happen and components can fail (by aging or by inappropriate operating conditions). In that case, isolation techniques must be put in place.
The challenge now is how to split the low power/low voltage part. The split must maintain the power transfer from the input stage to the output stage and the feedback control from the output to the input. In addition, the two portions cannot share the same power lines, including the ground.
In SMPS, proper galvanic disconnection must occur in four places: power path, feedback path, and supply lines.
Figure 5: Galvanic Isolation Made in Four Places
Isolation quality has standards (for example, ADC or DC, safety degree…). As power supply circuits become smaller and smaller, the distance between elements becomes shorter, and the risk of electrical shock is higher. Remember, an arc is produced between two air separated conductors and is produced if the electrical field reaches more than 30 kV per cm (this value decreases with air temperature and moisture).
Conclusion
We have learned what galvanic isolation is, as well as the risks associated with non-isolation, both for human safety and equipment protection. We then start to highlight what the challenges are when implementing isolation for power supply designs. In the following parts, we will discuss the various ways to materialize the isolation without jeopardizing the energy transfer and the regulation feedback.
Read all the blogs in the SMPS Galvanic Isolation series.