Published Date
Author Cadence PCB Solutions
Designing isolated systems, such as isolated DC/DC converters, is one area that many designers struggle with. Apparently, the idea of separating grounds brings up all sorts of strange PCB layout practices and circuit design practices, many of which are intended to control EMI. Unfortunately, isolated systems that are not designed properly are prone to EMI problems that are difficult for inexperienced designers.
One important component in isolated power systems is the use of safety capacitors, specifically Class Y safety capacitors. Also known as Y-type safety capacitors, these capacitors play a dual role of determining leakage current and suppressing radiated emissions. The EMI suppression function of these capacitors is quite simple, and will show you how it works in this article.
How Y-type Capacitors Are Placed
Y-type capacitors are not specialized capacitors that are manufactured in a specific way. The IEC standards contain definitions for Y-type capacitors, providing performance specifications for these components. Technically, any capacitor could be classified as a Y-type capacitor if it meets certain specifications.
With that out of the way, Y-type capacitors are used in two specific locations in isolated dc/dc converters:
EMI filters coming off of the line voltage
As a bridge between the isolated grounds
With a direct connection between the isolated grounds, the Y-type capacitor performs an important function that helps control EMI. This is achieved by maintaining the required isolation at very low frequencies or at DC, yet still providing a low impedance path between grounds at high frequencies.
Y-Type Capacitors and EMI
Y-type capacitors are able to provide some control over Emi due to establishment of a low impedance current path between the two grand regions. The circuit path between the two grounds passes across the parasitic capacitance between Transformer coils and the y-type capacitor. This path is shown in the schematic below.
The Y-type capacitor in this circuit (C13) bridges the primary and secondary grounds.
In this simple example, the ground nets are a ground plane on each side of the transformer, or at minimum very large copper pours that make up the ground nets. Because this circuit is a switching DC/DC converter, a switching waveform can induce its return path into the adjacent ground plane on each side of the transformer. With these currents in the ground planes, it is now possible for return current to pass between the planes via the Y-type capacitor.
Why does this primary to secondary ground connection help control EMI?
Y-type capacitors used in this way eliminate voltage offset between the two grounds at high frequencies. This suppresses high frequency radiation by dissipating noise currents between the two planes via a displacement current. Essentially, the Y-type capacitor acts like a filter that allows secondary side noise currents to return to the primary side and complete a circuit.
The Y-type capacitor in this circuit (C13) bridges the primary and secondary grounds.
This use of a Y-type capacitor requires that the capacitor have the following qualities:
Very high breakdown voltage, so the capacitor case can end up being quite large
Larger capacitance value than the winding-to-winding capacitance of the transformer
The capacitance value is not always very large, typically being only on the order of nF. The capacitance value represents a trade-off when we start to look at noise control versus safety, the latter of which relates to leakage current.
Y-Type Capacitors and Safety
Y-type capacitors can meet galvanic isolation requirements that ensure safety, and it is their capacitance value that controls the emissions. Larger capacitance values push the low impedance portion of the ground connection to lower frequencies such that the required frequency range can be covered for a typical DC/DC converter. The image below shows an example of a radiated emissions spectrum from a DC/DC converter; it should be clear that we would like to have as much noise control as possible at very high and very low frequencies.
Example radiated emissions spectrum from a DC/DC converter. (Image source)
Unfortunately, we cannot just place the largest possible Y-type capacitor across these grounds. There are two reasons for this:
Larger capacitance with high withstand voltage requires very large case capacitors
Larger capacitances in small cases start to have large DC leakage currents
The second reason is very important in some areas such as medical devices. The EMI standards on these devices limit leakage currents for human safety reasons. So as the Y-type capacitor size scales up, the leakage current also starts to scale up.
This is an aspect of emissions control that has to be verified thoroughly through testing. For example, leakage current could leave the system through any connectors or HMI components, and when a human touches any of these elements in the product they would be exposed to the leakage current. If there is also a low impedance path from secondary ground to chassis, the user would also experience the leakage current if they touch the product chassis. For example, the latter could happen if the chassis is tied to secondary ground using small capacitors.
Just like any engineering problem, these systems require a careful approach that balances multiple aspects of design for low EMI. Practices for layout of isolated systems must be followed to ensure the design can provide stable power with low conducted and radiated emissions.
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