PCB Tech - How to ground in an isolated power supply PCB layout

Most designers who use desktop power supplies will probably use an isolated regulated (switching) PSU that plugs into the wall. Everything needed to provide a stable power supply at a specific DC or AC level is built into the unit, and the noise is relatively low. As a designer, you don’t actually need to do it except for connecting some leads to the circuit board. anything. Unfortunately, real systems with integrated power sections, or even just power regulator modules that you want to integrate into a larger system, are not that simple, and some custom design is required to ensure they operate correctly.

An important aspect of integrating the power supply into the system is the correct setting and connection of grounding, even for isolated power supplies. If you integrate the isolated power supply with the rest of the main circuit on the circuit board, you still need to ground in the system. These rules even apply to the PCB of an isolated DC charger or DC power adapter, because the design may need to be connected back to the earth, depending on the application and safety issues. Since a poor ground connection can cause noise issues and even safety hazards, let us look at the best practices for creating a ground connection in the power conditioning section when converting AC power to DC power on the circuit board.

Grounding structure in isolated power supply

Suppose you are designing a system that needs to perform power conversion (AC to DC), conditioning, and transmission to the circuit in the design. If you consider the actual construction of the system, there are three different possible options for the ground:

Grounding: This is a true grounding electrical connection that exists as a safety wire (PE) on the 3-wire AC line.

Chassis grounding: This applies to enclosures with metal components, where the metal in the enclosure is used to create a ground connection.

Signal ground: This is sometimes incorrectly described as analog ground and digital ground (don't separate your ground like this). Signal ground usually refers to anything other than ground or chassis.

Power supplies built with transformer coupling (such as AC-DC converters, DC-DC switching converters, or a combination of these two systems) will be built with transformers to compensate for these gaps in the PCB layout. The reason is simple: unless you only operate at low voltage and low current, you usually want to isolate in your design to protect users from safety hazards.

For various reasons, these grounding systems are not always located on a single ground plane. This applies to switching power supplies, especially more complex power supplies, such as LLC resonant converters. The reason grounding is so important is that it defines the voltage measured when the component is operating in the system. When I write "the voltage measured by a component", it means that a 5V signal defined on a certain grounded area in the system may not be able to be measured at 5V when it is measured on some other grounded area in the system.

In this figure, if there is a potential difference between the two grounding areas, the signal from the left grounding area (GND1) may be incorrectly measured on the right grounding area (GND2).

This problem in isolated switching power supplies is called "ground offset" and can cause noise problems. This is very important because the ground offset in the system may be only a small part of the voltage you want to reliably provide in a transformer-coupled power supply.

Use capacitor grounding to maintain DC isolation

Fortunately, there is a simple solution: connect the planes together with capacitors. Y-class capacitors are a good choice for higher voltage/current designs. You can easily do this in the schematic: just find the components your capacitor needs, and then bridge the ground network by connecting directly. The typical location for doing this in the PCB layout is close to the transformer.

Although still effective in AC-DC conversion, a more complicated method is to use a capacitor between the power rail and the AC side of the system. Eliminate the ground offset between each side by drawing and releasing some displacement current.

About grounding in electronic design and PCB layout

Isolated and non-isolated power supplies: the right choice

Define power ground: system, chassis and ground in PCB

How to route through the ground plane gap

The power system implements the control algorithm and needs to allow feedback from the output back to the input so that the output power can be sensed. This means that you need to physically run a line from the output on the regulator side to the input side containing the switching elements. The question is: if your output is DC, but you want to maintain isolation, what is the best way to provide it?

The answer is to use optocouplers. It is not appropriate to place traces on the gap, because the traces will receive external noise, and the switching power supply will generate a lot of noise. Transformer coupling also cannot be used because you are adjusting the DC. In the schematic below, the optocoupler spans the isolation between the ground planes, so we have maintained the required isolation in this power supply.

Optocouplers allow you to send signals across the ground plane gap without the need for wiring.

After placing the optocoupler, you can route the output to the power controller. A microcontroller with a PWM output is a good choice for custom power supply controllers, although some companies produce MOSFET gate driver controllers that have feedback inputs and can be configured with some external resistors. If you are designing very precise power regulation or you are experimenting with control algorithms, this is a simple solution to achieve output detection. You can then use standard control algorithms to adjust the frequency of the PWM controller to ensure maximum efficiency or specifically track the required power output.

How to ground in the PCB layout of the isolated power supply is very important for safety