Power Tip 58: Power supply grounding – which camp are you in?

Power Tip 58: Power supply grounding – which camp are you in?

Technology News |
By eeNews Europe

If you want to start a heated discussion between a group of power supply engineers, all you need to do is ask them how they layout the grounds in a power supply. You will quickly learn there are two basic strategies. Each side will swear that the other approach has no chance of functioning as they smugly remember how well their last design worked.

The first approach is based on the concept of a single-point ground or star system (Figure 1). This idea steers the currents to control noise due to high di/dt in the conductors. A single-point ground is established at the ground of the control IC and all currents in the ground connection flow into that point. In this manner, high frequency, high slew-rate currents are not allowed to flow in sensitive paths such as the IC bypass capacitor, timing or analog circuit connections. Unfortunately, this approach can significantly degrade circuit performance due to increased inductances that result from the longer connections. For instance, in Figure 1, the star ground adds inductance to the transistor’s source connection. The transistor’s switching speed is related to this source inductance. As the transistor tries to turn off, the di/dt increases the source voltage and, hence, reduces the drive voltage gate-to-source. This slows the switching speed, which reduces efficiency. The additional inductance also distorts the current sense voltage, which can cause false tripping with peak current-mode control due to the leading edge spike.

Figure 1: Basic single-point grounding strategy puts significant inductance in source connection.
Click on image to enlarge.
A variant of the single-point ground is shown in Figure 2. Here, the power connections and analog low-level circuit connections are made separately and tied together at one point. This has a number of benefits:

  1. Minimizes high-power current flow in the ground connections;
  2. Reduces power path loop area to reduce electromagnetic interference (EMI) and improve switching speeds;
  3. Creates an analog ground to minimize noise in timing and control circuits.

When planning these two types of grounds, one thing people often overlook is that there can be more than one reason current flows in a system. There are the obvious power supply currents and the not so obvious ones, such as lightning testing, which can induce current to flow in unexpected portions of the circuit. These single-point ground approaches often are used in single-layer circuit boards and can result in significant inductance in ground paths, which can cause compliance problems. With high di/dt of lightning testing, significant voltages can be generated across the spindly ground connections of a star approach, which can damage power supply components.

Figure 2: A variant single-point approach constrains high-frequency currents.
Click on image to enlarge.

The second grounding approach involves the use of a ground plane and is appropriate when you have a multi-layer circuit card. With a ground plane, you simply put the plane down without any partitioning or separating of analog ground and power ground. The goal is to minimize the inductance and resistance in all grounds. With this approach, you get away from the careful planning of a single-point ground with the addition of printed wiring board (PWB) layers. The second benefit of the ground plane is that it significantly reduces the inductance of conductors above it by reducing the loop area. This in turn helps to increase switching speeds and reduces unwanted cross talk. The plane also reduces the proximity effects in high-frequency/high-current conductors as it spreads the currents across the faces of the conductors, rather than let it gather at the edges.

To summarize, a variant of the single-point ground is appropriate, particularly in a single-layer circuit board. Rather than blindly implementing it, you should consider constraining the high-frequency current paths. Additionally, you should consider current paths for lightning type testing as the single-point ground may have significant inductance associated with it. Single-layer boards pose a challenge in noise control, so you may have multiple revisions. If you have the luxury of a multi-layer board, use one or more ground planes and give some thought to minimizing the high-frequency currents on each, as well as minimizing the interconnect inductance. In general, you will have a much lower chance of board iterations with the multi-layer approach.

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