FAQ on high-side vs. low-side load switching: part 1

The difference between high-side and low-side power switching is not apparent to many new designers, but it is a critical aspect of a viable circuit.

Applying power (voltage and current) to a load and removing it using a mechanical or electronic switch is a requirement in most systems. Even if the system is powered on, subcircuits and subfunctions often have independent control of their local power (think of a car and its various functions such as electric starter, A/C, lights, and more). In principle, on/off power control is a straightforward function with simple implementation.

However, this basic function has subtleties in its execution. Many of these relate to the power-control load switch’s “high side” vs. “low side” positioning. In some situations, the decision to use one or the other switch location is at the designer’s discretion. Still, in many cases, it is determined by the circuit topology and application.

This FAQ will examine issues related to high-side and low-side power switching and control via load switches and the implications when choosing and using each type. It will also look briefly at the load switches that are used.

First, a note regarding terminology: technical literature, as well as formal and informal discussions about systems and circuitry, often use the word “ground” or phrase “circuit ground” even when there is no Earth ground involved, which is the situation when the unit is either portable (battery powered) or is electrically isolated from Earth ground via a transformer. The better term would be “common.”

While this misnaming may seem to be a minor point, it can actually distort clear thinking about the circuit topology. However, the use of “ground” is so pervasive that there is no point in trying to fight that battle. Therefore, we’ll continue using “ground” to minimize confusion with other references and sources, even though it is often incorrect. Further, many circuit schematics use the little triangle symbol for “common,” At the same time, they say “ground,” while others use the Earth ground symbol even if there is no such ground.

High-side versus low-side scenarios

Q: What’s the basic circuit on/off power arrangement?
A: It starts with a source (battery or AC line), an on/off switch, and a load; the switch can be a mechanical device with physical contacts or an electronic switch using a bipolar transistor (FET or IGBT) (Figure 1).

Figure 1. Whether the power switch is a mechanical device or a solid-state component, it can be placed between the power source and load or between the load and “ground.” While both topologies have the same operational effect, there are important circuitry and system implications between the approaches. (Image: Electro-Tech-Online.com/WTWH Media, LLC)

The source supplies current to the load, which returns to the source via the ground (common) path. The on/off switch can be placed between the source and load on the “high” side or in the low-side current-return path. Either way, the current flow can be switched on and off as needed without any apparent system-level implications.

Note that for simple circuits such as lighting an LED or bulb (battery, light source, switch in a single loop), the terms “high-side” and “low-side” are somewhat arbitrary and don’t convey any deeper meaning. However, most circuits and systems are much more complicated than that.

Q: What do we mean by a load here?
A: It can be a single tangible component, such as an LED or motor, or a circuit subfunction part of a larger circuit.

Q: What is a “switch” in this context?
A: switch is an electronically controllable current-pass element (usually a bipolar transistor or FET). The term usually encompasses the driver, which converts a logic-level control signal to the drive voltage and current needed to switch the current-control element between its conducting and non-conducting states. Note that some higher-current switches include the drive circuitry only, and their associated control element is an external discrete component.

Q: Are high-side/low-side designations irrelevant in all single-source/single-load arrangements?
A: No, it is relevant when the current-return path is not just a plain wire with convenient connections. For example, the body of a motor may be grounded for safety and connected to the current return path (often a safety mandate or mechanical arrangement necessity). In this case, the current path can only be switched on the high side, as low-side switching would require separating and isolating the return path from the motor body.

Q: This makes sense for a simple one-source/one-load scheme, but what does high- and low-side switching look like with a single source supplying multiple loads?
A: It gets more complicated. If all the loads share a common current-return path (ground), low-side switching is impossible, and high-side must be used instead (Figure 2).

Figure 2. In high-side switching of multiple loads connected to a common rail, each load has its own load switch between source and load; alternatively, each load can also have its own source. (Image: blog.naver.com)

Q: Why not?
A: Consider an automobile with a single 12-V battery directly supplying all the loads in the car—starter motor, various lights, radio, power windows, and more (this is admittedly a simplified case in today’s world of 48-V batteries, hybrid, and electric vehicles, and many electronic PC boards with their own local DC/DC regulators). For practical, safety, and current-path reasons, all these loads return their current to the battery via the chassis itself or a heavy-gauge ground cable routed around the car. It’s not practical or desirable to have these loads be separated from the true ground (0-V potential) by even a small resistance of the electronic switch (under one ohm), so high-side switching is mandated.

Q: How else are load switches used?
A: They can be used when each load has a different rail value. Each switch is placed between the load and common ground (Figure 3) or between the individual rails and their respective loads.

Figure 3. In low-side switching, shown here with each load having a different source, a switch between load and ground controls the power to each load. (Image: blog.naver.com)

Q: What about non-vehicle designs?
A: A similar situation applies to an electronic product such as a smartphone, which operates from one battery. That voltage is then regulated down to other voltages for use by different subfunctions, some of which may sometimes be turned off to save power. However, all these subfunctions return their current through a common ground path.

Q: Where else does the high-side/low-side situation arise?
A: It occurs in the standard half-bridge or H-bridge circuit, which controls and reverses a DC motor. In the H-bridge, Figure 4, the power can be switched through one path for one direction or a mirror-image alternate path to reverse the current path through the motor windings and so reverse motor direction. In this arrangement, two switches are on the low side, and two are on the high side, with one high/low-side pair used for one direction and the other for the reverse.

Figure 4. High- and low-side switches are often used in tandem to control the direction of current flow through a reversible motor. (Image: Kartik Mohta)

You can buy a H-bridge driver IC with high- and low-side switches (Figure 5). These devices usually provide many additional motor-related functions and various circuit and motor protection types.