H-bridge

Key takeaways

H-bridge belongs to engineering; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect H-bridge to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of H-bridge from memory before moving on to harder problems.

Reference excerpt

An H-bridge is an electronic circuit that switches the polarity of a voltage applied to a load. These circuits are often used in robotics and other applications to allow DC motors to run forwards or backwards. The name is derived from its common schematic diagram representation, with four switching elements configured as the branches of a letter "H" and the load connected as the cross-bar. Most DC-to-AC converters (power inverters), most AC/AC converters, the DC-to-DC push–pull converter, isolated DC-to-DC converter most motor controllers, and many other kinds of power electronics use H-bridges. In particular, a bipolar stepper motor is almost always driven by a motor controller containing two H-bridges.

General

H-bridges are available as integrated circuits, or can be built from discrete components. The term H-bridge is derived from the typical graphical representation of such a circuit. An H-bridge is built with four switches (solid-state or mechanical). When the switches S1 and S4 (according to the first figure) are closed (and S2 and S3 are open) a positive voltage is applied across the motor. By opening S1 and S4 switches and closing S2 and S3 switches, this voltage is reversed, allowing reverse operation of the motor. Using the nomenclature above, the switches S1 and S2 should never be closed at the same time, as this would cause a short circuit on the input voltage source. The same applies to the switches S3 and S4. This condition is known as shoot-through.

Common usage An H-bridge is used to supply power to a two terminal device. By proper arrangement of the switches, the polarity of the power to the device can be changed. Two examples are discussed below: DC motor Driver, and transformer for a switching regulator. Note that not every switching configuration is safe; the "short"(see below in "DC motor driver" section) cases are dangerous to the power source and to the switches.

DC motor driver

Changing the polarity of the power supply to DC motor is used to change the direction of rotation. Apart from changing the rotation direction, the H-bridge can provide additional operation modes, "brake" and "free run until frictional stop". The H-bridge arrangement is generally used to reverse the polarity/direction of the motor, but can also be used to 'brake' the motor, where the motor comes to a sudden stop when the motor's terminals are connected together. By connecting its terminals, the motor's kinetic energy is consumed rapidly in form of electrical current and causes the motor to slow down. Another case allows the motor to coast to a stop, as the motor is effectively disconnected from the circuit. The following table summarizes operation, with S1-S4 corresponding to the diagram above. In the table below, "1" is used to represent "on" state of the switch, "0" to represent the "off" state.

Primary coil driver of switching power converter Typical primary coil driver is to simply replace the two terminals of the DC motor by the two terminals of the primary coil. The switching current in the primary coil turns electrical energy into magnetic energy and transfers back to ac electrical energy in the secondary coil.

Construction

Relays One way to build an H-bridge is to use an array of relays from a relay board. A "double pole double throw" (DPDT) relay can generally achieve the same electrical functionality as an H-bridge (considering the usual function of the device). However a semiconductor-based H-bridge would be preferable to the relay where a smaller physical size, high speed switching, or low driving voltage (or low driving power) is needed, or where the wearing out of mechanical parts is undesirable. Another configuration is to have a DPDT relay to set the direction of current flow and a transistor to enable the current flow. This can extend the relay life, as the relay will be switched while the transistor is off and thereby there is no current flow. It also enables the use of PWM switching to control the current level.

N and P channel semiconductors A solid-state H-bridge is typically constructed using opposite polarity devices, such as PNP bipolar junction transistors (BJT) or P-channel MOSFETs connected to the high voltage bus and NPN BJTs or N-channel MOSFETs connected to the low voltage bus.

N channel-only semiconductors The most efficient MOSFET designs use N-channel MOSFETs on both the high side and low side because they typically have a third of the ON resistance of P-channel MOSFETs. This requires a more complex design since the gates of the high side MOSFETs must be driven positive with respect to the DC supply rail. Many integrated circuit MOSFET gate drivers include a charge pump within the device to achieve this. Alternatively, a switched-mode power supply DC–DC converter can be used to provide isolated ('floating') supplies to the gate drive circuitry. A multiple-output flyback converter is well-suited to this application. Another method for driving MOSFET-bridges is the use of a specialised transformer known as a GDT (gate drive transformer), which gives the isolated outputs for driving the upper FETs gates. The transformer core is usually a ferrite toroid, with 1:1 or 4:9 winding ratio. However, this method can only be used with high frequency signals. The design of the transformer is also very important, as the leakage inductance should be minimized, or cross conduction may occur. The outputs of the transformer are usually clamped by Zener diodes, because high voltage spikes could destroy the MOSFET gates.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with H-bridge

Start with the simplest possible case. Write down what H-bridge claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to H-bridge before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about H-bridge ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of H-bridge

In research: H-bridge appears in engineering research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses H-bridge in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: H-bridge is common in secondary-school and first-year university syllabi. It links to neighbouring topics Bridge circuits, so understanding it makes those chapters shorter.
In everyday life: Look for H-bridge outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study H-bridge in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what H-bridge means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain H-bridge out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is H-bridge in simple terms?

An H-bridge is an electronic circuit that switches the polarity of a voltage applied to a load. These circuits are often used in robotics and other applications to allow DC motors to run forwards or backwards.

Why does H-bridge matter?

Because it connects several engineering ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study H-bridge?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on H-bridge.