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H Bridge I - Basic Concepts and Ideal Simulation - Power Electronics

Updated on February 14, 2013

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This is Stage 1 of the design for a 50 V, 10 A DC motor drive. You can follow the link(s) below to the previous article(s) that this hub builds up on. Alternatively, you can navigate to hubs for the stages ahead.

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A Veroboard implementation of the H-Bridge
A Veroboard implementation of the H-Bridge

Stage 1: H Bridge I - Basic Concepts and Ideal Simulation

H Bridge is perhaps the most studied and most commonly used circuit in Power Electronics. It is indeed a blessing that such a massively utilized model is actually quite simple to understand and also to implement.

On the right is what I successfully implemented on a veroboard. The annotations NMOS and PMOSare visible and further in the hub I shall be explaining why each of the MOSFETs was chosen, what desired characteristics did it have and how was it implemented.

Introduction to H-Bridge

The figure below shows the simple H Bridge model. Do not worry because it is indeed as simple as it looks. The working may be explained with the help of the figure above and the labelling. At one instant, say t1, we configure the circuit such that transistors A1 and A2 are working as closed switches and transistors B1 and B2 are open. Hence, we can visualize the current to flow from the source through the transistor A1 to the load and sink to the ground via transistor A2.

Consider now the second instant t2 at which we design the transistors B1 and B2 to work as closed switches and transistors A1 and A2 to be open circuit. The result is, as anticipated, the opposite of the previous situation and the current flows from right to left through the load. This load in our case is a DC motor and most motors can be roughly represented by an inductive load. So far as the discussion in this hub goes, I will be analyzing and designing our motor drive considering an inductive load.


A typical H-Bridge
A typical H-Bridge

The specialty and the quality of interest of the H-Bridge is that it can deliver controlled power to the load. The net voltage/current can be applied to load as per the desire of the user.

The PWM wave can be quite crudely (in fact sometimes incorrectly) thought of a square wave. PWM are basically pulses with varying widths or duty cycles and hence deliver varying power for each duty cycle. Typical PWM are shown in the figure below for basic understanding and concept of PWM wave.In practice, all of the switches are operated through a single PWM wave, the duty cycle of the PWM wave is controlled and the processed wave is applied accordingly for each switch. For example, the transistors A1 and A2 shall be short-circuited when the PWM is 'high' and the transistors B1 and B2 shall be short-circuited when the PWM is 'low'.

The idea behind the speed control is to measure the motor’s speed via a feedback mechanism and compare the speed to a set reference value. If the motor’s current speed is lower than the reference value, we would increase the duty-cycle to increase power delivery thus raising the motor’s speed.

In our design, the duty cycle of 50% would lock the load (somewhat like when you apply equal force from both sides of an object and making net force zero!) while increasing the duty-cycle above 50% would increase speed in one direction while decreasing duty-cycle below 50% would increase speed in the other direction.

Pulse Width Modulated (PWM) Waveforms
Pulse Width Modulated (PWM) Waveforms

H Bridge Simulation in LTSpice (Ideal Switches)

To understand the working of the H-Bridge, we run a simulation of the bridge in LTSpice. In this preliminary simulation, we shall use ideal switches for our purpose. The simulated circuit is shown in the figure below.

The circuit for H-Bridge simulation with ideal switches in LTSpice.
The circuit for H-Bridge simulation with ideal switches in LTSpice.

I hope for an Electrical Engineer (or a student of EE), LTSpice is quite easy to understand and the above circuit is self explanatory. Source voltage is 12 V, the load (motor) is depicted as a 1.2 ohm resistor (for simplicity). Pulses are applied to base of the BJTs to control them as switches. Although in the real implementation we will be using MOSFETs, I initially simulated with BJTs merely because I felt comfortable with them.

In this simulation I applied the base pulses via voltage sources to merely understand the working of the H-Bridge. You can see that when switches Q3 and Q2 are on, current flows in the positive direction and for Q4 and Q1 on, the current flows in the negative direction. For the PNPs, pulses at the bases must be equal to (or higher than) the source voltage i.e. 12 V in order to stop current flow in the PNP.

I did a simple transient analysis on LTSpice and got the waveforms that verified my understanding. I have attached these waveforms in the figure below.

PWM signals to bases and current through load (ideal switches simulation)
PWM signals to bases and current through load (ideal switches simulation)

The first two waveforms are the PWM signals that I applied to the bases. The final waveform is the current through the load, which rose up to 10 A in the positive direction and -10 in the negative direction.

The next stage is the choice of components and simulation with real components with their parameters defined and the design of the H Bridge to meet our requirements of 10 A current and 50 V source voltage.

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Navigation

You can follow the link(s) below to the previous article(s) that this hub builds up on. Alternatively, you can navigate to hubs for the stages ahead.

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If you have any queries or want help on your project / design, fire away and I shall get back to you as soon as possible with as much help as I can provide.
Your comments are most appreciated and would be an enlightening beacon for my hubs to come.

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