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H Bridge II - Component Selection and Real Simulation - Power Electronics

Updated on February 14, 2013

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This is Stage 2 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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Stage 2: H Bridge II - Component Selection and Real Simulation

In this stage we shall build up on stage 1 and select the appropriate components for our H Bridge according to our requirements. Then the simulation is repeated again with real components and the issues that occur are discussed and we will look for a solution to the issues.

Component Selection

According to our specifications, the switches must

  • be able to withstand Vds of 50 V.
  • be able to withstand current of 10 A during ON time.
  • be able to operate within switching frequency of 5-15 kHz

For the switches to be connected to Vcc (source voltage), we shall look for PMOS devices. The decision is based on the fact that PMOS (or for this matter PNP) switches would be easier to switch ON and OFF when connected to Vcc. That is, to switch on the PMOS, we just have to apply a low signal at its gate. On the other hand, if we had chosen an NMOS or an NPN switch, we would require voltage greater than VCC to be applied to the gate/base in order to switch on the transistor. For the ‘lower’ switches (those connected between motor and ground), we shall use the NMOS switches. This is the main reason why PNP or PMOS are usually the choice when switching near the source.

The components finally selected are IRF9540 and IRF540N. The features of each are shown below:

Typical Specifications of selected switches
Typical Specifications of selected switches

The specifications for IRF9540 and IRF540N listed above are the most relevant and sensitive in nature to our goal of 50 V 10 A DC motor drive.

  1. Maximum Source to Drain Current
  2. Maximum Voltage
  3. Turn On Delay Time
  4. Turn Off Delay Time
  5. Presence of Free-Wheeling Diode
  6. Reverse Recovery Time

Some of these specifications may not make sense (such as the delay time, recovery time, free-wheeling diode) at this stage of the design but you will see that they are deadly important for the safety of the circuit and the user!

Testing of Components (IRF9540 and IRF540N)

To test the components, switches were connected independently at first and their switching characteristics were verified. A simple resistive load and a low voltage Vcc was connected and switching pulses were applied at their gates. These pulses were generated using the function generator only for testing purposes. The switches were checked at switching frequencies varying from 5 kHz to 12 kHz. The next step was to connect the switches as a ‘leg’ of the H-Bridge (the load between PMOS and NMOS) and the same procedure was repeated to verify the switching frequency of the components selected.

REMEMBER! Testing at lower voltages and under controlled conditions is a must for each component that you use in any electronic circuit. Even though the datasheet may suggest that your component is well capable of handling the conditions, you always have the slightest probability of having a faulty component. Always test your components beforehand and verify their characteristics before joining them in the final circuit.

H-Bridge Simulation (Real Components)

Now we simulate H-Bridge circuit with the real components selected. The simulated circuit is similar to the previous one but ideal transistors have been replaced with IRF9540 and IRF 540N.

We observe the waveform through the load resistor in this simulation and see that it is identical to the simulation for ideal switches. However when we see the current through the transistors, we see that huge currents flowing. This is shown in the figure below waveforms. This is because when we use real components, they cannot turn on and off instantly. However our PWM signals change values immediately. The result is that the PMOS has not switched off completely yet and the NMOS beneath it switches on. This shorts the supply Vcc to the ground. Notice in the waveforms that the current rises to 130 A ! This will not only burn your components but also damage the voltage supply and could cause heavy electrical hazard!

Current waveform through real switches
Current waveform through real switches

Hence we need to make sure that the NMOS does not switch on before the PMOS has switched off completely and vice versa. To achieve this, we have to introduce dead-time (or deadband) within the PWM wave. At this time all of the switches will be signaled to remain off.

This will be our next stage (Stage 3: Generation of Dead-Time / Dead-Band in Electronic Circuits). It will provide us with the solution to our problem.

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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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      Bogdan 21 months ago

      If I wanted to accurately depict the LMD18200 H-bridge model in spice, what would the schematic look like?