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Optical Isolation and Gate Drive Circuits for H Bridge - Power Electronics

Updated on February 14, 2013

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This is Stage 4 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 4: Optical Isolation and Gate Drive Circuits

Why do we need to provide isolation between different sections of electronic circuits? What is the easiest way to implement isolation?

What must be the characteristics of Gate Drive circuits for controlling the H Bridge switches?

All these questions are discussed in this stage of the design and finally circuit implementations are shown as simulated in Multisim and LTSpice to solidify the concepts presented in this hub.

Optical Isolation in Electronics

One important phenomenon in power electronics and circuits involving feedback and high voltages is of isolation. As the word itself suggest, one section of the circuit is isolated electrically from the other section to protect from delicate modules from interaction with the high voltage power modules. In our DC Motor Drive circuit, the feedback module consists of a microcontroller and several gates. These devices work on low voltages (5V) and are easily damaged if exposed to high voltages and currents. Sometimes, different sections are given different grounds to gain complete electrical isolation. In this design, isolation is achieved using opto-couplers. The opto-coupler IC used is 4N35.

The figure below shows an opto-coupler and also states its purpose.

Optical Isolation for protection
Optical Isolation for protection

The opto-coupler (as shown in the figure above) consists of a Light-Emitting-Diode and a photo-transistor. The photo-transistor switches on when the input signal can provide sufficient current to the LED.

In our circuit, the PWM waves from the buffer are connected as input signal to the opto-couplers. Gate-drives cannot be achieved simply but the PWM wave generated by the microcontroller because the microcontroller works on low voltage of 5V and generates pulses of that amplitude, while the PMOS and NMOS require pulses of amplitude VCC to operate. The photo-transistor is connected to VCC and is used in the gate-drives of the power transistors.

Gate Drive Circuit for Controlled H Bridge

The gate drive circuit for the power transistors must provide the gates with signals appropriate enough to switch ON and OFF the transistors and also maintain them in these states. The implemented circuit is simulated and simulation results are shown in this section. The gate-drive for one PMOS is shown below in the figure.

Gate-drive circuit for PMOS switch
Gate-drive circuit for PMOS switch

To see the figure before the buffer U6 visit Stage 3: Generation of Dead-Time / Dead-Band in Electronic Circuits. Similarly to understand the PMOS and NMOS used in the H Bridge visit Stage 2: H Bridge II - Component Selection and Real Simulation.

The figure above shows the opto-coupler receiving the input PWM wave from the buffer. When the high signal appears at the opto-coupler, the photo-transistor switches on and connects the base of Q3 to Vcc, hence Q3 switches on. This shorts the gate of the PMOS to ground and the PMOS switches on. The waveform below shows the signal at the gate of the PMOS.

Gate-Drive signal of PMOS switch
Gate-Drive signal of PMOS switch

A similar setup is done for the NMOS switches, the circuit and the corresponding final gate signal is shown below:

Gate-drive circuit for NMOS switch
Gate-drive circuit for NMOS switch

The magnitude of the gate-drive signal is decided by using the Vgs graphs for different Drain-to-Source currents. One such graph for IRF540N is shown in figure below taken from the datasheet. Since we have to pass maximum Id of 10 A, we check the value of Vgs from the graphs. We find that for voltages greater than 4.5 V our circuit will operate perfectly. Our applied gate-signals are of amplitude Vcc and thus lie within the working range.

Typical output characteristics graph for IRF540N
Typical output characteristics graph for IRF540N

The next stage will be Stage 5: Reading Motor Speed using Optical Encoder. This will used for the feedback from the motor to the microcontroller.

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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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    • profile image

      DAVID GUIMARÃES 21 months ago

      PRECISO DE UM PROJETO

      PARA CONTROLE DE VELOCIDADE DC DE 80V PARA ESTEIRA

      MOTOR DE CORRENTE CONTINUA.

      davidgtow@gmail.com

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