Isolated Gate Drive System
Original Publication Date: 2003-May-19
Included in the Prior Art Database: 2003-May-19
An isolated transistor gate drive circuit is presented whereby the energy to drive said device is transferred capacitively through an insulating medium such as a printed wiring board by way of modulating the gate drive logic level signal to a high frequency carrier.
Isolated Gate Drive System
Disclosed is a method and system whereby a transistor may be driven by an isolated source without the use of conventional technology such as reverse junction isolation, transformers, or opto-isolators. This system differs in that energy to drive the gate is supplied by a low power, high frequency oscillator. This energy passes through adjacent layers on a printed wiring board to provide gate drive, while maintaining high voltage isolation. Such a system is typically employed in full bridge switching converters and also in motor drive applications where a
H-Bridge is used.
In a full bridge configuration or three phase H-Bridge in a motor drive application, transistors are often arranged in a "stacked" configuration. This means that the same ground referenced controls that drive the bottom devices will not work for the top devices.
A transistor gate drive system composed of three components is presented. The three components are:
2.) Isolating medium and plates
3.) Demodulator Together these items form a method and system for transferring energy through a medium such as a printed wiring board with only minor loss of energy and radiated emissions for the sole purpose of driving a transistor in a bridge configuration where any one of the above mentioned current state of the art have been employed in the past.
MODULATOR: The first item in this system is the modulator. The purpose of the modulator is to modulate the gate drive "CONTROL" signal to a high frequency carrier so that it may be transmitted through a UL recognized insulating medium. This
modulator will consist of an oscillator of some type. For simplicity, the modulator shown here is a simple one transistor parallel resonant tank oscillator. This circuit is capable of producing one Watt of RF power at frequencies of up to 100MHz. This could be improved with a better transistor, but the two-cent 2N2222 was chosen due to its high availability.
Figure 2. Basic implementation.
In the above circuit, the control input directly alters the quiescent bias point of the circuit which affects output amplitude and to a greater extent, frequency. FYI this circuit is tuned to oscillate at approximately 100MHz for simple round numbers. It costs less than $0.25 to implement. What is important to notice here is that the amount of power and therefore Gate amplitude can be changed in a continuously variable fashion
while maintaining high voltage isolation.
INSULATING MEDIUM AND PLATES: The printed wiring board is used in this application to form a series of plates on adjacent layers, through which energy will pass when one plate is energized by the modulator and there as a RF return path present.
Figure 3. Printed wiring board pattern for
Red areas indicate bottom sid...