Driver circuits are most commonly used to amplify signals from controllers or microcontrollers in order to control power switches in semiconductor devices. Driver circuits often take on additional functions which include isolating the control circuit and the power circuit, detecting malfunctions, storing and reporting failures to the control system, serving as a precaution against failure, analyzing sensor signals, and creating auxiliary voltages.
Thyristor Driver Circuits
In thyristor driver circuits, appropriate control signals are used to generate gate current pulses in order to trigger the thyristor. A transformer often isolates the control circuit from the high voltages of the power circuit.
Figure 1. Principle of a thyristor driver circuit
The firing pulses are repeated several times in order to ensure that the pulses exceed the thyristor's latching current. The latching current is the minimum gate current required to trigger the thyristor. For the thyristor to turn on, the gate pulse must continue until the current through the thyristor reaches the holding current, the minimum current required for the thyristor to remain in the on-state.
MOSFET and IGBT Driver Circuits
IGBT and MOSFET drivers are very similar in that both components are controlled by voltage (charging the gate capacitor). Table 1 below shows typical control voltages for both types of drivers:
|Switch On||+10 V||+15 V|
|Switch Off||0 V||-8 V (-15 V)|
Table 1. Typical control voltages for MOSFETs and IGBTs
The IGBT gate voltage in switch off mode is generally approximately -8V to 15V in order to prevent an undesired capacitive switch on. The insulation between the gate and the emitter is made of thin silicon oxide. A maximum voltage of 20V to 25V must never be exceeded in order to ensure that the oxide layers remain intact.
Functions of Typical Driver Circuits
The diagram below shows an example of an IGBT half-bridge driver circuit.
Figure 2. Outline of IGBT half-bridge driver circuit
Primary side of driver circuit
On the primary side of the IGBT half-bridge driver circuit above, input signals are received and error signals are sent back to the controller.
Signal processing that takes place on the primary side of the driver circuit enables short pulse suppression in order to minimize glitches, prevention of both IGBTs in the half-bridge turning on simultaneously so to avoid short circuits, and monitoring of temperature and undervoltages.
Isolation between primary and secondary sides
Isolation of primary and secondary sides of the driver circuit is generally accomplished by use of transformers or optocouplers (LEDs and phototransistors). Typical driver circuits can withstand voltages of 2500 V between the primary and secondary side.
Some simple driver circuits forego the isolation between the primary and secondary sides, leaving this task in the hands of the user.
Secondary side of driver circuit
On the secondary side of the driver circuit, input signals are amplified and used to control the switching of the IGBTs. Overcurrents are monitored to detect shorts in the power circuit. This is done by either comparing the collector-emitter voltage VCE to a preset threshold or by monitoring the signal of a current sensor.
If overcurrent occurs, the secondary side of the driver switches all the IGBTs off and sends an error signal to the primary side.
Isolation of the triggering signal using an optocoupler
Optocouplers are often used in place of transformers to isolate signals. An optocoupler is made up of an LED (Light Emitting Diode) and a light sensitive transistor (phototransistor) all in one package.
In general, optocouplers are inferior to transformers since they display higher susceptibility than transformers, less durability, limited performance, and limited isolation voltage. These drawbacks are more apparent in low cost optocouplers.
Output Stage of Driver Circuits
The gate emitter capacitor is charged and discharged through the gate resistor by the driver. This determines the switching speed of the IGBT.
Figure 3. Output stage of a gate driver
In the diagram above, Rgon represents the external gate series resistance at switch-on and Rgoff the external gate series resistance at switch-off. The higher Rg the slower the switching process.
Soft turn off
For many driver circuits, a switch to a higher Rgoff is made in the case of a short circuit, leading to a slower, or soft, turn-off. In this way, overvoltages that can cause high currents and parasitic inductances which might otherwise damage the module are avoided.
Maximum and average gate current
High currents flowing within a short time are required in order to charge or discharge the gate capacitor. The higher the switching speed and the bigger the IGBT surface, the higher the peak gate current (up to 10A).
The average driver current depends additionally on the clock frequency, that is, the number of switching operations per second.
Maximum as well as average gate currents must be considered when selecting a driver circuit for a particular application.
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