Need For Isolated Gate-Control Signals For The Switches
Need for isolated Gate-control signals for the switches:
- The gate control signals are low voltage signals referred to the source (emitter) terminal of the switch. For n-channel IGBT and MOSFET switches, when gate to source voltage is more than threshold voltage for turn-on, the switch turns on and when it is less than threshold voltage the switch turns off.
- The threshold voltage is generally of the order of 5 volts but for quicker switching the turn-on gate voltage magnitude is kept around 15 volts whereas turn-off gate voltage is zero or little negative (around –5 volts).
- The two switches of an inverter-leg are controlled in a complementary manner. When the upper switch of any leg is ‘on’, the corresponding lower switch remains ‘off’ and vice-versa. When a switch is ‘on’ its emitter and collector terminals are virtually shorted. Thus with upper switch ‘on’, the emitter of the upper switch is at positive dc bus potential.
- Similarly with lower switch ‘on’, the emitter of upper switch of that leg is virtually at the negative dc bus potential. Emitters of all the lower switches are solidly connected to the negative line of the dc bus. Since gate control signals are applied with respect to the emitter terminals of the switches, the gate voltages of all the upper switches must be floating with respect to the dc bus line potentials.
- This calls for isolation between the gate control signals of upper switches and between upper and lower switches. Only the emitters of lower switches of all the legs are at the same potential (since all of them are solidly connected to the negative dc bus) and hence the gate control signals of lower switches need not be isolated among themselves.
- The isolation provided between upper and lower switches must withstand a peak voltage stress equal to dc bus voltage. Gate-signal isolation for inverter switches is generally achieved by means of optical-isolator (opto-isolator) circuits.
- Input stage of the IC is a light emitting diode (LED) that emits light when forward biased. The light output of the LED falls on reverse biased junction of an optical diode. The LED and the photo-diode are suitably positioned inside the opto-coupler chip to ensure that the light emitted by the LED falls on the photo-diode junction. The gate control pulses for the switch are applied to the input LED through a current limiting resistor of appropriate magnitude.
- These gate pulses, generated by the gate logic circuit, are essentially in the digital form. A high level of the gate signal may be taken as ‘on’ command and a low level (at ground level) may be taken as ‘off’ command.
- Under this assumption, the cathode of the LED is connected to the ground point of the gate-logic card and anode is fed with the logic card output. The control (logic card) supply ground is isolated from the floating-supply ground of the output. In the figure the two grounds have been shown by two different symbols.
- The schematic connection shown in the figure indicates that the photo-diode is reverse biased. A resistor in series with the diode indicates the magnitude of the reverse leakage current of the diode. When input signal to LED is high, LED conducts and the emitted light falls on the reverse biased p-n junction.
- Irradiation of light causes generation of significant number of electron-hole pairs in the depletion region of the reverse biased diode. As a result magnitude of reverse leakage current of the diode increases appreciably. The resistor connected in series with the photo-diode now has higher voltage drop due to the increased leakage current.
- A signal comparator circuit senses this condition and outputs a high level signal, which is amplified before being output. Thus an isolated and amplified gate signal is obtained and may directly be connected to the gate terminal of the switch (often a small series resistor, as suggested by the switch manufacturer, is put between the output signal and the gate terminal of the switch).