Introduction - Control of Power Inverters in Renewable Energy and Smart Grid Integration - page 25

Figure 1.24 Block diagram of an inverter system
1.3.1 Isolation
In order to guarantee proper operation, the low-power-low-voltage electronic part of an inverter
should be isolated from the high-power-high-voltage part. The isolation from the power part
to the electronic part is often taken care of by the sensors. The isolation from the electronic
part to the power part is often done to the PWM signals before entering the driving circuit, as
shown in Figure 1.24. This can be easily done with optocouplers.
TLP550 is an optocoupler commonly used in the drivers for IGBT (Toshiba 2002), with
a typical circuit shown in Figure 1.25. The PWM signal from a signal buffer/driver, e.g.
SN74AB541, which processes the PWM signals of the Digital Signal Processor (DSP) from
3.3 V to 5 V, is connected to the cathode of the diode of the optocoupler to generate an isolated
output signal PWM_Drive, which can be connected to the driving circuits. The truth table of
the circuit shown in Figure 1.25, together with the logic for the operation of the IGBT, are
given in Table 1.1, where PWM_DSP means the PWM signal from DSP ports. In order to
avoid the damage caused by the high-impedance state of the DSP ports during reset, the ports
can be pulled up with resistors. Hence, the high-impedance state of a port is equivalent to the
OFF state. Note that an IGBT is turned on when the PWM signal is 0 in this case.
Figure 1.25 Typical circuit for TLP550. Source: Toshiba 2002