Driver circuit for Voltage Source Inverter
Voltage Source Inverter:
A typical VSI consists of suitably connected power electronic switches (IGBT, MOSFET, GTO-
based thyristor, etc.) with antiparallel diodes, dc voltage source and coupling transformers/
transformer less. The VSI topology is chosen based on the application. The interface inductance
L, dc link capacitance Cdc. Figure 1 shows the circuit diagram of a VSI with H-bridge topology for
PV – Grid application.
Figure 1
Blanking Circuit:
The control algorithm loaded in the DSP generates the switching pulses for the VSI. These
pulses are not directly given to the IGBT switches in order to prevent the short circuit of the dc
capacitor during the turning ON and OFF of both the switches in the same leg. Thus a dead band
time is provided between the turning ON and turning OFF of the switches in the same leg. The
blanking circuit shown in Figure 2 is used for this purpose. Its inputs are the switching pulses Sa
and Sa generated from the DSP. The integrated circuit SN74LS123 has two identical units of a
monostable multivibrator, and the detailed circuit diagram for one channel of the blanking circuit
is shown in Figure 3. This unit generates a shot pulse Sashot with a duration of td. The gate signal
Ga is generated by AND-ing Sa and Sashot using IC 7408 and passing this signal through a buffer
(CD4050) and a transistor (CL100). Similarly the complementary gate signal Ga can be obtained.
Figure 4 illustrates the inclusion of the dead band in gate signals Ga and .
Figure 2
Figure 3
Figure 4
Opto-isolation Circuit:
The logical gate signals available from the blanking circuit cannot be directly applied to gates of
the IGBT switches which are a part of the high-power network. Thus, high speed optocouplers (IC
HCPL3120) with isolated dc power supplies are used for providing isolation between the logic
control circuits and the power network. The internal and external circuit details of the optocoupler
are shown in Figure 5.
Figure 5
Calculation of the Input Resistance:
The input resistance must be connected in series with the emitter of the optocoupler to limit its
input current.
Input forward current (IF) =10 mA
Input forward emitter voltage (VF) =1.4 V
High-level voltage of the switching pulse (VH) = 5 V
.
Ri = = 360Ω
Input resistance should be less than 360 Ω
Power Supply Modules:
The generalized hardware set-up requires regulated dc power supplies of different voltage levels.
Individual transformers and power supply units are used for each module in order to increase the
reliability of the set-up. To generate a 15 V bipolar dc regulated supply, a 230/18-0-18 V centre-
tapped step-down transformer of 500 mA rating is used as shown in Figure 6. A full bridge diode
rectifier (D1-D4, using IN4007) is used to convert ac to dc. Two identical electrolytic capacitors
(Cdc) of value 100 µF are used to smoothen the output voltage. A local ground (GND) is created
by connecting the centre tap of the transformer and mid-point of the capacitors. The unregulated
dc voltage is then given as input to the voltage regulator ICs, 7815 and 7915, to produce regulated
dc output voltages of +15 V and -15 V, respectively. The 5 V supply can be generated using a
fixed voltage regulator IC 7805.
Figure 6
