Voltage And Current Ratings Of Inverter Switches & Applications Of Square Wave Inverter
Voltage and Current Ratings of Inverter Switches:
- Switches in each leg of the inverter operate in a complementary manner. When upper switch of a leg is on the lower switch will need to block the entire dc bus voltage and vice versa. Thus the switches must be rated to block the worst-case instantaneous magnitude of dc bus voltage.
- In practical inverters the switch voltage ratings are taken to be somewhat higher than the worst-case dc voltage to account for stray voltages produced across stray inductances, the turn-on transient voltage of a power diode etc.
- For a well laid out circuit a 50% margin over the dc-bus voltage may be the optimum switch voltage rating. Each switch of the inverter carries load current during half of the current cycle. Hence the switches must be rated to withstand the peak magnitude of instantaneous load current.
- The semiconductor switches have very small thermal time constant and they cannot withstand overheating for more than a few milli-seconds. Thus even though the load current passes through the switches only in alternate half cycles, the thermal limit may be reached during half cycle of current itself.
- It may be pointed out that each inverter switch consists of a controlled switch in anti-parallel with a diode. The distribution of current between the diode and the controlled switch will depend on the load power factor at the operating frequency. In general cases both diode as well as the controlled switch should be rated to carry the peak load current.
Applications of Square Wave Inverter:
Fig: 100W square wave inverter
- The square wave voltage-source inverter has application in many low cost ac motor drives, uninterruptible power supply units and in circuits utilizing electrical resonance between an inductor and a capacitor.
- Some examples of circuits utilizing resonance phenomenon are induction heating units and electronic ballasts for fluorescent lamps.