Considerations On Switch Voltage And Current Ratings
Considerations on switch voltage and current ratings:
- As in square wave inverter the switches of PWM inverter must also be rated for the maximum dc link voltage. There will, however, be a significant difference in the switch current ratings of the square wave and PWM inverter for comparable magnitudes of inverters’ output current.
- This is due to the increased switching losses in the PWM inverter. Since the switches in PWM inverter operate at much higher frequencies than in square wave inverter, the switching losses in the former are comparable to the conduction losses.
- This calls for suitable de-rating of the switch current rating. For medium power rated inverters mostly IGBT switches (with fast acting anti-parallel diodes) are used.
- Generally molded blocks of six switches and six diodes, connected in bridge fashion with their power and control terminals brought out, are commercially available.
- These molded blocks come with isolated metallic case that need to be mounted on suitably sized heat sinks for dissipation of thermal losses in the switch.
- The switch manufacturers provide the turn-on and turn-off loss data for the switches for different magnitudes of dc link voltage, switch current and gate-to-emitter voltages. Similarly conduction loss data for the switches and the diodes are also provided.
- The thermal resistance data (thermal resistance between case and semiconductor-junction) for the switches and diodes are also provided.
- The heat-sink manufacturers provide data / guide lines for calculating the thermal resistance between heat sink and ambient.
- The inverter designer needs to do a detailed analysis of the worst-case thermal losses and temperature rise and need to limit the switch current accordingly.
- In PWM inverters, because of large number of switching per output cycle, the load current frequently jumps from controlled switch (say, IGBT) to diode and hence the diodes of the switches must also be rated to carry the peak magnitude of load current.
- It is to be kept in mind that in PWM inverters the load current polarity changes only according to the output frequency and not according to the switching frequency.
- For load power factor close to one, as the PWM inverter’s output voltage decreases the diode conduction duration increases. The worst-case diode losses also need to be determined for deciding on the de-rating factor for diode currents.