Why is inverter efficiency low at low power output?

[jeffwjz]

Hello,

Just a simple question when looking at an inverter efficiency profile. It is normal that the efficiency of an inverter rises initially with power output and then stablises. Can anyone explain why it is low at low power?

Thanks.

 

[red_alert]

That's because MOSFETs switching losses are almost constant (only conduction losses depend on load).

 

[jeffwjz]

That's because MOSFETs switching losses are almost constant (only conduction losses depend on load).

How about IGBT? I think the switching energy loss is dependent on current conducted, there is a plot of switching loss against current in the datasheet.

 

[FvM]

That's because MOSFETs switching losses are almost constant (only conduction losses depend on load).

How that? Switching losses are roughly proporional to switched voltage, current and rise/fall time. So they are proportional to load current. Similarly diode recovery losses are proportional to √Id, if MOSFET diode operation is involved or for an IGBT inverter.

A constant switching loss share is caused by the magnetizing current of output transformers or inductors and by the MOSFET capacitances. Additional constant losses are generated by the control circuit.

 

[red_alert]

For a MOSFET, the switching losses depend on total gate charge. Conduction losses are Rds(on) multiplied by the square of Ids. Moreover, the Rds(on) depends on Ids, too.

 

[jeffwjz]

How that? Switching losses are roughly proporional to switched voltage, current and rise/fall time. So they are proportional to load current. Similarly diode recovery losses are proportional to √Id, if MOSFET diode operation is involved or for an IGBT inverter.

A constant switching loss share is caused by the magnetizing current of output transformers or inductors and by the MOSFET capacitances. Additional constant losses are generated by the control circuit.

Hi, if both conduction loss and switching loss are proportional to load current, why is the inverter efficiency is lower at low power output, presumably the DC link voltage is fixed?

 

[FvM]

It's answered in the second part of my reply. A part of the switching losses is constant, you also have constant power consumption of the control circuit. Just calculate what it means in efficiency terms.

 

[red_alert]

Take it this way: when there's no load, the only power consumption is generated by the switching losses and the control/driver circuit. If you calculate the efficiency, the ratio of useful power (almost zero) to total power consumption is very small. As the useful power increase, the ratio is getting bigger.

 

[D.A.(Tony)Stewart]

50Hz Bridge rectifer inverters are very inefficient at full load and when running a dimmer to DC rectifier for low voltage even worse, but cheap.

THats why they invented PFC HF BUCK Boost regulators inverters.