Conduction,Blocking & Switching Losses of Semiconductors
Conduction Losses of Semiconductors:
- Conduction losses are caused by the forward voltage drop when the power semiconductor is on and can be described by (with reference to an IGBT).
where Ic is the current carried by the device and Vce(sat)(Ic) is the current dependent forward voltage drop. This drop may be expressed as
- This relation defines the forward drop of an IGBT in a similar manner to a diode. A part of the drop is constant while another part is collector current dependent.
- The given data should be used as follows: Using the numerical value is the simplest way to determine conduction losses. The numerical value can be applied if the current in the device is equal or close to the specified current - data sheet numerical values are specified for typical application currents.
- The graph most accurately determines conduction losses. The conditions in which the data are used should correspond to the application. To estimate if a power semiconductor rating is appropriate, usually the values valid for elevated temperature, close to the maximum junction temperature TJmax , should be used to calculate power losses because this is commonly the operating point at nominal load.
- Blocking losses are generated by a low leakage current through the device with a high blocking voltage.
WB = Vb(I).IL
Where IL is the leakage current and Vb(I) is the current dependent blocking voltage.
- Data sheets indicates leakage current at certain blocking voltage and temperature. The dependence between leakage current and applied voltage typically is exponential; this means that using a data sheet value given for a blocking voltage higher than applied overestimates blocking losses.
- However in general, blocking losses are small and can often, but not always, be neglected.
- For IGBTs they are specified as an amount of energy, Eon/off for a certain switching operation. Eon/off are the energy dissipated at turn-on/turn-off respectively. Using the numerical value is again the most simple way to determine switching losses.
- The numerical value can be applied if the switching operations are carried out at the same or similar conditions as indicated in the data sheet. Graphs for Eon(IC)/(RG ), Eoff (IC)/(RG) with collector current IC and gate resistance RG are provided.
- The graphs permit the most accurate determination of switching losses, given the parameters of the converter: RG and converter current IC.