Power Semiconductor Losses Dr John Fletcher John.Fletcher@unsw.edu.au

Losses Losses affect the operating temperature of the device junction. The junction temperature is important because If Tj(max) is exceeded the device performance is considerably degraded – or fails. Reliability is a function of junction temperature. Rule of thumb a 10oC increase in Tj halves the operating lifetime. Thermal considerations define equipment ratings. Therefore it is imperative that a design engineer can calculate the total losses in power semiconductor devices.

What are losses? Losses in switching converters fall into two categories Switching loss Conduction loss For a converter both losses must be calculated for each device. Using the losses and a model of the thermal system, the junction temperature of each device can be estimated.

Conduction loss Power dissipation during conduction is a function of voltage and current A component dissipates power when it supports a voltage whilst conducting a current. During the on-state, the current is conducted through the device with the device supporting a forward voltage drop – hence power is dissipated.

Diode conduction loss

MOSFET conduction loss

IGBT conduction loss In all cases, to calculate the average conduction (on state) loss in the device the instantaneous conduction loss must be averaged over an electrical cycle.

Switching Loss In switching converters devices must transit through their linear region. As they transit through this region they dissipate significant amounts of energy.

Switching Loss There is a dependence of Eon and Eoff on the load current and supply voltage that the device support – typical these are squared relationships. In order to calculate the switching loss, the voltage and current trajectory during the turn-on and turn-off transients must be known. From the trajectory the energy loss, E, during the transient can be calculated. And the total switching loss

Conduction Loss Example

Q. Calculate the on-state loss in the diode and in the switch Q. Calculate the on-state loss in the diode and in the switch. Assuming no switching loss, what is the efficiency of the converter?

IGBT On-state loss:

Diode on-state loss

Total conduction loss

Power delivered to load

Switching loss example In the previous example, switching losses were neglected. However, inspection of the actual IGBT transients during switching yield the following waveforms Q: Calculate the total switching loss in the converter and the overall converter efficiency. The diode turn-off loss is 600μJ with negligible turn-on loss.

Calculating turn-on and turn-off loss Eon is the shaded area Use the area of a triangle to calculate areas 1 and 2 Also, when IGBT turns-on, diode turns off dissipating 600μJ of energy. Total turn-on loss = 700 + 600μJ Similarly, IGBT turn-off loss calculates to be 1200 μJ. (Diode turn-on loss is negligible.)

Total Power Loss

Total Power Loss

Converter Efficiency If the load current is not constant, or the dc link voltage varies, the variations must be taken into account when calculating the conduction and switching loss.