Environmental Engineering Reference
In-Depth Information
8.2.2 Switching
In power electronic systems the switching loss accounts for a significant fraction of
inverter dissipation. For the power MOSFET and IGBT the turn-ON and turn-OFF
switching energy is calculated based on the dc link voltage and load current.
The switching power loss is then the switching energy times the switching fre-
quency as follows:
P sw ¼ f ð E ON þ E OFF Þ
1
6 U dc t r i
E ON ¼
ð 8 : 12 Þ
1
6 U dc t f i
E OFF ¼
where a hard switched inverter is assumed and the inverter current and voltage
during the transitions are triangular. Switching waveform rise time, t r , and fall time,
t f , determine the switching energy when the dc link potential is U dc . Figure 8.6
illustrates the derivation of (8.12) when the current and voltage transitions are
linear. A bipolar junction transistor (BJT) will have an additional turn-OFF
switching loss due to the phenomena of charge-storage 'walk-out', an effect of full
current being sustained even as the device voltage begins to rise. The impact of
temperature on the BJT stored charge is to support load current until the junction
charge is depleted, and then the current begins to tail off.
Since the link voltage is fixed, some investigators approximate the switching
loss using a pair of exponents on current as shown in (8.13), which may be useful in
U , I
U dc
i
Switching event
t r , t f
t
Figure 8.6 Switching waveforms for power semiconductors in hard
switched inverter
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