Power Electronics Test 2

• Power MOSFET has lower switching losses but its on-resistance and conduction losses are more. A BJT has higher switching losses but lower conduction losses.
• MOSFET is a voltage-controlled device whereas BJT is a current controlled device.
• MOSFET has positive temperature coefficient for resistance. This makes parallel operation of MOSFETs easy. If a MOSFET shares increased current initially, it heats up faster, its resistance rises, and this increased resistance causes this current to shift to other devices in parallel.
• A BJT has negative temperature coefficient, so current-sharing resistors are necessary during parallel operation of BJTs.
• In MOSFET, secondary breakdown does not occur, because it has positive temperature coefficient.
• As BJT has negative temperature coefficient, secondary breakdown occurs. With decrease in resistance, the current increases. This increased current over the same area results in hot spots and breakdown of the BJT.

• GTO is a four-layer, three junction semiconductor device with three external terminals, namely, the anode, the cathode and the gate.
• The main advantage of the GTO over the SCR is that it can be turned on or off by applying the proper pulse to the gate.
• The capability of the GTO to be turned on by a gate signal and also turned-off by a gate signal of negative polarity give it a unique capability within the thyristor family of devices.

Important formulas:

1) \({Q_{RR}} = \frac{1}{2} \times {I_{RR}} \times {t_{rr}}\)

2) \({I_{RR}} = {\left[ {2\;{Q_{RR}}\frac{{di}}{{dt}}} \right]^{1/2}}\)

3) \({t_{rr}} = {\left[ {\frac{{2{Q_{RR}}}}{{\frac{{di}}{{dt}}}}} \right]^{\frac{1}{2}}}\)

Calculation:

\({Q_{RR}} = \frac{1}{2}t_{rr}^2\;\left( {\frac{{di}}{{dt}}} \right)\)

\( = \frac{1}{2} \times {\left( {3 \times {{10}^{ - 6}}} \right)^2} \times 30\frac{A}{{\mu s}}\;\)

\({Q_{RR}} = 135\;\mu C\)

\({I_{RR}} = {\left( {2\;{Q_{RR}}\frac{{di}}{{dt}}} \right)^{\frac{1}{2}}} = 90\;A\)

IGBT has low input impedance compared to PMOSFET

IGBT and PMOSFET are both voltage controlled devices.

IGBT can be designed for higher voltages compared to PMOSFETS.

Concept:

The average on-state power loss per cycle is given by

\({P_{avg}} = I_{rms}^2{R_{on}}\)

Where I_rms is the rms value of current

R_on is the on-state resistance

Calculation:

\(i\left( t \right) = \left\{ {\begin{array}{*{20}{c}}{15,\;\;0 < t < \frac{T}{4}}\\{0,\;\;\frac{T}{4} < t < T}\end{array}} \right.\)

\({i_{rms}} = \sqrt {\frac{1}{T}\mathop \smallint \limits_0^T {{\left( {i\left( t \right)} \right)}^2}dt} \)

\( = \sqrt {\frac{1}{T}\mathop \smallint \limits_0^{\frac{T}{4}} {{\left( {15} \right)}^2}dt} \)

\( = \sqrt {\frac{1}{T}\left[ {225t} \right]_0^{\frac{T}{4}}} = \sqrt {\frac{{225}}{4}} = 7.5\;A\)

The average on-state power loss per cycle, P_avg = (7.5)² × 0.25 = 14.0625 W