General Flashcards
1
Q
MOSFET and IGBT have positive or negative T coefficient
A
Positive T coefficient
2
Q
Considerations for turning on SCR
A
- Remove gate signal after switch turns-on to avoid gate power losses
- Anode I should exceed latching I to make sure on-state and not being less than holding I
- Time of gate signal must be higher than SCR turn on time
3
Q
Ways to turn on SCR
A
- Heat
- Light
- Apply high forward voltage (not practical)
- Apply high dv/dt
- Apply gate pulse
4
Q
Effect of increasing gate I in SCR
A
- forward breakdon voltage decreases
- turn-on time decreases
5
Q
SCR needs to be protected against
A
- Hidh di/dt (hotspot): an L in series with SCR
- High dv/dt (unwanted turn-on): use RC snubber in parallel.
6
Q
Characteristics of GTO
A
- Turned-on by low + gate I
- Turned-off by high - gate I
- Higher on-state voltage than on-state voltage of SCR for same I
7
Q
What is a TRIAC
A
bidirectional SCR
8
Q
number of AC voltage levels for n-bridge
A
2n+1
9
Q
Forbidded states of CSI
A
- All upper and lower switches OFF: switches will be destroyed by overvoltage
- Two or more upper or lower switches conducting at the same time: short circuit of output.
10
Q
Saturation of inductor problem in boost converter
A
- More problematic since L decreases and behaves as short circuit
- When switch is closed voltage source is short circuited
11
Q
Why dv/dt in C of buck converter is high
A
- since it is directly connected to input voltage
- in boost L is charged first
12
Q
How an SCR is turned off?
A
- By making the I drop below the holding I
- By applying a negative voltage
13
Q
Why do we need freewheling diode for half-wave PCR rectifier
A
- for firing angles grater than pi/2, average output voltage becomes negative
- energy cannot be transferred to the source, it will be stored in L
- System will become unstable
14
Q
distorted reactive power refers to
A
power of all harmonics
15
Q
Protections of gate drive circuit
A
- Protection against miller clamp (use smaller Roff)
- Protection against overvoltage of gate (clamping diode to Vdd or from Gate to GND with series diode)
- Active clamping to protect against high drain-source overvoltage caused by stray L (TVS in series with diode)
- Desaturation protection against short-circuits (desaturation pin of gate driver connected to R and D, Vce proportional to I)
- UVLO ()
