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Power electronics > Q1 > Flashcards

Q1 Flashcards

1
Q

Definition of diode and main characteristics

A

Uncontrolled semiconductor device that conducts current in only one direction
- Blocking voltage and forward I ratings
- on-state voltage
- leakage current
- reverse recovery time
- negative T coefficient of R

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2
Q

Operations regions of diode

A
  • forward region
  • reverse region
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3
Q

Describe operation in forward region

A
  • Diode is forward biased with a voltage higher than built-in voltage
  • Diode starts conducting
  • Small forward voltage appears across diode
  • Current increases exponentially and can be represented as short circuit
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4
Q

Why is it difficult to parallel diodes?

A
  • They show negative T coefficient
  • As T increases forward voltage decreases
  • Higher I flow through the diode increasing power losses and T
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5
Q

Describe operation in reverse bias region

A
  • Diode blocks the I in reverse direction
  • Very small leakage I flows in reverse direction, as long as voltage applied is less than breakdown voltage
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6
Q

types of diodes

A
  • Line-frequency diode
  • Fast-recovery
  • Schottky
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7
Q

characteristics of line-frequency diode

A
  • very low forward voltage (reduced conduction losses)
  • high stored charge and reverse recovery time
  • high voltages and I ratings
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8
Q

characteristics of fast-recovery diode

A
  • low stored charge and recovery time
  • higher forward voltage (higher conduction losses)
  • medium voltages and I ratings
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9
Q

characteristics of schottky diode

A
  • low forward voltage drop (lower conduction losses)
  • no reverse recovery time
  • low voltages ratings
  • higher leakage I
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10
Q

Explain turn-on process of diode

A
  • When diode is forward biased I starts increasing and V decreases
  • Positive overvoltage Vfp appear across diode before dropping to forward voltage
  • Overvoltage caused by effect ohmic R of drift region and L of Si chip and bond wires (larger if di/dt are applied)
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11
Q

Explain turn-off process of diode

A
  • Forward I starts decreasing and becomes negative
  • Negative I is called reverse recovery I needed to remove excess carrier stored in drift region (diode is not in blocking state)
  • When excess carriers are removed (at t4) reverse I decreases and voltage across diode increases rapidly.
  • When inductive loads or stray inductance is in series with the diode, and overvoltage appears across its terminals
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12
Q

Explain and characterize reverse recovery behavior of diode

A
  • reverse recovery time is the sum of the times t4 and t5
  • peak of reverse recovery I is t4*di/dt
  • Stored charge can be approximated as Irr*trr/2
  • Softness factor is t5/t4, can be used to categorize the diode as soft-recovery or snappy recovery if S is close to zero
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13
Q

Q1. slide 3 main ideas

A
  • Need of series connection of diodes (increase blocking voltage)
  • Problems of connecting diodes in series
  • Explain figure
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14
Q

explain problem of connecting diodes in series

A
  • every diode has different I-V characteristics
  • In the figure when two diodes are in blocking state, leakage I is same for both diodes
  • Voltage drop across each diode is different and cause breakdown of diode with higher voltage drop
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15
Q

describe static solution for series diode connection

A
  • a R is connected in parallel to each diode to balance voltage drops.
  • we obtain equations considering that V1+V2 is the total voltage
  • Total I is the I through D1 and R1 or D2 and R2.
  • To obtain same voltage across diodes, we set V1=V2 and solve equations for R1 for given leakage currents and R2.
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16
Q

considerations of resistors in static solution of series diode connection

A
  • If we use large values of R1 and R2, power loss is reduced, but we have poor balance
  • at limit when R1 and R2 are too large, they can be considered as open circuit and i1 and i2 will be equal
17
Q

problems of static solution in series connection of diodes

A
  • simple but if reverse recovery time of D1 is shorter, it will recovering blocking state first and must support total voltage.
  • D2 recovers slowly since recovery is limited by the low leakage I of D1.
18
Q

Explain dynamic solution of series connection of diodes and advantages

A
  • Connect an RC network in parallel to diode
  • With RC network voltage across diode increases gradually with time constant RsCs and can also help to protect the diode from voltage spikes
  • The RC network provides transient I path to allow D2 to recover faster instead of being limited by the low leakage I of D1
19
Q

Consideration for RsCs in dynamic solution of series diode connections

A
  • The time constant RsCs must be much lower than switching period, so that turn-on speed of diode is not affected.
  • If RsCs is small the circuit can respond quickly to transients.
  • At turn on capacitor voltage is equal to diode voltage, if discharging takes much time voltage across diode decreases slowly and turn on speed is affected (switching losses increase).
20
Q

explain problem of parallel diodes

A

Due to different I-V characteristics of diodes
- voltage across diodes is the same but I can be different
- since pn junction diodes have negative T coefficient power dissipation of diode conducting higher I increases
- Due to power dissipation T increases, it reduces even more R of diode allowing more I to flow and power losses and T increases even more
- If not controlled diode might be destroyed due to high I and power dissipation

21
Q

explain static solution of parallel diodes

A
  • we add a R in series with each diode
  • we get the equations, considering I=i1+i2 and sum of voltage across the diodes and R is the total voltage.
  • To obtain equal I through the diodes, we set I1=I2 and solve for R1 for given values of V1, V2 and R2.
22
Q

considerations for static solution of parallel diodes

A
  • lower R values reduce power losses but produce poor balance
  • at the limit when R are close to 0, V1 is equal to V2.
23
Q

Problem of static solution for parallel diodes

A
  • simple but not enough to protect diodes against dynamic changes of I
  • i.e. if T of one diode increases by external factors, its R decreases and higher I will flow through the diode (voltage drop of series R compensates reduction of R of diode)
24
Q

Explain dynamic solution of parallel diodes

A
  • we add two coupled L with reverse polarity
  • i.e. when I through D1 increases the voltage across L1 increases to oppose I flow through D1.
  • at the same time voltage with opposite polarity appears across L2, increasing voltage across D2 and R2 thus current I2 increases.
25
Q

Selection of power diodes is mostly based on

A
  • voltage rating
  • current rating
  • switching speed (switching losses)
  • on-state voltage (conduction losses)
26
Q

Advantages/disadvantages of soft-recovery behavior

A
  • overvoltage peak during turn-off is reduced (important with L loads)
  • longer reverse recovery time (increases switching losses)
  • turn-off transient is not abrupt reducing di/dt and EMI
27
Q

How R of drift region contributes to overvoltage peak during turn on

A
  • R of drift region is not constant, it decreases with conductivity modulation
  • R is large due to lack of conductivity modulation until space charge region is discharged.
  • The large R causes higher voltage drop until conductivity modulation reduces R.

Power electronics (8 decks)

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