Q1 Flashcards
Definition of diode and main characteristics
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
Operations regions of diode
- forward region
- reverse region
Describe operation in forward region
- 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
Why is it difficult to parallel diodes?
- They show negative T coefficient
- As T increases forward voltage decreases
- Higher I flow through the diode increasing power losses and T
Describe operation in reverse bias region
- 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
types of diodes
- Line-frequency diode
- Fast-recovery
- Schottky
characteristics of line-frequency diode
- very low forward voltage (reduced conduction losses)
- high stored charge and reverse recovery time
- high voltages and I ratings
characteristics of fast-recovery diode
- low stored charge and recovery time
- higher forward voltage (higher conduction losses)
- medium voltages and I ratings
characteristics of schottky diode
- low forward voltage drop (lower conduction losses)
- no reverse recovery time
- low voltages ratings
- higher leakage I
Explain turn-on process of diode
- 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)
Explain turn-off process of diode
- 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
Explain and characterize reverse recovery behavior of diode
- 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
Q1. slide 3 main ideas
- Need of series connection of diodes (increase blocking voltage)
- Problems of connecting diodes in series
- Explain figure
explain problem of connecting diodes in series
- 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
describe static solution for series diode connection
- 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.
considerations of resistors in static solution of series diode connection
- 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
problems of static solution in series connection of diodes
- 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.
Explain dynamic solution of series connection of diodes and advantages
- 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
Consideration for RsCs in dynamic solution of series diode connections
- 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).
explain problem of parallel diodes
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
explain static solution of parallel diodes
- 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.
considerations for static solution of parallel diodes
- lower R values reduce power losses but produce poor balance
- at the limit when R are close to 0, V1 is equal to V2.
Problem of static solution for parallel diodes
- 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)
Explain dynamic solution of parallel diodes
- 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.
Selection of power diodes is mostly based on
- voltage rating
- current rating
- switching speed (switching losses)
- on-state voltage (conduction losses)
Advantages/disadvantages of soft-recovery behavior
- 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
How R of drift region contributes to overvoltage peak during turn on
- 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.
