lec2 power diode
DESCRIPTION
power diodesTRANSCRIPT
Power Diodes
Chapter No. 2
Power Diode
• Role in Power Electronics• Uncontrolled Rectifiers– AC to fixed DC voltages
• Freewheeling Diode– To provide a path for the current in inductive loads
The PN junction Diode
• Power diodes- Silicon/Germanium • Silicon diodes– Operate at higher currents– Higher junction temperature– Greater reverse resistance
• Forward bias• Reverse bias
Zero Bias Condition
Forward Biased
Reverse Biased
VI characteristics of Diode
Schottky Diode
• Low-voltage, high speed device• A thin metal is interfaced with N-type
semiconductor• Metal- chromium, platinum, or tungsten• Low on state voltage• Turn off much faster• Higher reverse leakage current• Reverse breakdown voltage is lower
Example
• A Schottky diode rated at 40V and 25A has an on-state voltage of 0.5V and a reverse leakage current of 50nA. Find the on-state and off-state power loss at rated conditions.
• On-state power loss = ID * Von = 12.5W
• Off-state power loss = Ileakage * Vs = 2μW
Example
• Find power losses for a PN junction diode rated at 40V and 25A, with an on-state voltage of 1.1V and a reverse leakage current of 0.5nA.
• On-state power loss = ID * VD(on) = 27.5W
• Off-state power loss = Ileakage * Vs = 0.02μW• (The on-state power loss of Schottky diode is
less than half that of a PN junction diode)
Diode Rating
• Peak Inverse Voltage (PIV, PRV, V(BR))– Maximum reverse voltage that can be connected across diode without
breakdown– Ranges from tens of volts to several thousands
• Maximum Average Forward Current (If(avg)max)– Ratings from few amperes to several hundred amperes
• Maximum Junction Temperature (Tj(max))– Max temperature diode can withstand without failure– -40oC to +200oC
• Maximum Surge Current (IFSM)– Max current diode can handle as an occasional transient or from a
circuit fault
Reverse Recovery Time (trr)
• No instantaneous switching (on → off)
• Reverse current flows• Time during which reverse
current flows is reverse recovery time
• Charge carriers are removed• Fast recovery, slow recovery• Few microseconds to several
hundred nanoseconds
Diode Protection
• Overvoltage– If reverse voltage exceeds breakover voltage,
diode breaks down– Large current, power dissipation …. Destroy diode– PIV rating is 1.2 times higher than normal rating
• Overcurrent– Current rating based on max junction temperature
produced by junction losses– Fuse to ensure current protection
Diode Protection
• Transients– Leads to higher-than-normal voltages across diode– Snubber circuits for protection– RC series circuit– Snubs the rate of change of voltage
Testing a Diode
• Ohmmeter• Forward biased: low resistance• Reverse biased: high resistance• Open diode• Shorted diode
Diode Circuit Analysis
• Diodes in DC circuit– State of diode must be found (on or off)– Diode is replaced by switch-equivalent circuit– Helpful to replace diode mentally with a resistive
element and note resulting current direction
Diode Circuit Analysis
• Diodes in AC circuits– Voltage varies– Circuit analysis can be done separately for positive
and negative half-cycles– Diodes can be replaced with switch-equivalent
circuits accordingly
Diode Losses
• PT= PON + POFF + PSW
• POFF = VR * IR *
• PON = VF* IF * d
• PSW = (max) (max) ( ) ( )
1* * *[ ]*6 R F F ON R OFFV I t t f
( )OFFt
t
Power Rating and Manufacturing Parameters
Power Ratings
Series and Parallel Operation of Diodes
• Maximum power diode can handle:– Rated Reverse Voltage– Rated Forward Current
• Diode with insufficient power handling capability– Connected in series to increase voltage rating– Connected in parallel to increase current rating
Series Connection of Diodes
• In high-voltage application:– Insufficient reverse voltage rating
• Series connection of two or diodes to increase voltage rating
• Problem: Reverse voltage may not be equally divided– Diode with lower Ileak can have excessive reverse
voltage
Series Connection of Diode
Forced Voltage Sharing
• Voltage-sharing resistors• Resistors must conduct greater current than
leakage current of diodes• Resistors will consume power during reverse-
bias• It is important to use high value resistors
Protection against Voltage Transient
• There can be excessive reverse voltage due to different reverse recovery times
• A capacitor connected in parallel with each diode will protect the diode from voltage transients
Forced Voltage Sharing
Voltage-Sharing Resistor
1 21 2
D Ds D D
V VI I I
R R
1 2
2 1
D D
D D
V VR
I I
2 21 2* *R R RP I R I R
Example• Two diodes with voltage ratings
of 800V and reverse leakage currents of 1mA are connected in series across an AC source whose peak value is Vs(max)= 980V. The reverse characteristics are shown in the figure. Determine:– Reverse voltage across each diode– The value of the voltage-sharing
resistor, so that the voltage across any diode is no more than 55% of Vs(max)
– Total source current and power loss in resistors
Parallel Connections of Diodes
• Load current is greater than current rating of single diode
• Diodes can be connected in parallel– To achieve higher forward current rating
• Diodes do not share current equally– Due to difference in forward-bias characteristics
• Diode with lowest forward voltage drop will try to carry larger current and overheat
Parallel Connection of Diodes
Forced Current Sharing
• By connecting a very small resistance in series with each diode
• Current-sharing resistor establishes values of ID1 and ID2 nearly equal
• Drawbacks:– Power loss in the resistors in very high– An increase in voltage across the combination
• Unless absolutely necessary parallel arrangement must be avoided
Current-Sharing Resistor
Current-Sharing Resistor
1 1 2 2* *D D D DV V I R V I R
2 1
1 2
D D
D D
V VR
I I
2 21 2* *R D DP I R I R
Example• Two diodes having the
characteristics as shown in the figure are connected in parallel. The total current through diodes is 50A. Determine:
• The resistance of current-sharing resistor, so that the current through any diode is no more than 55% of I
• Total power loss in the resistors• Voltage drop across the diode
combination (V)