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)