Power Diodes

Department of Mechatronics Engineering

MCT 4333Power Electronics

Power Diodes


Content

Diode characteristics,Power Diode types, Series–connected diode parallel-connected diodes, diodes with RC and RL loads, diodes with LC and RLC loads, freewheeling diodes.


Diode Characteristics

Reverse 　Leakage Current



)1( / TD nVVsD eII

Shockley Diode Equation:

ID= current through the Diode, Amp;VD= Diode voltage with Anode positive;Is= Leakage (reverse saturation) current (10-6 to 10-15), A;n= empirical const., emission coefficient or ideality factor

(1.1 to 1.8 for silicon diode)

VT= Thermal voltage,At 25OC VT=25.7mV

q

kTVT

Find the saturation current, diode current, etc. using above equation.



Reverse recovery characteristicsReverse recovery time, trr

Due to charge in depletion layer, ta

Due to charge storage in semiconductor materials, tb

During trr, the diode behaves as a short circuit, not capable of blocking reverse voltage, allowing reverse current flow, and then suddenly disrupting current.


Power Semiconductor Diode

Power Diode Types:1. General purpose diode2. Fast recovery diode3. Schottky diode


• Suitable for low frequency operation: up to 1KHz.

• High reverse recovery time, 25sDiffusion type:• Current ratings from: <1A to nKA• Voltage rating: 50V to 5KVAlloyed type:• Current ratings from: 400A• Voltage rating: 1500V• Most cost effective and rugged

General purpose diodes


Suitable for dc-dc converter, where speed of recovery time is often of critical importance.

low reverse recovery time, <5s Current ratings from: <1A to n100A Voltage rating: 50V to 3KV Diffusion type but recovery time

controlled by platinum or gold diffusion

Fast Recovery Diodes


Made by the contact of metal and semiconductor.

No charge storage in the junction. No minority carrier in rectifying actions Low forward voltage and high Leakage

current, vice versa. Current ratings from: 1A to 400A Voltage rating: up to 100V Ideal for high current low voltage dc

supplies.

Schottky Diodes


Series-Connected Diodes

In High voltage applications, when single diode cannot meet the voltage rating, multiple diodes are connected in series to increase reverse blocking capabilities.

Same type of diodes do not share the same reverse voltage due to mismatches in their reverse v-i characteristics. Voltage sharing networks are needed to equalize the voltage sharing.


Series-Connected DiodesForcing equal voltage sharing by connecting resistors across each diode, then leakage current will vary through the diodes but shared by the diode and resistor.

SDD

Ds

Ds

RsRss

VVVR

VI

R

VI

IIIII

21

22

11

2211

21


Series Connected Diodes

Voltage sharing under transient conditions (due to mismatch in reverse recovery times). Rs limits the rate of rise of the blocking voltage.


Parallel-Connected Diodes

In High power applications, diodes are connected in parallel to increase current carrying capabilities.

They must have the same forward voltage drop.They should be mounted on the same heat sink or cooled equally.

A current sharing network is necessary to equalize the current sharing of each diodes.


Parallel-Connected Diodes

Current sharing by series resistors is not practical due to power loss.

Mutually coupled inductors is the best option to equalize the current sharing.


Diodes with RC Load

Assuming ideal diode with trr=0; VD=0

RC

V

dt

dv

eVtv

eR

Vti

s

t

t

sc

RCt

s

0

),1()(

)(

Current does not change the polarity


Diodes with RL Load

L

V

dt

di

eVtv

eR

Vti

s

t

t

sL

LtR

s

0

,)(

)1()(

Energy stored in the inductor = 0.5 Li2. An attempt to open the switch will transfer the energy to the switch and diode in the form of high reverse voltage.

A diode is needed to overcome this situation.

Current does not change the polarity


Diodes with LC Load

L

V

dt

di

LC

tItL

CVti

s

t

Ps

0

0

00

1

sinsin)(


Diodes with RLC Load

))cos()cos(()()3

)()2

)()()1

21

21

21

21

tAtAeti

eAeAti

etAAti

rrt

tsts

st

Three cases:

R


Free Wheeling Diode

Dm: The diode is used to provide path for current to continue through an inductive load when conducting switch closes.


Conclusions

•Diode Characteristics, recovery time, •Diode types•Series and parallel connected diodes with protecting network•Diode with RC, RL, LC and RLC loads•Freewheeling diode, trapped energy.•Related problems

Power Diodes

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