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Diode Reverse Recovery and its Effect onSwitching Losses

Peter Haaf, Senior Field Applications EngineerJon Harper, Market Development ManagerNovember 2006

2

Agenda

1. Basics2. Mathematical Estimations3. Comparison of the Estimations with real measurements4. Switching Losses vs. Voltage5. Switching Losses vs. Current6. EON Losses during Hard Switching with different Diode

Technologies7. Effect of parallel Caps on Switching Losses8. Switching Losses vs. rise and fall time9. Summary

3

Diode charge distribution in conductingand non-conducting states

P-type N-type

xx=0

Minority carrier concentrationnear the junction

x

Minority carrier concentrationnear the junction

Electronconcentrationin P-type region

Holeconcentrationin N-type region

Electronconcentrationin P-type region

Holeconcentrationin N-type region

P-type N-type

x=0

Diode conducting Diode blocking

4

Diode Forced Commutation Behavior

VSWITCH

Step 1: Switch is turned onCurrent rises

Step 2: Switch is turned offCurrent is circulating

Step 3: Switch is turned on again, Diode is recovering and current continues rising

DC Bus

Reference GND

Switch

IL IDIODE

ISWITCH

VDD

VDIODE+

+

-

-

1 3

2

5

Switching loss calculations

t

V

I

E=(1/2)*V*I*t E=(1/6)*V*I*tE=(1/3)*V*I*t

t t t

• Definition ofPower Losses

P = 1/T* ∫ V(t) * I(t) dt= mean (V(t) * I(t))

E = P * t= ∫ V(t) * I(t) dt= area (V(t) * I(t))

Pon = EON * f ; Poff = EOFF * f

6

IRRM

VCE

IL assumed constant during switching time

t0

IC

dIC/dt

tBtA

tF

VF

IF

VRM

IRRM

t1 t2 t3

tR

dIF/dt

o

t = t0 IGBT turns on

switching time: tR+tA+tB

Eon1 2RLout tIV **=

Turn On Loss Due to Diode Recovery (Phase tR)

dI/dt = IL / tR

Eon1 2

2Lout

dI/dtIV

**=

with

7

IRRM

VCE

t0

IC

dIC/dt

tBtA

tF

VF

IF

VRM

IRRM

t1 t2 t3

tR

dIF/dt

t = t0 IGBT turns on

switching time: tR+tA+tB

Turn On Loss Due to Diode Recovery (Phase tA)

ARRM

Louton2 tIIE *⎟⎠⎞

⎜⎝⎛

+*=2

V

RRML

II ⎟⎠⎞

⎜⎝⎛ +* 2outon2E = V *

2IRRMdt

2 dI**

dI/dt RRMI=

t A

with

IL assumed constant during switching time

8

on3E = BRRML

out tIIV *⎟⎠⎞

⎜⎝⎛

+* 32

lossDiode 6BRRMout tIV **=

Turn On Loss Due to Diode Recovery (Phase tB)

IRRM

VCE

IL assumed constant during switching time

t0

IC

dIC/dt

tBtA

tF

VF

IF

VRM

IRRM

t1 t2 t3

tR

dIF/dt

At t=to IGBT turns on

switching time: tR+tA+tB

tF = tB

9

Double check of the formulas:Eon calculation vs. measurement

Eon = 32.67 uJ Pon = 1.63 W 50 (kHz) Frequency

Eon1 = 11.20 uJ Pon = 0.56 W 4 (A) CurrentEon2 = 14.00 uJ Pon = 0.70 W 280.00 (V) UdcEon3 = 7.47 uJ Pon = 0.37 W

2.00E+08 (A/s) dI/dtDiode: 2 (A) Irr; DiodeEoff = 9.33E-01 uJ Poff = 0.05 W 1.00E-08 (s) tf fall time

Ic = 4 A

VCE

EON

10

8A Stealth II versus Stealth™ comparison

FFP08S60S ISL9R860P2

TC=25ºC TC=125ºC TC=25ºC TC=125ºC

tA / ns (typ) 11.9 25.2

32.8

IRRM / A (typ) 2.2 4.3 3.4 6.5

QRR / nC (typ) 21 125 150 190

Switch losses

example calculation / µJ118 232 246 220

1.6

16.4 15.1

tB / ns (typ) 7.1 60.6 37.9

2.0VF / V (typical) 2.1 1.6

Specification

Measured with di/dt=200A/us, see datasheets for full detailsExample: Loss in switch for 8A, di/dt=200A/us, VDD=390VEquations in Power Seminar 2007 documentation

Loss calculation 25 °C and 125 °C

11

8A Stealth II versus Stealth™ comparison

FFP08S60S ISL9R860P2

TC=75ºC TC=100ºC TC=75ºC TC=100ºC

tA / ns (typ) 18.5 21.9

26.4

IRRM / A (typ) 3.3 3.8 5.0 5.7

VF / V (typical) 1.85 1.725 1.8 1.7

Switching loss @ 100 kHz / W 17.2 20.1 23.5 22.8

Switch losses

example calculation / µJ172 201 235 228

15.8 15.5

tB / ns (typ) 20 49.2 43,5

Specification

6.3 W difference on switching losses

Calculated with di/dt=200A/us, see datasheets for full detailsExample: Loss in switch for 8A, di/dt=200A/us, VDD=390VEquations in Power Seminar 2007 documentation Linear approximation: of ta, tb, Irrm and Vf

Loss calculation 75 °C and 100 °C

12

8A Stealth II versus Stealth™ comparison

Loss measurements

FFP08S60SFFP08S60S

Vds:100V/div

Id:2A/div Eon : 106.2uJ

Idiode:2A/div

Vdiode:100V/div

20ns/div 20ns/div

Vds:100V/div

Id:2A/div Eon : 129.2uJ

Idiode:2A/div

Vdiode:100V/div

20ns/div

ISL9R860P2ISL9R860P2

DUTs Ta TMOSFET Tdiode dTMOSFET dTdiode Pin Vout Iout Pout Efficienccy PF

ISL9R860P2 26.2 120.2 76.7 94.0 50.5 431.2 401.240 0.984 394.70 91.54 0.999

FFP08S60S 26.2 113.3 70.1 87.1 43.9 426.0 401.240 0.984 394.70 92.65 0.999

Test condition : Vin=220Vac, Pout=400V/1A(400W ), Fs=100kHz5.2 W difference in input power

20ns/div

13

Test circuits

Test Circuits which are used for the following measurements

Ids Ids

Vds Vds

14

Waveforms and loss definition

td off: 90 % Vge => 90 % Ice

tf: 90 % Ice => 10 % Ice

td on: 10 % Vge = > 10 % Ice

tr: 10 % Ice => 90 % Ice

Switch off lossesSwitch on losses

15

Switching Losses vs. Voltage FQP9N50C + ISL9R460

EON / EOFFlosses

VIN = 100V

EON = 8.7uJEOFF = 9.5uJ

VIN = 300V

EON = 32.3uJEOFF = 23.1uJ

EON EOFF

16

Switching Losses vs. Voltage: EON and EOFF losses

Comparison oftwo Stealth™ diodes, which are optimized for hard switching

Eon and Eoff losses of the FET - FQP9N50C vs. Input Voltage

0

5

10

15

20

25

30

35

40

45

50

0 50 100 150 200 250 300 350Input Voltage [V]

Eon

and

Eoff

Loss

es [u

J]

Eon @ ISL9R1560

Eoff @ ISL9R1560

Eon @ ISL9R460

Eoff @ ISL9R460

FQP9N50CHigher Current rating of the Diode will increase Eon, but decrease Eoff (Diode capacitance acts as a snubber). Eon is dominating!

17

Switching Losses vs. CurrentFQP9N50C + ISL9R460 @ VIN = 300V

I = 2A, EON = 16.7uJ I = 4A, EON = 33.4uJ I =6A, EON = 54.8uJ

I = 2A, EOFF = 9.7uJ I = 4A, EOFF = 24.1uJ I =6A, EOFF = 43.1uJ(nearly) Linear relation between current and losses.

18

EON = f (Ice) and EOFF = f (Ice) for different diode technologies and ratings

Eon and Eoff losses of the FET - FQP9N50C vs. Current

0

20

40

60

80

100

120

140

160

180

200

0 1 2 3 4 5 6 7

Current [A]

Eon

and

Eoff

Loss

es [u

J]

Eon @FCP11N60FEon @ FQPF5N50CFEon @ RURD660Eon @ RHRP860Eon @ ISL9R460Eoff @ ISL9R460

FQP9N50C

Technologies as well as rating will have a big impact on the Eon losses. Fast recovery FETs will lead to significant higher Eon losses compared to single diode technologies. => Sometimes the reason for external fast recovery diodes.

19

Variation of IRRM with load current for different diode technologies

Irr, Reverse Recovery Peak Current of the Diode vs. Current

0

2

4

6

8

10

12

14

0 1 2 3 4 5 6 7

Current [A]

Rev

erse

Rec

over

y C

urre

nt [

A]

Irr @ FCP11N60FIrr @ FQPF5N50CFIrr @ RURD660Irr @ RHRP860Irr @ ISL9R460

FQP9N50C

Irr values are a good indicator for a loss comparison of diodes.

Only Irr’s measured at the same dI/dt are comparable!

20

EON Losses at Hard Switching with different Diode Technology @ VIN = 300V @ I = 4A

ISL9R1560; EON = 42.9uJ ISL9R860; EON = 33.1uJ ISL9R460; EON = 32.3uJ

MUR1560; EON = 77.7uJ RURD660; EON = 60.1uJ RHRP860; EON = 37.9uJ

21

Variation of the EON Losses with input voltage for different diode technologies and ratings

Eon losses of the FET - FQP9N50C vs. Input Voltage

0

10

20

30

40

50

60

70

80

90

0 50 100 150 200 250 300 350

Input Voltage [V]

Eon

Loss

es [u

J]

Eon @ MUR1560Eon @ RURP860Eon @ RURD660Eon @ FFPF10UP60Eon @ ISL9R1560Eon @ RHRP860Eon @ ISL9R860Eon @ ISL9R460Eon @ SIC 6A

FQP9N50CEspecially in hard switching applications the diode technology will have a significant impact on the Eon losses of the switch.

22

Variation of IRRM with input voltage for different diode technologies

Irr, Reverse Recovery Peak Current of the Diode vs. Input Voltage

0

1

2

3

4

5

6

7

0 50 100 150 200 250 300 350

Input Voltage [V]

Rev

erse

Rec

over

y C

urre

nt I

rr

[A]

Irr @ MUR1560Irr @ RURD660Irr @ FFPF10UP60Irr @ ISL9R1560Irr @ RHRP860Irr @ ISL9R860Irr @ ISL9R460Irr @ SIC 6A

FQP9N50C

The Irr value is a good parameter to estimate the switching losses of different technologies.

Only Irr’s measured at the same dI/dt are comparable!

23

Effect of temperature on reverse recovery

Results for Tj = 25°C

Small difference

dI/dt = 200A/ms, Vdd = 400V, If = 8A, Tj = 25°C and Tj = 125 °CTwo industry standard diodes

Results for Tj = 125°C

Big difference

The difference between low and high temperature reverse recoverybehavior is not the same for all technologies. Be careful if you compare only at low temperatures.

24

Switching Losses @ increasing switching speed

• Switching off:Same FET and Diode, reducing Rg:

EOFF = 22.8uJ 16.7uJDrawback: ringing due to parasitic Ind. & Caps

All measurements: FDD6N50 + ISL9R460, U = 300V, I = 4A

Recommended Rg

Good switching performance, no ringingLow Rg

Bad switching performance, ringing, but lower EOFF

25

Switching Losses @ increasing switching speed- Same MOSFET, different Rg -

Diode = ISL9R460, U = 300V, I = 4A

FDD6N50, Rg = 10 OhmEON = 8 uJdI/dt = 1400A/us IRRM = 6.2A

Right : FDD6N50, Rg = 3 OhmEON = 4 uJdI/dt = 1600A/usIRRM = 7.4A

26

Switching Losses @ increasing switching speed - Different MOSFET Technologies -

FQP9N50C, Rg = 30 OhmEON = 23.2 uJdI/dt = 400 A/usIRRM = 2.6 A

FDD6N50, Rg = 30 OhmEON = 15.3 uJdI/dt = 640 A/usIRRM = 3.9 A

Diode = ISL9R460, U = 300V, I = 4A

27

Variation of IRRM with dI/dt for different diode technologies

Reverse Recovery Current Irr of the Diode vs. dI/dt @ V = 300V @ I = 4A

0

1

2

3

4

5

6

7

8

9

10

0 200 400 600 800 1000 1200 1400 1600

dI/dt [A/us]

Rev

erse

Rec

over

y C

urre

nt Ir

r [A

]

Irr @ FFP08H60SIrr @ ISL9R860Irr @ ISL9R460

A higher dI/dt will increase the reverse recovery current, but…

28

Variation of EON losses with di/dt for different diode technologies

Eon losses of the FET vs. dI/dt @ V = 300V @ dI/dt = 4A

0

5

10

15

20

25

30

35

0 200 400 600 800 1000 1200 1400 1600

dI/dt [A/us]

Eon

Loss

es [u

J]

Eon @ FFP08H60SEon @ ISL9R860Eon @ ISL9R460

A higher dI/dt will decrease the Eon losses.

29

Effect of parallel caps on switching losses@ VIN = 300V @ I = 4A Diode = ISL9R460

Increase of the Eon losses due to the parallel Capacitance.Advantages in switching offOverall losses? …

No parallel Capacitance Cpar = 470pF Cpar = 1nF

EON = 32 uJ EON = 39 uJ EON = 57 uJ

EOFF = 27 uJ EOFF = 7.8 uJ EOFF = 5.47 uJ

30

Effect of parallel caps on switching losses

Eon and Eoff losses with a snubber Capacitance

0

10

20

30

40

50

60

70

80

90

100

0 100 200 300 400 500 600 700 800 900 1000

Capacitance parallel to the Diode [pF]

Etot

/ Eo

n / E

off l

osse

s [u

J] Etot at 300VEtot at 200VEtot at 100VEoff at 300VEon at 300VEoff at 200VEon at 200VEoff at 100VEon at 100V

A small capacitance can help to reduce the overall switching losses

80 pF @ 300 V

As higher the voltage, as smaller the cap to decrease the overall losses.

200 pF @ 200 V 450 pF @ 100 V

31

Summary

Reverse recovery in diodes in half-bridge structures causessmall losses in the diodeslarger losses in the MOSFET/IGBT

IRRM and tRR increase with temperaturedi/dtcurrent (less dominant)

Larger current rated diodes of the same familyhave higher IRRM resulting in higher EON (measured at the same dI/dt for comparison)have larger capacitance, resulting in lower EOFFcause higher total switching losses

Higher di/dt results in lower EON losses, but also in a higher IRRM

Addition of extra capacitanceincreases EON losses but decreases EOFF lossesaddition of extra capacitance could reduce total losses.