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www.fairchildsemi.com
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.