Diode Reverse Recovery and its Effect on
Switching Losses
Peter Haaf, Senior Field Applications Engineer
Jon Harper, Market Development Manager
November 2006
www.fairchildsemi.com
Agenda
1.
2.
3.
4.
5.
6.
Basics
Mathematical Estimations
Comparison of the Estimations with real measurements
Switching Losses vs. Voltage
Switching Losses vs. Current
EON Losses during Hard Switching with different Diode
Technologies
7. Effect of parallel Caps on Switching Losses
8. Switching Losses vs. rise and fall time
9. Summary
2
Diode charge distribution in conducting
and non-conducting states
Minority carrier concentration
near the junction
Hole
concentration
in N-type region
Electron
concentration
in P-type region
Electron
concentration
in P-type region
x
x=0
P-type
Minority carrier concentration
near the junction
Hole
concentration
in N-type region
x
x=0
N-type
P-type
N-type
Diode blocking
Diode conducting
3
Diode Forced Commutation Behavior
Step 1: Switch is turned on
Current rises
DC Bus
IL
2
VDD
1
Switch
Reference GND
Step 2: Switch is turned off
Current is circulating
IDIODE
VDIODE
+
3
+
VSWITCH
-
Step 3: Switch is turned on
again, Diode is
recovering and
current continues
rising
ISWITCH
4
Switching loss calculations
• Definition of
Power Losses
P = 1/T* ∫ V(t) * I(t) dt
= mean (V(t) * I(t))
I
V
E =P*t
= ∫ V(t) * I(t) dt
= area (V(t) * I(t))
Pon = EON * f ;
Poff = EOFF * f
t
E=(1/3)*V*I*t
E=(1/2)*V*I*t
t
t
5
E=(1/6)*V*I*t
t
Turn On Loss Due to Diode Recovery (Phase tR)
VCE
IRRM
IC
IL assumed constant during switching time
dIC/dt
Eon1
tA
dIF/dt
tB
with
Eon1
VF
Vout * I L * t R
2
dI/dt = IL / tR
tF
IF
=
=
Vout * I L2
2 * dI/dt
IRRM
switching time: tR+tA+tB
tR
o
t = t0 IGBT turns on
t0
t1
t2
t3
VRM
6
Turn On Loss Due to Diode Recovery (Phase tA)
VCE
IL assumed constant during switching time
IRRM
IC
⎛
⎞
I
E on2 = Vout * ⎜ I L + RRM ⎟ * t A
2 ⎠
⎝
dIC/dt
tA
dIF/dt
with
tB
tF
dt
⎛ I
2 ⎞
E on2 = Vout* ⎜I L* RRM + IRRM⎟*
2
⎠ 2 *dI
⎝
IF
VF
IRRM
tR
t = t0 IGBT turns on
t0
t1
t2
I RRM
dI/dt =
tA
t3
VRM
7
switching time:
tR+tA+tB
Turn On Loss Due to Diode Recovery (Phase tB)
VCE
IRRM
IC
IL assumed constant during switching time
dIC/dt
E on3
tF = tB
tA
dIF/dt
⎞
⎛I
I
= Vout * ⎜ L + RRM ⎟ * t B
3 ⎠
⎝ 2
tB
tF
switching time:
tR+tA+tB
IF
VF
IRRM
tR
At t=to IGBT turns on
t0
t1
t2
Diodeloss
t3
VRM
8
Vout * I RRM * t B
=
6
Double check of the formulas:
Eon calculation vs. measurement
VCE
EON
Ic = 4 A
Eon =
32.67 uJ
Pon =
1.63 W
Eon1 =
Eon2 =
Eon3 =
11.20 uJ
14.00 uJ
7.47 uJ
Pon =
Pon =
Pon =
0.56 W
0.70 W
0.37 W
Diode:
Eoff =
9.33E-01 uJ
Poff =
0.05 W
9
50 (kHz)
4 (A)
280.00 (V)
2.00E+08 (A/s)
2 (A)
1.00E-08 (s)
Frequency
Current
Udc
dI/dt
Irr; Diode
tf fall time
8A Stealth II versus Stealth™ comparison
Loss calculation 25 °C and 125 °C
FFP08S60S
ISL9R860P2
Specification
TC=25ºC
TC=125ºC
TC=25ºC
TC=125ºC
tA / ns (typ)
11.9
25.2
16.4
15.1
tB / ns (typ)
7.1
32.8
60.6
37.9
IRRM / A (typ)
2.2
4.3
3.4
6.5
QRR / nC (typ)
21
125
150
190
118
232
246
220
2.1
1.6
2.0
1.6
Switch losses
example calculation / µJ
VF / V (typical)
Measured with di/dt=200A/us, see datasheets for full details
Example: Loss in switch for 8A, di/dt=200A/us, VDD=390V
Equations in Power Seminar 2007 documentation
10
8A Stealth II versus Stealth™ comparison
Loss calculation 75 °C and 100 °C
FFP08S60S
ISL9R860P2
Specification
TC=75ºC
TC=100ºC
TC=75ºC
TC=100ºC
tA / ns (typ)
18.5
21.9
15.8
15.5
tB / ns (typ)
20
26.4
49.2
43,5
IRRM / A (typ)
3.3
3.8
5.0
5.7
172
201
235
228
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 / µJ
Calculated with di/dt=200A/us, see datasheets for full details
Example: Loss in switch for 8A, di/dt=200A/us, VDD=390V
Equations in Power Seminar 2007 documentation
Linear approximation: of ta, tb, Irrm and Vf
6.3 W difference on switching losses
11
8A Stealth II versus Stealth™ comparison
Loss measurements
FFP08S60S
ISL9R860P2
Idiode:2A/div
Vds:100V/div
Idiode:2A/div
Vds:100V/div
Vdiode:100V/div
Id:2A/div
Vdiode:100V/div
Id:2A/div
Eon : 106.2uJ
Eon : 129.2uJ
20ns/div
20ns/div
20ns/div
20ns/div
DUTs
Ta
TMOSFET
Tdiode
dTMOSFET dTdiode
ISL9R860P2
26.2
120.2
76.7
94.0
FFP08S60S
26.2
113.3
70.1
87.1
Pin
Vout
Iout
Pout
Efficienccy
PF
50.5
431.2
401.240
0.984
394.70
91.54
0.999
43.9
426.0
401.240
0.984
394.70
92.65
0.999
Test co nditio n : Vin=2 2 0 Vac , Po ut=4 0 0 V/ 1 A( 4 0 0 W) , Fs=1 0 0 kHz
12
5.2 W difference in input power
Test circuits
Ids
Ids
Vds
Vds
Test Circuits which are used for the following measurements
13
Waveforms and loss definition
Switch off losses
Switch on losses
td off: 90 % Vge => 90 % Ice
tf: 90 % Ice => 10 % Ice
td on: 10 % Vge = > 10 % Ice
tr: 10 % Ice => 90 % Ice
14
Switching Losses vs. Voltage
FQP9N50C + ISL9R460
EON
EON / EOFF
losses
VIN = 100V
EON = 8.7uJ
EOFF = 9.5uJ
VIN = 300V
EON = 32.3uJ
EOFF = 23.1uJ
15
EOFF
Switching Losses vs. Voltage: EON and EOFF losses
Eon and Eoff losses of the FET - FQP9N50C vs. Input Voltage
50
Comparison of
two Stealth™ diodes,
which are optimized for
hard switching
45
40
Eon and Eoff Losses [uJ]
35
Eon @ ISL9R1560
30
Eoff @ ISL9R1560
25
Eon @ ISL9R460
Eoff @ ISL9R460
20
15
10
5
0
0
50
100
150
200
250
300
350
Input Voltage [V]
Higher Current rating of the Diode will increase
Eon, but decrease Eoff (Diode capacitance acts as
a snubber). Eon is dominating!
16
FQP9N50C
Switching Losses vs. Current
FQP9N50C + 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.
17
EON = f (Ice) and EOFF = f (Ice) for different diode
technologies and ratings
Eon and Eoff losses of the FET - FQP9N50C vs. Current
200
Eon and Eoff Losses [uJ]
180
160
Eon @FCP11N60F
Eon @ FQPF5N50CF
Eon @ RURD660
Eon @ RHRP860
Eon @ ISL9R460
Eoff @ ISL9R460
140
120
100
80
60
40
20
0
0
1
2
3
4
5
6
Current [A]
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.18
7
FQP9N50C
Variation of IRRM with load current for different diode
technologies
Irr, Reverse Recovery Peak Current of the Diode vs. Current
R e v e rs e R e c o v e ry C u rre n t [A ]
14
12
Irr @ FCP11N60F
Irr @ FQPF5N50CF
Irr @ RURD660
Irr @ RHRP860
Irr @ ISL9R460
10
8
FQP9N50C
6
4
2
0
0
1
2
3
4
5
6
Current [A]
Irr values are a good indicator for a loss comparison
of diodes.
Only Irr’s measured at the same dI/dt are comparable!
19
7
EON Losses at Hard Switching with different
Diode Technology @ VIN = 300V @ I = 4A
MUR1560; EON = 77.7uJ
RURD660; EON = 60.1uJ
RHRP860; EON = 37.9uJ
ISL9R1560; EON = 42.9uJ
ISL9R860; EON = 33.1uJ
ISL9R460; EON = 32.3uJ
20
Variation of the EON Losses with input voltage for
different diode technologies and ratings
Eon losses of the FET - FQP9N50C vs. Input Voltage
90
80
Eon Losses [uJ]
70
60
50
40
Eon @ MUR1560
Eon @ RURP860
Eon @ RURD660
Eon @ FFPF10UP60
Eon @ ISL9R1560
Eon @ RHRP860
Eon @ ISL9R860
Eon @ ISL9R460
Eon @ SIC 6A
30
20
10
0
0
50
100
150
200
250
300
350
Input Voltage [V]
Especially in hard switching applications the diode
technology will have a significant impact on the Eon
losses of the switch.
21
FQP9N50C
Variation of IRRM with input voltage for different diode
technologies
Irr, Reverse Recovery Peak Current of the Diode vs. Input Voltage
Irr @ MUR1560
Irr @ RURD660
Irr @ FFPF10UP60
Irr @ ISL9R1560
Irr @ RHRP860
Irr @ ISL9R860
Irr @ ISL9R460
Irr @ SIC 6A
Reverse Recovery Current Irr
[A]
7
6
5
4
3
2
1
0
0
50
100
150
200
250
300
350
Input Voltage [V]
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!
22
FQP9N50C
Effect of temperature on reverse recovery
dI/dt = 200A/ms, Vdd = 400V, If = 8A, Tj = 25°C and Tj = 125 °C
Two industry standard diodes
Results for Tj = 25°C
Results for Tj = 125°C
Small difference
Big difference
The difference between low and high temperature reverse recovery
behavior is not the same for all technologies. Be careful if you compare
only at low temperatures.
23
Switching Losses @ increasing switching speed
•
Switching off:
Same FET and Diode, reducing Rg:
EOFF = 22.8uJ Ù 16.7uJ
Drawback: ringing due to parasitic Ind. & Caps
All measurements: FDD6N50 + ISL9R460, U = 300V, I = 4A
Recommended Rg
Low Rg
Good switching performance, no ringing
Bad switching performance, ringing, but lower EOFF
24
Switching Losses @ increasing switching speed
- Same MOSFET, different Rg Diode = ISL9R460, U = 300V, I = 4A
FDD6N50, Rg = 10 Ohm
EON = 8 uJ
dI/dt = 1400A/us
IRRM = 6.2A
Right : FDD6N50, Rg = 3 Ohm
EON = 4 uJ
dI/dt = 1600A/us
IRRM = 7.4A
25
Switching Losses @ increasing switching speed
- Different MOSFET Technologies Diode = ISL9R460, U = 300V, I = 4A
FQP9N50C, Rg = 30 Ohm
EON = 23.2 uJ
dI/dt = 400 A/us
IRRM = 2.6 A
FDD6N50, Rg = 30 Ohm
EON = 15.3 uJ
dI/dt = 640 A/us
IRRM = 3.9 A
26
Variation of IRRM with dI/dt for different diode
technologies
Reverse Recovery Current Irr of the Diode vs. dI/dt @ V = 300V @ I = 4A
10
9
Reverse Recovery Current Irr [A]
8
7
6
Irr @ FFP08H60S
Irr @ ISL9R860
Irr @ ISL9R460
5
4
3
2
1
0
0
200
400
600
800
1000
1200
1400
1600
dI/dt [A/us]
A higher dI/dt will increase the reverse recovery current, but…
27
Variation of EON losses with di/dt for different diode
technologies
Eon losses of the FET vs. dI/dt @ V = 300V @ dI/dt = 4A
35
30
Eon Losses [uJ]
25
20
Eon @ FFP08H60S
Eon @ ISL9R860
Eon @ ISL9R460
15
10
5
0
0
200
400
600
800
1000
1200
dI/dt [A/us]
A higher dI/dt will decrease the Eon losses.
28
1400
1600
Effect of parallel caps on switching losses
@ VIN = 300V @ I = 4A Diode = ISL9R460
EON = 32 uJ
No parallel Capacitance
EOFF = 27 uJ
EON = 39 uJ
Cpar = 470pF
EOFF = 7.8 uJ
¾ Increase of the Eon losses due to the parallel Capacitance.
¾ Advantages in switching off
¾ Overall losses? …
29
EON = 57 uJ
Cpar = 1nF
EOFF = 5.47 uJ
Effect of parallel caps on switching losses
Eon and Eoff losses with a snubber Capacitance
100
A small capacitance can help to reduce the overall switching losses
E to t / E o n / E o ff lo s s e s [u J ]
90
Etot at 300V
80
Etot at 200V
Etot at 100V
Eoff at 300V
70
60
Eon at 300V
Eoff at 200V
Eon at 200V
50
40
Eoff at 100V
30
Eon at 100V
20
10
0
0
100
80 pF @ 300 V
200
300
400
500
600
700
800
900
Capacitance parallel to the Diode [pF]
200 pF @ 200 V
450 pF @ 100 V
As higher the voltage, as smaller the cap to decrease
the overall losses.
30
1000
Summary
¾ Reverse recovery in diodes in half-bridge structures causes
ƒ
ƒ
small losses in the diodes
larger losses in the MOSFET/IGBT
¾ IRRM and tRR increase with
ƒ temperature
ƒ di/dt
ƒ current (less dominant)
¾ Larger current rated diodes of the same family
ƒ have higher IRRM resulting in higher EON (measured at the same dI/dt for comparison)
ƒ have larger capacitance, resulting in lower EOFF
ƒ cause higher total switching losses
¾ Higher di/dt results in lower EON losses, but also in a higher IRRM
¾ Addition of extra capacitance
ƒ increases EON losses but decreases EOFF losses
ƒ addition of extra capacitance could reduce total losses.
31