Diode Reverse Recovery and its Effect on Switching Losses

<strong>Diode</strong> <strong>Reverse</strong> <strong>Recovery</strong> <strong>and</strong> <strong>its</strong> <strong>Effect</strong> on
Switching Losses
Peter Haaf, Senior Field Applications Engineer
Jon Harper, Market Development Manager
November 2006
www.fairchildsemi.com

Agenda
1. Basics
2. Mathematical Estimations
3. Comparison of the Estimations with real measurements
4. Switching Losses vs. Voltage
5. Switching Losses vs. Current
6. E ON Losses during Hard Switching with different <strong>Diode</strong>
Technologies
7. <strong>Effect</strong> of parallel Caps on Switching Losses
8. Switching Losses vs. rise <strong>and</strong> fall time
9. Summary
2

<strong>Diode</strong> charge distribution in conducting
<strong>and</strong> non-conducting states
Minority carrier concentration
near the junction
Minority carrier concentration
near the junction
Electron
concentration
in P-type region
Hole
concentration
in N-type region
Electron
concentration
in P-type region
Hole
concentration
in N-type region
x=0
x
x=0
x
P-type
N-type
P-type
N-type
<strong>Diode</strong> conducting
<strong>Diode</strong> blocking
3

<strong>Diode</strong> Forced Commutation Behavior
V DD
DC Bus
I L
Switch
I DIODE
-
2
1
+
+
3
-
V DIODE
V SWITCH
Step 1: Switch is turned on
Current rises
Step 2:
Switch is turned off
Current is circulating
Step 3: Switch is turned on
again, <strong>Diode</strong> is
recovering <strong>and</strong>
current continues
rising
Reference GND
I SWITCH
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))
t
Pon = E ON
* f ;
Poff = E OFF
* f
E=(1/2)*V*I*t E=(1/3)*V*I*t E=(1/6)*V*I*t
t t t
5

Turn On Loss Due to <strong>Diode</strong> <strong>Recovery</strong> (Phase t R )
V CE
t = t 0
IGBT turns on
o
dI C /dt
dI F /dt
t 0
I C
t A
t B
t F
I RRM
V F
I F
V RM
I RRM
t R
t 1 t 2 t 3
I L assumed constant during switching time
with
V out I L t
E on1
= * *
2
dI/dt = I L / t R
2
Vout
I L
E on1
= *
2 * dI/dt
switching time: t R
+t A
+t B
R
6

Turn On Loss Due to <strong>Diode</strong> <strong>Recovery</strong> (Phase t A )
V CE
t = t 0
IGBT turns on
dI C /dt
dI F /dt
t 0
I C
I RRM
t A t B
t F
V F
I F
V RM
I RRM
t R
t 1 t 2 t 3
I L assumed constant during switching time
=
RRM
E on2 Vout
* I L + *
with
E =
on2
V out*
*
⎝
switching time:
⎛
⎜
⎝
dI/dt
I
t R +t A +t B
2
⎛ I RRM 2 ⎞
⎜I L + I RRM
⎟
⎠
2
I RRM
= t A
⎞
⎟
⎠
t
A
dt
*
2dI *
7

Turn On Loss Due to <strong>Diode</strong> <strong>Recovery</strong> (Phase t B )
V CE
t F = t B
dI C /dt
dI F /dt
t 0
I C
I RRM
t A t B
t F
V F
I F
V RM
I RRM
At t=t o IGBT turns on
t R
t 1 t 2 t 3
I L assumed constant during switching time
⎛ I
E =
L
I
RRM
⎞
on3
V
out * ⎜ + ⎟ * tB
⎝ 2 3 ⎠
switching time: t R +t A +t B
V out I RRM t
<strong>Diode</strong> loss = * *
6
B
8

Double check of the formulas:
Eon calculation vs. measurement
V CE
E ON
Ic = 4 A
Eon = 32.67 uJ Pon = 1.63 W 50 (kHz) Frequency
Eon1 = 11.20 uJ Pon = 0.56 W 4 (A) Current
Eon2 = 14.00 uJ Pon = 0.70 W 280.00 (V) Udc
Eon3 = 7.47 uJ Pon = 0.37 W
2.00E+08 (A/s) dI/dt
<strong>Diode</strong>: 2(A) Irr; <strong>Diode</strong>
Eoff = 9.33E-01 uJ Poff = 0.05 W 1.00E-08 (s) tf fall time
9

8A Stealth II versus Stealth comparison
Loss calculation 25 °C <strong>and</strong> 125 °C
Specification
FFP08S60S
ISL9R860P2
T C =25ºC T C =125ºC T C =25ºC T C =125ºC
t A / ns (typ) 11.9 25.2
16.4 15.1
t B / ns (typ) 7.1 32.8
60.6 37.9
I RRM / A (typ) 2.2 4.3 3.4 6.5
Q RR / nC (typ) 21 125 150 190
Switch losses
example calculation / µJ
118 232 246 220
V F / V (typical) 2.1 1.6
2.0
1.6
Measured with di/dt=200A/us, see datasheets for full details
Example: Loss in switch for 8A, di/dt=200A/us, V DD
=390V
Equations in Power Seminar 2007 documentation
10

8A Stealth II versus Stealth comparison
Loss calculation 75 °C <strong>and</strong> 100 °C
Specification
FFP08S60S
ISL9R860P2
T C =75ºC T C =100ºC T C =75ºC T C =100ºC
t A / ns (typ) 18.5 21.9
15.8 15.5
t B / ns (typ) 20 26.4
49.2 43,5
I RRM / A (typ) 3.3 3.8 5.0 5.7
Switch losses
example calculation / µJ
172 201 235 228
V F / V (typical) 1.85 1.725 1.8 1.7
Switching loss @ 100 kHz / W 17.2 20.1 23.5 22.8
Calculated with di/dt=200A/us, see datasheets for full details
Example: Loss in switch for 8A, di/dt=200A/us, V DD
=390V
Equations in Power Seminar 2007 documentation
Linear approximation: of ta, tb, Irrm <strong>and</strong> Vf
6.3 W difference on switching losses
11

8A Stealth II versus Stealth comparison
Loss measurements
FFP08S60S
ISL9R860P2
Vds:100V/div
Idiode:2A/div
Vds:100V/div
Idiode:2A/div
Vdiode:100V/div
Vdiode:100V/div
Id:2A/div
Eon : 106.2uJ
Id:2A/div
Eon : 129.2uJ
20ns/div
20ns/div
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=100kHz
5.2 W difference in input power
12

Test circu<strong>its</strong>
Ids
Ids
Vds
Vds
Test Circu<strong>its</strong> which are used for the following measurements
13

Waveforms <strong>and</strong> 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
E ON
E ON
/ E OFF
losses
E OFF
V IN
= 100V
E ON
= 8.7uJ
E OFF
= 9.5uJ
V IN
= 300V
E ON
= 32.3uJ
E OFF
= 23.1uJ
15

Switching Losses vs. Voltage: E ON <strong>and</strong> E OFF losses
Eon <strong>and</strong> Eoff losses of the FET - FQP9N50C vs. Input Voltage
Eon <strong>and</strong> Eoff Losses [uJ]
50
45
40
35
30
25
20
15
Comparison of
two Stealth diodes,
which are optimized for
hard switching
Eon @ ISL9R1560
Eoff @ ISL9R1560
Eon @ ISL9R460
Eoff @ ISL9R460
10
5
0
0 50 100 150 200 250 300 350
Input Voltage [V]
Higher Current rating of the <strong>Diode</strong> will increase
Eon, but decrease Eoff (<strong>Diode</strong> capacitance acts as
a snubber). Eon is dominating!
16
FQP9N50C

Switching Losses vs. Current
FQP9N50C + ISL9R460 @ V IN = 300V
I = 2A, E ON
= 16.7uJ I = 4A, E ON
= 33.4uJ I =6A, E ON
= 54.8uJ
I = 2A, E OFF
= 9.7uJ I = 4A, E OFF
= 24.1uJ I =6A, E OFF
= 43.1uJ
(nearly) Linear relation between current <strong>and</strong> losses.
17

E ON = f (Ice) <strong>and</strong> E OFF = f (Ice) for different diode
technologies <strong>and</strong> ratings
200
Eon <strong>and</strong> Eoff losses of the FET - FQP9N50C vs. Current
180
Eon <strong>and</strong> Eoff Losses [uJ]
160
140
120
100
80
60
40
Eon @FCP11N60F
Eon @ FQPF5N50CF
Eon @ RURD660
Eon @ RHRP860
Eon @ ISL9R460
Eoff @ ISL9R460
FQP9N50C
20
0
0 1 2 3 4 5 6 7
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

Variation of I RRM with load current for different diode
technologies
Irr, <strong>Reverse</strong> <strong>Recovery</strong> Peak Current of the <strong>Diode</strong> vs. Current
14
<strong>Reverse</strong> <strong>Recovery</strong> Current [A]
12
10
8
6
4
2
Irr @ FCP11N60F
Irr @ FQPF5N50CF
Irr @ RURD660
Irr @ RHRP860
Irr @ ISL9R460
FQP9N50C
0
0 1 2 3 4 5 6 7
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

E ON Losses at Hard Switching with different
<strong>Diode</strong> Technology @ V IN = 300V @ I = 4A
MUR1560; E ON = 77.7uJ RURD660; E ON = 60.1uJ
RHRP860; E ON = 37.9uJ
ISL9R1560; E ON = 42.9uJ ISL9R860; E ON = 33.1uJ ISL9R460; E ON = 32.3uJ
20

Variation of the E ON Losses with input voltage for
different diode technologies <strong>and</strong> ratings
Eon Losses [uJ]
90
80
70
60
50
40
30
20
Eon losses of the FET - FQP9N50C vs. Input Voltage
Eon @ MUR1560
Eon @ RURP860
Eon @ RURD660
Eon @ FFPF10UP60
Eon @ ISL9R1560
Eon @ RHRP860
Eon @ ISL9R860
Eon @ ISL9R460
Eon @ SIC 6A
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.
FQP9N50C
21

Variation of I RRM with input voltage for different diode
technologies
<strong>Reverse</strong> <strong>Recovery</strong> Current Irr
[A]
7
6
5
4
3
2
1
Irr, <strong>Reverse</strong> <strong>Recovery</strong> Peak Current of the <strong>Diode</strong> vs. Input Voltage
Irr @ MUR1560
Irr @ RURD660
Irr @ FFPF10UP60
Irr @ ISL9R1560
Irr @ RHRP860
Irr @ ISL9R860
Irr @ ISL9R460
Irr @ SIC 6A
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!
FQP9N50C
22

<strong>Effect</strong> of temperature on reverse recovery
dI/dt = 200A/ms, Vdd = 400V, If = 8A, Tj = 25°C <strong>and</strong> Tj = 125 °C
Two industry st<strong>and</strong>ard diodes
Results for Tj = 25°C
Small difference
Results for Tj = 125°C
Big difference
The difference between low <strong>and</strong> 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 <strong>and</strong> <strong>Diode</strong>, reducing Rg:
E OFF
= 22.8uJ 16.7uJ
Drawback: ringing due to parasitic Ind. & Caps
All measurements: FDD6N50 + ISL9R460, U = 300V, I = 4A
Recommended Rg
Good switching performance, no ringing
Low Rg
Bad switching performance, ringing, but lower E OFF
24

Switching Losses @ increasing switching speed
- Same MOSFET, different Rg -
<strong>Diode</strong> = ISL9R460, U = 300V, I = 4A
FDD6N50, Rg = 10 Ohm
E ON
= 8 uJ
dI/dt = 1400A/us
I RRM
= 6.2A
Right : FDD6N50, Rg = 3 Ohm
E ON
= 4 uJ
dI/dt = 1600A/us
I RRM
= 7.4A
25

Switching Losses @ increasing switching speed
- Different MOSFET Technologies -
<strong>Diode</strong> = ISL9R460, U = 300V, I = 4A
FQP9N50C, Rg = 30 Ohm
E ON
= 23.2 uJ
dI/dt = 400 A/us
I RRM
= 2.6 A
FDD6N50, Rg = 30 Ohm
E ON
= 15.3 uJ
dI/dt = 640 A/us
I RRM
= 3.9 A
26

Variation of I RRM with dI/dt for different diode
technologies
<strong>Reverse</strong> <strong>Recovery</strong> Current Irr of the <strong>Diode</strong> vs. dI/dt @ V = 300V @ I = 4A
10
9
8
<strong>Reverse</strong> <strong>Recovery</strong> Current Irr [A]
7
6
5
4
3
Irr @ FFP08H60S
Irr @ ISL9R860
Irr @ ISL9R460
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 E ON losses with di/dt for different diode
technologies
Eon losses of the FET vs. dI/dt @ V = 300V @ dI/dt = 4A
35
30
25
Eon Losses [uJ]
20
15
Eon @ FFP08H60S
Eon @ ISL9R860
Eon @ ISL9R460
10
5
0
0 200 400 600 800 1000 1200 1400 1600
dI/dt [A/us]
A higher dI/dt will decrease the Eon losses.
28

<strong>Effect</strong> of parallel caps on switching losses
@ V IN = 300V @ I = 4A <strong>Diode</strong> = ISL9R460
E ON
= 32 uJ E ON
= 39 uJ E ON
= 57 uJ
No parallel Capacitance Cpar = 470pF Cpar = 1nF
E OFF
= 27 uJ E OFF
= 7.8 uJ E OFF
= 5.47 uJ
‣ Increase of the Eon losses due to the parallel Capacitance.
‣ Advantages in switching off
‣ Overall losses? …
29

<strong>Effect</strong> of parallel caps on switching losses
Eon <strong>and</strong> Eoff losses with a snubber Capacitance
Etot / Eon / Eoff losses [uJ]
100
90
80
70
60
50
40
30
20
10
A small capacitance can help to reduce the overall switching losses
Etot at 300V
Etot at 200V
Etot at 100V
Eoff at 300V
Eon at 300V
Eoff at 200V
Eon at 200V
Eoff at 100V
Eon at 100V
0
0 100 200 300 400 500 600 700 800 900 1000
80 pF @ 300 V
Capacitance parallel to the <strong>Diode</strong> [pF]
200 pF @ 200 V 450 pF @ 100 V
As higher the voltage, as smaller the cap to decrease the overall losses.
30

Summary
‣ <strong>Reverse</strong> recovery in diodes in half-bridge structures causes
• small losses in the diodes
• larger losses in the MOSFET/IGBT
‣ I RRM
<strong>and</strong> t RR
increase with
• temperature
• di/dt
• current (less dominant)
‣ Larger current rated diodes of the same family
• have higher I RRM resulting in higher E ON (measured at the same dI/dt for comparison)
• have larger capacitance, resulting in lower E OFF
• cause higher total switching losses
‣ Higher di/dt results in lower E ON
losses, but also in a higher I RRM
‣ Addition of extra capacitance
• increases E ON losses but decreases E OFF losses
• addition of extra capacitance could reduce total losses.
31