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 small losses in the diodes larger losses in the MOSFET/IGBT ¾ IRRM and tRR increase with temperature di/dt current (less dominant) ¾ Larger current rated diodes of the same family have higher IRRM resulting in higher EON (measured at the same dI/dt for comparison) have larger capacitance, resulting in lower EOFF 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 increases EON losses but decreases EOFF losses addition of extra capacitance could reduce total losses. 31
