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I have made a simple circuit in LTSpice to test MOSFET turn-off time.

Schematic

Inductance is changed from 1.5nH to 1.5mH in a few steps. The next picture shows gate-to-source voltage and drain-to-source voltages.

Top: Gate-to-source voltage, Bottom: Drain-to-source voltage; Green is for 1.5nH, cyan for 1.5mH Top: Gate-to-source voltage, Bottom: Drain-to-source voltage; Green is for 1.5nH, cyan for 1.5mH

Turn of time increases dramatically with increase of inductance L1.

Why? What phenomenon is behind this?

Cheers

EDIT: I have measured currents for diode, inductor and transistor. Simulation was done for L=1.5mH

vaveforms Top: RED-Inductor current; Purple-Transistor current; Cyan-Diode current; Dark Green-Gate-source voltage; Bottom: Drain-source voltage

After gate voltage goes to zero, transistor keeps conducting the whole current (transistor current is equal to inductor current). Diode starts conducting much later, taking the current over from transistor.

So the question still holds.

EDIT2: Here is the simulation file:

Version 4
SHEET 1 3688 1024
WIRE 576 -32 352 -32
WIRE 720 -32 640 -32
WIRE 880 -32 720 -32
WIRE -512 -16 -912 -16
WIRE -368 -16 -512 -16
WIRE 176 -16 -368 -16
WIRE -368 0 -368 -16
WIRE 720 0 720 -32
WIRE 880 16 880 -32
WIRE -512 32 -512 -16
WIRE 176 32 176 -16
WIRE -912 80 -912 -16
WIRE -368 80 -368 64
WIRE 352 80 352 -32
WIRE -512 96 -512 80
WIRE -512 160 -512 96
WIRE -368 160 -368 144
WIRE -368 160 -512 160
WIRE 720 160 720 64
WIRE -512 192 -512 160
WIRE 880 224 880 96
WIRE -912 240 -912 160
WIRE 352 240 352 160
WIRE -512 272 -512 256
WIRE -512 368 -512 352
WIRE 176 368 176 112
WIRE 176 368 -512 368
WIRE 176 416 176 368
WIRE 128 496 80 496
WIRE 80 544 80 496
WIRE 80 544 -1200 544
WIRE 176 688 176 512
WIRE -1328 704 -1392 704
WIRE -1200 704 -1200 544
WIRE -1200 704 -1248 704
WIRE -1168 704 -1200 704
WIRE -1392 784 -1392 704
WIRE -1392 896 -1392 864
FLAG -912 240 0
FLAG -1392 896 0
FLAG 720 160 0
FLAG 880 224 0
FLAG 352 240 0
FLAG 880 -32 Vout
FLAG 176 688 0
SYMBOL voltage -912 64 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value 900
SYMBOL res -1232 688 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R2
SYMATTR Value 10
SYMBOL voltage -1392 768 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value PULSE(0 15 0 2n 2n {D/f-td} {1/f})
SYMBOL ind2 160 16 R0
SYMATTR InstName L1
SYMATTR Value 1.5mH
SYMATTR Type ind
SYMBOL cap 704 0 R0
SYMATTR InstName C1
SYMATTR Value 20µF
SYMBOL ind2 368 176 R180
WINDOW 0 36 80 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName L2
SYMATTR Value 47.83µH
SYMATTR Type ind
SYMBOL current 880 16 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName I1
SYMATTR Value {I}
SYMBOL diode 576 -16 R270
WINDOW 0 32 32 VTop 2
WINDOW 3 0 32 VBottom 2
SYMATTR InstName D6
SYMATTR Value MyDiode
SYMBOL zener -528 32 R0
SYMATTR InstName D3
SYMATTR Value Zener1
SYMBOL cap -384 0 R0
SYMATTR InstName C2
SYMATTR Value 2.2n
SYMBOL cap -384 80 R0
SYMATTR InstName C3
SYMATTR Value 2.2n
SYMBOL res -528 256 R0
SYMATTR InstName R1
SYMATTR Value {R4}
SYMBOL diode -496 256 R180
WINDOW 0 24 64 Left 2
WINDOW 3 24 0 Left 2
SYMATTR InstName D1
SYMATTR Value Diode1
SYMBOL nmos 128 416 R0
SYMATTR InstName M1
SYMATTR Value STH2N120K5-2AG
TEXT -488 664 Left 2 ;.param f=130k D=0.08024
TEXT 1816 224 Left 2 !.tran 0 3.5m 0 20n
TEXT 1712 520 Left 2 !.model STH2N120K5-2AG VDMOS(Rg=16 Rd=1m Rs=1m Vto=3 Kp=3.5 Cgdmax=0.5p Cgdmin=0.5p Cgs=124p Cjo=1n Is=2.3p Rb=6m mfg=Fairchild Vds=1200 Ron=10 Qg=5.3n)
TEXT 224 8 Left 2 !K L1 L2 1
TEXT 1568 384 Left 2 !.MODEL DI_BAV20W D  ( IS=1.09u RS=0.105 BV=150 IBV=100n CJO=5.00p  M=0.333 N=3.29 TT=72.0n )
TEXT -480 704 Left 2 ;.param f=130k D=0.0655
TEXT 160 -80 Left 2 ;Transformer
TEXT 1568 640 Left 2 !.MODEL DI_US1M D  ( IS=709n RS=82.3m BV=1.00k IBV=5.00u CJO=18.5p  M=0.333 N=3.23 TT=108n )
TEXT 1520 720 Left 2 !.MODEL SM6T200A D (IS =1.0e-6 RS=13.5 N=1 IBV=1m BV=200 CJO=0)
TEXT 1328 128 Left 2 !.MODEL BAV20W D (Is=15u RS=5 BV=200 IBV=1m CJO=1.5p TT=50n)
TEXT 1328 224 Left 2 !.ic V(Vout)=12V
TEXT 1168 952 Left 2 ;.MODEL STH2N120K5-2AG VDMOS(KP=2.4957 RS=1.676 RD=5.4696 RG=16.0 VTO=2.4 \n+LAMBDA=0.001 CGDMAX=196p CGDMIN=1p CGS=114p TT=72n \n+IS=1.23E-09 N=1.492 RB=0.048707 m=1.491 Vj=22.72 Cjo=1041.54pF)
TEXT 792 512 Left 2 ;.step param D list 0.038 0.06 0.08
TEXT 808 592 Left 2 !.param C2 = 10p
TEXT 832 640 Left 2 !.param R4=47
TEXT 1056 368 Left 2 !.model Diode1 D ( Vfwd=0.7 )
TEXT 1056 408 Left 2 !.model Zener1 D (Vrev=200 Vfwd=1)
TEXT -504 752 Left 2 !.param f=130k D=0.038
TEXT 504 704 Left 2 ;.MODEL STH2N120K5-2AG VDMOS(KP=.001 RS=1.676 RD=5.4696 RG=16.0 VTO=2.4 \n+CGS=114p  \n+RB=0.048707 m=1.491 Vj=22.72 Cjo=341.54pF)
TEXT 496 296 Left 2 !.model MyDiode D(Ron=.001 Roff=1Meg Vfwd=.4)
TEXT 1032 168 Left 2 !.param I=0.25
TEXT 1056 -40 Left 2 ;.step param I list 0.1 0.3 0.5 0.7 1 1.5 2 3
TEXT -16 864 Left 2 !.param td=130n
TEXT 408 392 Left 2 !.step param td list 0n 130n 160n
RECTANGLE Normal 384 208 -176 -128 2
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  • \$\begingroup\$ Stored energy in the inductor has to be released somehow. \$\endgroup\$ – Peter Smith Oct 4 at 13:55
  • \$\begingroup\$ If you post the ascii text of the circuit (the .asc file is text) I can take a look - I have a suspicion as to what it is. \$\endgroup\$ – Peter Smith Oct 4 at 14:50
  • \$\begingroup\$ I've added the simulation ascii as an edit. \$\endgroup\$ – Velibor Simovic Oct 6 at 11:59
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During turn on, the FET is actively sinking current and therefore the turn on time won't vary very much.

During turn off, the FET is simply an open circuit and the inductor will release the stored energy via D1 and R3 and that will take time.

The FET itself will be OFF, but the inductor will hold the drain voltage low for a while as it discharges.

I suggest probing the current in L1 to get an insight into this.

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