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ISO721 Digital isolator in use in SMPS isolated feedback

treez
Intellectual 970 points
Other Parts Discussed in Thread: ISO721

Hello,

Do you see any problems with this (non-optocoupler based) isolated feedback method for offline SMPS’s? Why does nobody do it like this?

The attached schematic shows what surely must be a   total revolution  in isolated feedback for offline SMPS’s? –It allows the feedback signal to cross the isolation boundary without recourse to an optocoupler. (instead , a digital isolator is used, freeing the engineer from the opto with all its inherent problems such as poor parameter tolerance and ageing problems )

With the error amplifier on the secondary side, the feedback signal takes the form of a  variable duty cycle square wave, produced by  a simple PWM controller IC. This is passed  across the isolation boundary by the digital isolator. On the primary  side, the PWM error signal is filtered and then fed to the PWM comparator in the primary side PWM controller IC. –Hence the SMPS output is regulated

There is also the LTspice simulation attached.

What’s wrong with this way of doing it?
Why does nobody do this?

SI8410 Digital isolator datasheet:
https://www.silabs.com/Support%20Doc...ocs/si8410.pdf

LT1243 datasheet:
http://cds.linear.com/docs/en/datasheet/1241fa.pdf

LTspice simulation here...

Version 4
SHEET 1 4756 4968
WIRE -176 -224 -2272 -224
WIRE 576 -224 -176 -224
WIRE 800 -224 720 -224
WIRE 976 -224 864 -224
WIRE 2496 -224 976 -224
WIRE 2720 -224 2496 -224
WIRE 2992 -224 2720 -224
WIRE 2992 -208 3168 -256
WIRE 2992 -208 2992 -224
WIRE -176 -192 -176 -224
WIRE 1616 -176 1504 -176
WIRE 2048 -176 1760 -176
WIRE 1504 -144 1504 -176
WIRE -1376 -112 -1920 -112
WIRE -1248 -112 -1376 -112
WIRE -864 -112 -1248 -112
WIRE 1936 -112 1824 -112
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WIRE 720 -96 720 -224
WIRE 2496 -96 2496 -224
WIRE 2576 -96 2496 -96
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WIRE 1824 -80 1824 -112
WIRE 1936 -80 1936 -112
WIRE 2048 -80 2048 -176
WIRE 2576 -64 2576 -96
WIRE 2720 -64 2720 -224
WIRE 0 -48 -80 -48
WIRE 352 -48 208 -48
WIRE 2784 -48 2768 -48
WIRE 1504 -32 1504 -64
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WIRE 2800 0 2768 0
WIRE 2896 0 2880 0
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WIRE 1760 32 1760 -176
WIRE 1824 32 1824 0
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FLAG -80 608 0
FLAG 0 80 0
FLAG 352 80 0
FLAG -176 -96 0
FLAG 1504 -32 0
FLAG 1200 64 PWM
FLAG -1248 32 0
FLAG 2784 -48 0
FLAG 2896 0 0
FLAG 3056 448 0
FLAG 3168 496 0
FLAG 3168 -256 VOUT_P
SYMBOL PowerProducts\\LT1241 -80 272 R0
SYMATTR InstName U1
SYMBOL cap -432 480 R0
SYMATTR InstName C1
SYMATTR Value 3n
SYMATTR SpiceLine Rser=0.03
SYMBOL res -432 320 R0
SYMATTR InstName R2
SYMATTR Value 2330
SYMBOL cap -304 480 R0
SYMATTR InstName C2
SYMATTR Value 4µ7
SYMATTR SpiceLine Rser=0.03
SYMBOL voltage 0 -32 R0
WINDOW 123 0 0 Left 2
WINDOW 39 24 44 Left 2
SYMATTR SpiceLine Rser=0.1
SYMATTR InstName V1
SYMATTR Value 11
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WINDOW 123 0 0 Left 2
WINDOW 39 24 44 Left 2
SYMATTR SpiceLine Rser=0.03
SYMATTR InstName V8
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SYMBOL res 240 416 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 0 56 VBottom 2
SYMATTR InstName R3
SYMATTR Value 700
SYMBOL res 192 16 R0
SYMATTR InstName R4
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SYMBOL cap 192 448 R0
SYMATTR InstName C3
SYMATTR Value 330p
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WINDOW 3 -99 -21 Left 2
SYMATTR Value STW11NM80
SYMATTR InstName M1
SYMBOL res 336 224 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 0 56 VBottom 2
SYMATTR InstName R1
SYMATTR Value 1
SYMBOL res 560 432 R0
SYMATTR InstName R6
SYMATTR Value 0.36
SYMBOL res 448 432 R0
SYMATTR InstName R8
SYMATTR Value 33k
SYMBOL ind2 560 -112 R0
WINDOW 0 52 44 Left 2
WINDOW 3 -71 60 Left 2
SYMATTR InstName L1
SYMATTR Value 1.5e-3
SYMATTR Type ind
SYMATTR SpiceLine Rser=0.1
SYMBOL ind2 736 0 R180
WINDOW 0 46 69 Left 2
WINDOW 3 -54 31 Left 2
SYMATTR InstName L2
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SYMATTR Type ind
SYMATTR SpiceLine Rser=0.05
SYMBOL schottky 800 -208 R270
WINDOW 0 32 32 VTop 2
WINDOW 3 0 32 VBottom 2
SYMATTR InstName D1
SYMATTR Value MBRS1100
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL cap 960 96 R0
SYMATTR InstName C4
SYMATTR Value 220µ
SYMATTR SpiceLine Rser=0.03
SYMBOL cap -192 -192 R0
SYMATTR InstName C5
SYMATTR Value 2µ1
SYMATTR SpiceLine Rser=0.03
SYMBOL voltage -2272 32 R0
WINDOW 123 0 0 Left 2
WINDOW 39 24 124 Left 2
SYMATTR SpiceLine Rser=0.03
SYMATTR InstName V2
SYMATTR Value 250
SYMBOL res 2480 448 R0
SYMATTR InstName R5
SYMATTR Value 2k5
SYMBOL res 2480 -48 R0
SYMATTR InstName R7
SYMATTR Value 14k5
SYMBOL cap 2560 -64 R0
SYMATTR InstName C8
SYMATTR Value 100n
SYMATTR SpiceLine Rser=0.03
SYMBOL res 2560 16 R0
SYMATTR InstName R10
SYMATTR Value 100
SYMBOL res 2320 128 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R11
SYMATTR Value 33k
SYMBOL res 1808 -96 R0
SYMATTR InstName R9
SYMATTR Value 10k
SYMBOL cap 1920 -80 R0
SYMATTR InstName C6
SYMATTR Value 10n
SYMATTR SpiceLine Rser=0.03
SYMBOL cap 1792 432 R0
SYMATTR InstName C7
SYMATTR Value 4µ7
SYMATTR SpiceLine Rser=0.03
SYMBOL res 1904 272 R0
SYMATTR InstName R12
SYMATTR Value 3300
SYMBOL cap 1904 464 R0
SYMATTR InstName C9
SYMATTR Value 3n3
SYMATTR SpiceLine Rser=0.03
SYMBOL npn 2096 336 R0
SYMATTR InstName Q1
SYMATTR Value 2N3904
SYMBOL res 2096 368 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R13
SYMATTR Value 100
SYMBOL res 2144 448 R0
SYMATTR InstName R14
SYMATTR Value 1k9
SYMBOL res 2288 448 R0
SYMATTR InstName R15
SYMATTR Value 1k4
SYMBOL voltage 1504 -160 R0
WINDOW 123 0 0 Left 2
WINDOW 39 24 44 Left 2
SYMATTR SpiceLine Rser=0.1
SYMATTR InstName V4
SYMATTR Value 11
SYMBOL res 1280 368 R0
SYMATTR InstName R16
SYMATTR Value 3k
SYMBOL voltage -2048 368 R0
SYMATTR InstName V3
SYMATTR Value V=V(PWM)
SYMATTR Prefix B
SYMBOL res -400 224 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R17
SYMATTR Value 22k
SYMBOL res -560 224 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R18
SYMATTR Value 10k
SYMBOL res -1696 224 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R19
SYMATTR Value 1k
SYMBOL cap -1696 368 R0
SYMATTR InstName C10
SYMATTR Value 1n
SYMATTR SpiceLine Rser=0.03
SYMBOL res -1520 224 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R20
SYMATTR Value 1k
SYMBOL cap -1536 368 R0
SYMATTR InstName C11
SYMATTR Value 1n
SYMATTR SpiceLine Rser=0.03
SYMBOL Opamps\\LT1006 -864 176 R0
SYMATTR InstName U3
SYMBOL voltage -1248 -96 R0
WINDOW 123 0 0 Left 2
WINDOW 39 24 44 Left 2
SYMATTR SpiceLine Rser=0.1
SYMATTR InstName V5
SYMATTR Value 10
SYMBOL res -880 128 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R21
SYMATTR Value 5k
SYMBOL res -1200 240 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 0 56 VBottom 2
SYMATTR InstName R22
SYMATTR Value 3k3
SYMBOL voltage -960 368 R0
WINDOW 123 0 0 Left 2
WINDOW 39 -111 117 Left 2
SYMATTR SpiceLine Rser=0.1
SYMATTR InstName V6
SYMATTR Value 1.5
SYMBOL PowerProducts\\LT1243 1616 240 M0
SYMATTR InstName U2
SYMBOL Opamps\\LT1006 -1376 160 R0
SYMATTR InstName U4
SYMBOL res 1280 208 R0
SYMATTR InstName R23
SYMATTR Value 8k
SYMBOL cap -496 304 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C12
SYMATTR Value 4n7
SYMATTR SpiceLine Rser=0.03
SYMBOL cap -912 0 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C13
SYMATTR Value 220p
SYMATTR SpiceLine Rser=0.03
SYMBOL cap 2032 -80 R0
SYMATTR InstName C14
SYMATTR Value 220p
SYMATTR SpiceLine Rser=0.03
SYMBOL Opamps\\LT6220 -1920 240 R0
SYMATTR InstName U5
SYMBOL res 2704 352 R0
SYMATTR InstName R26
SYMATTR Value 17
SYMBOL sw 2720 -80 M0
SYMATTR InstName S1
SYMBOL voltage 2784 0 M90
WINDOW 3 -288 -234 VRight 2
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR Value PWL(0 +1 10m +1 10001u -1 15m -1 15001u +1)
SYMATTR InstName V7
SYMBOL sw 2992 416 M0
SYMATTR InstName S2
SYMBOL voltage 3056 496 M90
WINDOW 3 135 -293 VRight 2
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR Value PWL(0 -1 25m -1 25001u +1 25500u +1 25501u -1)
SYMATTR InstName V9
TEXT 288 -192 Left 3 ;omit leakage\nfor simulation\nspeed.
TEXT 568 -288 Left 3 !K L1 L2 1
TEXT 776 -152 Left 2 ;Higher voltage\ndiode would be\nused.
TEXT 608 640 Left 2 !.tran 0 50m 0 startup
TEXT 2888 -256 Right 2 !.model sw sw(Ron=1m)
TEXT 3144 664 Right 2 !.model sw sw(Ron=1m)
TEXT -1280 304 Left 2 ;0 to 2v5
TEXT 2760 -152 Left 4 ;LOAD\nSWITCH
TEXT 3024 272 Left 4 ;SHORT\nCIRCUIT\nAT 15ms.
TEXT -1544 760 Left 6 ;PWM ERROR SIGNAL PASSED TO PRIMARY BY DIGITAL ISOLATOR
TEXT -8 -632 Left 7 ;17W offline flyback with digital \nisolator feedback.
TEXT -2136 -320 Left 5 ;DC BUS\nPost-Rectifier.
TEXT -1632 160 Left 5 ;Filter
TEXT -1904 104 Left 4 ;BUFFER
TEXT -856 -24 Left 4 ;INVERTER
TEXT -1192 424 Left 4 ;Reference
TEXT 1864 -264 Left 5 ;Vout = 17V, 17W
LINE Normal 1120 192 1216 192 2
LINE Normal 1120 768 1120 192 2
LINE Normal 1008 768 1120 768 2
LINE Normal 1072 736 1008 768 2
LINE Normal 1072 800 1072 736 2
LINE Normal 1008 768 1072 800 2
LINE Normal -2224 752 -1568 752 2
LINE Normal -2224 256 -2224 752 2
LINE Normal -2080 256 -2224 256 2
LINE Normal -2176 224 -2080 256 2
LINE Normal -2176 288 -2176 224 2
LINE Normal -2080 256 -2176 288 2

  • 0
    TI__Mastermind 20820 points

    Hi Treez,


    I am moving this post over to the industrial interface forum as they support the isolation ICs.

  • 0 in reply to Michael Peffers
    TI__Mastermind 21769 points

    Hi Treez,

    Can you attach the schematic that you are referring to in your post? I was not able to see on in the original post.

    Thanks,

    John

  • 0 in reply to treez
    TI__Mastermind 21769 points

    Hi Treez,

    From a functional point of view, there is no reason our digital isolators would not work for this type of application as long as the frequency of your PWM signal and the minimum pulse width that you are trying to transmit is compatible with the device. All of our isolators can handle DC high, DC low and data being transmitted up to their specified max data-rate.

    From a quick look at the LT124x devices, the output is clamped at 18 volts, and the oscillator frequency can be either 250kHz, or 50KHz.  

    The high output voltage you look like you are going to handle this with a resistor divider of 3/11, which works out to be 4.9 volts on the input of the isolator. Therefore, if you supply the ISO721 with 5 volts on the input side this should not be an issue.

    The ISO721 device has a input filter that blocks all input pulses that are shorter than 2ns in duration. Since the oscillator frequencies seem so slow in relation to this family of devices that can handle 150MBPS, I do not think you should have any issues.

    Can you verify the frequency and the minimum pulse width you are trying to achieve with your system?

    Thanks,

    John

  • 0 in reply to John Griffith
    Intellectual 970 points

    Thanks, that's interesting, I am limited in the frequency by the bandwidth of the LT6220 opamp buffer. So I cannot use more than about 130KHz square wave through the digital isolator.

    the minimum pulse width would be  say , well , definitely well above 2ns. -say about 100ns or so.

    To be honest, the problem is with the filtration of the pwm after and the poles that this puts into the transfer function. I am seeing a lot of undershoot on the output voltage after a no load to full load transient, and I think the rc filter is responsible.

    I am sure you realise how "awful" optoisolators are in smps isolated feedback, and most people, as you know, are desperate to find an alternative.

  • 0 in reply to treez
    TI__Mastermind 21769 points

    Hi Treez,

    Have you looked into any type of active filter (use an op amp with some single or double pole filter network).

    Thanks,

    John

    PS: I am happy to see you using our capacitive isolation as opposed to the optos. :)