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undervoltage lockout for flyback powersupply input

J

Jamie Morken

Jan 1, 1970
0
Hi,

I have a flyback powersupply that operates from about 20V to 450V+ (AC
or DC input) and I would like to reduce the operation to 75V to 450V,
what is a possible circuit to use on its input to keep it from starting
up until this threshold is reached?

cheers,
Jamie
 
E

Eeyore

Jan 1, 1970
0
Jamie said:
Hi,

I have a flyback powersupply that operates from about 20V to 450V+ (AC
or DC input) and I would like to reduce the operation to 75V to 450V,
what is a possible circuit to use on its input to keep it from starting
up until this threshold is reached?

A comparator that detects the voltage on the bulk reservoir cap and
inhibits switching below a certain voltage seems to be the way.

What's your existing circuit ?

Graham
 
J

Jamie

Jan 1, 1970
0
Jamie said:
Hi,

I have a flyback powersupply that operates from about 20V to 450V+ (AC
or DC input) and I would like to reduce the operation to 75V to 450V,
what is a possible circuit to use on its input to keep it from starting
up until this threshold is reached?

cheers,
Jamie
For simplicity reasons, Get your self a DIAC rated around 75 volts
connect one end to the cathode of D34 and the other end will be
the V+ supply for R5 instead of connecting R5 directly to V+.

Using a comparator circuit would also be fine how ever, you would need
to use a voltage divider and protection zener to make sure you don't
over do the input. The output could then simply control the base of
that transistor switch you have.
 
E

Eeyore

Jan 1, 1970
0
J

Jamie Morken

Jan 1, 1970
0
Eeyore said:
Neat idea actually Jamie. 30-40V diacs are more commonly available though
(I've never met a 75V diac) so perhaps 2 of those in series.

http://uk.farnell.com/jsp/search/browse.jsp?N=500004+1000013&Ntk=gensearch_001&Ntt=diac&Ntx=

Graham

Thanks for the ideas guys, also is there a way to shut off the circuit
when the voltage drops below 75V? I think the DIAC and comparator
circuits will only prevent startup until 75V, but won't disable the
circuit if the voltage drops below 75V.

cheers,
Jamie
 
E

Eeyore

Jan 1, 1970
0
Jamie said:
Thanks for the ideas guys, also is there a way to shut off the circuit
when the voltage drops below 75V? I think the DIAC and comparator
circuits will only prevent startup until 75V, but won't disable the
circuit if the voltage drops below 75V.

Correct.

Stopping it again is slightly more tricky, since once started up the switcher IC gets its supply
from the 'boost in' winding.

Maybe something could be kludged using the Isense input to inhibit switching ?

Graham
 
J

Jamie Morken

Jan 1, 1970
0
Jamie said:
For simplicity reasons, Get your self a DIAC rated around 75 volts
connect one end to the cathode of D34 and the other end will be
the V+ supply for R5 instead of connecting R5 directly to V+.

Never used Diac's before, from what I read it looks equivalent to
back to back zeners except the DIAC stays on until the current
drops below a threshold, is this correct? Would back to back
zeners, work for this as well?

cheers,
Jamie
 
J

Jamie Morken

Jan 1, 1970
0
Jamie said:
Never used Diac's before, from what I read it looks equivalent to
back to back zeners except the DIAC stays on until the current
drops below a threshold, is this correct? Would back to back
zeners, work for this as well?

Oh nevermind, I read a diac is just like a triac but without the gate.

cheers,
Jamie
 
E

Eeyore

Jan 1, 1970
0
Jamie said:
Never used Diac's before, from what I read it looks equivalent to
back to back zeners except the DIAC stays on until the current
drops below a threshold, is this correct?

Not quite. Once it 'fires' the voltage across it drops to a low level and it
acts like a thyristor thereafter.

Would back to back zeners, work for this as well?

Actually just one zener might work (no need for back to back) but I'm a bit
tentative since the startup supply wouldn't snap on in the same way.

Graham
 
L

legg

Jan 1, 1970
0
Correct.

Stopping it again is slightly more tricky, since once started up the switcher IC gets its supply
from the 'boost in' winding.

Maybe something could be kludged using the Isense input to inhibit switching ?

There are a number of ways to inhibit the 384x:

- Pull the Ct pin low (z dependant on Rt)
- pull the Comp pin below 1V2 (<800uA)
- pull the Fb pin above 2V5 (z dependant on V-divider)
- pull the Isense pin above 1V (z dependent on sensor decoupling )
- crowbar the Vref line.
- crowbar the Vcc line.

Most of these show up in app notes for the part.They all have
different advantages and disadvantages. Normally you'd want to tie
inhibition into a slow-start circuit, but I don't see one in your
schematic. At this power level, you may think you don't need one.

By the way, what prevents R4 (or anything else) from popping under
overload conditions? You'd have to disable the start-up bias it if you
wanted to prevent the rest circuit from drawing juice early, or
intended it to hiccough under overload - the latter being the usual
cheap method of overload protection with simpler flybacks.

It's beginning to look complicated, isn't it? That's one of the
reasons you won't see it used often, any more, at low power levels.
Simplification would involve combining circuit functions to reduce
parts count, instead of just continuing to add more parts every time
you think of something new that may or may not need attending to.

RL
 
J

John Fields

Jan 1, 1970
0
Thanks for the ideas guys, also is there a way to shut off the circuit
when the voltage drops below 75V? I think the DIAC and comparator
circuits will only prevent startup until 75V, but won't disable the
circuit if the voltage drops below 75V.

---
This ought to turn it on at 75V on the way up and turn it off at 75V
on the way down. View in Courier:

..FROM +V>---+----------------+
.. |5%,2W |1%
.. [150K] [7.5M]
.. | |
.. +----------------|-----+ TO Q3 BASE
.. | | | |
.. | | 5 |
.. +--[909K]--+-----|---4|-\LT1716 C
.. | 1% | | | >1----[100K]--B 2N3904
.. |K |1% +---3|+/ 5% E
.. [LN4040] [100K] |1% 2 |
.. |10V | [100K] | |
.. | | | | |
..GND>-------+----------+-----+-----+-----------------+

At least, according to this:

Version 4
SHEET 1 1276 680
WIRE -32 32 -432 32
WIRE 528 32 -32 32
WIRE 576 32 528 32
WIRE -32 64 -32 32
WIRE 528 64 528 32
WIRE 96 176 -336 176
WIRE 528 208 528 144
WIRE 528 208 352 208
WIRE 96 224 96 176
WIRE -32 240 -32 144
WIRE 64 240 -32 240
WIRE 528 240 528 208
WIRE 640 240 528 240
WIRE 176 256 128 256
WIRE 288 256 256 256
WIRE -336 272 -336 176
WIRE -288 272 -336 272
WIRE -160 272 -208 272
WIRE 64 272 -160 272
WIRE 528 304 528 240
WIRE -160 336 -160 272
WIRE -32 336 -32 240
WIRE -432 352 -432 32
WIRE -336 352 -336 272
WIRE 528 384 528 368
WIRE -432 480 -432 432
WIRE -336 480 -336 432
WIRE -336 480 -432 480
WIRE -160 480 -160 416
WIRE -160 480 -336 480
WIRE -32 480 -32 416
WIRE -32 480 -160 480
WIRE 96 480 96 288
WIRE 96 480 -32 480
WIRE 352 480 352 304
WIRE 352 480 96 480
WIRE 528 480 528 448
WIRE 528 480 352 480
WIRE -432 544 -432 480
FLAG -432 544 0
FLAG 576 32 V+
SYMBOL Comparators\\LT1716 96 256 R0
WINDOW 0 33 -64 Left 0
WINDOW 3 20 -36 Left 0
SYMATTR InstName U1
SYMBOL res -48 48 R0
SYMATTR InstName R1
SYMATTR Value 7.5e6
SYMBOL res -48 320 R0
SYMATTR InstName R2
SYMATTR Value 1e5
SYMBOL voltage -336 336 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value 10
SYMBOL res -176 320 R0
SYMATTR InstName R3
SYMATTR Value 1e5
SYMBOL res -192 256 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R4
SYMATTR Value 9.09e5
SYMBOL voltage -432 336 R0
WINDOW 3 24 104 Invisible 0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value PULSE(0 450 0 1s 1s 1s 4s)
SYMBOL res 512 48 R0
SYMATTR InstName R7
SYMATTR Value 1e6
SYMBOL zener 544 368 R180
WINDOW 0 46 32 Left 0
WINDOW 3 32 -1 Left 0
SYMATTR InstName D1
SYMATTR Value BZX84C10L
SYMBOL zener 544 448 R180
WINDOW 0 47 30 Left 0
WINDOW 3 24 0 Left 0
SYMATTR InstName D2
SYMATTR Value BZX84C10L
SYMBOL npn 288 208 R0
SYMATTR InstName Q1
SYMATTR Value 2N3904
SYMBOL res 272 240 R90
WINDOW 0 75 61 VBottom 0
WINDOW 3 83 58 VTop 0
SYMATTR InstName R5
SYMATTR Value 100k
TEXT -402 514 Left 0 !.tran 5
TEXT 640 240 Left 0 ;>TO Q3 BASE
TEXT 600 104 Left 0 ;YOUR R5
TEXT 576 376 Left 0 ;YOUR D11
 
J

John Fields

Jan 1, 1970
0
.FROM +V>---+----------------+
. |5%,2W |1%
. [150K] [7.5M]
. | |
. +----------------|-----+ TO Q3 BASE
. | | | |
. | | 5 |
. +--[909K]--+-----|---4|-\LT1716 C
. | 1% | | | >1----[100K]--B 2N3904
. |K |1% +---3|+/ 5% E
. [LN4040] [100K] |1% 2 |
. |10V | [100K] | |
. | | | | |
.GND>-------+----------+-----+-----+-----------------+
 
J

Jamie

Jan 1, 1970
0
Jamie said:
Never used Diac's before, from what I read it looks equivalent to
back to back zeners except the DIAC stays on until the current
drops below a threshold, is this correct? Would back to back
zeners, work for this as well?

cheers,
Jamie
Yes, you have it correct, the DIAC are a cheap way of closing a
circuit and opening it how ever, depending on the spec's, You
need to tailor the circuit so that the holding current will
drop below spec at the desired point to have it open again.
 
L

legg

Jan 1, 1970
0
.FROM +V>---+----------------+
. |5%,2W |1%
. [150K] [7.5M]
. | |
. +----------------|-----+ TO Q3 BASE
. | | | |
. | | 5 |
. +--[909K]--+-----|---4|-\LT1716 C
. | 1% | | | >1----[100K]--B 2N3904
. |K |1% +---3|+/ 5% E
. [LN4040] [100K] |1% 2 |
. |10V | [100K] | |
. | | | | |
.GND>-------+----------+-----+-----+-----------------+

You'd need hysterisis, to account for loading effects and ripple on
the line.

RL
 
J

John Fields

Jan 1, 1970
0
.FROM +V>---+----------------+
. |5%,2W |1%
. [150K] [7.5M]
. | |
. +----------------|-----+ TO Q3 BASE
. | | | |
. | | 5 |
. +--[909K]--+-----|---4|-\LT1716 C
. | 1% | | | >1----[100K]--B 2N3904
. |K |1% +---3|+/ 5% E
. [LN4040] [100K] |1% 2 |
. |10V | [100K] | |
. | | | | |
.GND>-------+----------+-----+-----+-----------------+

---
Better yet:


..FROM +V>---+----------------+
.. |5%,1W |1%
.. [300K] [7.5M]
.. | |
.. +----------------|-----+
.. | | |
.. | | 5
.. +--[909K]--+-----|---4|-\LT1716
.. | 1% | | | >1----> TO Q3 BASE
.. |K |1% +---3|+/
.. [LM4040] [100K] |1% 2
.. |10V | [100K] |
.. | | | |
..GND>-------+----------+-----+-----+
 
J

John Fields

Jan 1, 1970
0
.FROM +V>---+----------------+
. |5%,2W |1%
. [150K] [7.5M]
. | |
. +----------------|-----+ TO Q3 BASE
. | | | |
. | | 5 |
. +--[909K]--+-----|---4|-\LT1716 C
. | 1% | | | >1----[100K]--B 2N3904
. |K |1% +---3|+/ 5% E
. [LN4040] [100K] |1% 2 |
. |10V | [100K] | |
. | | | | |
.GND>-------+----------+-----+-----+-----------------+

You'd need hysterisis, to account for loading effects and ripple on
the line.

---
Not necessarily, since that depends on the amplitude and frequency
of the ripple and how long it takes +V to get through the 75V
threshold, neither of which the OP specified.

Also, note that the ripple on +V will be attenuated about 37.5dB by
the divider on the non-inverting input of the comparator.
 
E

Eeyore

Jan 1, 1970
0
legg said:
There are a number of ways to inhibit the 384x:

- Pull the Ct pin low (z dependant on Rt)
- pull the Comp pin below 1V2 (<800uA)
- pull the Fb pin above 2V5 (z dependant on V-divider)
- pull the Isense pin above 1V (z dependent on sensor decoupling )
- crowbar the Vref line.
- crowbar the Vcc line.

I had a feeling an expert on the part might eventually show up !

Graham
 
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