help with foldback current limit

OOPS -- that s/b 3*60v zener

how do you calculate the values for R6,R7 and R8 figured Imax @ 250ma and I short limit @ 125ma

Q1 irf840 +240 o-----+---+----- D S --+-- R8 ---+-----+----+--o +180V +-5V | | G | | | | 200mA R5 | | R7 | _|_+ | | '-- R6-- | ----+ | --- | | | | | | R for min +-----R------+ B Q2 | gnd | load | +---C E-------+ | | '-----|

THANK YOU FRED for the very indepth analysis

i've seen R6 tied to ground in lv regulators the 2 hi voltage regulators had R6 tied back to Q1 drain thought that's the way it should be done what is the reason for tying back to Q1 drain?

during power-up is there a chance of going into current limit under normal operating conditions?

thanks for the help, barney

By tying R6 to the drain, the foldback trip point now becomes a function of Vin-Vo, where Vin is the unregulated 240V, and this makes the trip point a function of the power dissipation in the MOSFET. The ground referenced R6 circuit assumes that power dissipation has been accounted for in setting the trip point limits and does not adapt as Vin varies. The drain referenced R6 circuit varies the trip point was Vin varies so that power dissipation influences the trip point. You can compute the new R6,7,8 values using the same equations by substituting -(Vin-Vo) for Vo. This puts -60V and -240V in the array in place of 180V and 0V respectively. The new circuit becomes R8=1.8, R6=110K, and R7=220 like so: View in a fixed-width font such as Courier.

--I-->

irf840 Q1 Vs 1.8 1/4W Vo +240 o-----+----+---- D S --+-- R8 ---+-----+----+--o +180V +-5V | | G | | | | 200mA R5 | 110K | R7 220 | _|_+ | | '-- R6--|-----+ | --- | | 1W | | | | R for min | 100K | B Q2 | gnd | load +--R---------+---C E-------+ | | 1W '-----|

hi fred thank you again for the explanation i can see the difference in fix and changing limit

want to make sure i haven't missed anything in your first post lower righthand corner of the schematic: R7 - I*R8 * ----- R6+R7 does this belong to?

R7 0.0008333 there is >= in front of ----- = --------- = .005 R6+R7 0.1666667 is something missing here?

again thanks for all the help, barney

Nothing- it is something I should have erased after cutting and pasting character blocks around.

That should be '=>', an arrow- which means "leads to" or "rearranges to" or "is equivalent to" etc..

More information on those two circuits is that the drain referenced R6 circuit will start up with much more current than the gnd referenced R6 circuit. The drain referenced short circuit limit is 300mA at low voltages and tapers down to 180mA at Vin=180V for a Pd,max=32W. The gnd referenced R6 circuit will short circuit limit at 100mA at low voltage rising to maybe 110mA at Vin=180. The max Pd is Vin,max*110mA which is not too far removed from 32W for heat sinking purposes. If you have some kind of troublesome nonlinear load that causes the foldback to stay on with the output at somewhat less than full voltage, then you would want to use the drain referenced circuit with a 32W heatsink for protection against turning on into a dead short. This is one advantage it has over the fixed limit gnd referenced R6 circuit.

I suppose this circuit is spiffier:

View in a fixed-width font such as Courier.

. . SPICE FOLDBACK CHARACTERISTIC PLOT . . --I-->

. irf840 . +240 Q1 Vs 1.8 1/4W Vo . o-----+----+---- D S --+-- R8 ---+--+--------. . | | G | | | | . R5 | 110K | R7 220 | | | . | '-- R6--|-----+ | | | . | 1W | | | | +-------+ . | 100K | B Q2 | | |AMMETER| . +--R---------+---C E-------+ | +-------+ . | 1W '-----|--' . | 5w zener| .---|/ . ----------- Vsrc| + IDEAL OA . | --- . | - . gnd | . | . gnd . . | . A 250m+ - - / |

hi fred

that helps pretty sure the the load is linear i going with the ground referenced R6

turn-on into dead or ?? anything can happen the radio is over 50 30+ tube set with the ss regulator 5 tubes eleminated along with the heat

do you see any problems here want to force equal voltage drop across Q1 and Q2

Q1 Q2 vin --+--- D S --+-- D S --+-- vo | G | G | | | | | | | +-|

R2 should be returned to vo, not gnd. Watch out for the minimum-load requirement due to R2's current.

hi win i've re-drawn the the schematic changed some the references i assume this is what you mean a more less realworld example will help my understanding how do i calculate R1-R4?

Q1,Q2 vds 40v irf840s 300v Q1 Q2 vin --+--- D S --+-- D S --+----+-- vo 220v | G | G | | | | | | | | | +-|

I don't know what you're aiming at, but for series FETs the top one wants to sit at half the total i/o difference voltage and you control the bottom one, like this...

. irf840s . 300v Q1 Q2 . vin --+--- D S --+-- D S --+-----+---+--- vo 220v . | G | G | | | . | | | | | | min . | +-|

hi win thanks for the info the idea was to spead power dissipation between the 2 there's 1 condition that over 30w would be dissipated by a single fet thanks, barney