Reducing voltage, diodes in series, dual PSU

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My SMPS PSU is very near the voltage limit of my LM3886 chipamp. +84v (42,0,42)

I assume there will be some voltage sag, even when idle. I tested the PSU under various resistor loads...it does sag, though not greatly.
I'm not sure what the idle-load will be of the amp circuit.
(Datasheet says quiescent is "50mA typ. / 85mA limit"....I think this per LM3886, using their test circuit #2)


If I wanted to create a slightly lower and safer voltage limit, is my drawing one way to do this ?

Are the violet rectifiers necessary at all ?

img024_zps122f81e8.jpg


I
 
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You could use a rectifier diode to drop ~600mV, or a string of diodes to drop more voltage. Or replace the string with a lowish voltage power Zener.
Or add a CRCRC filter between the SMPS and the Chipamps to clean up the supply to the analogue stages.

Note that 84Vdc is the absolute max for a signal present condition. If no signal is present then upto 90Vdc can be safely survived (and a little bit to spare).
 
You are right, the voltage drop of a silicon diode is between 500mV and 700mV.

I knew this was the typical value for such diodes, but I wonder why the datasheet would say Vf is 1.3v ??
Maybe I'm not understanding how they arrive at that figure.
I'm guessing it's because that's the "worst case"......The Vf spec says "Maximum instantaneous forward voltage at 8.0A".

....and I just tested these rect. diodes.....indeed, a 0.6v drop.
So five of them in series would give a 3v drop.
 
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I wonder why the datasheet would say Vf is 1.3v ??

Take a look at figure 3 on pg. 3:

Vishay - datasheet pdf (opens PDF)

Vf = 1.5V maximum (for the FES8JT from the table on pg. 2) is what happens when you hit it with a 40 amp pulse for 300 microseconds at 1% duty cycle. :) Lower forward currents produce a lower Vf, as per the graph. 8 amps is the maximum continuous (non-pulsed, average, DC) current for the part.
 
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On the FES8JT datasheet, the Maximum Ratings section says that junction temperature (operating) must be in the range -55C < TJ < +150C.

They give IF versus VF at the upper end of the temperature range but not the lower end. Shifting the curves over by a distance that looks plausible, I get the red curve shown below. At IF= 8 amperes, VF is 1.3 volts, for the 600-volt rated FES8JT when TJ = -50C.
 

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I did all the mods to the SMPS as seen here....Hi

Reducing the voltage via Diodes is becoming not an option.....they will get way too hot, and I'm out of heatsink space.

Plus, the SMPS won't "start-up"....it seems to need:
A) needs enough resistance to trigger start-up.
B) smaller capacitance to help start-up. (large capacitance prevents start-up)

I've reduced the cap values of the chipamp from 6600uf per rail to 660uf per rail....no help.
...plus I tried adding resistance to the SMPS output to help start-up....no help.

My linear Bench PSU powers the chipamp fine.
Problem is getting these SMPS's to gosh-darn produce voltage to the chipamp......the SMPS keeps trying to start-up, but refuse to.

I've built the case around the SMPS's (one per channel).
Very frustrating.

I need to do one or two of these things....
1) Change to a linear supply (making this portable amp very heavy, opposite of my goal)
2) Or adding a "soft start". (still not sure that will work.....plus means 2 more circuit boards)
3) Asking for help/advice.

Do I eliminate nearly all of the chipamp's rail capacitance ??....Does it even need any when using an SMPS ? (except for typical decoupling caps ?....example 1uf + 0.1uf ?)
EDIT: switched to 200uf per rail....still no SMPS turn-on. Does that seem right ?
 
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It seems as though you might be able to employ a voltage adjust control pot at the junction of R5 & R6 with the wiper going to pin 2/4 (depending on what package is being used) NFB of the PWM IC UC3842.

Maybe a 470ohm or 1Kohm.

FWIW

jer :)

Geraldfryjr.....I've nearly given-up on reducing the voltage any more than I have. But I'll look into what you are suggesting.

I just can't believe (though I have to) that these SMPS units will refuse to turn-on at all, with 200uf per rail. They start-up fine on their own, with 1.2kohm rail-to-rail resistors, no capacitance.

I will try with 1uf just to see if I am headed down the wrong path.
If they don't start-up with 1uf......something else is my problem.

Do you think the mods I did per the "HI" website, made these units extremely sensitive to "turn=on ".

info....I have a Clarostat 240 power decade resistor...to easily test what these SMPS need for turn=on, resistance-wise.
 
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I have had a few that wouldn't turn on at all unless there was a load on them.

It is possible that the mods have offset the duty cycle range making it not want to start up.
I know how these things work but I don't have any experience modifying them, I have a few laying around that I want to experiment with sometime.

The reference voltage in the chip is 2.5V so due to the feedback circuit it tries to adjust the NFB input to match the reference voltage.

I tried to widen the range on one of my supply's by using a higher value resistor on one side of the pot but it didn't give me anywhere near what I was expecting.
I forget what I was using it for (CPU overclocking I think) and I found that the supply was drooping quite a bit so I needed to crank it up a bit.

Since then I just use it to power my stack of SCSI drives.

jer :)

P.S. I Remember, I was using it to power my overclocked 7600GS video card. :)
 
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