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DC/DC converter question

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Plasma

Tue Oct 09 2018, 06:47AM

Registered Member #61406 Joined: Thu Jan 05 2017, 11:31PM
Location:
Posts: 268

Hi I've got multiple TEG 5 in series and would like to connect them to a DC/ DC converter like

Would it be alright to parrellel the converter's ?
Should I have the current limiting resistor on the output or input
The data sheet say's a max capacitive load of 1000uF, does parallel converter s output capacitive add.

Thanks

1539067665 61406 FT0 Img 20181009 193809

Each TEG can reach a max of 4.8 volt 650 mA ,with five they can put out 15.6 Watts.
There's these

zener 12 volt 5 Watts, they have a voltage tolerance of 5% would six of them in parallel have enough buffer to protect the input to the DC converter

hen918

Tue Oct 09 2018, 09:24AM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

Your diagram looks OK, whilst it is generally not recommended to parallel DC/DC converters unless they are designed for it, your setup should be OK, especially if you can trim the output voltages of each converter so they are the same. You could probably reduce the ballasting resistors, and yes, paralleled capacitors do add, but the max capacitance is per converter, so you should be alright.

WaveRider

Tue Oct 09 2018, 09:54AM

Registered Member #29 Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500

Generally, redundant power supplies use a series diode on the output to prevent reverse current from flowing in the event of failure of one power supply. No current limiting resistors should be needed as this defeats the purpose of using a high efficiency DC/DC converter! Good DC/DC converters have current limiting built in.

Sulaiman

Tue Oct 09 2018, 04:30PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Paralleling voltage regulators always risks oscillations,
and
60 Ohms x 250 mA = 15 V .... very inefficient,
I would probably aim to drop a voltage across each resistor equivalent to five or more times the voltage output differences,
i.e. if all outputs are X volts +/- Y volts, drop about 5.Y volts across the resistors.
e.g. in this case, 12 V +/- 1% = +/- 0.12 v
5 x 0.12 V = 0.6 V
0.6 V / 0.25 A = 2.4 Ohms ... use 2.7 Ohms each.
As each output has >=2 Ohms resistive in series there should be stability,
so you can have as much output capacitance as you like / can afford.

one diode per output to protect against a failed module is a good precaution.

I have to ask ...
the most efficient approach would be to control the heat/temperature difference applied to the TEGs and use the electrical output(s) directly to the load.
Or in your case, the speed of TEG cooling fans could be controlled ?

As a repair engineer, one of my first suspects on PCBs are dc:dc converters.
They live much longer if not stressed to maximum specifications.
For hobby use I recommend at least a 20% safety margin,
i.e. use 250 mA converters as if they are 200 mA converters.

Plasma

Tue Oct 09 2018, 11:49PM

Registered Member #61406 Joined: Thu Jan 05 2017, 11:31PM
Location:
Posts: 268

Thanks you three, updated the circuit. It mentioned in a aluminium electylight capacitor datasheet a life expected time of 1000 hours and then you need to refresh them.
How do you do that, 25 volt 100uF 280mA ripple current.

1539128990 61406 FT183071 Img 20181010 124206

Sulaiman

Wed Oct 10 2018, 02:06AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I think that you need to replace, not refresh, after the rated hours.
Lifetime is increased by reducing temperature, voltage and ripple current to below rated.
I usually use 5,000 or 10,000 hour, 105 C rated capacitors,
for hobby purposes the cost difference is usually negligible compared to overall cost, time, effort ....

The first '5A 0.65V' diode is not required;
if the TEG string fails then the entire module will fail anyway,
the 100 uF capacitor would not supply useful power for long even with the diode, if the TEGs fail.

5W zener diodes get extremely hot when dissipating 5 W.

Sometimes it is cheaper and easier to use a power transistor with a lower power zener diode to act as a high power zener diode,
e.g.

The simplest method is to not use more TEGs in series than the input of the dc:dc converter is rated for :)
TEGs can not give over-voltage spikes etc. worst case is open-circuit voltage per datasheets.

Patrick

Thu Oct 11 2018, 01:24AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Current-mode-control is the only way to really be sure of equal load sharing. Voltage-mode-control is just too likely to cause load shedding frome the many throttling back, and loading up the few or single one.

2Spoons

Thu Oct 11 2018, 02:34AM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615

electrolytic capacitor lifetime roughly doubles for every 10C drop in temperature from rated temp. E.g a 2000hr, 85C cap would have an expected life of 4000hr at 75C, 8000 hr at 65C, 16000 hr at 55C etc.
This is just a guideline - nothing more.

Plasma

Thu Oct 11 2018, 07:55PM

Registered Member #61406 Joined: Thu Jan 05 2017, 11:31PM
Location:
Posts: 268

Did a test for the first time with five in series, Patrick is current mode control built into the converter?
Thanks Suleiman for the help.
2Spoons selected a 8000 hour at 75 degrees

Patrick

Fri Oct 12 2018, 03:22AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

It's a integrated circuit each company has a series and so on. But the converter is built or modified to do it. It's generally done to ease component cost, size, and ensure reliability over years, and abuse.

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