Cascading DC/DC Converters

Thread Starter

ulrichmuc

Joined Dec 1, 2021
4
I need two voltages +5 V and -5 V to power an analog multiplexer. Can I achieve this by cascading two
DC/DC converters as shown in the attachedcascade.jpg picture?
 

dl324

Joined Mar 30, 2015
14,712
I need two voltages +5 V and -5 V to power an analog multiplexer. Can I achieve this by cascading two
DC/DC converters as shown in the attached
No. These are drop-in replacements for 78xx linear regulators. The current coming out of the ground terminal will be low.

What is the current requirement from the -5V supply? Have you considered using an inverting switching regulator?

From Motorola:
1638399722685.png
You just adjust the feedback to get -5V.
 

ronsimpson

Joined Oct 7, 2019
2,173
What you are trying to do in #1 is cleaver but it will not work.
The bottom regulator needs to get power from +12V.
The top regulator needs to have it GND pin connected to some voltage. Usually ground. It will work if you connect to +5V to get +10V out.
The red arrows shows that current can flow from 10V to 0V or 5V to 0V but the blue arrow is not allowed.
1638402767788.png
 

DickCappels

Joined Aug 21, 2008
8,858
If you don’t need too much current for the “Negative” power supply the current can come from a load resistor. Maybe no efficient, but can you beat it for parts count?
 

ronsimpson

Joined Oct 7, 2019
2,173
You must be using a old or small Raspberry Pi. 5V 1A? I think you need a larger supply.
Next if looks like the -5V needs only 100uA at the most.
I see that Traco Power makes some supplies with +/-5V out. But the price is not good. I am using a +/- 5V supply from another company.

I think the next two supplies are interesting. Use one of these to make the -5V.
I found two power supplies like you wan to use but they can make a negative output. (most supplies you can not do this with)
1638412689641.png
1638412879233.png

A different company1638412924597.png
 

Thread Starter

ulrichmuc

Joined Dec 1, 2021
4
I now found an easy solution: there are DC/DC converters with two output voltages:
e.g. :
DPUN02M-05, 12 V -> +-5 V, 200 mA , 4,31 €
DLW05A-05 9-18 V ->+-5V, 500 mA, 13,60 €

Also, I could use a Pico instead of a PI to reduce power requirement.
 
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