Author Topic: Isolated 5V 5-10ma supply  (Read 4649 times)

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Offline ali_asadzadehTopic starter

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Isolated 5V 5-10ma supply
« on: December 05, 2019, 08:33:53 pm »
Hi,
I need to have 16 channel, isolated 5V 5-10ma supply, what's the cheapest solution to do that?
any Ideas? the input can be flexible,like 12V or 5V or 3.3V, the design should be small efficient and cheap, I was thinking about using EFD15 ferrite core with a 10 pin bobbin and generating 3-4 channel isolated voltage on each transformer, But I think maybe we have something better and smaller.
« Last Edit: December 05, 2019, 09:11:43 pm by ali_asadzadeh »
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Offline David Hess

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Re: Isolated 5V 5-10ma supply
« Reply #1 on: December 06, 2019, 05:04:42 pm »
A flyback switching regulator with multiple output windings is one way.  Better I think is to use a square wave inverter because then current is continuous allowing for a smaller transformer and lower noise.  If the lowest noise is required, then a sine wave inverter can be used with the associated loss of efficiency.
 

Offline ali_asadzadehTopic starter

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Re: Isolated 5V 5-10ma supply
« Reply #2 on: December 07, 2019, 06:17:13 am »
Since the output current is so small, I was thinking of a simple circuit with open loop, and use low cost post LDO's to regulate the outputs. in this way maybe I could save one aux winding and add one more output.
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Offline TurboTom

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Re: Isolated 5V 5-10ma supply
« Reply #3 on: December 07, 2019, 07:37:24 am »
Isolation requirements? Capacitive coupling? Depending on the application, there are many options. I once played with the idea to use the smallest available mains-rated symmetric common mode chokes with a common exciter to provide a multitude of well-isolated outputs. The sensitivity of the application required a sine drive and as little stray capacitance as possible. A few quick-and-dirty proof of concept tests showed this approach to be quite promising but since the expense of the PSU was of little importance, finally the decision was reached to use off-the-shelf DC/DC converters (which coused their own, different problems...of course  ::)).

If you plan to follow this road, please be aware that the exciter has to be perfectly symmetric (capacitive coupling) to avoid core saturation (there is no air gap in the ferrite cores) and that the secondary rectifier also has to provide a symmetrical load (full bridge or voltage doubler). Otherwise, this concept works really well and requires little post-regulation since it's a pure forward converter with good output voltage stability. If a single exciter is going to be used for all channels, it's good practice to use an individual DC blocking capacitor for each transformer to minimize mutual coupling.
 

Offline JohnnyBerg

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Re: Isolated 5V 5-10ma supply
« Reply #4 on: December 07, 2019, 08:42:56 am »
You can buy Isolated DC DC converters ready made:

https://www.digikey.com/product-detail/en/recom-power/RFM-0505S/945-3159-ND/8550767

On EBay or Aliexpress even cheaper. No need to reinvent the wheel.
 

Offline TurboTom

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Re: Isolated 5V 5-10ma supply
« Reply #5 on: December 07, 2019, 09:28:30 am »
You can buy Isolated DC DC converters ready made:

https://www.digikey.com/product-detail/en/recom-power/RFM-0505S/945-3159-ND/8550767

On EBay or Aliexpress even cheaper. No need to reinvent the wheel.

Depends on the requirements. If you are involved in designing an electrical measurement system with multiple channels (cannot go into more detail) that's supposed to match CAT3 / 1000V or possibly higher, your choices are getting sparse and expensive. Other requirements may make spinning your own design also appear to be not such a bad choice. Just look at the isolation clearance and the data (test isolation voltage 1kV for 1s, less voltage for repeated or prolonged tests...) of the device you linked. It's also interesting to read publications on long-term behavior and deterioration of isolation barriers. All this puts designing high-reliability devices as mentioned above under a completly different light, not only regarding isolated power supplies but also the data links.
 

Offline JohnnyBerg

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Re: Isolated 5V 5-10ma supply
« Reply #6 on: December 07, 2019, 09:52:33 am »
Depends on the requirements.  ...

Op had a DIY solution with EFD15 ferrite core with a 10 pin bobbin in mind .. Any industrial DC DC converter will beat that.
 

Offline David Hess

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Re: Isolated 5V 5-10ma supply
« Reply #7 on: December 07, 2019, 05:29:09 pm »
Since the output current is so small, I was thinking of a simple circuit with open loop, and use low cost post LDO's to regulate the outputs. in this way maybe I could save one aux winding and add one more output.

With an inverter, there is a good chance you can get away with either no regulation or only primary side regulation.  If diodes are used for rectification of the secondaries, then primary side regulation can be done by including compensating diodes in the feedback loop.  Alternatively with only a 5 volt output and square wave drive, bipolar transistors or MOSFETs can synchronously rectify the outputs without special drivers.

For those who like to abuse parts, CMOS inverters can be wired backward to make a full wave synchronous rectifier.  Drive goes to the inputs and outputs and rectified DC is pulled from the supply pins.  Use several gates in parallel to decrease series resistance.

The flyback configuration always requires regulation of the pulse width modulator and the outputs roughly track each other.
 

Offline ali_asadzadehTopic starter

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Re: Isolated 5V 5-10ma supply
« Reply #8 on: December 08, 2019, 06:11:31 am »
Quote
Isolation requirements? Capacitive coupling? Depending on the application, there are many options. I once played with the idea to use the smallest available mains-rated symmetric common mode chokes with a common exciter to provide a multitude of well-isolated outputs. The sensitivity of the application required a sine drive and as little stray capacitance as possible. A few quick-and-dirty proof of concept tests showed this approach to be quite promising but since the expense of the PSU was of little importance, finally the decision was reached to use off-the-shelf DC/DC converters (which coused their own, different problems...of course  ::)).

If you plan to follow this road, please be aware that the exciter has to be perfectly symmetric (capacitive coupling) to avoid core saturation (there is no air gap in the ferrite cores) and that the secondary rectifier also has to provide a symmetrical load (full bridge or voltage doubler). Otherwise, this concept works really well and requires little post-regulation since it's a pure forward converter with good output voltage stability. If a single exciter is going to be used for all channels, it's good practice to use an individual DC blocking capacitor for each transformer to minimize mutual coupling.
Sound a good plan, is there any schematics or info that you could share?
Quote
With an inverter, there is a good chance you can get away with either no regulation or only primary side regulation.  If diodes are used for rectification of the secondaries, then primary side regulation can be done by including compensating diodes in the feedback loop.  Alternatively with only a 5 volt output and square wave drive, bipolar transistors or MOSFETs can synchronously rectify the outputs without special drivers.

For those who like to abuse parts, CMOS inverters can be wired backward to make a full wave synchronous rectifier.  Drive goes to the inputs and outputs and rectified DC is pulled from the supply pins.  Use several gates in parallel to decrease series resistance.

The flyback configuration always requires regulation of the pulse width modulator and the outputs roughly track each other.
Thanks david, I was thinking of driving the primary with a dirt cheap DC motor driver part, something like this
https://lcsc.com/product-detail/Motor-Drivers_MX116_C113342.html
do you suggest any circuit for your idea? since my product would be manufactured in volumes and each single unit is cost sensitive and should have 16 channel of isolated 5V supplies, I should design something reliable and dirt cheap( maybe with all chinese parts from lcsc)
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Offline TurboTom

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Re: Isolated 5V 5-10ma supply
« Reply #9 on: December 08, 2019, 10:22:15 am »
There isn't much of a schematic regarding this experiment -- I used an AWG to generate the drive signal, amplified it by a DIY wide-band amplifier (LT1210 based, an off-the-shelf unit would have done just as well), and through a coupling capacitor (MLCC 4.7µF) to a common mode choke (CoilCraft SBU9-2820R5L_ would be a decent choice). On the secondary, a schottky bridge rectifier was used along with some RFI reduction and voltage post processing stuff (the PSU had to power a sensitive analog front end with considerable power consumption -- some 5W IIRC).

In your case, you could well use the mentioned MX116 motor driver to power the system, but I would use it single-ended and utilize the capacitive coupling mentioned above. If your power requirements are really only 10mA@5V, you can use a single driver to feed all 16 transformers (8 on each output) and driving both inputs with a 70~100kHz square wave of opposite polarity. Use a voltage-doubling rectifier (2* small Schottky, 2* 1µF MLCC) and if the stability and load regulation isn't too important, you can adjust the supply voltage of your driver to provide the proper output voltage (loaded). If inrush/switching currents are a problem, you may want to place inductances in series with the diodes, but due to the very small required power, I'ld initially try without.

Of course, that's just a quick-and-dirty suggestion that would probably be a reasonable starting point that will provide very good high-pot isolating characteristics.
« Last Edit: December 08, 2019, 10:28:32 am by TurboTom »
 

Offline SiliconWizard

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Re: Isolated 5V 5-10ma supply
« Reply #10 on: December 08, 2019, 04:45:28 pm »
Also, maybe you could specify your isolation requirements (voltage) first (primary to secondaries and secondary to secondary...)

Then of course the solution would also depend on factors like cost vs. quantity, area/volume taken, EMI, ...
 

Offline David Hess

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Re: Isolated 5V 5-10ma supply
« Reply #11 on: December 09, 2019, 02:45:53 am »
Thanks david, I was thinking of driving the primary with a dirt cheap DC motor driver part, something like this
https://lcsc.com/product-detail/Motor-Drivers_MX116_C113342.html

I do not see any specifications for speed.  Above 20 kHz is typical.  Some gate driver ICs are suitable.

Quote
do you suggest any circuit for your idea? since my product would be manufactured in volumes and each single unit is cost sensitive and should have 16 channel of isolated 5V supplies, I should design something reliable and dirt cheap( maybe with all chinese parts from lcsc)

The transformer will be the difficult part.  Gate drive and pulse transformers are suitable if you want to get something from Coilcraft or someone to experiment with.
 

Offline rvalente

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Re: Isolated 5V 5-10ma supply
« Reply #12 on: December 10, 2019, 12:10:48 am »
This is siemens S7 1200 RS485 board, you can see their aproach using the MAX253 to drive the trafo.

You coud do something similar, with multiple winding in the secondary. Lets say 4 outputs per trafo.



https://datasheets.maximintegrated.com/en/ds/MAX253.pdf

The transformer will be the difficult part.  Gate drive and pulse transformers are suitable if you want to get something from Coilcraft or someone to experiment with.


Using pulse transformers would be very elegant design, since some of them are 1:1

« Last Edit: December 10, 2019, 12:12:51 am by rvalente »
 

Offline David Hess

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Re: Isolated 5V 5-10ma supply
« Reply #13 on: December 10, 2019, 12:31:36 am »
Using pulse transformers would be very elegant design, since some of them are 1:1

A lot of standard pulse transformers are trifillar 1:1:1 and some are quadfillar 1:1:1:1.

https://www.mouser.com/Power/Transformers/Pulse-Transformers/_/N-8u9sj?P=1yqdy9r
 

Offline rvalente

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Re: Isolated 5V 5-10ma supply
« Reply #14 on: December 10, 2019, 01:54:26 am »
Ethernet transformer could be pretty cheap due to the high production volumes
 

Offline ali_asadzadehTopic starter

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Re: Isolated 5V 5-10ma supply
« Reply #15 on: December 10, 2019, 06:41:38 am »
Thanks for suggestions, The isolation from primary to secondary is about 1Kv and secondary to secondary is about 500V, the links for the transformers are useful, but they would fail the production due to the cost! who would pay 7.7$ on a single transformer |O |O |O we have 0.03MUC and 0.07 DC motor driver, we should be able to find some sub 0.1$ transformer too.
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Online Marco

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Re: Isolated 5V 5-10ma supply
« Reply #17 on: December 10, 2019, 08:54:17 am »
we should be able to find some sub 0.1$ transformer too.

Chinese UU9.8 common mode filters seem to get close to that.
 

Offline David Hess

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Re: Isolated 5V 5-10ma supply
« Reply #18 on: December 10, 2019, 08:27:09 pm »
Ethernet transformer could be pretty cheap due to the high production volumes

Ethernet transformers could work but I have not tried them myself.  The inverter frequency would need to be higher but should still be feasible.
 
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Offline GreggD

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Re: Isolated 5V 5-10ma supply
« Reply #19 on: December 10, 2019, 09:01:00 pm »
B0505S-1W

200mA out
About 68% eff
Requires a 10% load
Keep load capacitors below 47uF

10pcs for $7.20 USD, free shipping

https://www.aliexpress.com/wholesale?catId=0&initiative_id=SB_20191210125104&SearchText=b0505s

I just ordered boards today from JLCPCB that have one on it as well as the mentioned UU9.8 choke.
 

Offline ali_asadzadehTopic starter

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Re: Isolated 5V 5-10ma supply
« Reply #20 on: December 11, 2019, 09:27:58 am »
Quote
Chinese UU9.8 common mode filters seem to get close to that.
Do you have any link?
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Online Marco

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Re: Isolated 5V 5-10ma supply
« Reply #21 on: December 11, 2019, 12:56:03 pm »
On phone now and not too going to look for it, but yesterday I saw an ebay offer for 100 pcs for 13$.

These should be able to work at relatively low frequencies.
 

Online Gyro

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Re: Isolated 5V 5-10ma supply
« Reply #22 on: December 11, 2019, 01:16:11 pm »
This is siemens S7 1200 RS485 board, you can see their aproach using the MAX253 to drive the trafo.

You coud do something similar, with multiple winding in the secondary. Lets say 4 outputs per trafo.



https://datasheets.maximintegrated.com/en/ds/MAX253.pdf

The transformer will be the difficult part.  Gate drive and pulse transformers are suitable if you want to get something from Coilcraft or someone to experiment with.


Using pulse transformers would be very elegant design, since some of them are 1:1

The MAX253 is a very handy little chip for such applications. It's also that rarest of beasts, a Maxim chip that has been around for at least 20 years!  ;D
« Last Edit: December 11, 2019, 01:18:12 pm by Gyro »
Best Regards, Chris
 

Offline ali_asadzadehTopic starter

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Re: Isolated 5V 5-10ma supply
« Reply #23 on: December 11, 2019, 03:15:20 pm »
I have done it! and it's working better than expected for the first try! the efficiency is around 50-60%, the output with no load is around 12V and with a 7ma load, it would drop to 7V, so Now I should find a cheap SOT23-6 LDO that would generate 5V and could accept input up to 20V, and a working temp of -40~85C

The whole thing cost is under 0.5USD in single unit QTY ;)



The bonus point for this circuit is, that I have another none isolated boosted output too! so I have 4 separate isolated outputs and a single none-isolated output ;)
« Last Edit: December 11, 2019, 03:25:32 pm by ali_asadzadeh »
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Online Marco

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Re: Isolated 5V 5-10ma supply
« Reply #24 on: December 11, 2019, 07:54:46 pm »
Here's the ebay listing for the common mode chokes.
 


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