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    low power high efficiency switching regulator (5V to -28V)

    Hello, I have a 5V power supply from which I want to produce -28V, @500uA max load current.
    I've tried using lt1617-1 from linear technology but the simulation at LTspice shows an RMS current consumption of about 7-8mA, which is not acceptable for my application.
    Any ideas on improving the circuit attached to lower overall current consumption (ideally around 1-2mA) or suggestions of any other implementations would be really helpful.
    Thank you.
    Click image for larger version. 

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    Re: low power high efficiency switching regulator (5V to -28V)

    Hi,

    but the simulation at LTspice shows an RMS current consumption of about 7-8mA,
    RMS current isn´t valid for a swtiching regulator´s (input or output) power consumption --> use average current.

    (RMS only makes sense for the inductor to calculate it´s power loss.)

    Klaus
    Please don´t contact me via PM, because there is no time to respond to them. No friend requests. Thank you.


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    •   Alt7th February 2018, 14:11

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    Re: low power high efficiency switching regulator (5V to -28V)

    If you want -28V 500uA then your 5V supply must provide at least 3mA average. Probably twice that to make up for losses.



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    Re: low power high efficiency switching regulator (5V to -28V)

    you could use a 555 CMOS timer and external fet to run this ckt at 100kHz say, this would give you the lowest losses in the driving / control ckt... the chokes are often a source of loss in these ckts - for lowest losses you need to specify an RM6 say of N87 or better material run at low peak flux 150mT max say...



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    Re: low power high efficiency switching regulator (5V to -28V)

    Here is a buck-boost converter, twin interleaved, astable operation. It draws average 3.7 mA from the 5v supply. It ignores losses due to parasitic resistances. The load is calculated as 56k ohms. Output is -28v to the load.

    Click image for larger version. 

Name:	twin astable buck-boost 5v to -28v 500uA.png 
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    •   Alt9th February 2018, 12:50

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    Re: low power high efficiency switching regulator (5V to -28V)

    Do you need -regulation- or just a roughly right
    negative voltage?

    My last design at my first employer was a DC-DC
    chip for "ringing SLIC" applications (negative boost
    converter). Meant to produce -48V but could be
    set arbitrarily. You might find similar and better
    (been over 15 years since then) options in the TI,
    Intersil/Renesas power management product lines'
    telecom offerings. If you wanted regulation.



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    Re: low power high efficiency switching regulator (5V to -28V)

    Here is a simple design based on an off-the-shelf gate drive transformer that gives should give good efficiency (~75 %). You will have some control over the output voltage using pot P1. The efficiency can be increased (~78 %) by omitting the snubbers, but you will have more EMC issues. The DC average value shown is with both snubbers in the circuit.



    •   Alt14th February 2018, 19:30

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    Re: low power high efficiency switching regulator (5V to -28V)

    Module selling for $10 (1Kpcs)
    https://www.eetimes.com/document.asp?doc_id=1287903

    Gonna be tough to beat that with a discrete parts-pile
    and PCB of your own cobbling.

    https://www.mouser.com/Maxim-Integra...oZwnwwZ1yxz1ec

    Roughly appropriate piece-part (uses a fancy looking
    xfmr) at $4.60 (qty=1)

    Maxim inverting dc-dc selector guide - w/ budgetary pricing even
    https://para.maximintegrated.com/en/...eg&tree=master

    I'm sure you can find others of similar description
    (Mr. Googlez was happy to cough up many hits for
    "slic dc-dc").



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    Re: low power high efficiency switching regulator (5V to -28V)

    E-design, RC snubber directly across T1 might release the need for D2. What do you think?



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    Re: low power high efficiency switching regulator (5V to -28V)

    The IC solution is obviously the best by far. The transformer in the design I posted cost $3 for one piece ($1.80/100pcs) from Digikey, and the rest should cost maybe $1-2 more, which can be a low-cost roll-your-own option. The OP never supplied more information regarding regulation requirement, which may make my proposal unacceptable anyway



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    Re: low power high efficiency switching regulator (5V to -28V)

    A sepic, per post #1 at 70-100kHz driving a mosfet from simple CMOS 555 control, will give low cost and very high efficiency for low loss chokes, and 45V schottky diode.
    Last edited by Easy peasy; 14th February 2018 at 21:28.



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    Re: low power high efficiency switching regulator (5V to -28V)

    Quote Originally Posted by CataM View Post
    E-design, RC snubber directly across T1 might release the need for D2. What do you think?
    It should not make a significant difference. Below are both results (with D2 out), and it is hard to spot any difference.

    In general, we can do away with D2 when using the gate transformer with guaranteed known maximum leakage inductance, but otherwise with sloppy construction you can cause a reverse breakdown in T1. I don't think it hurts to keep D2 as it consumes less than 40 uW of power in this instance.


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    Re: low power high efficiency switching regulator (5V to -28V)

    Quote Originally Posted by Easy peasy View Post
    A sepic, per post #1 at 70-100kHz driving a mosfet from simple CMOS 555 control, will give low cost and very high efficiency for low loss chokes, and 45V schottky diode.
    SEPIC can't produce negative outputs, maybe a buck-boost.



  14. #14
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    Re: low power high efficiency switching regulator (5V to -28V)

    Many of the chips available have high efficiency at rated full load, but not at very light loads.



    •   Alt14th February 2018, 22:39

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    Re: low power high efficiency switching regulator (5V to -28V)

    CataM: the sepic with o/p diode and L swapped over, per the fig in post #1 sometimes called a Zeta converter, does produce a neg output.

    Sepic, stands for Single ended primary inductance converter, and can be used to describe all such topologies, Sepic, Zeta, Cuk, etc


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    Re: low power high efficiency switching regulator (5V to -28V)

    If regulation is required we can use the spare gate to implement some burst-mode regulation. This will push the efficiency near/over 85%.



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