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Achieving 98% efficiency in a boost converter

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  Achieving 98% efficiency in a boost converter Winfield Hill <hill@rowland.harvard.edu> - 2016-11-09 11:00 -0800
    Re: Achieving 98% efficiency in a boost converter sea moss <danluster81@gmail.com> - 2016-11-09 20:12 -0800
    Re: Achieving 98% efficiency in a boost converter "Tim Williams" <tiwill@seventransistorlabs.com> - 2016-11-09 22:27 -0600
      Re: Achieving 98% efficiency in a boost converter bill.sloman@ieee.org - 2016-11-10 04:07 -0800
    Re: Achieving 98% efficiency in a boost converter Winfield Hill <hill@rowland.harvard.edu> - 2016-11-11 02:08 -0800
    Re: Achieving 98% efficiency in a boost converter dagmargoodboat@yahoo.com - 2016-11-11 13:44 -0800
      Re: Achieving 98% efficiency in a boost converter, inducement Winfield Hill <hill@rowland.harvard.edu> - 2016-11-11 19:05 -0800
        Re: Achieving 98% efficiency in a boost converter, datasheets Winfield Hill <hill@rowland.harvard.edu> - 2016-11-12 04:06 -0800
          Re: Achieving 98% efficiency in a boost converter, datasheets dagmargoodboat@yahoo.com - 2016-11-12 05:40 -0800
            Re: Achieving 98% efficiency in a boost converter, datasheets dagmargoodboat@yahoo.com - 2016-11-12 05:54 -0800
            Re: Achieving 98% efficiency in a boost converter, datasheets Winfield Hill <hill@rowland.harvard.edu> - 2016-11-12 06:33 -0800
              Re: Achieving 98% efficiency in a boost converter, datasheets Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> - 2016-11-12 07:58 -0700
                Re: Achieving 98% efficiency in a boost converter, datasheets Winfield Hill <hill@rowland.harvard.edu> - 2016-11-15 05:29 -0800
              Re: Achieving 98% efficiency in a boost converter, datasheets bill.sloman@ieee.org - 2016-11-12 20:08 -0800
                Re: Achieving 98% efficiency in a boost converter, datasheets Winfield Hill <hill@rowland.harvard.edu> - 2016-11-13 18:29 -0800
            Re: Achieving 98% efficiency in a boost converter, datasheets Clifford Heath <no.spam@please.net> - 2016-11-13 09:08 +1100
              Re: Achieving 98% efficiency in a boost converter, datasheets dagmargoodboat@yahoo.com - 2016-11-12 16:48 -0800
                Re: Achieving 98% efficiency in a boost converter, datasheets Winfield Hill <hill@rowland.harvard.edu> - 2016-11-16 18:00 -0800
                  Re: Achieving 98% efficiency in a boost converter, datasheets bill.sloman@ieee.org - 2016-11-17 18:41 -0800
                  Re: Achieving 98% efficiency in a boost converter, datasheets dagmargoodboat@yahoo.com - 2016-11-18 09:37 -0800
          Re: Achieving 98% efficiency in a boost converter, datasheets Winfield Hill <hill@rowland.harvard.edu> - 2016-11-15 05:05 -0800
            Re: Achieving 98% efficiency in a boost converter, datasheets Winfield Hill <hill@rowland.harvard.edu> - 2016-11-18 09:13 -0800

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#434559 — Achieving 98% efficiency in a boost converter

Anybody care to speculate about the most important aspects to consider when
trying to achieve 98% efficiency in a low-power (20mW) boost converter?  Most
available step-up converter circuits seem to perform in the 85 to 90% region.

I'm finishing a PCB "test bed" layout, which allows jumper-selection of
different inductors, synchronous switches, and adjustable controller parameters.
I'm hoping not to forget something important before sending it off to the PCB
house.  Just for fun I thought I'd make it in the form of an Arduino shield;
giving an option for the processor to control the parameters and take
measurements.


-- 
 Thanks,
    - Win

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#434630

For that efficiency and low power level, I would throw away regulation and go with a switched cap converter.  I'm guessing that's out of the question here though.

For this board my first two guesses would be switches' gate charge losses and the inductor's DCR.

Sounds like a cool project!

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#434636

Sadly, your approach has an irony: the jumpers will add leakage inductance, 
which you'll have to identify as a source of loss. :-(

The quiescent current of the control circuit itself will be a considerable 
fraction of that, even for very-fine-feature-size ICs.  Perhaps there are 
some energy-harvesting controllers that idle that low, but gee.

Is this from scratch, or testing commercial ICs?

Note that a synchronous converter needs to be very closely tuned.  Nearly 
zero dead time.  No time to let body diodes conduct (that's loss, and 
probably step recovery behavior too), no time to draw shoot-through current 
(which you'll want to assist by adding supply inductance).  Few commercial 
controllers and regulators are designed this way, instead using a cautious 
~30ns or so.

Tim

-- 
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
Website: http://seventransistorlabs.com

"Winfield Hill" <hill@rowland.harvard.edu> wrote in message 
news:nvvrni02lg2@drn.newsguy.com...
> Anybody care to speculate about the most important aspects to consider 
> when
> trying to achieve 98% efficiency in a low-power (20mW) boost converter? 
> Most
> available step-up converter circuits seem to perform in the 85 to 90% 
> region.
>
> I'm finishing a PCB "test bed" layout, which allows jumper-selection of
> different inductors, synchronous switches, and adjustable controller 
> parameters.
> I'm hoping not to forget something important before sending it off to the 
> PCB
> house.  Just for fun I thought I'd make it in the form of an Arduino 
> shield;
> giving an option for the processor to control the parameters and take
> measurements.
>
>
> -- 
> Thanks,
>    - Win 

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#434668

On Thursday, November 10, 2016 at 3:27:37 PM UTC+11, Tim Williams wrote:
> Sadly, your approach has an irony: the jumpers will add leakage inductance, 
> which you'll have to identify as a source of loss. :-(
> 
> The quiescent current of the control circuit itself will be a considerable 
> fraction of that, even for very-fine-feature-size ICs.  Perhaps there are 
> some energy-harvesting controllers that idle that low, but gee.
> 
> Is this from scratch, or testing commercial ICs?
> 
> Note that a synchronous converter needs to be very closely tuned.  Nearly 
> zero dead time.  No time to let body diodes conduct (that's loss, and 
> probably step recovery behavior too), no time to draw shoot-through current 
> (which you'll want to assist by adding supply inductance).  Few commercial 
> controllers and regulators are designed this way, instead using a cautious 
> ~30ns or so.

The Baxendall Class-D oscillator is self-resonant, so doesn't have to be "closely tuned" and switches when there is zero current through the switches, eliminating switching losses.

In the rudimentary MOS-FET-driven version 

http://sophia-electronica.com/Baxandall_parallel-resonant_Class-D_oscillator1.htm

about the only place to waste power is in the damping resistors on the MOS-FET gates, and the series resistance of the inductor and the transformer windings.

Keeping those losses low might bulk up the cores a bit.

Leakage inductance doesn't come into it

I've no idea whether it could make 98%, but Jim Williams got close to 95% with messier and cheaper circuits.

You might have have floating windings to drive synchronous inverter switches on the output, and might all end up a bit bulkier than Win has in mind.

-- 
Bill Sloman, Sydney

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#434823

Winfield Hill wrote...
>
> Anybody care to speculate about the most important aspects
> to consider when trying to achieve 98% efficiency in a
> low-power (20mW) boost converter?  Most available step-up
> converter circuits seem to perform in the 85 to 90% region.
>
> I'm finishing a PCB "test bed" layout, which allows jumper-
> selection of different inductors, synchronous switches, and
> adjustable controller parameters.  I'm hoping not to forget
> something important before sending it off to the PCB house.
> Just for fun I thought I'd make it in the form of an Arduino
> shield; giving an option for the processor to control the
> parameters and take measurements.

 Here's my draft schematic and PCB layout.
 https://www.dropbox.com/sh/aj1tlto0fa6w6ty/AADJ9BWmCQE_ed7NHp60PgMja?dl=0


-- 
 Thanks,
    - Win

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#434963

On Wednesday, November 9, 2016 at 6:54:04 PM UTC-5, Winfield Hill wrote:
> Anybody care to speculate about the most important aspects to consider when
> trying to achieve 98% efficiency in a low-power (20mW) boost converter?  Most
> available step-up converter circuits seem to perform in the 85 to 90% region.
> 
> I'm finishing a PCB "test bed" layout, which allows jumper-selection of
> different inductors, synchronous switches, and adjustable controller parameters.
> I'm hoping not to forget something important before sending it off to the PCB
> house.  Just for fun I thought I'd make it in the form of an Arduino shield;
> giving an option for the processor to control the parameters and take
> measurements.
> 
> 
> -- 
>  Thanks,
>     - Win

Too many degrees of freedom to answer you Win--it depends very much on
the Vout/Vin ratio, for one, and the absolute values of Vin and Vout,
too.

E.g., I designed a 3uW boost (0.6-to-3V @ 1uA). I had to use all jelly-bean
discretes since there were no Iq-suitable ICs, and BJTs due to the low
input voltage.  Efficiency was ~70%, IIRC.

Where the input voltage is low and currents high, switch saturation voltage
is limiting.  Where the output voltage is low, rectification losses are a
pain; a Schottky diode's 5% loss was usually better than a synchronous
rectifier at 6V output, after switching losses and rectifier drive and
added complexity to gain perhaps 2% extra efficiency were factored in.

Breaking your problem into pieces, fundamentally, getting 98% overall
efficiency is a matter of storing over 99% of the input energy in the
inductor each time you charge it, and delivering over 99% of the
stored energy to the load each flyback cycle.

Reducing further, charging, the inductor has to store at least 99 times
the switch's gate drive, switching loss, and conduction losses.  The
inductor's d.c.r. loss factors in too.  Core losses were minor, IME.

The same analysis can be applied to the flyback portion of the cycle, and
the 2% loss budget can be distributed across the two phases as you see
fit.

I suspect you'll find your parameters produce optimal efficiency with
MOSFET switches infrequently operated, large-cored high-valued inductors,
short bursts or single-cycles of activity, and long periods quiescent.
But YMMV.

I hope these musings are of some use...

Cheers,
James Arthur

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#435022 — Re: Achieving 98% efficiency in a boost converter, inducement

dagmargoodboat@yahoo.com wrote...
>
>On Wednesday, November 9, 2016 at 6:54:04 PM UTC-5, Winfield Hill wrote:
>> Anybody care to speculate about the most important aspects to consider when
>> trying to achieve 98% efficiency in a low-power (20mW) boost converter?  Most
>> available step-up converter circuits seem to perform in the 85 to 90% region.
>> 
>> I'm finishing a PCB "test bed" layout, which allows jumper-selection of
>>different inductors, synchronous switches, and adjustable controller parameters.
>> I'm hoping not to forget something important before sending it off to the PCB
>> house.  Just for fun I thought I'd make it in the form of an Arduino shield;
>> giving an option for the processor to control the parameters and take
>> measurements.
>
> Too many degrees of freedom to answer you Win [ snip ]

 Thanks, your comments are right on target and can kick
 off the discussion I was hoping to see.  I'll get into
 that conversation tomorrow.  But in the meantime I've
 updated my draft schematic and PCB layout, adding more
 features and items relevant to the discussion.
 https://www.dropbox.com/sh/aj1tlto0fa6w6ty/AADJ9BWmCQE_ed7NHp60PgMja?dl=0

 Here's an inducement: anyone who'd like to experiment
 with issues and concepts embedded in the RIS-767
 board, I'll send you two blank PCBs to play with.
 You just have to talk about what you learn, should
 you get around to learning anything!


-- 
 Thanks,
    - Win

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#435063 — Re: Achieving 98% efficiency in a boost converter, datasheets

Winfield Hill wrote...
> dagmargoodboat@yahoo.com wrote...
>>
>> Too many degrees of freedom to answer you Win [ snip ]
>
> Thanks, your comments are right on target and can kick
> off the discussion I was hoping to see.  I'll get into
> that conversation tomorrow.  But in the meantime I've
> updated my draft schematic and PCB layout, adding more
> features and items relevant to the discussion.
> https://www.dropbox.com/sh/aj1tlto0fa6w6ty/AADJ9BWmCQE_ed7NHp60PgMja?dl=0
>
> Here's an inducement: anyone who'd like to experiment
> with issues and concepts embedded in the RIS-767
> board, I'll send you two blank PCBs to play with.
> You just have to talk about what you learn, should
> you get around to learning anything!

 I've added a folder of datasheets to dropbox.


-- 
 Thanks,
    - Win

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#435072 — Re: Achieving 98% efficiency in a boost converter, datasheets

On Saturday, November 12, 2016 at 7:06:58 AM UTC-5, Winfield Hill wrote:
> Winfield Hill wrote...
> > dagmargoodboat@yahoo.com wrote...
> >>
> >> Too many degrees of freedom to answer you Win [ snip ]
> >
> > Thanks, your comments are right on target and can kick
> > off the discussion I was hoping to see.  I'll get into
> > that conversation tomorrow.  But in the meantime I've
> > updated my draft schematic and PCB layout, adding more
> > features and items relevant to the discussion.
> > https://www.dropbox.com/sh/aj1tlto0fa6w6ty/AADJ9BWmCQE_ed7NHp60PgMja?dl=0
> >
> > Here's an inducement: anyone who'd like to experiment
> > with issues and concepts embedded in the RIS-767
> > board, I'll send you two blank PCBs to play with.
> > You just have to talk about what you learn, should
> > you get around to learning anything!
> 
>  I've added a folder of datasheets to dropbox.
> 
> 
> -- 
>  Thanks,
>     - Win

You're going to wind up with a while bunch of trade-offs, such as larger and
larger FETs reduce conduction loss, but take more power to drive;
higher inductance lowers i^2*r losses from peak currents, but increases
the parasitic capacitance you have to drive, etc.

I designed a ~120W two-stage switcher a couple years ago where maximum
efficiency was essential for heat management, as the unit was in still
air in a small, sealed box, & subject to heating by direct sunlight.  I
wrote a spreadsheet to model the losses given the various parameters,
which helped me choose FETs & make other decisions.  A little messy and
crude, but predicted actual losses quite accurately.

At 20mW you'll have the advantage of almost ideal capacitors and plenty
of capacitance for storage.

At Rds(on)=.065 ohms, the FDN337 looks like overkill.

Please pardon me as I doodle here...

A few calculations:
1) Conduction loss. For 20mW @ n=98%, total loss budget is 400uW.  Assuming
 Vin=4V and n=98%, i.in(avg)=5mA; i^2 * Rds(on) = 1.6uW for a perfect,
 zero-ripple-current infinitely-fast continuous-mode switcher, or four
 times that for a boundary-mode design with twice the peak iL.

2) Gate drive loss: If the gate charge is 7nC driven at 4V, and we start
 off allocating it 1/10th the total loss budget...that's 40uW @ 4V, or
 10uA(avg), which means we could afford to drive it at f=i/2q=700Hz. Ballpark.

3) Output capacitance: Assuming Vout=8V (I don't know the actual), 20mW is
 2.5mA load.  For 50mV output ripple and 700Hz update rate, we calculate
 the necessary filter cap:
     C = i * dt / dV = 2.5mA*(1/700)s / 50mV = 70uF.

4) If we budgeted a maximum permissible conduction loss, i^2 * ( Vds(on) +
 dcr(L) ), that fixes a max. peak current i.pk. From i.pk and Vin and our
 output ripple voltage spec. and the output capacitance, we can compute
 the minimum inductor value.  The inductor value and i.pk then give us
 the inductor charging time, i.e., the switch on-time.

Interesting--the ripple voltage depends ultimately on the available
output capacitance.  After picking a maximal practical capacitor, given
the output capacitance available, the maximum allowable output ripple
voltage, and the inductor value, you can calculate the peak inductor
current needed for any given switching frequency, and from that, the FET
conduction losses.

98% is going to be fun!

Cheers,
James Arthur

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#435074 — Re: Achieving 98% efficiency in a boost converter, datasheets

On Saturday, November 12, 2016 at 8:40:48 AM UTC-5, dagmarg...@yahoo.com wrote:
> On Saturday, November 12, 2016 at 7:06:58 AM UTC-5, Winfield Hill wrote:
> > Winfield Hill wrote...
> > > dagmargoodboat@yahoo.com wrote...
> > >>
> > >> Too many degrees of freedom to answer you Win [ snip ]
> > >
> > > Thanks, your comments are right on target and can kick
> > > off the discussion I was hoping to see.  I'll get into
> > > that conversation tomorrow.  But in the meantime I've
> > > updated my draft schematic and PCB layout, adding more
> > > features and items relevant to the discussion.
> > > https://www.dropbox.com/sh/aj1tlto0fa6w6ty/AADJ9BWmCQE_ed7NHp60PgMja?dl=0
> > >
> > > Here's an inducement: anyone who'd like to experiment
> > > with issues and concepts embedded in the RIS-767
> > > board, I'll send you two blank PCBs to play with.
> > > You just have to talk about what you learn, should
> > > you get around to learning anything!
> > 
> >  I've added a folder of datasheets to dropbox.
> > 
> > 
> > -- 
> >  Thanks,
> >     - Win
> 
> You're going to wind up with a bunch of trade-offs, such as larger and
> larger FETs reduce conduction loss, but take more power to drive;
> higher inductance lowers i^2*r losses from peak currents, but increases
> the parasitic capacitance you have to drive, etc.
> 
> I designed a ~120W two-stage switcher a couple years ago where maximum
> efficiency was essential for heat management, as the unit was in still
> air in a small, sealed box, & subject to heating by direct sunlight.  I
> wrote a spreadsheet to model the losses given the various parameters,
> which helped me choose FETs & make other decisions.  A little messy and
> crude, but predicted actual losses quite accurately.
> 
> At 20mW you'll have the advantage of almost ideal capacitors and plenty
> of capacitance for storage.
> 
> At Rds(on)=.065 ohms, the FDN337 looks like overkill.
> 
> Please pardon me as I doodle here...
> 
> A few calculations:
> 1) Conduction loss. For 20mW @ n=98%, total loss budget is 400uW.  Assuming
>  Vin=4V and n=98%, i.in(avg)=5mA; i^2 * Rds(on) = 1.6uW for a perfect,
>  zero-ripple-current infinitely-fast continuous-mode switcher, or four
>  times that for a boundary-mode design with twice the peak iL.
> 
> 2) Gate drive loss: If the gate charge is 7nC driven at 4V, and we start
>  off allocating it 1/10th the total loss budget...that's 40uW @ 4V, or
>  10uA(avg), which means we could afford to drive it at f=i/2q=700Hz. Ballpark.
> 
> 3) Output capacitance: Assuming Vout=8V (I don't know the actual), 20mW is
>  2.5mA load.  For 50mV output ripple and 700Hz update rate, we calculate
>  the necessary filter cap:
>      C = i * dt / dV = 2.5mA*(1/700)s / 50mV = 70uF.
> 
> 4) If we budgeted a maximum permissible conduction loss, i^2 * ( Vds(on) +
>  dcr(L) ), that fixes a max. peak current i.pk. From i.pk and Vin and our
>  output ripple voltage spec. and the output capacitance, we can compute
>  the minimum inductor value.  The inductor value and i.pk then give us
>  the inductor charging time, i.e., the switch on-time.
> 
> Interesting--the ripple voltage depends ultimately on the available
> output capacitance.  After picking a maximal practical capacitor, given
> the output capacitance available, the maximum allowable output ripple
> voltage, and the inductor value, you can calculate the peak inductor
> current needed for any given switching frequency, and from that, the FET
> conduction losses.
> 
> 98% is going to be fun!
> 
> Cheers,
> James Arthur

Oh, I forgot to factor in that FET conduction losses are reduced by the
duty factor, both for the main switch and the synchronous rectifier;
dcr(L) matters twice, both for charging and discharging.

All this doodling suggests the start of a spreadsheet to me with Vin,
Vout, n, P, Vripple, Rds(on), Qg, dcr(L), and a few other inputs...

Cheers,
James Arthur

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#435082 — Re: Achieving 98% efficiency in a boost converter, datasheets

dagmargoodboat@yahoo.com wrote...
>
> At Rds(on)=.065 ohms, the FDN337 looks like overkill.

 I'm sorry for not yet taking the time to introduce
 and discuss my project in more detail.  I'm still
 postponing that, but will say a few quick things:

 1) My initial and primary goal is to optimize a
 2.0 to 2.4-volt boost converter running at 10mA,
 or 24mW, with no more than 0.5mW of loss.  For
 that purpose the small TS5A3159 spdt switch,
 0.8 ohms and low capacitance, should be good.
 There are also even smaller parts in the same
 sot-23 package layout.

 2) Other choices on the PCB are to allow for
 playing around with higher optimal currents.
 I figured, once the generalized PWM hardware
 was in place, why not add a provision for
 larger switches and inductors.  I'm going to
 add to the discrete MOSFET footprint choices.

 3) Since this is just a test-bed breadboard,
 I separated the power consumption for PWM
 control and driving FETs, from the inductor
 and switch losses.  The switch losses still
 include f C V^2 of switch capacitance loss,
 but not the gate-driving power consumption.

 4) I'm still looking hard for good inductor
 candidates to add to the PCB.  Most have
 appallingly-high losses, and unfortunately
 these are not detailed on their datasheets.


-- 
 Thanks,
    - Win

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#435089 — Re: Achieving 98% efficiency in a boost converter, datasheets

On 12 Nov 2016 06:33:28 -0800, Winfield Hill
<hill@rowland.harvard.edu> wrote:

>dagmargoodboat@yahoo.com wrote...
>>
>> At Rds(on)=.065 ohms, the FDN337 looks like overkill.
>
> I'm sorry for not yet taking the time to introduce
> and discuss my project in more detail.  I'm still
> postponing that, but will say a few quick things:
>
> 1) My initial and primary goal is to optimize a
> 2.0 to 2.4-volt boost converter running at 10mA,
> or 24mW, with no more than 0.5mW of loss.  For
> that purpose the small TS5A3159 spdt switch,
> 0.8 ohms and low capacitance, should be good.
> There are also even smaller parts in the same
> sot-23 package layout.
>
> 2) Other choices on the PCB are to allow for
> playing around with higher optimal currents.
> I figured, once the generalized PWM hardware
> was in place, why not add a provision for
> larger switches and inductors.  I'm going to
> add to the discrete MOSFET footprint choices.
>
> 3) Since this is just a test-bed breadboard,
> I separated the power consumption for PWM
> control and driving FETs, from the inductor
> and switch losses.  The switch losses still
> include f C V^2 of switch capacitance loss,
> but not the gate-driving power consumption.
>
> 4) I'm still looking hard for good inductor
> candidates to add to the PCB.  Most have
> appallingly-high losses, and unfortunately
> these are not detailed on their datasheets.

Large inductor and slow switching speed?
		
                                        ...Jim Thompson
-- 
| James E.Thompson                                 |    mens     |
| Analog Innovations                               |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| STV, Queen Creek, AZ 85142    Skype: skypeanalog |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
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"Those who expect to reap the blessings of freedom, must, like
 men, undergo the fatigues of supporting it."
                --Thomas Paine, "The Crisis," Sept. 12th, 1777

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#435721 — Re: Achieving 98% efficiency in a boost converter, datasheets

Jim Thompson wrote...
>
> On 12 Nov 2016, Winfield Hill wrote:
>
>> 4) I'm still looking hard for good inductor
>> candidates to add to the PCB.  Most have
>> appallingly-high losses, and unfortunately
>> these are not detailed on their datasheets.
>
> Large inductor and slow switching speed?

 So far measuring commercial inductors, that's
 all I've come up with.  Most have unacceptable
 losses at high frequencies.  One that should
 work is a TDK SL1720, a large 17mm D x 20mm H
 size.  The 1.5mH part is rated at 1 amp, and
 DCR = 0.86 ohms.  Its AC resistance measures
 3 ohms at 20kHz.  With D = 17% and ON time
 8.4us, dI = 8.4us 2.0V/1.5mH = 11 mApp, and
 the I^2 R loss would be about 0.125W, or 0.6%
 of my 24mW power.  Wait, that seems too low.

 I'd like to be able to scale this to higher
 frequencies, e.g. 100uH at 300kHz.  Could
 be, I have some candidate parts to test.


-- 
 Thanks,
    - Win

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#435284 — Re: Achieving 98% efficiency in a boost converter, datasheets

On Sunday, November 13, 2016 at 1:33:37 AM UTC+11, Winfield Hill wrote:
> dagmargoodboat@yahoo.com wrote...

<snip>
 
>  4) I'm still looking hard for good inductor
>  candidates to add to the PCB.  Most have
>  appallingly-high losses, and unfortunately
>  these are not detailed on their datasheets.

The usual way of getting a better inductor is to wind your own, but manufacturers are better at doing banked (low capacitance) windings.

And your foot-print is a bit on the small side to offer you much choice of ferrite.

-- 
Bill Sloman, Sydney

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#435497 — Re: Achieving 98% efficiency in a boost converter, datasheets

bill.sloman@ieee.org wrote...
>
> The usual way of getting a better inductor is to wind
> your own ...

 Yes.

> but manufacturers are better at doing banked
> (low capacitance) windings.

 Well, with laser-cut coil forms, etc., this
 could be reversed.

> And your foot-print is a bit on the small
> side to offer you much choice of ferrite.

 Good point. RM8 is big for a 24mW converter,
 but I'm adding larger footprints to the pcb.


-- 
 Thanks,
    - Win

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#435203 — Re: Achieving 98% efficiency in a boost converter, datasheets

On 13/11/16 00:40, dagmargoodboat@yahoo.com wrote:
> On Saturday, November 12, 2016 at 7:06:58 AM UTC-5, Winfield Hill wrote:
>> Winfield Hill wrote...
>>> dagmargoodboat@yahoo.com wrote...
>>>>
>>>> Too many degrees of freedom to answer you Win [ snip ]
>>>
>>> Thanks, your comments are right on target and can kick
>>> off the discussion I was hoping to see.  I'll get into
>>> that conversation tomorrow.  But in the meantime I've
>>> updated my draft schematic and PCB layout, adding more
>>> features and items relevant to the discussion.
>>> https://www.dropbox.com/sh/aj1tlto0fa6w6ty/AADJ9BWmCQE_ed7NHp60PgMja?dl=0
>>>
>>> Here's an inducement: anyone who'd like to experiment
>>> with issues and concepts embedded in the RIS-767
>>> board, I'll send you two blank PCBs to play with.
>>> You just have to talk about what you learn, should
>>> you get around to learning anything!
>>
>>  I've added a folder of datasheets to dropbox.
>>
>>
>> --
>>  Thanks,
>>     - Win
>
> You're going to wind up with a while bunch of trade-offs, such as larger and
> larger FETs reduce conduction loss, but take more power to drive;
> higher inductance lowers i^2*r losses from peak currents, but increases
> the parasitic capacitance you have to drive, etc.
>
> I designed a ~120W two-stage switcher a couple years ago where maximum
> efficiency was essential for heat management, as the unit was in still
> air in a small, sealed box, & subject to heating by direct sunlight.  I
> wrote a spreadsheet to model the losses given the various parameters,
> which helped me choose FETs & make other decisions.  A little messy and
> crude, but predicted actual losses quite accurately.
>
> At 20mW you'll have the advantage of almost ideal capacitors and plenty
> of capacitance for storage.
>
> At Rds(on)=.065 ohms, the FDN337 looks like overkill.
>
> Please pardon me as I doodle here...
>
> A few calculations:
> 1) Conduction loss. For 20mW @ n=98%, total loss budget is 400uW.  Assuming
>  Vin=4V and n=98%, i.in(avg)=5mA; i^2 * Rds(on) = 1.6uW for a perfect,
>  zero-ripple-current infinitely-fast continuous-mode switcher, or four
>  times that for a boundary-mode design with twice the peak iL.
>
> 2) Gate drive loss: If the gate charge is 7nC driven at 4V, and we start
>  off allocating it 1/10th the total loss budget...that's 40uW @ 4V, or
>  10uA(avg), which means we could afford to drive it at f=i/2q=700Hz. Ballpark.

What if the gate is in a series-resonant tank? The L stores
the necessary energy for the next phase, instead of it being
lost. You only lose due to the R(L) and the gate resistance.

> 3) Output capacitance: Assuming Vout=8V (I don't know the actual), 20mW is
>  2.5mA load.  For 50mV output ripple and 700Hz update rate, we calculate
>  the necessary filter cap:
>      C = i * dt / dV = 2.5mA*(1/700)s / 50mV = 70uF.
>
> 4) If we budgeted a maximum permissible conduction loss, i^2 * ( Vds(on) +
>  dcr(L) ), that fixes a max. peak current i.pk. From i.pk and Vin and our
>  output ripple voltage spec. and the output capacitance, we can compute
>  the minimum inductor value.  The inductor value and i.pk then give us
>  the inductor charging time, i.e., the switch on-time.
>
> Interesting--the ripple voltage depends ultimately on the available
> output capacitance.  After picking a maximal practical capacitor, given
> the output capacitance available, the maximum allowable output ripple
> voltage, and the inductor value, you can calculate the peak inductor
> current needed for any given switching frequency, and from that, the FET
> conduction losses.
>
> 98% is going to be fun!
>
> Cheers,
> James Arthur
>

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#435248 — Re: Achieving 98% efficiency in a boost converter, datasheets

On Saturday, November 12, 2016 at 5:09:03 PM UTC-5, Clifford Heath wrote:
> On 13/11/16 00:40, dagmargoodboat@yahoo.com wrote:
> > On Saturday, November 12, 2016 at 7:06:58 AM UTC-5, Winfield Hill wrote:
> >> Winfield Hill wrote...
> >>> dagmargoodboat@yahoo.com wrote...
> >>>>
> >>>> Too many degrees of freedom to answer you Win [ snip ]
> >>>
> >>> Thanks, your comments are right on target and can kick
> >>> off the discussion I was hoping to see.  I'll get into
> >>> that conversation tomorrow.  But in the meantime I've
> >>> updated my draft schematic and PCB layout, adding more
> >>> features and items relevant to the discussion.
> >>> https://www.dropbox.com/sh/aj1tlto0fa6w6ty/AADJ9BWmCQE_ed7NHp60PgMja?dl=0
> >>>
> >>> Here's an inducement: anyone who'd like to experiment
> >>> with issues and concepts embedded in the RIS-767
> >>> board, I'll send you two blank PCBs to play with.
> >>> You just have to talk about what you learn, should
> >>> you get around to learning anything!
> >>
> >>  I've added a folder of datasheets to dropbox.
> >>
> >>
> >> --
> >>  Thanks,
> >>     - Win
> >
> > You're going to wind up with a while bunch of trade-offs, such as larger and
> > larger FETs reduce conduction loss, but take more power to drive;
> > higher inductance lowers i^2*r losses from peak currents, but increases
> > the parasitic capacitance you have to drive, etc.
> >
> > I designed a ~120W two-stage switcher a couple years ago where maximum
> > efficiency was essential for heat management, as the unit was in still
> > air in a small, sealed box, & subject to heating by direct sunlight.  I
> > wrote a spreadsheet to model the losses given the various parameters,
> > which helped me choose FETs & make other decisions.  A little messy and
> > crude, but predicted actual losses quite accurately.
> >
> > At 20mW you'll have the advantage of almost ideal capacitors and plenty
> > of capacitance for storage.
> >
> > At Rds(on)=.065 ohms, the FDN337 looks like overkill.
> >
> > Please pardon me as I doodle here...
> >
> > A few calculations:
> > 1) Conduction loss. For 20mW @ n=98%, total loss budget is 400uW.  Assuming
> >  Vin=4V and n=98%, i.in(avg)=5mA; i^2 * Rds(on) = 1.6uW for a perfect,
> >  zero-ripple-current infinitely-fast continuous-mode switcher, or four
> >  times that for a boundary-mode design with twice the peak iL.
> >
> > 2) Gate drive loss: If the gate charge is 7nC driven at 4V, and we start
> >  off allocating it 1/10th the total loss budget...that's 40uW @ 4V, or
> >  10uA(avg), which means we could afford to drive it at f=i/2q=700Hz. Ballpark.
> 
> What if the gate is in a series-resonant tank? The L stores
> the necessary energy for the next phase, instead of it being
> lost. You only lose due to the R(L) and the gate resistance.

Yes, you can recycle the gate drive that way.  But if the gate drive
loss is small, that would add a lot of complexity. High-power converters
can even recycle their leakage inductance's stored energy--Tim Wescott
gave it a whirl a ways back IIRC.

I've not thought about resonant converters for decades--don't even
they want a square gate drive, to keep the switches from going linear?

I'm curious about Win's application.  Normally 20mW is not valuable enough
an amount of energy to throw a lot of expensive parts at.  His must be a
cubesat, or some super-demanding application.

Cheers,
James Arthur

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#435985 — Re: Achieving 98% efficiency in a boost converter, datasheets

dagmargoodboat@yahoo.com wrote...
>
> I'm curious about Win's application.  Normally 20mW
> is not valuable enough an amount of energy to throw
> a lot of expensive parts at.  

 In the world of solar-energy-conversion research,
 every percent counts.  Players struggle for every
 1 to 2% advantage.  If a test system size is 20mW,
 then 1% is 200uW.  My associate's lab system is
 at that level.  I'm fighting losses converting
 0.7 - 2.0V to 2.40V for an electrochemical cell.
 So if I take more than 2% away from his process,
 that's serious business!  But in the meantime,
 98% is also interesting challenge.**  If we can
 do this without special stuff, that'll be great!

 ** Certainly many others must have done this.


-- 
 Thanks,
    - Win

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#436087 — Re: Achieving 98% efficiency in a boost converter, datasheets

On Thursday, November 17, 2016 at 1:00:32 PM UTC+11, Winfield Hill wrote:
> dagmargoodboat@yahoo.com wrote...
> >
> > I'm curious about Win's application.  Normally 20mW
> > is not valuable enough an amount of energy to throw
> > a lot of expensive parts at.  
> 
>  In the world of solar-energy-conversion research,
>  every percent counts.  Players struggle for every
>  1 to 2% advantage.  If a test system size is 20mW,
>  then 1% is 200uW.  My associate's lab system is
>  at that level.  I'm fighting losses converting
>  0.7 - 2.0V to 2.40V for an electrochemical cell.
>  So if I take more than 2% away from his process,
>  that's serious business!  But in the meantime,
>  98% is also interesting challenge.**  If we can
>  do this without special stuff, that'll be great!
> 
>  ** Certainly many others must have done this.

I'm not so sure about that. There are easier ways of getting more power out of setup than raising the efficiency from 95% to 98%, and usually easier ways of limiting the temperature rise in the inverter. Diminishing returns definitely start setting in hard when you get to 90% efficiency.

-- 
Bill Sloman, Sydney

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#436157 — Re: Achieving 98% efficiency in a boost converter, datasheets

On Wednesday, November 16, 2016 at 9:00:32 PM UTC-5, Winfield Hill wrote:
> dagmargoodboat@yahoo.com wrote...
> >
> > I'm curious about Win's application.  Normally 20mW
> > is not valuable enough an amount of energy to throw
> > a lot of expensive parts at.  
> 
>  In the world of solar-energy-conversion research,
>  every percent counts.  Players struggle for every
>  1 to 2% advantage.  If a test system size is 20mW,
>  then 1% is 200uW.  My associate's lab system is
>  at that level.  I'm fighting losses converting
>  0.7 - 2.0V to 2.40V for an electrochemical cell.
>  So if I take more than 2% away from his process,
>  that's serious business!

Sure. I'd meant to point out that a) components for super-high
efficiency are a lot harder to come by at the 20mW power level, and
that is partly because b) it's an uncommon requirement: you don't
often throw 10x$ in parts to harvest an extra x$ worth of power.
Maybe in spacecraft. On Earth you'd just use a 2% larger solar cell.

IOW, I suspect it's a lot easier, with COTS components, to get 98%
efficiency in a 20W converter than at the 20mW level.

>  But in the meantime,
>  98% is also interesting challenge.**  If we can
>  do this without special stuff, that'll be great!
> 
>  ** Certainly many others must have done this.

Fun project, as usual.  Thanks Win.

Cheers,
James Arthur

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