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Re: Boost converter wisdom
On 08/26/2018 09:54 PM, Winfield Hill wrote:
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Sorry, I was unclear.  That's the spec for the buck driving the Class H  
amp.  Pop option #1 will be to run that off the +5V, but pop option #2  
(hopefully) will be to run it off the boost converter, which will put  
out +12 to +14.  That way I can use either kind of TEC.

Cheers

Phil Hobbs

--  
Dr Philip C D Hobbs
Principal Consultant
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Re: Boost converter wisdom
On Fri, 24 Aug 2018 15:10:00 -0400, Phil Hobbs

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I don't understand "Class-H" in this context.  Why not a four switch
buck-boost?

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I do them all the time.  No big deal, other than there is a DC path
from the input to the output to remember.  Startup can be a real
bitch!

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Ah, do you mean an "H bridge" or "full bridge"?  "Class-H" means
something completely different to me.


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No problem as long as you have enough input capacitance on the second
converter.  People often forget this little detail.  A boost after a
buck is easier than the other way around, too, because the boost's
inductor is in the right place.  

Your efficiency is going to suffer, though.  A four-switch buck-boost
might be a better plan, or even a SEPIC?  A SEPIC buys some isolation,
as well.

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This tool is rather useful for anyone playing with switchers.

http://www.ti.com/tool/powerstage-designer?keyMatch=power%20stage%20designer&tisearch=Search-EN-Everything

Re: Boost converter wisdom
On 08/24/2018 10:57 PM, snipped-for-privacy@notreal.com wrote:
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Noise.  Putting a Class D driver on a TEC is a recipe for all sorts of  
birdies on the laser bias, because the capacitance from the TEC to the  
cold plate is surprisingly large (nanofarads).

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No, it's Class H--that is, you servo the SMPS output to get the minimum  
supply headroom for the (linear) output stage to work properly.  That  
preserves the low noise of the linear stage while avoiding excess  
dissipation.  The output stage is an asymmetric BJT bridge--one side is  
a current conveyor and the other side is a normal emitter follower type.  
  Because I have a higher supply voltage available for the bridge  
driver, I avoid the V_BE drop on the emitter follower side, at least in  
the cooling direction, which is what I mostly care about.

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Takes more board space, though, doesn't it?  I really only have space  
for an integrated solution.  Keeping the dissipation down is also mainly  
a board space issue, since the thing is mains powered.

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Thanks

Phil Hobbs


--  
Dr Philip C D Hobbs
Principal Consultant
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Re: Boost converter wisdom
On Sat, 25 Aug 2018 01:40:58 -0400, Phil Hobbs

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If the switcher keeps the amp's supply rail just above the load
voltage, doesn't the amp degenerate into a filter, specifically a
c-multiplier?

Why not let the switcher be the control element? You don't need
bandwidth.

A buck or buck-boost switcher followed by a DPDT switch would be
simple. Merge the c-multiplier into that somehow.


--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: Boost converter wisdom
On 08/25/2018 01:18 PM, John Larkin wrote:
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It needs to be bipolar, so the output amp is an asymmetric bridge:  
class-B current conveyor on one side, emitter followers on the other.  I  
have a higher supply voltage available for the drivers, so the emitter  
follower doesn't lose me a V_BE drop in the direction I mostly care  
about, namely cooling.

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Yup, that would work OK with a digital control loop, which this version  
will have.  The little TEC driver board is designed already, and there's  
no NRE budget for doing it over.  It doesn't make too much difference in  
board area since class H doesn't need significant thermal pours.

Cheers

Phil Hobbs

--  
Dr Philip C D Hobbs
Principal Consultant
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Re: Boost converter wisdom
On Sat, 25 Aug 2018 13:48:12 -0400, Phil Hobbs

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Ah, now I understand where you're going.  I didn't get that you had a
linear after the switcher.  I do that fairly often for fixed supplies
where I care about ripple.  I haven't had need for a variable supply,
though.
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So use two cap multipliers and two low-side switches as your DPDT
switch.
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Yeah, I'm familiar with that management edict, "fix it but don't
change anything!"


Re: Boost converter wisdom
On Sat, 25 Aug 2018 20:40:13 -0400, snipped-for-privacy@notreal.com wrote:

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Are there any high-current DPDT solid-state switches, or SSRs?


--  

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  

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Re: Boost converter wisdom

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something like this? http://www.ti.com/product/DRV8839
  


Re: Boost converter wisdom
On Sun, 26 Aug 2018 14:07:05 -0700 (PDT), Lasse Langwadt Christensen


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Something like that.  

I was thinking of a tiny buck switcher (I like TPS54302) then a noise
filter then the h-bridge into the TEC.


--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: Boost converter wisdom
On Monday, August 27, 2018 at 9:41:55 AM UTC+10, John Larkin wrote:
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It seems a bit silly to have a set of four switches in the H-bridge and add
 more to drive a separate buck converter. If you use the H-bridge switches  
to activate say, a Cuk converter, and filter the output (whose polarity wou
ld depend on which switches you activated) before it goes into the TEC you  
should end up with the same result from fewer switching elements.

That circuit would have to be designed, rather than evolved by glomming tog
ether conceptually simpler elements, so it isn't surprising that John Larki
n couldn't come up with it.

--  
Bill Sloman, Sydney

--  
Bill Sloman, Sydney

Re: Boost converter wisdom
mandag den 27. august 2018 kl. 01.41.55 UTC+2 skrev John Larkin:
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that DRV8839 is a h-bridge and the datasheet specify 0-11V motor supply  
and separate 1.8-7V logic supply

Re: Boost converter wisdom
On Mon, 27 Aug 2018 09:13:15 -0700 (PDT), Lasse Langwadt Christensen

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Right. The biggest part would be the inductor.


--  

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  

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Re: Boost converter wisdom
On 2018-08-24 22:40, Phil Hobbs wrote:
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[...]

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There is the occasional "synchronous SEPIC", meaning no big diode, but  
have you considered buck-boost chips? In your case it probably has to be  
something in the MHz range so that the inductor can be tiny. EMC is  
another story.

[...]

--  
Regards, Joerg

http://www.analogconsultants.com/

Re: Boost converter wisdom
On 08/25/2018 03:52 PM, Joerg wrote:
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The EMC spec is DC-2MHz. ;)

Cheers

Phil Hobbs


--  
Dr Philip C D Hobbs
Principal Consultant
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Re: Boost converter wisdom
On Sat, 25 Aug 2018 16:11:11 -0400, Phil Hobbs

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So use a 2.2MHz switcher?  How much current do you need?

Re: Boost converter wisdom
On Saturday, August 25, 2018 at 3:41:11 PM UTC+10, Phil Hobbs wrote:
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snip>

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You do have to make sure that the class D waveform doesn't get as far as th
e TEC. The TEC response is non-linear and cycle-to-cycle variation in the c
urrent going through it generates more ohmic heating than a more nearly sta
ble current.

Sloman A.W., Buggs P., Molloy J., and Stewart D. ?A microcontroller
-based driver to stabilise the temperature of an optical stage to 1mK in th
e range 4C to 38C, using a Peltier heat pump and a thermistor sensor?
? Measurement Science and Technology, 7 1653-64 (1996)  

used a thumping great toroidal common mode inductor with capacitors to filt
er the class D switching component out of the current going through the TEC
. I also messed with the switching waveform to minimise the lower frequency
 harder-to-filter components.  

<snip>

--  
Bill Sloman, Sydney

Re: Boost converter wisdom
Phil Hobbs wrote:
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  Color me stupid and ignorant, but what eXplicitly wrong with using  
your standard, known-characteristics 24V supply?


Re: Boost converter wisdom
On 08/28/2018 01:43 AM, Robert Baer wrote:
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It isn't available in the instruments they want to retrofit.

Cheers

Phil Hobbs


--  
Dr Philip C D Hobbs
Principal Consultant
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