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Charge controller - power diode question

Started by Unknown December 26, 2012
On Wednesday, December 26, 2012 2:30:18 PM UTC-5, John Larkin wrote:

> > If you look at actual diode curves, diode TC is negative at low > > currents and positive at higher currents. That's because the ohmic > > component of VF dominates at high current, and it has a positive TC. > > It's generally safe to parallel identical diodes on the same heat > > sink.
Ummm- that observation only applies to the small signal stuff you work with. The power Schottky's stay negative TC all the way: see Fig. 3 http://www.vishay.com/docs/88953/m2035s.pdf
> > Couldn't you do a series switcher with no inductor and no catch diode? > > Just connect the solar array to the battery through a mosfet or SSR, > > on/off, from a comparator with maybe a little timing, like a clocked > > d-flop in the path. It's simple and power dissipation would be very > > low.
Yeah- if the battery was the only energy load on the system, but there might be other things like an MPP inverter which will get all confused with that scheme.
On Wednesday, December 26, 2012 2:12:32 PM UTC-5, upsid...@downunder.com wrote:

> Why not use three separate insulated conductors ...
That suggestion is insane for a professional project.
On Wed, 26 Dec 2012 14:38:17 -0800 (PST),
bloggs.fredbloggs.fred@gmail.com wrote:

>On Wednesday, December 26, 2012 2:30:18 PM UTC-5, John Larkin wrote: > >> >> If you look at actual diode curves, diode TC is negative at low >> >> currents and positive at higher currents. That's because the ohmic >> >> component of VF dominates at high current, and it has a positive TC. >> >> It's generally safe to parallel identical diodes on the same heat >> >> sink. > >Ummm- that observation only applies to the small signal stuff you work with. The power Schottky's stay negative TC all the way: see Fig. 3 http://www.vishay.com/docs/88953/m2035s.pdf
All diodes eventually get to positive TC. Sometimes that point is above the rated max current, sometimes it isn't. But it's still usually safe to parallel identical diodes on the same heat sink. It's really not much different dynamics than one big slab of diode which, in theory, can have one zone hog the current. But check the data sheet and do the math, of course.
> > >> >> Couldn't you do a series switcher with no inductor and no catch diode? >> >> Just connect the solar array to the battery through a mosfet or SSR, >> >> on/off, from a comparator with maybe a little timing, like a clocked >> >> d-flop in the path. It's simple and power dissipation would be very >> >> low. > >Yeah- if the battery was the only energy load on the system, but there might be other things like an MPP inverter which will get all confused with that scheme.
If it won't work for you, don't do it.
"George Herold" <gherold@teachspin.com> wrote in message 
news:2ca7c3bb-b288-4661-bf0c-6cea49321852@i1g2000vbp.googlegroups.com...
> On Dec 26, 2:32 pm, John S <Soph...@invalid.org> wrote: >> On 12/26/2012 5:44 AM, mike wrote: >> >> > But there's a more basic question. Many solar panels have built-in >> > diodes. Are you sure yours don't? >> >> Acutually, the solar panels I have in front of me have the diodes in >> _parallel_ with the panel output. >> >> The purpose, AIUI, is to provide a path for the current of shaded >> panels >> when wired in series. >> >> In that case, the provided diodes will not prevent current back into >> the >> battery. > > Hmm, well first I know squat about solar panels. But that seems > wasteful. > Doesn't it cost a bit more than one solar panel 'photovoltage' to > overcome the > diode drop of the panel in the shade? > Or is there a series stack of 'PV's with one diode across the whole > lot? > > George H.
It sounds like a 24vdc PV panel. usually they are wired up in series for 600vdc to run a Grid-tie inverter. The shading Diode sounds plausable and may be in there for other issues;) It's your standard ~0.5v drop power diode. If you partially shade an array the GTI's tend to shutdown altogether, so I'm not sure there is any benefit. Cheers
On Wednesday, December 26, 2012 7:03:35 PM UTC-5, John Larkin wrote:
> On Wed, 26 Dec 2012 14:38:17 -0800 (PST), > > bloggs.fredbloggs.fred@gmail.com wrote: > > > > >On Wednesday, December 26, 2012 2:30:18 PM UTC-5, John Larkin wrote: > > > > > >> > > >> If you look at actual diode curves, diode TC is negative at low > > >> > > >> currents and positive at higher currents. That's because the ohmic > > >> > > >> component of VF dominates at high current, and it has a positive TC. > > >> > > >> It's generally safe to parallel identical diodes on the same heat > > >> > > >> sink. > > > > > >Ummm- that observation only applies to the small signal stuff you work with. The power Schottky's stay negative TC all the way: see Fig. 3 http://www.vishay.com/docs/88953/m2035s.pdf > > > > > > All diodes eventually get to positive TC. Sometimes that point is > > above the rated max current, sometimes it isn't. But it's still > > usually safe to parallel identical diodes on the same heat sink. It's > > really not much different dynamics than one big slab of diode which, > > in theory, can have one zone hog the current. But check the data sheet > > and do the math, of course.
As a rule, if there is a single component rated to handle the job, it is always more economical than using multiple copies of a component that can't handle the job. There may be exceptions.
> > > > > > > > > > > > >> > > >> Couldn't you do a series switcher with no inductor and no catch diode? > > >> > > >> Just connect the solar array to the battery through a mosfet or SSR, > > >> > > >> on/off, from a comparator with maybe a little timing, like a clocked > > >> > > >> d-flop in the path. It's simple and power dissipation would be very > > >> > > >> low. > > > > > >Yeah- if the battery was the only energy load on the system, but there might be other things like an MPP inverter which will get all confused with that scheme. > > > > If it won't work for you, don't do it.
I know for a fact the MPP in-/con-verter will not like you periodically clamping its input to Vbatt, so you will have to think of something else.
On Wed, 26 Dec 2012 17:03:22 -0800 (PST),
bloggs.fredbloggs.fred@gmail.com wrote:

>On Wednesday, December 26, 2012 7:03:35 PM UTC-5, John Larkin wrote: >> On Wed, 26 Dec 2012 14:38:17 -0800 (PST), >> >> bloggs.fredbloggs.fred@gmail.com wrote: >> >> >> >> >On Wednesday, December 26, 2012 2:30:18 PM UTC-5, John Larkin wrote: >> >> > >> >> >> >> >> >> If you look at actual diode curves, diode TC is negative at low >> >> >> >> >> >> currents and positive at higher currents. That's because the ohmic >> >> >> >> >> >> component of VF dominates at high current, and it has a positive TC. >> >> >> >> >> >> It's generally safe to parallel identical diodes on the same heat >> >> >> >> >> >> sink. >> >> > >> >> >Ummm- that observation only applies to the small signal stuff you work with. The power Schottky's stay negative TC all the way: see Fig. 3 http://www.vishay.com/docs/88953/m2035s.pdf >> >> >> >> >> >> All diodes eventually get to positive TC. Sometimes that point is >> >> above the rated max current, sometimes it isn't. But it's still >> >> usually safe to parallel identical diodes on the same heat sink. It's >> >> really not much different dynamics than one big slab of diode which, >> >> in theory, can have one zone hog the current. But check the data sheet >> >> and do the math, of course. > >As a rule, if there is a single component rated to handle the job, it is always more economical than using multiple copies of a component that can't handle the job. There may be exceptions. > >> >> >> >> >> >> > >> >> > >> >> >> >> >> >> Couldn't you do a series switcher with no inductor and no catch diode? >> >> >> >> >> >> Just connect the solar array to the battery through a mosfet or SSR, >> >> >> >> >> >> on/off, from a comparator with maybe a little timing, like a clocked >> >> >> >> >> >> d-flop in the path. It's simple and power dissipation would be very >> >> >> >> >> >> low. >> >> > >> >> >Yeah- if the battery was the only energy load on the system, but there might be other things like an MPP inverter which will get all confused with that scheme. >> >> >> >> If it won't work for you, don't do it. > >I know for a fact the MPP in-/con-verter will not like you periodically clamping its input to Vbatt, so you will have to think of something else.
I didn't have in mind using an MPP inverter, but something much simpler to regulate the battery charging. All I was suggesting was: solar panel, series switch, battery. The switch is the only high-current part, and maybe a series diode if that's needed. The control is a simple, low-power comparator thing. One could use a single bidirectional SSR or equivalent as the switch, to handle the daytime and nighttime cases. Saves the loss of a series diode. If nobody likes the idea, don't use it.