Hi,

That ringing is the diodes capacitance in series with the 33uH inductor.
This is very very typical for a buck, and i've seen it many times in the past.
If you add more capacitance (a little more) across the diode, you see the
frequency of the ringing go lower, and if you calculate the resonate frequency
lo and behold it's the same as the ringing :-)
It happens because the diode stops conducting, and once it stops conducting
it's like an open circuit and all that is left is the capacitance in series with the
inductor. The mosfets capacitance may contribute a little too during that time.
It's not too bad as long as it doesnt go too high and thus exceed some components
voltage rating.

I almost forgot to comment on the inductor value itself...
33uH does sound quite low for 30kHz so it is good that the frequency was increased to
100kHz. The lower inductor value as well as possibly saturating too soon
also puts a strain on the output capacitor, and the
output capacitor needs to be able to handle the rms ripple current without any problem.
At 100kHz that situation gets better too.

 

You are going to need a good low Rs cap of about 1000 uF on output to get to your full power goal. That's at 100 KHz switching speed.

At your final power target, coil inductance is too high for even 30 Khz operation. At 30 KHz should use about 15 uH, at 100 kHz use about 5 uH.

Peak coil current is going to be upwards of 6-8 amps. It's more difficult to keep core out of saturation at larger inductance value. You can use larger inductance but the coil core size is going to get a lot bigger.

Input capacitor should handle supplying 4-5 amps for 75% of period. At 30 kHz this would be about 800-1000 uF. At 100 kHz would be 250-500 uF.

If your going to hunt for maximum power point on panel you need to keep ripple voltage on panel to minimum otherwise it will fake out your search.

Remember panel should get constant current load. Input cap has to supply the additional coil peak current otherwise panel voltage drops out.

Also need a backwash switch or diode when panel illumination drops out.

 

Here is couple of candidates for both input 470uf and 1000uf output caps from Rubycon. May substitute a general purpose 105C cap for output and swap the current output over to the input, until order arrives.

Here is a couple of inductor candidates. J.W. Miller 5uf and 10uf. The previously mentioned 33uf is pretty beefy, and not sure if it would saturate or not, seemed to be pretty conservative. How I would tell if coil is saturating or not? Still hunting for the sweet spot, i.e. frequency vs. inductor value, but willing to try a few combos. Before this conversation, I was actually was thinking of trying to raise the inductor value, and lower the frequency to lessen switching losses. Those type of inductors start getting expensive though.

The output voltage a-d was worked out in the microcontroller section. It can be sampled every 4th, 8th pwm (rising edge) interrupt to give consistent results. There is an Alleggro hall effect sensor for measuring current. So mppt will be looked at in the future. The hall effect output needs to be buffered though, because a direct connect to the micro a-d seems to suck the life out of sensor.

I was going to try and set the control loop up as a constant voltage device for the battery, and drop the converter out if panel current goes below a set minimum. In low light situations have seen the voltage collapse, I know this will be a problem, and need to come to grips with.

Yes there is a blocking diode on the input after the fuse and on/off switch. I need to update the schematic to show what all is involved.

Scope probe is Tek 6105A 10x.

 

Hi again,


The site for the 1000uf cap was doing maintenance so i couldnt look at that cap yet.
The inductor however you are right is pretty beefy. As i said before though, the
inductor saturation is not the only issue here. The other is the ripple current rating
of the output cap(s). Using one cap, a 30uH inductor, a 21v supply, 12v output,
the cap ripple peak to peak current can be calculated:
Ipp=v*dt/L
where dt approximately comes out to 12us for a 50kHz frequency, so we get
Ipp=3.6 amps peak to peak, or 1.8 amps peak or 1.04 amps rms, so the cap
would have to be able to handle that ripple current. If the battery voltage
drops down a little the ripple will go up a little, and also with current due
to the resistive and other losses in the circuit.
With 30kHz that ripple current will go up to about 1.65 amps rms.
With 100kHz that ripple current will go down to about 0.5 amps rms.
That shows the effect of the frequency on the ripple current and thus the required
ripple current rating of the capacitor.
The voltage rating of the capacitor is usually recommended to be 1.5 times the output
voltage, so the cap should have a voltage rating of at least 18v, but i have seen
16v caps on well regulated power supplies. Even better would be 25v of course.

How many amps do you expect to output to the battery in the
final design? We need this info in order to determine if your inductor will saturate
or not for a given frequency and value. If you only need to push 4 amps out,
that inductor should be able to handle that as the max current will be up around
6.4 amps during turn on surge and slightly less than 6 amps during normal operation.
That leaves plenty of headway too which is certainly a good thing.
You'll also need to implement current limit somehow if you havent already done that.

Also, what kind of ripple voltage are you looking for? The cap ESR has a very big effect
on this specification, although a workaround is to use a post filter with a small inductance
(even air core hand wound) and a second capacitor but not in the feedback path.
You can get super smooth output that way for little extra cost and less headaches after
some normal capacitor aging.

Lastly, how does the AD converter "suck the life out of" the hall effect sensor?
What is the symptom when this happens?

 

I disagree. In a continuous buck converter, the first indication that the inductor is too small for the operating frequency is when you see the diode stop conducting!

33kHz is unusually slow for a switcher these days, so I dug out the design guide for the (50kHz) LM5576. For 20-25V in, 12V out, 1A they recommend 470 to 680µH. Lower currents need even more inductance. 5µH is too small unless you can guarantee currents of 20A or more. Simulation should confirm this. Maybe tomorrow unless someone else wants to do it.

@nickelflippr, the easiest solution appears to be: keep your frequency high. Those low ESR capacitors will also help.

__________________
de KI6RWX

 

Just a quick correction: Inductors don't come in uF (microfarads). It's microhenries (uH.)

__________________
de KI6RWX

 

Hi again,

I agree that 5uH is way too small, and so is 15uH really. For 50kHz 100uH is
more typical, but i've used 50uH many times with good results. It has a lot
to do with input to output differential too.

Yeah that must have been a simple oversight...stating uf instead of uH he he.
I think he went with the inductor stated in previous posts because they are
on sale right now for about 2 dollars, while the others seem to be over 4 dollars
each and are rated for less current with the higher inductance.
BTW, two 39uH inductors in series gives 78uH but that would cost 4 dollars
too :-) That would provide for a 78uH 11 amp inductor though...not too shabby.

 

I think you were influenced by the OP's drawing, which shows a 33uf inductor.

__________________
You don't need a quadraphonic Blaupunkt -- you need a curve
ball.

 

Dunce cap award on inductor values, uH not uf.

Actually the current 330uf/50V output cap is probably doing the job at 100khz. It was picked for a previous small switcher project based on the L4972A chip. It has a low esr 0.054Ω and ripple of 1.26A. For the previously mentioned Rubycon of 470uf/50V and 1000uf/50V the specs are .027Ω/2.05A and .021Ω/3.01A respectively.

As far as amps were concerned, was originally thinking a 10A input from solar. As it is, probably won't be getting any more solar power, so lets stick with the 4.4A for now. Not sure why I would need a current limit, the 12V lead acid battery will just stop accepting current, voltage goes up, and need to prevent over voltage. The blocking diode would protect from hooking up backwards.

The current ripple voltage "appears" to be around 100-150mv, and would seem acceptable. Possibly willing to go for less.

Hall sensor problem solved. AN7 wasn't working, so put it on AN1 of the PIC micro. This was a software problem.

Sounds like the consensus is to leave off the 5uH and 15uH inductors from the shopping list. Have added another 33uH, because they are cheap like me. Don't know where I will find the room on the board.

Thanks again for everyone's suggestions. Still need to update schematic and work on a charging algorithm.

Initial and rough power calculations had this around 85% efficient, does that sound right? Does it make sense to change this into a synchronous buck regulator?

 

If you keep the frequency at 100kHz, the extra inductor shouldn't be needed.

The blocking diode has to prevent the battery from supplying the panel. Draw the circuit, including body diodes, and confirm there is no leakage path. If the loss in the blocking diode concern you, use a MOSFET. If you have the freedom to put the blocking in the (-) line, it can be N-channel.

As you know, synchronous rectification only eliminates the losses in the freewheeling diode. Calculate these losses and judge for yourself whether the added parts are worth the trouble. It would be an N-channel, so it's cheap.

__________________
de KI6RWX

 

From your original schematic it looks like you have a simulation program. You can determine the values from it. Do think about sweeps in conditions that may apply.

The coil charge L*di/dt supports 5 volt drop. Coil discharge supports L* di/dt of 12.5 vdc (added some for negative going diode).

What is not likely in your simulation is satuaration characteristics on your inductor. From your sim get the peak coil current and check it against specs on the coil.

Higher inductance puts the switcher in continuous conduction mode. This is okay as long as coil core can handle it. In continuous conduction mode the coil is not allowed to fully discharge its field through diode before the next MOSFET charge cycle starts. Duty cycle of switcher would be roughly 75%. There is little to no ringing in this mode. Ringing occurs in discontinuous mode when the diode conduction ceases at its diode drop voltage but there is still a small amount of energy left in the coil with nowhere to go.

If you want to better represent the PV panel in your sim replace the 17v voltage source with a variable current source in parallel with 34 diodes in series. You can add a little series resistance between this PV model and your circuit to represent the Rs of the PV panel. Varying the current source represents illumination level on the panel. The main thing is to see how your circuit reacts when illumination drops and the source become a high impedance current source. The input cap has to provide the switcher current peaks.

 

Hi again,


Your choice of capacitor sounds good at 100kHz. I agree that you probably
wont need an extra inductor at that frequency.

85 percent sounds normal yes.

I'd like to hear who selected your initial inductor choice, because the sat
rating is very good. Because the input can be as high as 4.4 amps and
the input is as high as 21v (rounding off) and output 12v, the output current
(in a buck circuit like this one) could be higher than the input current with
the same ratio of input to output voltage, so that means the output current
can be as high as 7.7 amps average, and the peak of say 1 amp added
to that means the max output current at any time can be close to 9 amps.
Thus, 11 amps is a good choice for the inductor rating so i wonder who
selected that now :-) Did you? If so, good choice :-)

The efficiency without a diode would be slightly greater at least theoretically
but this is more of an issue with very low output voltages. At 12v output
the difference would be the difference between 12v output at say 7 amps
and 12.5v output at 7 amps, times an approximate factor of 0.5 for a
50 percent duty cycle, which comes out to about 2 percent, which isnt
a huge difference for the trouble of going to a synchronous converter
with extra transistor. The diode in the non sync converter of course should
be a Schottky type or similar low voltage drop type.

Let us know how this project works out ok...

 

Hi MrAl et al,

Got around to updating the schematic. Comments welcome. Have started a LTspice schematic, but hung up with some of the details, as this is a first go for me. May have to ask for some help on how to do some of the models and simulation. Have the info plugged in for the inductor, diode and caps. The input and output caps are based on those arriving from the Newark order, so not in circuit yet, later this week hopefully.

The inductor was a lucky guess, as described in earlier post, with the idea that it was the highest readily available value at the rated output. The buck converter idea is from a schematic detailed by the Microchip dsPICDEM SMPS Buck Development Board. Other resources are Microchip SMPS appnotes AN114 and AN1207.

Considering that zero amps are produced at the open circuit voltage of the solar panel (21V), the peak amps you describe seems high. If the rated voltage of 17V is used, then maybe only 5.5 amps peak or so?

Anxious to hook up the battery, but must wait to get feedback loops and failsafes figured out. This will take some time, but will keep progress updates.

Attached Thumbnails

 

 

Hi again,


Well with 17v input and 4.4 amps from that supply the output can go to approx
4.4*17/12 amps which is about 6.2 amps.

I guess you dont need current limit if your battery can take the full output current.
I wasnt sure what size battery you were using.

I guess the battery will be hard wired into the system too? No possibility of
reverse connection there then.

It sounds like your project is coming along nicely and i cant wait to hear how it
works out after you get all the parts.

 

Make a note that the SFP9540 might not handle your power without a heat sink (5A in 0.2 ohms = 5W). Consider two or three in parallel instead. Each should have its own gate resistor. This sounds extravagant, but in addition to saving power it ends up a lot smaller than a heat sink. (5A in 0.067 ohms is 1.67W total shared by three devices, or 0.56W each.)

With 4.4A from the panel, the input SB560 is approaching its 5A rating. While not a big deal reliability wise, again you might consider that it'll also be at the top end of its voltage drop at 0.7V. This is more important than the freewheeling diode because it's in the circuit at 100% duty cycle. Another diode in parallel would reduce the voltage drop and save a little power. A MOSFET switch it could save even more power. A TO-220 standing up is about the same footprint as the DO-201A diode package.

__________________
de KI6RWX