SZBK07 300W 20A Buck converter

Dear reader,
For an esp8266 project I have a question and I hope one of you can help me I want to use this SZBK07 300W 20A Buck module ( see below) on a 72Cell 300Watt solar panels with a Open voltage Voc of 43V. This module is rated 40V input max. That is just outside the absolute maximum ratings of the TL8711. This TL8711 seem to be, but is not for sure , the 20Pin control chip at the back of the module. Can any one of you tell me if this control chip is fed directly from this input or is there a regulator in between the input voltage and the control chip Vcc. These are my questions:

Does any one have experience with this module?
Is the control chip a TL8711?
Can I use this module for this panel with a Voc of 43 Volt ?.

Thanks for the help
https://www.youtube.com/watch?v=R3s_g25Gw68
**broken link removed**

Hi,

what do you expect?
That anybody here can guarantee it works outside specified range?

Or that anybody just says there is a good chance that it will survive? And what if it fails then? What if it works for a week and then fails? or a year?

--> It´s always your own risk. And if you test it ... you can never be sure it will still work one second later.

***
And who guarantees that the 43V are the absolute top limit? Don´t forget your wiring, with induced additional voltage and maybe a spike during operation caused by the stray inductance?

My recommendation: If you want safe operation, then use a device within it´s specifications, with all the additional errors that may arise.


Klaus

I have no experience with this device or affiliation to the manufacturer.

Point 1: that module looks rather small to regulate a 300W load. Chinese manufacturers have a habit of 'stretching' the specifications so be aware of potential cooling problems. You might have to force cool it with a small fan. The amount of heat it produces will depend on the load you apply to it.

Point 2: the Voc of your solar panels is the voltage they produce with no load on them. Connecting a load will drop the voltage so you probably won't reach the module maximum input anyway. If in doubt, add a clamp circuit across the PV output that limits the maximum voltage to say 35V.

Brian.

betwixt said:

I have no experience with this device or affiliation to the manufacturer.

Point 1: that module looks rather small to regulate a 300W load. Chinese manufacturers have a habit of 'stretching' the specifications so be aware of potential cooling problems. You might have to force cool it with a small fan. The amount of heat it produces will depend on the load you apply to it.

Point 2: the Voc of your solar panels is the voltage they produce with no load on them. Connecting a load will drop the voltage so you probably won't reach the module maximum input anyway. If in doubt, add a clamp circuit across the PV output that limits the maximum voltage to say 35V.

Brian.

Click to expand...

Hi Betwixt,
Thanks for your response, your remark on 1) is correct but on the other hand there is a guy that has tested it in a video with a load of 18Amps if i remember well and that worked. But again a fan is probably needed and that's not a problem. Your point 2) is also correct the work point is below that voltage but under the condition there is a load, I can't guaranty that. So there is a moment that the is above 43V. The idea of a clamp to protect the module might be a good idea i'll consider that, good point didn't think of that.

Can I use this module for this panel with a Voc of 43 Volt ?.

Click to expand...

Open circuit voltage specifications are often useless because the source impedance is not specified.

You add 25% of the specified /expected load and measure the actual voltage.

Please also examine the open circuit voltage specification - was it under the max (vs typ) illumination? I expect the manufacturer will specify the max attainable voltage that you may never be able to see!!

What will be connected to the regulator? A const ot var load?

Well , it does say 40V input so your falling short with that 43V input, as you say, there will be a short period where it sees the 43v.
You could always add a big input capacitor to slow up the voltage rise, then put an in-line fet in there to switch out the input when it goes over 39V…Thus protecting the unit from 40V….
Tl8711 is not on findchips.com so I don’t think it exists……maybe you can get a schem and show us, then we can see?

Just to be safe I would use the clamp idea. Set it to prevent the input voltage going above about 35V or whatever the PV produces in your location when not loaded. The clamp might dissipate a lot of power but probably only when the PSU itself is not loaded, Voc is just that, it drops, maybe substantially even under light load. If it drops below clamp voltage, the clamp circuit ceases to have any effect and runs cold.

Brian.

yes, a clamp comprising a divider feeding into a opamp error amplifier, which feeds into a shunt transistor which shunt regulates the input down to "safe voltage" would be good.
Avoid temptation to use tvs clamp as there voltage varies too much with the current flowing in them.

I would use a much simpler approach. The voltage doesn't need regulating, just limiting and the current available at peak voltage would be relatively small anyway. I would use a cheap NPN power transistor, fit it to a heat sink, wire a 1K resistor across base and emitter and a 36V Zener between base and collector. Three components!
Basically an amplified Zener but importantly, with the majority of heat dissipation in the transistor where it is easy to keep it cool.

Brian.

That sounds great, i wonder sometimes why such things are not used as TVS replacements when more of a "brick wall" response is required..(maybe they are(?))

TVS are good for clamping short surges but they can not sustain high currents for long. They are also bi-directional. An amplified Zener using a power transistor can clamp high currents for long periods but only in one polarity. Each has their benefits and drawbacks.

I have used amplified Zener configuration across the center arm of bridge rectifier before now to clamp both polarities but that was in a special situation, the cost and effectiveness starts to creep up and other alternatives become more attractive.

Brian.

Thanks, often times, Power BJTs are more often available as Darlingtons...so i wonder, would wogoos be able to use a Darlington (2 X npn) for this "amplified zener"?

A Darlington would work but it would make little difference to the efficiency. For a given Zener, the clamp voltage will be higher by about 0.7V but the Zener current would be lower. Overall dissipation would be exactly the same.

Brian.

Sorry folks I made a typo its not the TL8711 but the LT8711
Hope this helps.

I also investigate this board a little more and found the following info. I hope there is some one who can solve some of the question marks.

Your guesses at part numbers seems reasonable. Generic 80N08 MOSFETS are made by several manufacturers. The IC is best discovered by tracing out the schematic around it. If it wanted to boast of a particular type it wouldn't have the number sanded off.

Brian.

Tracing a few pins on the photo clarifies that the controller is surely not LT8711. Generally I would rather expect a cheap Chinese SMPS controller.

The 5V regulator cannot take more than 40V (most, at least) and will be responsible for the start up. I guess the 40V input limit comes from there...

betwixt said:

Your guesses at part numbers seems reasonable. Generic 80N08 MOSFETS are made by several manufacturers. The IC is best discovered by tracing out the schematic around it. If it wanted to boast of a particular type it wouldn't have the number sanded off.

Brian.

Click to expand...

Thanks Brian, I did have my doubt about the LT8711 because I can trace one of the gate drivers from the photo and that one doesn't line up with the pinning. One of the next responses confirms that. Beside the fact that the LT8711 application notes work with external PMOS and NMOSFETS and that would mean that the MOSFET must be of different types. I had this LT8711 and MOSFET data from and other guy at the internet and it seems to be be incorrect.
regards Oscar

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Hi FvM
Thanks, I came to the same conclusion it's not a LT8711 as far as I can see. I did have my doubt about the LT8711 because I can trace one of the gate drivers from the photo and that one doesn't line up with the pinning. Beside the fact that the LT8711 application notes work with external PMOS and NMOSFETS and that would mean that the MOSFET must be of different types. I had this LT8711 and MOSFET data from and other guy at the internet and it seems to be be incorrect.
regards Oscar

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Hi c_mitra
Thanks, I saw that to, it might be the reason of the max input limit I agree. If that is the limiting factor its possible to replace it with a higher voltage one and the problem is solved. A LM314 has a differential in out voltage of 40V that means a 45V input would be possible. That's why its nice to know the controller chip because I assume it's fed directly from the Line. Analog Devices who makes LT and LTC controllers has types that can have 60Volts. If that would be a 60V version the module can be changed in a 60V version. Imaginable 60V 20 Amp. So the question stays what is that controller chip?
regards Oscar Goos