[SOLVED] Diode and tunnel diode in parallel? Weird Chinese Vsupply schematic!

[Tuppe]

Tunnel diode in Vsupply?

Hello!
I came across this PCB schematic of SIM5320E board, that has some weird diode configurations going on. There seems to tunnel diode to ground? What do you think?

The module in this schematic is rated "4.6V-5.2V" input, although the SIM5320E Vcc(after the diodes) is rated 3.3V-4.2V, and peak current 2A. The 1N5408 seems to do some crude voltage conversion.

Does it make sense to have tunnel diode anyway? Is it actually just Zener? Schottky?

I hope someone can make sense of this configuration! Thank you for reading!

 

[betwixt]

I think it is far more likely that is a 5V Zener diode or other device for clamping excess voltage. A Tunnel diode would simply burn out and would cerainly not do anything useful in that circuit configuration. Besides, they are VERY expensive and quite difficult to source.

The 1N5408 would drop around 0.7V to 1.2V depending on the current so with the maximum 5.2V input there would be at least 4.5V at VCC_4 so a 5V clamp would be a suitable protection device.

Brian.

 

[schmitt trigger]

It is most likely a badly drawn Tranzorb diode.

Can you take a picture of the diode? Well focused, such that the lettering can be read?

 

[Tuppe]

It is most likely a badly drawn Tranzorb diode.

Can you take a picture of the diode? Well focused, such that the lettering can be read?

I don't have the module yet. I was asking so that I'd know that the power layout isn't rubbish. Now that I verified the purpose of those diodes, I will buy the module.
I will just bypass the useless dropout diode(I need low power consumption) and put some proper 3.7V input voltage directly. I would leave the Zener there just for safety, it shouldn't matter.

I have no idea why they design the input so poorly, without just routing the input voltage directly. I guess it's this "Arduino is 5V" thing. There's no reason for me to burn power in that diode when I have input protected 3.7V buck-converter in my project, right?
The datasheet of SIM5320 doesn't suggest any voltage input diode anyways.

Here is image of the module, the diodes are those 2 huge beasts near the input pins:

 

[dick_freebird]

Big diode is probably meant to be inert in normal operation
and protect against some class of potential faults. That
it doesn't do anything under normal conditions, is good.
The small diode may be for power supply "ORing" that
serves some redundancy or switchover (using internal
caps for supply hold-up) interest, like hot-plugging an
interrupting adaptor onto one running off battery?

 

[Tuppe]

The small diode may be for power supply "ORing" that
serves some redundancy or switchover (using internal
caps for supply hold-up) interest, like hot-plugging an
interrupting adaptor onto one running off battery?

So it prevents power supply capacitor(those big tantalums) charge flowing back into the battery? Did I understood correctly?

I'm going to power this using SMPS(12V car battery to 3.7V buck-converter), do you think the series diode is necessary for that? I don't think it's necessary, so I plan to bypass it.

Thank you!

 

[dick_freebird]

3.7V input may embed the diode forward drop - check the
core parts ratings and any info on the inboard intended supply
voltage. You might be wanting a 3.3V supply for a direct
feed inside the forward diode, or something.

If you have one load that can be fed by two separate
sources, you do not want to short those two sources
together. You can switch or you can steer, steering
(diode "OR") is easier but imposes inefficiencies; a
switching scheme may be needier of contril circuits
and/or auxiliary supplies.