Feedback-less SMPS

[eccerr0r]

I've been staring at two switch mode power supplies that apparently run open loop, at least with respect to the primary side - there's no optoisolator. One is a "6-8 V" that I was using for a cell phone, and this other more recent one is a Lithium Ion standalone battery charger for a digital camera. The first one for the cell phone I guess it's OK if the cell phone itself has a SMPS that's closed loop, but this camera battery charger, not only it runs open loop, appears to have a linear feedback current/voltage limiting (and this is the closed loop section)!

So I'd imagine and expect the series pass transistor to get somewhat warm... It's a TO-92 SS8050 that "can pass 1.5A".

How pervasive is this? And I suppose these SMPS can have pitiful efficiency ratings though the devices are very lightweight.

(Both are MiC but the digital camera charger is Nikon branded - a MH-24... Maybe I have a phony charger? The two electrolytic caps on it look like they're made by Jwco, but I was trying to investigate why it was charging so slowly...)

[Edit]
Hmm... Maybe this Nikon charger isn't completely without feedback, there are three windings on the transformer coil and there is a zener on the primary side of it... perhaps it's a winding based primary side feedback solution...

[eccerr0r]

Re: Feedback-less SMPS

Ahh I think I got a phony charger... This is not an authentic Nikon charger, no wonder it's so bad...

[kaboom]

Re: Feedback-less SMPS

I've been staring at two switch mode power supplies that apparently run open loop, at least with respect to the primary side - there's no optoisolator.

No they're not...

These use the aux voltage (primary side) to determine, via turns ratio, the secondary voltage. Since Np/Ns/Naux is fixed, so would be the voltages between them. This is never the case due to leakage reactance (mostly) and other losses.

IOW, the feedback point is not the secondary via an opto, but the aux voltage instead. By keeping Vaux the same, the secondary voltage is more-or-less the same. The aux winding has poor coupling/leakage reactance to the secondary, hence voltage regulation is quite poor. 6V+ at no load is rather typical. The "spikes" from Lleak being pumped cause quite a bit of ringing, this can push the output voltage higher still. Say maybe 8V, dropping to 5-6 with a few miliamps. After that ringing-induced voltage subsides, Vout wont drop as much as fast with increasing load. It'll still drop though, again because of Lleak.

Ahh I think I got a phony charger... This is not an authentic Nikon charger, no wonder it's so bad...

Oh well. At least you know how it works now!

Connect it to a 10 ohm, 10W resistor and check the voltage.

[japlytic]

Re: Feedback-less SMPS

An example of switching power supplies without a feedback loop can be dimmable electronic halogen/LED lighting transformers, as far as I know.

[eccerr0r]

Re: Feedback-less SMPS

Yeah I realized there was a third winding on the transformer and that indeed is feedback. So there is feedback for this phony charger (the other phone charger however only has two windings on the main transformer so it really does not have feedback).

But this charger still sucks. It uses a series pass transistor (which I don't quite yet understand the topology, looks like emitter follower) and a 1/8W 51 ohm resistor as the current limit(!!!). It has two metal oxide resistors and a TIL431 to do voltage regulation.

Lots of complaints about this phony charger online, I wonder if I can make this into a better charger. Voltage regulation looks OK, just that it doesn't pass enough current...

[eccerr0r]

Re: Feedback-less SMPS

Hmm... Looks like 9-10V output on secondary regulation, 8.4V output series pass. At 6.4V "dead" li-ion there would be 3.6V drop across resistor and pass transistor... that would mean only 70mA to start charging with. The normal cell capacity is 1Ah thus taking about a day to charge a battery to full! Wow... Awful, Awful. This thing is near useless!

I wonder if I increase the secondary voltage and reduce the current limit resistor resistance... hmmm...

[lti]

Re: Feedback-less SMPS

Post some pictures of this mess.

[eccerr0r]

Re: Feedback-less SMPS

Dang... I finally have cameras that exceed the limits of forum software... heh
Well, megapixels doesn't mean that much, these were taking by my Canon SD780IS.




Looks like standard cheap 2-transistor primary with zener + aux loop feedback. Secondary is measured around 10V and there's a series pass regulator - Q4 (front and silk not visible) is the series pass transistor and yes, it is a TO-92 (SS8050).

The other PSU I don't know if I have a pic of it before trying to mod it, but it is silked for closed loop so I was trying to populate it for closed loop... but haven't finished...

[eccerr0r]

Re: Feedback-less SMPS

Aha!

Open loop SMPS (taken by my Canon A60 prior to sensor death):



And yes, this SMPS regulation STINKS. Badly.

[eccerr0r]

Re: Feedback-less SMPS

Hmm... no, that pinout problem was incorrect. It is correctly E-B-C after all, must have been some side branch - testing in circuit sometimes can be deceiving...

[ben7]

Re: Feedback-less SMPS

Wow, what POS's

I do know someone who has developed a way to have a primary side feedback, with great output regulation, but he needs to have a few million dollars to design an IC to control it. He needs an IC because the circuit has to be very compact or else it won't work right.

[eccerr0r]

Re: Feedback-less SMPS

Primary side, as in possible to use a 2-winding transformer?
Will the resultant design still be cheaper (with patent royalties) than using an optoisolator?

[ben7]

Re: Feedback-less SMPS

Primary side, as in possible to use a 2-winding transformer?
Will the resultant design still be cheaper (with patent royalties) than using an optoisolator?

It wouldn't require compensation, which can be very tricky to get right, and it wouldn't need optoisolators, which will increase lifetime of the PSU.