CC/CV Feedback for LNK364

[fatalelieberi]

Hi,

I'm modifying the reference design for low power SMPS of LNK364. The design complements the VR1, R2, and R3 of the reference design, and should turn on the opto coupler if the drawn current exceeds 500mA or the output voltage is over 6.2 (supposedly). However, I run into a problem with the op amps. The op amps don't work if the op amps use Vin as its power source as shown in my simulation results. But, if I use an external voltage source of 7V, it will work, but below that they don't work. However, I don't really understand why it wouldn't work.

I was thinking of creating an additional winding for the flyback transformer with higher voltage output to drive the op amp, but that kinda defeats the purpose of optocoupler. I can just use the additional winding to do the feedback but it would force me to design a new circuit. Can someone give me a suggestion how to handle this problem? Thanks in advance.

[fatalelieberi]

Stupid me. I found out that my op amp requires minimum 4.75V. After changing using LTC1366 the problem is solved.

[T3sl4co1l]

You might be better off with a traditional UC3842 or the like, just because it's simpler --> you have less special conditions to worry about.

In a supply with auxiliary windings for controller power, you can only expect maybe a 50% variable range in output.  More than that, and you're either overvolting the controller (or need a regulator to stabilize it -- a hack), or running out of voltage and it turns off (which is why almost all power supplies go into a burst / auto-restart mode, rather than a continuous current limit).

For wide control range, you really need a separate auxiliary supply.  AC-DC and DC-DC (mains) supplies are cheap and plentiful -- well, not as cheap as a mere aux winding, but $10 is better than some alternatives.  It's the only way to maintain control down to zero.

If you only need primary side power, then this can be as simple as a high voltage tolerant (linear) regulator, or a buck or DC-DC converter.  If you need secondary side as well, you need another DC-DC converter, or a dual winding supply, or something like that.  Or it might be easier to eliminate the primary side supply, in which case you need isolation (usually a gate drive transformer) to drive the primary side switches.

A GDT benefits from balanced PWM, as you'd see in a bridge or push-pull forward converter.  This is why a lot of wide range SMPS use a forward converter design, such as TL494 (which is also a very simple controller).  The secondary side inductor current can also be sensed directly, allowing average current mode control -- a vast improvement on the dangerous voltage mode control the TL494 is typically found with.

Tim

[fatalelieberi]

Hi T3sl4co1l,

Thanks for responding. I'm actually a really newbie in SMPS. I've been reading a book titled Switching Power Supply Design on how the theory works. It is a great book, the theory is explained clearly. However, it doesn't explain on how to put it into practice. I've looked up many PWM controller chips for SMPS, but many have their power input to be less than line voltage. But, there is a chip which can take direct line voltage, the LNK364. Sure it's for lower power application, but for testing the theory from the book, I think that's enough. So, I'm building right now a low power battery charger for 4V lead acid battery that has an output of 4.5V and 500mA, and utilises an isolated flyback topology. That's why I need to design a CC/CV feedback for the chip, to ensure the battery doesn't draw too much current.

Actually, there's another chip that might be used specifically for CC/CV application from the same manufacturer, LNK616. However, this IC is not widely available in my country. I need to import it from China, along a custom flyback transformer for the feedback loop. But, if this modified CC/CV circuit isn't working, I might buy the 616 chip from China though.

Since you're an expert, I think I might learn from you. There are several questions about SMPS in my mind:

• How do you power up SMPS controller ICs that have much lower voltage that the line voltage (110V/220V)? Is a voltage divider a good solution? Or should an auxiliary power, maybe using the LNK chip again, be used?
• Is limiting inrush current with NTC thermistor a really good solution? Is there better solution without using NTC?
• You said "...to eliminate the primary side supply...", does this mean the controller is put into the secondary side? How do you power the controller? Because you see, it's like a chicken and an egg problem. The secondary side gets its power from driving the primary side, but to have the controller to drive the primary side switches, you need to have power from the secondary side. 

Sorry for asking so many questions. But, I'm really interested in SMPS right now and want to learn from the experts in this field. 

[T3sl4co1l]

Sure it's for lower power application, but for testing the theory from the book, I think that's enough. So, I'm building right now a low power battery charger for 4V lead acid battery that has an output of 4.5V and 500mA, and utilises an isolated flyback topology. That's why I need to design a CC/CV feedback for the chip, to ensure the battery doesn't draw too much current.

Well, if you just start with a current mode flyback controller, the output current will be limited (if crudely) simply by virtue that the controller cannot deliver any more current than its maximum rating.  The crudeness occurs because, as the output voltage decreases, the duty cycle increases, so the output current increases.  If the input waveform remained constant, it would actually be a constant power relationship.  But that doesn't happen, so it's kind of something inbetween.

Ah, I forgot that LNK364 is one that's self-powered without an aux winding.  This suggests it'll operate down to low output voltages... but, it also has a "fault detect", which they don't happen to say how it's detecting a fault condition.

So LNK364 won't actually go to low output voltages while current limiting.

As for the load: are you expecting to charge batteries that are completely flat (~0V)?  Maybe it's an advantage, that the output current will be very low (because of the low duty cycle of the fault counter) until the controller leaves fault mode.  Gentle charging.  Maybe not necessary for lead acid.

How do you power up SMPS controller ICs that have much lower voltage that the line voltage (110V/220V)? Is a voltage divider a good solution? Or should an auxiliary power, maybe using the LNK chip again, be used?

If not for the internal fault circuit, the LNK alone would be fine -- it has a "voltage divider" sort of supply (well, an active one, a regulator, but it's burning a lot of power in the process, as a resistor does), and runs from that, all the time.

A traditional one like UC3842 needs an external supply.  It could be, yes, another LNK chip, or an off the shelf module (saves you the trouble of specifying another transformer and building the circuitry), or a buck regulator like one of these,
https://www.monolithicpower.com/DesktopModules/DocumentManage/API/Document/GetDocument?id=5

Is limiting inrush current with NTC thermistor a really good solution? Is there better solution without using NTC?

I'm not very worried about inrush on small things.  Tossing an NTC on there is a cheap way to obtain it.  Small supplies (like most LNK applications) don't even need an NTC, a regular resistor can be used (without costing much efficiency).  The resistor can also be a fusible type, eliminating the fuse component too.

If you're designing for CE market, you'll want to make sure all the EMC (including inrush, dips and swells, EMI...) are in order.

You said "...to eliminate the primary side supply...", does this mean the controller is put into the secondary side? How do you power the controller? Because you see, it's like a chicken and an egg problem. The secondary side gets its power from driving the primary side, but to have the controller to drive the primary side switches, you need to have power from the secondary side. 

You still need some initial power source, but as above, it can be a small PSU module, or a LNK circuit or whatever.

If you need controls on both primary and secondary side, it's more reasonable to eliminate the primary side controls (using a gate drive transformer to pass signals to the primary side switch(es)), than to eliminate the secondary side controls (because you still need some kind of power source to sense the output voltage and current, all the way down to zero output).

Tim