LinkSwitch-HP remote on/off function
I have a working design very similar to that of RDR-313 and I am looking at ways to incorporate a remote on/off function, to be controlled by our microcontroller on the secondary (load) side. Since I need to maintain galvanic isolation between primary and secondary, I have no way of pulling up pin 1 to >1.3V (as described on p4 of the LinkSwitch-HP family datasheet) from a voltage source on the secondary side (other than by using a small pulse transformer as an isolated supply). I could develop 1.3V from the 325V DC bus by using a zener reference and current limiting resistor. This voltage could then be switched onto pin 1 using an optocoupler as in the attached schematic.
A simpler alternative I have found is that by pulling pin 3 (CP) to GND using an optocoupler, the output is disabled, as in the second diagram in the attachment. Every 1.3 seconds the LinkSwitch device checks to see if it can start switching again. If pin 3 is not pulled low, the device resumes switching. I have not detected any power loss within the LinkSwitch device when it is disabled, nor noted any other problems with this scheme.
1. Can you please advise if there is a better method than what I have outlined for pulling pin 1 to >1.3V?
2. Can you please advise if there is any reason why pulling pin 3 low to disable the output is not advisable?
Thank you in advance,
-Sebastien.
Comments
"You might be able to use the shunt regulator supplying the BP pin to provide the voltage needed to bring the PD pin > 1.3V."
That's a good idea. If your colleague agrees that it would not be problematic/injurous to the IC, I'd like to try it.
In order to do so, I would need to know if it is OK to directly connect the BP pin to the PD pin via the opto to drag it to >1.3V, or if I would need to insert a current limiting resistor between the two pins.
I presume that if this worked, the internal regulator bleeding energy from the DC bus (whilst the output is disabled) would be dissipating power. We only need to disable the output for a few hours at a time when the product is in operation. Is there any risk of irreperable device failure under these circumstances, as a result of deriving power from the internal regulator over an extended period, rather than from the bias/feedback winding?
Thanks.
So it looks like there are a couple ways you can configure remote on/off with LinkSwitch-HP. You can bring CP down to SRC as you're doing now but it's not preferred as it could affect the control loop. Another method would be to use the BP pin with a 100k resister to bring the CP pin voltage up and disable switching. This isn't as likely to affect the control loop because of the 100k resister. You can also use the BP pin with an opto and current limiting resister to the PD pin. You shouldn't need much current to bring PD up > 1.3V.
Whichever method you use, the IC will be dissipating some power. The IC has an internal shunt regulator which provides the BP pin with current so that the controller logic is continuously working. While the IC isn't switching though, this current is small. Use of a current limiting resister from the BP pin to CP or PD will keep the current draw small and total power dissipation down as well.
I hope that helps. Let me know if you have any other questions.
-The Traveler
The CP pin is the output of the error amplifier and is provided so that you can change the loop compensation. Adding an opto onto this pin might change the loop characteristics and I'd be wary of that solution.
Let me double check with one of the LNK-HP guys and see if he has any suggestions. You might be able to use the shunt regulator supplying the BP pin to provide the voltage needed to bring the PD pin > 1.3V. Let me double check this first though.
-The Traveler