Brown-out functionality using LNK or TNY devices

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Which of the low power AC-DC CV devices lends itself best to implementation of mains brown-out detection?
Our standard products use TNY284DG-TL or LNK624DG neither of which seems really well suited to this.
Its a requirement for emergency lighting applications that the emergency function (ie mains fail) is triggered somewhere between 60% and 85% of the nominal mains voltage.
Basic designs with the chips listed tend to shut down in the range 60 to 70 VAC - which for 230 VAC mains is way too low.
Is there a better choice of device?
Its likely this function has been added many times - is there a conventional way of doing it for a very low cost?

Hi Mike at ABB,

Thanks for using Power Integrations' products.

For TNY284DG-TL, you can utilize the EN/UV pin to set the brown-in voltage 60% to 85% of your main input (230VAC) by adding typically two line resistors from DC bus to pin EN/UV. To calculate for the line resistors, use the formula below:

Line Resistor Value = (V_brown in required*sqrt(2))/I_LUV

V_brown in required = the desired voltage for brown-in/VAC_min
I_LUV = brown-in threshold

You can also check the datasheet of TinySwitch-4 datasheet in our website or use Power Integrations PI Expert Online. Please see attached schematic for 195VAC-265VAC with line resistors R1 and R2.

LNK624DG does not have this kind of feature, thus you can add additional circuit to enable brown in/brown out.

Other device for lighting application with line voltage sensing is LYTSwitch-6. You can also check this product in the website. Thank you. I hope this is helpful.

https://led-driver.power.com/products/lytswitch-family/lytswitch-6/

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Hi PI-Zek,
thanks for the response. Actually I had a suggestion from another group in our company to use a simple additional cct connected to the BP pin of an LNK623. All the cct is is a PNP BJT pulling that pin down via a resistor to the source. The base is driven from the rectified mains input. Its not clear from the data sheet that this is a supported function though its apparent that it works OK. Its clearly intended to keep the device from switching below a set mains voltage by overloading the regulator supplying that pin. Do you have additional documentation of any sort that addresses that behavior in any more detail?

Hi Mike,

You can read the function of the of line sensing in the datasheet.

https://led-driver.power.com/sites/default/files/product-docs/tinyswitch-4_family_datasheet.pdf

A design example with undervoltage lockout option is in RD-839:

https://ac-dc.power.com/sites/default/files/product_document/design_example/rdr-839_12watt_power_supply_using_tinyswitch-4.pdf

If you want to verify the values of the line resistors of undervoltage sensing, you can use the PI Expert tool.

https://ac-dc.power.com/design-support/pi-expert/pi-expert-online/

Thank you.

Hi PI-Zek - thanks for responding. We built some prototypes using the TNY284 and implemented the line undervoltage function as outlined. It did not function the way we hoped. It did prevent startup under low mains (brown-in) but did not shut the switcher down once running when mains dropped (brown-out). Actually the datasheet is a little unclear in this area or I was reading it and interpreting it in a way that gave me the functionality I was looking for. So is there a way of implementing brown out with this chip - by adding extra components? We want the unit to stop switch when the mains falls below 60 to 85% of the rated value of 230VAC ie around 167VAC

Hi Mike,

The UV sensing should work for brown-out as well. Please see page 44 of RDR-839 and check the waveform. Please double check the values of your line sensing resistors.

Hi PI-Zek - well that's encouraging to hear that but there does seem to be a problem with our cct in the light of that. We have 9.4Mohms as our res value - so with I_LUV at 25uA we expect about 167VAC as the brown out value. In fact we get 176VAC brown-in and 78VAC brown-out. I have attached a copy of the cct (Mains Power.pdf). As far as I can tell we have followed the directions in the datasheet closely - and it matches the PI expert file. Any suggestions?

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You can try reducing the value to 7.8M to 8Moms. The current in EN/UV pin must exceed 25uA to initiate switching/disable switching. If the brown-out is not the desired turn off voltage, an additional circuit may be required. Let me check if there are existing projects with the same application. Thanks.

Hi PI-Zek - thanks. I tried reducing the value from 9.4 to 4.7 Mohms. It dramatically reduced the brown-in value (by half) and had no effect on the brown out value. It suggests the under voltage function works in one direction only. As I mentioned in an earlier post one of our other R&D groups uses an additional cct to pull down the BP pin on an LNK623. It may be that approach works with the TNY284 too - by pulling the BP/M pin below 4.9V when the mains is below the desired threshold.. Its not clear from the data just how much current one should pull out to have this effect though.
Any help appreciated ...

Can you verify if your test setup is full-load or no-load? If the set up is at no-load, the V sensing may not work properly when the output is not fully discharged.

If this still does not work, maybe an additional circuit is needed. Okay I will let you know if I come across other applications for brown out functionality. Thanks.

Hi PI-Zek - yes the test was done with cct fully loaded.