100-200ns comparator needed for DIY Constant Off Time controller - Page 1
Forum section: electronics / projects

author: ocset
author_rank: Super Contributor
timestamp: September 28, 2017, 07:45:26 pm
content: Hello,We are doing a 230VAC offline, 150W, Power Factor corrected, single stage LED driver.We need constant off time control. However, there are no OffTheShelf control chips available for this purpose. Therefore , we have to make our own as in the attached schematic.As you can see, the DIY constant off time controller unfortunately consists of three separate IC’s and a signal FET….As you can see, there are also a number of resistors involved there. There are fast, high rise time signals within the bounds of the controller, and shielding this lot of components from noise will be a challenge.As such, we need to use hysteresis with the PWM comparator. The LT1713 comparator gives a very convenient Q! signal which is ideal for giving hysteresis. However, the LT1713 comparator is way too fast for our purpose, (it’s a 7ns comparator). We need a 100-200ns comparator instead, to make noise free operation easier to achieve. However, there are no OffTheShelf comparators which have the Q and Q! outputs that the LT1713 does. Can you confirm this? Do you know of a 100-200ns comparator with Q and Q! outputs?Schematic and LTspice sim attached.

author: wraper
author_rank: Unknown
timestamp: September 28, 2017, 08:10:03 pm
content: I don't see why hysteresis in this circuit would ever require inverted output. IMO this is invented problem to increase BOM cost. And even if you desperate for it, you could use cheap dual comparator and use second comparator as inverter.

author: mikerj
author_rank: Super Contributor
timestamp: September 29, 2017, 02:32:13 pm
content: Quote from: treez on September 28, 2017, 07:45:26 pmWhat wrong with NCL30100 or NCL30105?

author: ocset
author_rank: Super Contributor
timestamp: September 29, 2017, 07:38:37 pm
content: Thanks but to get PFC we need all that architecture in the chip. (as in the top post).The NCL parts just dont allow us to mold the current sense threshold to the sine of the mains input.

author: Marco
author_rank: Super Contributor
timestamp: September 29, 2017, 11:50:57 pm
content: It all looks a bit indulgent ... by the time you have the level shifter and regulators for the two extra rails in there it's going to be a tad complex and expensive. Even for a student project where the whole point is to avoid a COTS chip this seems excessive.For instance, do you really need a monostable? Can a simple RC network which is charged to a constant voltage when gate is high and discharges after that not suffice for the constant off time using the comparator?

author: ocset
author_rank: Super Contributor
timestamp: September 30, 2017, 06:55:46 am
content: Thanks you're right the component count is high because there are no chips that do this in integrated form.The RC offtime sounds good , but as you know , even that needs extra "bells and whistles" to get it "Plumbed in".As you may notice from the first post, the inductor of the buckboost converter carries peak current up to 4.7A and is 470uH in value. Its RMS current is 1.8A (average current is 1.3A).Instead of using a single large custom wound, 470uH inductor on a PTH former with a gapped ETD59 ferrite core, our contractor wants to “daisy chain” ten MSS1078T-473ML (47uH) inductors In a series chain to make up the 470uH.We have explained that such a long series chain of inductors is unwise because…1….It results in multiple switching nodes on the PCB, which can cause noise problems.2….It results in a large loop of power switching current with high di/dt in it, which again can cause noise problems.Do you agree?MSS1278T-473ML (47uH inductor):http://www.coilcraft.com/pdfs/mss1278t.pdf