Help with the OnSemi NCP1252 - Page 1
Forum section: electronics / projects

author: chintal
author_rank: Newbie
timestamp: October 09, 2020, 07:59:34 am
content: I'm trying to use the NCP1252 to build a DC/DC converter. Included here is the schematicand the associated calculationscalculations.pdf(48.29 kB - downloaded 391 times.) I have made in the design.The circuit is intended to accept 24VDC input and generate isolated 15V and 5V rails. The circuit has a flyback topology with secondary side synchronous rectification using the NCP4306A. Feedback is generated though the 5V secondary.What I am seeing is there are no pulses on the drive output of the NCP1252 at all, not even during and immediately after startup.Vcc of the chip is at 24V DC, BO is at ~1.6V, FB is at 0V. As far as I can tell, this should be sufficient to get the drive pulsing irrespective of anything on the secondary. However, it does not.The load I intend to use this for is ~1-2A into a DC motor on the 15V rail and ~0.5-1A of digital circuits, fans, etc on the 5V rail. For testing, I have placed a 25ohm load on the 5V output to keep the feedback valid.Any assistance, insight, or advice you can provide on this would be greatly appreciated. I have spent the last week poking at this thing every way I can think of and have made absolutely no progress.

author: perieanuo
author_rank: Frequent Contributor
timestamp: October 09, 2020, 02:53:13 pm
content: hi,R20 is not necessary, you have internal pull-upchech soft-start capacitor value, maybe eliminate him for testingyou may increase maybe Vbo a little bitbut Vfb=0V, why??? U5 should be opened... search here firstregards,pierre

author: perieanuo
author_rank: Frequent Contributor
timestamp: October 09, 2020, 03:01:21 pm
content: put some load also directly on secondary for 5V (xf1 1-12) in case your 5V regulator is dead and you have no load at alledit: it appears your IC is in the mode "Skip cycle feature" because your Vfb is 0, as I mentioned beforeat start-up, your opto-coupler transistor (the output side) is opened so you can't have 0V there, check the schematic and maybe your oc is shorted?

author: chintal
author_rank: Newbie
timestamp: October 09, 2020, 03:28:01 pm
content: Quote from: perieanuo on October 09, 2020, 03:01:21 pmDo you mean on the secondary directly, bypassing the rectification? I have placed load on the final 5V output, but not directly on the secondary.Also, wouldn't I still see atleast a few pulses on the primary drive during startup even without load and broken feedback?

author: chintal
author_rank: Newbie
timestamp: October 09, 2020, 03:33:01 pm
content: Quote from: perieanuo on October 09, 2020, 02:53:13 pmI have not soldered R20, left it in there in case the pullup wasn't enough.Quote from: perieanuo on October 09, 2020, 02:53:13 pmI will try to do that. I actually have a good feeling about this, the soft start was one of the things I just guesstimated, and now that I think about it, maybe removing it could atleast make it startup.Quote from: perieanuo on October 09, 2020, 02:53:13 pmMy mistake, Vfb is 6.1V approximately. When typing out the message here it somehow slipped my mind and I wrote 0 V. Terribly sorry about that.

author: perieanuo
author_rank: Frequent Contributor
timestamp: October 09, 2020, 03:39:56 pm
content: exactly, 'bypass' the rectification, put the load in AC side, it's ok for starting the oscillationby the way, like old 3842 chips, you can detach the drain from the control ic and put a scope on ic output, but anyway the ic has overload protection.as you said, he must generate output pulses

author: chintal
author_rank: Newbie
timestamp: October 19, 2020, 04:59:07 pm
content: Update and Bump :Removing the SS capacitor has had no effect. The voltage on the SS pin is still stuck at zero (0.04 mV on my DMM), and I see no output pulses from the NCP1252I expect the NCP1252 is fried. I am considering replacing it with another, but I do not have any reasonable or even unreasonable explanation for why the current one is dead. As such, I'm not sure if replacing it with a new one is going to help. The only change I have made to the circuit is fixing the Vbo network, which originally had the chip held in brownout.