flyback smps using lm2585-adj
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From: johnny.dr on 18 Jul 2005 16:08 I may be somewhat off topic but i've ran into a mountain i cannot seem to get over regarding the hardware design of a switching mode power supply. I'm using an LM2585-ADJ simple switcher chip from National in a non-isolated flyback configuration to generate 3 output voltages: 3.3, 15, and -15. The monitored (feedback) output, 3.3V, has a lot (1.3 V pk-pk) of transient noise at and only at every transition of the primary switch (off to on, on to off). Elsewhere, it is very clean. If anyone has any experience in this area and would be so kind to offer any help I would greatly appreciate it. If so, I'll be happy to post all the gory details. Thanks!
From: Jim Stewart on 18 Jul 2005 16:23 johnny.dr(a)gmail.com wrote: > I may be somewhat off topic but i've ran into a mountain i cannot seem > to get over regarding the hardware design of a switching mode power > supply. > > I'm using an LM2585-ADJ simple switcher chip from National in a > non-isolated flyback configuration to generate 3 output voltages: 3.3, > 15, and -15. The monitored (feedback) output, 3.3V, has a lot (1.3 V > pk-pk) of transient noise at and only at every transition of the > primary switch (off to on, on to off). Elsewhere, it is very clean. > > If anyone has any experience in this area and would be so kind to offer > any help I would greatly appreciate it. If so, I'll be happy to post > all the gory details. Thanks! > I'm not sure I could help, but I'd sure like to see a schematic.
From: johnny.dr on 18 Jul 2005 16:32 Jim Stewart wrote: > > I'm not sure I could help, but I'd > sure like to see a schematic. no problem. http://www.thistopicsucks.com/smps.pdf
From: Jim Stewart on 18 Jul 2005 19:18 Jim Stewart wrote: > johnny.dr(a)gmail.com wrote: > >> I may be somewhat off topic but i've ran into a mountain i cannot seem >> to get over regarding the hardware design of a switching mode power >> supply. >> >> I'm using an LM2585-ADJ simple switcher chip from National in a >> non-isolated flyback configuration to generate 3 output voltages: 3.3, >> 15, and -15. The monitored (feedback) output, 3.3V, has a lot (1.3 V >> pk-pk) of transient noise at and only at every transition of the >> primary switch (off to on, on to off). Elsewhere, it is very clean. >> >> If anyone has any experience in this area and would be so kind to offer >> any help I would greatly appreciate it. If so, I'll be happy to post >> all the gory details. Thanks! >> > > I'm not sure I could help, but I'd > sure like to see a schematic. Try moving C8 over to the far side of the inductor, along with the feedback source. My gut tells me the inductor should be closer to 470uh and the output cap more like 330ufd. Without knowing your current requirements, it's just a stab in the dark though.
From: Bob on 18 Jul 2005 19:26
<johnny.dr(a)gmail.com> wrote in message news:1121717330.849331.281990(a)g47g2000cwa.googlegroups.com... > I may be somewhat off topic but i've ran into a mountain i cannot seem > to get over regarding the hardware design of a switching mode power > supply. we've all been there... > I'm using an LM2585-ADJ simple switcher chip from National in a > non-isolated flyback configuration to generate 3 output voltages: 3.3, > 15, and -15. The monitored (feedback) output, 3.3V, has a lot (1.3 V > pk-pk) of transient noise at and only at every transition of the > primary switch (off to on, on to off). Elsewhere, it is very clean. 1: is the noise really there? you can't probe a SMPS using the alligator ground lead and the normal snout of you scope probe. Pull the snout off the probe. Now you can see the tip and the shield of the probe. If your probe comes with a low inductance ground clip (springy wire ~2cm long) then slide that on to the sheild. Otherwise, use a jewler's screwdriver to make the ground connection by holding the probe and the scewdriver in a little triangle on top of the output cap. It's tricky to make all 3 connections simultaneously - having someone else there to handle the scope helps a lot. The output cap is the only place to look for the noise - anything else is common-mode and can be managed at the system interconnect level (careful return path design). 2: If the noise is still there when properly probed, then the problem is poor circuit layout. The LM2585 data sheet should have a section on layout - follow the advice on keeping the output current loop small. HTH, Bob |