Flyback PSU for op amp?  - Page 1
Forum section: electronics / general

author: Feyman_Follower
author_rank: Contributor
timestamp: June 20, 2023, 09:06:09 pm
content: Hello!In my next project I will use an op amp and I had the idea to design a flyback converter with two secondary coils to supply ±15 V; I have a question: am I right or it could be too noisy or full of ripple for a precision op amp?I am planning to use an AD797 audio op amp but I want a general solution for all types.I will use high quality tantalum filter caps and a switching frequency between 250kHz-1MHz.Thanks, as a student any hint is welcome

author: f4eru
author_rank: Super Contributor
timestamp: June 28, 2023, 09:05:54 pm
content: you can filter anything, but the effort increases for very smaller signals.For Audio, if your SMPS PWM is much higher in freq than Audio signals, should be no problem, as long as a PWM amp does not beat with the SMPS to an audio freq.PSU regulation loop frequencies could be a problem, because typically in the kHz region, make sure to have good stability.common mode pickup, GND loops can affect also.For low currents, RC filters can be better than LC.

author: Faringdon
author_rank: Super Contributor
timestamp: June 28, 2023, 11:16:30 pm
content: If your vin is mains, the 250khz to 1MHz is too high frequency.Also, the opamp will be a load of some few mA?So we are talking very very low power...why not just use a 1W recom or TDK module, or murata module, to give you an isolated +/-15v...take a look on digikey.But yes you can always just filter out power supply vout ripple, as said in previous post.The noisiest thing about SMPS is the drain node dv/dt...(and the associated fast di/dt's) so slow that up to make it less noisy.

author: Phoenix
author_rank: Frequent Contributor
timestamp: June 29, 2023, 12:45:21 am
content: A common solution for ripple on the voltage rail is to use linear regulators after the switch mode. Set your flyback to generate say 18V and then linearly regulate it down to 15V. It could be a simple zener+BJT regulator at such low currents.For higher frequency ripple/noise outside the bandwidth of the linear regulator you'll need to use LC or RC as low pass filters.

author: Faringdon
author_rank: Super Contributor
timestamp: June 29, 2023, 07:22:00 am
content: Dave Jones has a vid on use of linear regs for that.As i rem from the vid, lin regs to iron out ripple is no great shakes.

author: Konkedout
author_rank: Frequent Contributor
timestamp: July 02, 2023, 01:26:42 am
content: What is your input voltage source? Recommend using cheap wall wart or an old/extra laptop power supply to handle non-SELV AC mains voltage. I do not recommend that you take mains input voltage directly. Flyback can work but is probably not the best unless you need voltage isolation.For making +/- 15VDC output when starting with 12V - 24V input, my preferred converter type is a combined SEPIC-Cuk converter using a coupled inductor. For the coupled inductor, you can use a Coilcraft "Hexapath", or Wurth "Flex" or similar from TDK or (what used to be called Cooper Coiltronics).Note: I have built a number of these and it is a good circuit. But you will need to engineer it.If your current is only a few mA, you might be able to start with 12V input and make +/-15V with a boost/pump combination. The -15 will be a bit lower, so you could make +18 for example, run that through a linear regulator, and be able to get -15 off of the pump output. Cheaper and simpler but not suitable for a lot of load current.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: July 02, 2023, 04:07:42 am
content: Take a look at TPS65131. Output ripple is 10mV, @ ~1.2MHz. An Additional LC filter can suppress it further.