High voltage 150W LLC switch mode power supply. - Page 1
Forum section: electronics / general

author: Enceladus
author_rank: Contributor
timestamp: June 03, 2019, 05:38:06 am
content: Hello!Usually a simple flyback converter is used for high voltage (300-500V) power supplies. It is pretty straight forward, although it produces a lot of noise. I was unable to get a <500mV ripple at 300V full load (~50W) no matter what kind of LC filter I used. The output signal always have a high frequency component as well (see attached, output voltage, AC coupled).It seems that LLC topology is less noisy than a standard flyback so I would like to redesign my power supply. Unfortunately I haven't found any high voltage LLC power supplies schematics, so I have a small question: is there a particular reason why there are no such examples? I found several designing guides and I'm thinking of using some reference design (e.ghttp://www.ti.com/tool/PMP10283) and change the output stage components to achieve 300V 150W output.Thank you!

author: f4eru
author_rank: Super Contributor
timestamp: June 03, 2019, 03:55:08 pm
content: HelloDo you mean 300-500V output or input ?A resonant architecture has a much cleaner specrtrum on the EMC side, so it´s generally easier to filter, as the PSU already incorporates a filter.A full wave rectifier at the output also helps to reduce ripple when using a forward, because of the higher on times.150W is at the upper edge where flybacks make sense, YMMV.But there´s more than just output ripple.Any SMPS will radiate, couple to input in a differential manner, output differential, and in-to-out common mode. That is 4 was to output EMC energy, if there´s no ground.The ripple you measure after your filter is probably coupled to your oscilloscope probe or wiring via inductive or capacitive coupling, bypasssing your filter, so perhaps you don´t really measure what you intend.The ESR of your filter cap may also play a significant role.Try shielding, magnetically and capacitively, reducing transformer through capacitance or improcing differential mode couplöing into this cap by winding layout, or increasing the Y-cap if possible.Also a good advice : look at the ripple with a spectrum analyzer instead of a scope, it really helps to understand and diagnose the problem. With a scope you can only vaguely guess. Don´t use the scope´s FFT, it´s useless. Protect the specan input with a 20dB attenuator.Also a way to improve EMC is to spread the chop frequency. This will not reduce the noise, but improve your EMC compliance significantly on the coupled range if done right.

author: Enceladus
author_rank: Contributor
timestamp: June 04, 2019, 12:20:16 pm
content: Quote from: f4eru on June 03, 2019, 03:55:08 pmNope, 300V outputQuote from: f4eru on June 03, 2019, 03:55:08 pmYou were right, the form of that high frequency ripple was due to the improper measuring technique. I used a probe spring and got a much better result. PIC_58_3 and PIC_58_4 show the output voltage ripple at the output capacitor (before LC filter) while PIC_58_8 and PIC_58_9 show the waveform of the ripple after a 470uH+22uF LC filter. Looks better but still has a 100mV ripple amplitude. I'll try to put a second LC in series and have a look at it.Nevertheless, the power supply is pretty noisy. If you bring the probe close (~1cm) to the output transformer then you will see something like the waveform on figure PIC_58_11. It seems that the only way to reduce the EMI noise is to put the whole power supply into a shield.Try shielding, magnetically and capacitively, reducing transformer through capacitance or improcing differential mode couplöing into this cap by winding layout, or increasing the Y-cap if possible.Also a good advice : look at the ripple with a spectrum analyzer instead of a scope, it really helps to understand and diagnose the problem. With a scope you can only vaguely guess. Don´t use the scope´s FFT, it´s useless. Protect the specan input with a 20dB attenuator.Also a way to improve EMC is to spread the chop frequency. This will not reduce the noise, but improve your EMC compliance significantly on the coupled range if done right.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 04, 2019, 08:48:12 pm
content: If probe grounding makes a difference, then the problem is common mode. Add a filter choke and Y caps.Tim

author: ocset
author_rank: Super Contributor
timestamp: June 09, 2019, 08:03:23 pm
content: You are measuring output voltager ripple?.....use a coaxial probe.....ie not the normal scope probe with ground lead...this will educe pickup into the scope probe leads......splay out the ground mesh of the coax and twizzle it into a "wire", and solder it to the ground, then solder the centre conductor to the rail...then put a scoep connector on the other end of the coax and put into your scope...(beware isolation etc.)