EEVblog #1294 - LLC Resonant Mode Converter Design

A brief look at how LLC resonant mode converters work and their advantages. A spin-off from the Rohde & Schwarz NGP800 power supply video.

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[EEVblog]

NOTE: There appears to be quite some debate as to whether this NGP800 PSU is actually a resonant converter topology, and yes, I need to take a closer look at this. But just go with the flow and pretend it is for the purposes of this video.

[bobcunningham]

It's been 25 years since I worked on one of these! It was for a US Navy submarine reactor control system, where space and heat were both huge issues given the power density we needed to meet (there was no room for conventional supplies). After a failed prototype followed by a failed tweak to it, we got a successful design only after we gave up trying to use pre-approved MIL-spec magnetics and made our own. Literally: We hand-wound the first two sets.

It was a design competition between two companies, and the Navy was impressed by how much smaller and cooler our supplies were compared to our competitor. In the end, we didn't get the contract. But we sold our supplies (and other tech we pioneered) to our competitor for an eye-watering markup. Turns out we made the profit we hoped to earn on the entire project just by selling some parts and licences. The engineering sadness at the loss was soon overcome by the financial win, and the sense that we had ducked a bullet: We likely would have lost money had we won!

[testep]

There's no where else on Earth that a hobbyist can get content this damn good! I paid $40 for a Beginner's Electronics class for my nephew and the content in that class isn't even remotely as good or educational as the information on this channel. Dave, you sir are a legend! It can't be overstated how much we hobbyists appreciate the information you provide.

[MetalheadAndNerd]

These application notes are better than some books! I imagine an engineer putting all his teaching skills into the papers since none of his younger colleagues is interested anymore into how the stuff works they are making.

[scor]

I’m speechless) The most complete analysis of an LLC architecture what so ever! There are two transformers because they took two standard half-bridge transformers and connected primaries in series and secondaries in parallel (there is no need to calculate a new transformer, primary L is doubled, secondary L is cut a half win-win)

[BrianCairns]

LLC resonant converters are actually getting pretty common in the PC power supply space.


Nearly all 80 Plus Gold/Platinum/Titanium models are use a LLC resonant primary. Many also use synchronous rectification on the secondary side.
Even mid-range models like the Corsair CX450 (which is "only" 80 Plus Bronze) have moved to LLC resonant topologies.

[EnergyWell]

Dave, you remind me of all the old hat professors I had in college, worn smooth from years of experience in industry. You are a wonderful and entertaining teacher, and a treasure amongst makers and engineers. Thank you for everything!

[marcosaraque]

The first assignment in my resonant converters class was to design one of these, and you got it spot on. And you're right about it being hard, it was painfully difficult, especially if you are designing controls as well

[MarcosChaparro]

My team and I designed a bidirectional LLC converter for an EV, getting it working both ways was crazy. Custom magnetics, custom math.
You end up with LLC because of EMC and efficiency targets, and they suck at regulating fast moving targets.

[byronwatkins]

Actually, it is the high-current switching transients that generate high-energy noise. Resonant mode supplies switch at near zero current and thus generate much less noise. Resonant mode supplies DO vary the driving frequency. This is how they change the current that flows from a constant input applied voltage amplitude. This does not present a noise problem and, in fact, is equivalent to dithering a microprocessor clock to spread the noise over a broader spectrum

[pilotmeh]

You are a legend, Dave! I should start studying EEVBLOG instead of going to uni!

[flomojou]

Nice! Brings to mind some of the complexity of a dual-resonant solid state Tesla coil, requiring a lot of careful design and tuning.

[byronwatkins]

Normal switches' drains begin at the input power voltage and must conduct current until they are open again at the input power voltage. At the beginning and end of these time intervals, they also have non-zero drain-source voltages. Since P=VI, there are substantial power pulses at the beginnings and ends of these pulses. In resonant designs the series capacitor stops the current every half cycle and allows the switches to change state without dissipating these pulses of power.

[pocoapoco]

I guess I’ve graduated from Fundmentals Friday to Sunday.

[khronscave]

16:37 What're the odds those two are both transformers, but the secondaries are wired in parallel for the 32V outputs, and in series for the 64V outputs?

16:55 A big fat inductor (on a *black* core) with a MOSFET next to it, takes my mind towards a mag-amp type regulator. Can those be made with an adjustable / variable output voltage, or do they need to be designed for a fixed value?

[egbertgroot]

Very well explained! Now i know where losses come from. Forget the square wave!

[byronwatkins]

The resonant circuit filters out the higher harmonics of the square wave drive so only the fundamental is a pretty good approximation.

[poptartmcjelly]

That bigass inductor is probably a secondary filter and that MOSFET next to it is to replace the freewheeling diode that you'd normally see there.

[antoniomonteiro]

The final MOSFET you found may be to have a linear regulator after the switching one. This would allow very precise output control and could be made to take only a few volts across. Makes sense?

[StreuB]

These videos right here are worth the price of admission. A+ for guys like me who are looking to learn. Thank you Dave for all the work you put into all of this as well as your enthusiasm. It really means a lot.