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Old 3rd May 2005, 02:03 PM   #1
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Default 700W ZVS switchmode converter

Hi all

I've started this thread because interest has been shown in ZVS converters in threads about other stuff. The circuit I've posted is that for the the main converter for my PowerDAC 1 digital amplifier, and was designed about 5 years ago.

The converter is designed to run on the non-isolated +385V-400V dc bus from a PFC preconverter. Design minimum was 350V. A non-isolated +18V supervisory supply referred to COM, is required
The converter uses a full bridge with ZVS switching to deliver 75V 700W at a measured 91% efficiency, using an ETD39 coreset, gapped. Switching frequency is 176.4kHz and there is provision for locking this to an external sync signal through an isolated input. Another isolated input accepts a shutdown signal, which can be fed to a suitable PFC shutdown input on the same side of the isolation barrier.

There is a cycle by cycle current limit, and conventional voltage mode control is used. The three phat Transzorbs are to limit voltage buildup when a downstream adjustable synchronous regulator in digital amp has it's output voltage reduced rapidly.


Some suggestions for improvement: There is no independent OverVoltage Protection mechanism (OVP) and this should be added in the interests of safety. The ML4818 is rather long in the tooth now, and better, less power-hungry chips exist, such as the UCC3895. It would also be better to use current mode control and remove the transformer's primary coupling capacitor.
Another efficiency-boosting addition would be the use of MOSFETs as synchronous rectifiers in a current doubling topology as advocated by Unitrode's Lazlo Baloch in several appnotes from TI.

I've often wondered on the feasibility of using a (distributed gap) big MPP toroid for the main transformer. Losses may be too high, but it may be worth doing a few sums, as the gapped ETD39 is likely to generate considerable EMI.

I would hope that this circuit may be of interest to members and might stimulate discussion on improvements and possibly be evolved with group effort to a really neat SMPS PSU. The circuit posted has been in use for about 4-5 years with no failures.

NOTE!! This circuit, although isolated, relies on construction of an SMPS transformer to achieve this isolation. Anyone who hasn't constructed offline SMPS transformers before, or is not familiar with creepage and clearance distances and other safety aspects found in UL1950, should not attempt to build something like this. There are high voltagees involved and great care must be taken.


Regards


John H
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Old 11th May 2005, 05:21 AM   #2
Alme is offline Alme  Ukraine
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John, thanks a lot for sharing such an experience. After reviewing your design and ML4818 datasheet (I wasn't familiar to it before), I would ask you some questions, more as DIYer than engineer
From the first sight it seems not easy to get how zero-voltage switching is organized.
Is it about that we adjust time delay between upper and lower side FETs on-state, so called deadtime - 250ns specified on your schematic, and also transformer inductance value, which deals with pulse rise/fall time together with capacitances in full bridge? Is it that why magnetizing/stray inductances also specified on schematic? If I understand it right, then supposedly I need to adjust deadtime for any new similar design to get ZVS operation, due to different FET type/transformer, etc., right?
Can you please explain why we need here gapped ETD core for transformer. Anyway core operation seem to be like in PWM mode, from the transformer's "point of view", and there is decoupling capacitor C10. Or is it just to get such quite low inductance value to allow such high-frequency ZV switching. Please clarify this.
And regarding your idea to replace ETD39 for MPP toroid.
I've had not much experience about using toroid transformers for mains power supplies, just made a couple of forward type 300-400W converters with toroid transformer. There's been everything well about principal operation, but terrible problem with HF noise and switching spikes on output DC rails. I traced that problem to transformer primary-to-secondary capacitance. After replacing transformer with ETD type, having single turn of copper foil shield between primary and secondary, I had solved the problem. Later, I was not quite successful about making some shield winding on toroid transformer. Also efficiency was about the same with toroid and ETD transformer, so I gave up using toroids for mains converters.
And one more question. Is it good idea to move driver part to secondary side, together with all logic signals and its supply (as we use transformers to drive gates anyway), and thus omit transfer of feedback analog signal thru exotic double-photosensor optocoupler?

Thanks,
Alex
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Old 11th May 2005, 06:04 PM   #3
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Default ZVS operation

Hi Alme:

Yes, I reckon you just about got it! The clearest reference I could find was MicoLinear Application Note 19, and even that
I had to read several times. Another good one is 'Designing a Phase Shifted Zero Voltage Transition (ZVT) Power Converter' by Bill Andreycak of TI/Unitrode. Then there's 'Design Review: 500W 40W/in3 Phase Shifted ZVT Power Converter', also by Bill Andreycak. I had the good fortune of attending a seminar presented by Bill today, as it happens. These appnotes have no identifying numbers on them, and I think they're from the Unitrode Power Supply Design Seminar series, which is on the TI site; like looking for a needle in a haystack,I know.

As I recall, the dead time is adjusted equal to one quarter of the period of the resonant frequency of the FET's Cds and the L(leakage) of the transformer. I set this up on a scope.
The gapped core is to control the L(magnetising) of the transformer to achieve a desired rate of rise of current. (Gapping doesn't significantly affect L(leakage)). The ZVS requires a certain minimum current, either from the load or from a deliberately high magnetising current. If the I(mag)
is too low, then there's a significant minimum load requirement on the converter for proper ZVS operation.

I have found my transformer design notes for the ZVS. They're in pencil on paper (proper stuff, eh, no computers req'd). Over the next few days I will scan them in and try make a pdf file of them,
so I can post them on the forum.

Cheers


John Hope
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Old 11th May 2005, 06:15 PM   #4
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Hi,

I hope this link helps a little.

Best regards,

Jaka Racman
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Old 12th May 2005, 05:07 PM   #5
Alme is offline Alme  Ukraine
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Thanks, John; thanks, Jaka. Things become clear step by step

Another question: how do I know transformer stray inductance value before actual transformer making? It doesn't seem apt to calculation
And if so then what is easy way to measure it and distinguish from magnetizing inductance (which is maybe easy to measure) ? Before closer look at ZVS, I never had the need to define it...
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Old 12th May 2005, 08:04 PM   #6
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Hi,

calculating stray or leakage inductance is difficult, but measurement is easy. Just measure with shorted secondary. When you measure with open secondary, you are actually measuring sum of leakage and magnetizing inductance which are series connected. By shortcircuiting secondary you are actually bypassing magnetizing inductance and all that is left is leakage inductance.

Best regards,

Jaka Racman
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Old 12th May 2005, 09:49 PM   #7
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Default Resonant parameters

Jaka: Snap, you took the words right out of my mouth!
Alme: This is one of those 'chicken and the egg things'. You don't actually know the L(leakage) until you've wound the transformer. You know what ballpark it will be: 5-50uH is typical. But you don't need the L(leakage) to be known to design the transformer, so you can do this first and then calc the resonant parameters afterwards. If you have to mod the transformer the L(leakage) might change a bit, but you can accomodate this with the dead time.

I've posted my design calcs for the resonant parameters of this converter; they were done after the xfmr was finished. These formulae were taken directly from ML appnote no. 19. What's interesting is that even with a L(mag) as low as 250uH, you still need a minimum load to guarantee ZVS action. You can't decrease L(mag) too much (I wouldn't go below 250uH) because you will then have high primary currents flowing back and forth that will increase copper losses and make your trafo run hot. But a minimum load isn't such a big deal; in the given design the quiescent power drawn by the amplifier circuitry was such that the 'deliberate' load resistor I needed to make up the required minimum load was only a couple of watts. It may not be enough to guarantee full 100% ZVS action, but even 'near ZVS' switching is far preferable to hard switching.

Although I previously said that it would be an improvement to go to current mode control, I'm no longer so convinced of this. CM is at a disadvantage at 0 load and very light loads because the current feedback signal corresponding to the very low currents at low loads tends to get swamped by noise. Audio amps in a domestic environment spend a lot of time at light load and it might be wiser to stick with the voltage mode control if this is the target application.

I totally ruined the family kettle testing this powerr supply, but that's for another forum.

I will post the transformer design details soon.

Regards

John H
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Old 14th May 2005, 06:19 PM   #8
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Default ZVS Transformer design

Here's file 1 of my design notes for the transformer design.
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Old 14th May 2005, 06:41 PM   #9
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Default ZVS transformer design

Here's file 2.

Some postscrips to the attachments:

1. During development I found it difficult to fit all the windings AND screens comfortably on the bobbin, and my final transfomer design was modified to use 81/0.1118mm Litz wire for the secondaries. (I somehow acquired a big roll of this. . .) Using this in practice resulted in a higher measured temperature rise, but still acceptable (35C as measured on the trafo outer winding tape, if I recall)

2. The design notes are based on 2mm spacing tape against each wall of the bobbin, giving a 4mm creepage in total. (For 400Vdc primary bus and with secondary earthed to mains earth. In my experience this has generally satisfied CE safety agencies, but UL can be more sticky and require 5mm which would mean 2.5mm tape on each side. (Squish!!). Even as a DIYer, you should adhere to proper safety guidelines when constructing offline supplies.

3. Gapping was done by inserting a small fibreglass shim of about 0.8mm between each of the outer leg faces of the ETD core. One has to push quite hard to get the core clamps to 'click' into position. (More Squish!)

Regards

John Hope
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Old 15th May 2005, 02:48 PM   #10
Alme is offline Alme  Ukraine
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again, thank you Jaka and John, I got those things.
John, I'm now going thru your transformer design notes. I begin to feel now what is different from hard-switching transformer design. Looks more and more attractive and worth trying
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