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Old 10th October 2002, 01:41 AM   #1
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Lightbulb High end Switch Mode Power supply for Audio

I am in the process of doing a high-end SMPS design for Audio power amps. Specifications should typically be 800W capable at around +-60V. I have gotten some great help from a member on this forum already but thought a collaborative effort might work better still. One of the main goals is to have a high switching frequency (say 200KHz or better). Being technology oriented, it should ideally use state of the art components as well

To the non-believers, I have heard a power amp with a low-tech SMPS (operated as low as 50KHz SIC!) and non-impressive amplifier stage. You should consider trying before dismissing

Configurations considered to date:

1. polyphase controller designed for PC motherboards -- ideally full 12 phase setup . This would probably yield the "strongest" supply -- as these in 4 phase mode yield 90 Amps for CPU power. The apparrent problem is that the output voltage sensor is optimized for low to very low voltage and this troubles me. I am seriously considering building this (transformer based) with magnetics (slightly modified), semiconductors and glue logic as per the Infineon reference design below (as distinct 2 transistor forward converters using high side signal to switch). To date it appears that www.linear.com have the "best" controllers. Would require separate PFC controller. With this approach, just about any frequency can be programmed up to 1MHz or so. Still 200KHz per phase at 12 phases would be very very cool indeed.

2. ZVT design. This is more complicated (theory of it), particularly as switching losses can go high when power drawn is low. Again, I have found an interesting chip from Linear Technologies http://www.linear.com/prod/datasheet.html?datasheet=601 which does simplify design, particularly balancing of current. Main problem seems that this type of design is sensitive to output load. If we could base this on an actual Linear design that would be good even though I am not all that chuffed about the gate drive circuitry that Linear uses (transformers everywhere). Still this is the most modern topology and yields less EMI.

3. Infineon's 200W reference design http://www.infineon.com/cgi/ecrm.dll...ent_oid=-8681. This is very very cool. Way to go German dudes! Simple 2 transistor forward converter. Problem: It is simply too small. I have solved scaling the PFC section, but am struggling with the power transformer, reason being that larger pot cores do not seem to be available in low loss materials required for high operating frequency. Still, I am considering taking a multiple of the output transormers (say 4) and using 4 paralell sections of wire with a quarter of the primary turns per unit -- all wired in series. Secondaries with foil wiring to suit. Now, all I have to do is reduce the current sense by the same factor and I verify that switches are capable of driving the thing.
Based on the quality of the reference design document -- and the usage of state of the art components just about everywhere this should be the simplest way to get there -- if I am doing the magnetics work right. Still, larger transformers would look a little neater ... One other problem ... Epcos have REALLY LONG delivery times and minimum orders of 60 cores which is a little painful. Cores are inexpensive, though.

Learnings to date: My understanding is that Infineon are basically the switching FET and diode masters of this universe (pricing of FET's are most reasonable (Euro 1.5 each), diodes are highly priced (Euro 8 each) but they are single sourced and truly radical). Linear Technologies seem to have just about the best regulator chipsets. www.mag-inc.com have some great cores, particularly well suited to PFC section. Pot cores seem to have maximum shielding but are problematic to get in large form factors. Toroids are probably not optimal for power transformer. Winding the transformer, secondary might well be best foil wound. Planar magnetics (if available) might also be very useful, particularly if we get into a ZVS design. Synchronous rectification considered but believed to be primarily for very low voltages. Current doubling outputs still considered.

How about it guys, anybody have deep knowledge in magnetics design for high frequency SMPS???

Petter
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Old 10th October 2002, 02:20 AM   #2
jcarr is offline jcarr  United States
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Petter:

Try: http://www.keisoku.co.jp/pw/scatfaq/index.html

At the bottom of the page you can find a "Tesla converter" by Dr. Cuk (modeling by Shin Nakagawa). The conversion efficiency is about 95%, which should be close to state-of-the-art. More circuit data is here: http://www.keisoku.co.jp/pw/scatfaq/tesla.exe

The circuit is quite new, and will be the topic of an upcoming feature article in "Transistor Technology", one of Japan's better-known EE magazines.

If you find it interesting, I may have another article or two on this Tesla converter that I could upload.

hth, jonathan carr
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Old 10th October 2002, 07:32 PM   #3
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I have checked the link out, and it was very very interesting. Thanks! I am kind of leaning towards doing either:

a) Polyphase with Infineon output stage and magnetics
b) Multiple transformers on the Infieon reference design for a forward converter.

That is of course unless the Japanese come up with something totally radical which is quite possible! If they do, I hope it is of a type which is not sensitive to output load to function.

Please keep be informed of developments, Jcarr

Petter
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Old 11th October 2002, 02:11 AM   #4
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search on Linear (www.linear.com)'s site for "low noise" switchers and drivers. I will caveat any comments here by saying that I don't design SMPS power supplies for a living, but I have built many and consider it the most intriguing and potentially elegant application of the DIY craft. Perhaps it's just my foibles, but I have found Linear and Nat Semi to be most supportive. Read the ap-notes at Linear and you'll get the message that it's all about a little calculus with the slew rate.
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Old 11th October 2002, 04:41 AM   #5
alvaius is offline alvaius  Canada
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I am curious as to what your design goals are, low ripple, high efficiency? I have nothing against switch mode for power supplies. My concern would not be the switching noise, so much as the loop reponse having a response well into the audio band. Increasing the switching frequency will certainly help. Multi-phase controllers can certainly help if you have a good controller.

My main concern with switch mode supplies is the jitter they can create in digital equipment. To that end, the Multi-phase structure will significantly cut down on your input ripple and hence effect on other audio pieces in your system.

For AC\DC supplies, you should probably look at ST also for FETs, and Controllers. ST has some nice PFC correction chips. It's FETS are top notch as well. Fairchild also has very good FETS especially with all the acquisitions they have made.

Very interested in how things go!

Alvaius
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Old 11th October 2002, 12:04 PM   #6
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the switching chips I am looking at operate at 48kHz to 100kHz.
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Old 15th October 2002, 10:12 AM   #7
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I am basically considering anything under 200KHz unsuitable by default (unless it is multiphase).

Check out the app note from Infineon (200W PSU) and see for yourself how cool it can be. Now imagine running a 12 phase unit in that mode ...

I have found large cores from Epcos now so I am kind of thinking that using the learnings from that application note and possibly considering another controller (for ZVS say) with the larger core HF transformer would be suitable.

My design goals are to create a low impedance power supply that interfers as little as possible with the sound. That means putting the feedback below the audio band ("strong psu" that inherently does not drop + large holdup capacitance), or above the audio band (fast feedback, also with large holdup capacitanc ...).

I will be looking more into this soon. Thanks for all the great responses so far!

Petter
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Old 15th October 2002, 12:37 PM   #8
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What a helpful ap-note -- worth reading if only for the transformer design section.

I feel sorry for anyone with "dial-up" connection, however, as the color in the PDF takes a long time to download.
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Old 15th October 2002, 07:16 PM   #9
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I am currently working on a switching power supply for an audio amp of the same power range. It consists of an off-line ZVS active PFC front end with a ZVS converter stage. The secondaries include current doubler topology with synchronous rectification. Pot cores are the wrong direction. Pot cores are not a good choice when using high currents. In this case it could be about 10-20A secondaries (max). Pot cores are not good for this application because of the increased copper cross section needed to carry this current and the limited amount of "opening" to allow wires to escape from the core. If the primary is around 400V from a PFC then arcing to the core is most likely and undesirable. The use of current doubler and synchronous rectification extends the ZVS load range into lower current loads and thus increasing efficiency and performance over the entire range. For those less knowledgable about Zero-Voltage Switching, the transistors in a full bridge arangement are controlled via phase-shifted gate drives and allow the transistors to "turn-on" when zero voltage is across them (due to parasitic components utilized within the MOSFETS). Because there are no volts across the transistor when it is turned "on", there will be nearly zero noise in switching. However, this effect is dependant on the amount of current which flows in the primary and under light load conditions there is not enough energy to place zero volts across the transistor. Thus a leakage spike will occur. But it will be small because there are not many volts across the transistor and there is not much energy in the cycle. So the trade off at light load is easily made for the benefits at full load current. I highly recommend the Unitrode series of controllers from TI. They are outstanding performers. I also recommend MOSFETs from International Rectifier and cores from Magnetics Inc. available from Allstar Magnetics on the web. The frequency of such a ZVS supply will be around 150kHZ to 200kHz because it is a quasi-resonant topology. Also it is recommended to use magnetics amplifier post-regulators for the auxiliary outputs which are not directly in the feedback loop. Current mode control is stongly advised!!!! Would you like me to go on? This message is rather lengthy......

BeanZ
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Old 16th October 2002, 08:11 PM   #10
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Please do go on.

However regarding current I am not sure that I agree with you. The Infineon application note has as I recall 20A at 5V and 8A or so at 12V from one small pot core using tape secondaries -- all from a 360V primary. The pot cores used have pretty good opening slots.

The pot core I am considering using has 10 times the mass and is made of the same material.

I have looked at Unitrode controllers but find the application notes lacking compared with what to me is the reference to date (Infineon). Regarding transistors, I am sure IRF make great devices and have standardized on diodes and transistors from them. However, it was Infineon who changed the HV on resistance from quadratic to linear function of voltage and they still appear to be way behind Infineon in this particular area. They are also driving the field with diodes. Infineon uses a PFC core from Magnetics made out of KoolMu and i have been using magnetics from them for years. Very good company, very helpful as well.

As I said, please do go on. If you can provide us with information, app notes, ideas, guidelines, device numbers etc. I would be particularly chuffed.

Thanks so much. I will re-read your posting tomorrow to attempt to get the full benefit of your contribution.

Petter
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