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Old 13th May 2019, 9:30 pm   #41
julie_m
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Default Re: Power MOSFETs

I'd be tempted to use a cheap microcontroller to generate a PWM signal by simple bit-banging, and a pair of TDA2030-alikes with no feedback to drive the gates of the MOSFETs, for the current sourcing and sinking ability. For 3-phase, make the table with a multiple of 3 entries and start reading 1/3 and 2/3 of the way through to drive the other phases. Software timing should be accurate enough, you know how many cycles the lookups and writes will take, and the only decisions will be in the idle loop.
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Old 14th May 2019, 1:34 am   #42
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Default Re: Power MOSFETs

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Originally Posted by julie_m View Post
I'd be tempted to use a cheap microcontroller to generate a PWM signal by simple bit-banging, and a pair of TDA2030-alikes with no feedback to drive the gates of the MOSFETs, for the current sourcing and sinking ability. For 3-phase, make the table with a multiple of 3 entries and start reading 1/3 and 2/3 of the way through to drive the other phases. Software timing should be accurate enough, you know how many cycles the lookups and writes will take, and the only decisions will be in the idle loop.
All true, but it might pay to consider these remarks from my article on my 400Hz 3 phase power source:

I looked at the notion of designing my own inverter and I’m not shy to take on a difficult task. However, it is not a trivial task to design and build a three phase power inverter capable of some hundreds of watts output. Generally, one would start with a DC supply derived from the mains power. Then create three sine wave oscillator outputs, separated by 120 degrees for each phase at the “base” frequency, in this case 400Hz.(of course you don't need that for single phase)

Those outputs would pass to comparators being fed a high frequency switching or “carrier” signal to generate a PWM (pulse width modulated signal). This would then pass to a three phase bridge output stage, typically with IGBT’s.

While the basic design of an inverter like this seems simple enough, there are very important issues in these circuits with isolation required between stages and this often requires an array of separate DC supplies and isolation devices which add to the complexity. There are also the issues of transient snubbing to protect the IGBT’s and often a dead-band circuit is also required to prevent simultaneous conduction and catastrophic failure of the IGBT’s in the bridge.

I also noticed that this sort of project has been presented as a topic for an Electronics Masters thesis by some, giving an indication of the scale of the task, to do it properly that is.

So its a bit more of a task than meets the eye, it is "doable" but not easily if you want it reliable.

(Also about the remark that its not necessary to have a feedback loop leveling the output if it was a power amp system with a sine wave drive signal, I still think it is especially if a home constructed device, that has the potential for more mishaps, maybe not for the work of the professional power engineer)
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Old 14th May 2019, 4:35 am   #43
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Default Re: Power MOSFETs

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By Argus25: if an amplifier is used for the task (be it any class of amplifier A,B,D) it will be very important to not run it open ended, meaning from some uncontrolled amplitude sine wave signal source. It ideally would have feedback around it that samples the output voltage and gain levels it to some particular output voltage. Or it could be a recipe for a very nasty over-voltage event...

Julesomega: I don't think a power engineer working at the CEGB would agree, they do not take feedback from customer loads, even the biggest loads, to control the network voltage
I don't really understand your objection. Every well designed power supply unit, analog or switching type, samples its own output and feeds it back to the drive circuitry to make sure its voltage is regulated and stays stable under varying loads and many have overvoltage and some undervoltage and many current overload protection too. These are features expected in every well designed supply. Really, these features should be added if you feed a sine wave source into an amplifier module to derive the power.

Perhaps this might not apply if you are a power grid generator (I assume that is what CEGB is) sending power down a street to a remote location where is not possible or even useful to sense the voltage easily, at least at the source you can still sense the current.
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Old 14th May 2019, 6:09 am   #44
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National power grids regulate the output of their turbo-alternators by varting the exciter drave to their fields to control the output voltage at the machine, and to balance the load current between machines. Voltage control around the network to handle usage variations and network losses is done in multiple places by transformers and tap changers. The limitation of a feedback loop is that it can control the amplitude in only one place, while a power distribution system has many places, so voltage controllers have to be distributed around the network, and in view of the power levels, have to be very efficient.

Back to aircraft.... The system needing to be substituted is a variable speed engine, running with about a 2:1 rev range, an alternator and a carbon pile regulator.

The carbon pile regulator uses a solenoid versus a spring to apply pressure to a stack of carbon discs operating as a variable resistor. The spring applies pressure to the discs. The output voltage creates current in the solenoid, and that current creates force opposing the spring force, letting the disc stack resistance rise to reduce the output voltage. It's a series-pass regulator done electro-mechanically. It isn't efficient by any stretch of the imagination, particularly in view of its limited ability in the low resistance direction.

With the change in revs, the EMF of an alternator varies. I don't know the system in the aircraft involved, but I suspect the carbon pile reg wasn't in series with the alternator output, but would have been in series with the field current feed from a metal rectifier. This is much more efficient, and the solenoid can still sense the alternator output voltage. The carbon pile can be a lot smaller, lighter and need less cooling. Valuable factors on an aircraft.

If you have a search around, Hugo has designed a number of solid state substitutes for the old magnetic (vibrating contact) controllers for battery-charging dynamos for period vehicles. Back in their day those things had a limited life. One RB340 seemed to have thelife of two sets of dynamo brushes.

Now back to the problem in hand...

There is another possible approach, a resonant inverter.

Neville Mapham at General Electric (USA) Wrote a paper on SCRs in a resonant power converter. One of his diagrams was picked up by SW Amos and published in an early Illiffe book on semiconductor applications - can't remember the tiitle but I had a copy in about '73. It had a red cover and was by Amos.

Mapham's resonant bridge circuit was used By HP for power distribution within a modular frame... but at tens of kHz. Interestingly for the TV people reading this, they used the fast SCRs that Philips developed for the Syclops line output/power supply system. In this case it gave 240W of rated output, but there was about 1.2kW circulating in the resonator.

They stopped building Syclops TVs, Philips took the SCRs out of production and HP bought a lifetime buy of the things for their Mapham inverters. These modular instruments had a rather long production life being used in military and three-letter government agencies. Life-buys ran low, as life-buys do. Muggins here got landed with the job of redesigning the thing. "But I'm an RF engineer!" got met with "Think of it as a longwave transmitter!"

Siemens had been a second source of fast SCRs, but they too had pulled them long ago. SCRs were out as an approach. Mapham's resonant bridge circuit provided a reverse voltage period to turn off the scrs. I analysed its operation in more detail but without the assumption of using a latching device like an SCR. If I had to turn off my devices myself, when would I have to do it? Well, it turned out to be surprisingly non-critical. I didn't believe my luck. I could drop IGBTs into the Mapham circuit and transformer drive their gates with opposing phases of a square wave. Simples! it was efficient as well, and left behind all the reprehensible habits of the SCRs (self-triggering on dv/dt worse if hot and causing a damaging failure).
So my modified converter was capable of higher power and we upped the rating for the frame distribution to 320W. Oh, the AC output was regulated by controlling the DC supply voltage to the inverter from an SMPS.

I think a 1600Hz inverter could be made to handle several hundred watts of load. The output could be regulated and the whole thing could get into the 80% efficiency stakes. The resonator inductors would need to be specially made and finding reliable resonator capacitors is not going to be easy. The wrong ones glow red and flash into flame.

There may be more development work in this than the OP would want, but I thought it might be interesting to show that others have been down this path before... only I had contracts from US admirals and spooks to contend with, so no pressure, then Trapped between Popeye and 'Get Smart'

If I had to do it myself right now for the OP, I'd phone Stewart of Reading and ask if they happened to have an old 'Elgar' AC supply amongst their stock (BIG audio power amp good for a kilowatt)

David
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Old 14th May 2019, 8:49 am   #45
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Default Re: Power MOSFETs

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There is another possible approach, a resonant inverter.

Neville Mapham at General Electric (USA) Wrote a paper on SCRs in a resonant power converter.

...only I had contracts from US admirals and spooks to contend with, so no pressure, then Trapped between Popeye and 'Get Smart'


David
Interesting about those resonant power converters. And two of my favorite shows !

In the aviation world there are "Static Inverters" which are just DC to AC converters, typically 12 or 24V that generate 400 Hz at over 100V in single or three phase. They are used in the Cessna and other small aircraft to run the avionics instruments, the power output is around 40W. I think these could be modified to up shift the frequency to 1600Hz quite easily. Many types are discrete component 70's-80's vintage and easier to work on and modify than new ones with firmware.

When I looked at 400Hz three phase power units for ground testing of avionics gear it frightened me away with the prices, which is why I made my own with the VFD. Its a shame the VFD's won't go to 1600Hz, as they are a really cheap and workable solution.

I saw this which looks like a 1.5kW device that runs at 1kHz (it appears), that might be close enough and its output could be transformed down:

https://www.ebay.com/itm/Static-inve...oAAOSw6AlbQs-U

So it might be worth hunting out these types of devices for use or modification. I think this sort of thing would be easy to modify to 1600Hz:

https://www.ebay.com/itm/250-VA-STAT...AAAOSw6EhUNDXs

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Old 14th May 2019, 10:06 am   #46
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Default Re: Power MOSFETs

I think all this discussion supports my conclusion, that to design and build a reliable electronic source of 80v 1600Hz power at maybe 600 watts is very much a non-trivial task!
Certainly it can be done, but it probably requires someone with expertise and experience in the relevant disciplines to devote time and effort to the problem, and that's unlikely to happen unless they also have an interest in powering WW2 aircraft radar systems, and I don't think anyone has 'a foot in both camps'!

I'm not sure what the way forward is, other than to stick with rotary machines...…

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Old 14th May 2019, 11:15 am   #47
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Default Re: Power MOSFETs

I think that's a fair synopsis.

Rolling your own would be a big undertaking
Throwing money at it would get a purpose made AC supply, a big Elgar amp. We also used to use AMCRON DC300 amplifiers channels strapped, driving transvormers to simulate military supplies.
Have a look around and see if there are any bargains going.

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Old 14th May 2019, 1:10 pm   #48
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Default Re: Power MOSFETs

This thread could focus on why some amp modules have blown up - as per post #5. If they hadn't blown up, then this thread may not exist.

Post #5 indicates a concern about poor thermal transfer between fet and heatsink. That raises alarm bells for me, as those fets use a very common package, with what should be very standard heat transfer design. I reckon the flatness of the base etc are red herrings.

I don't see any information about fet current and voltage waveforms, and designed junction and case temperatures - all very basic design and faultfinding aspects.
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Old 14th May 2019, 1:53 pm   #49
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You're right, we need the pal who's been experimenting (and blowing FETs!) to join in, he's here on the forum,

Come on, Nick!

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Old 14th May 2019, 6:04 pm   #50
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Default Re: Power MOSFETs

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> I reckon the flatness of the base etc are red herrings.
Really? Is that the case?

I know one audio company that went bust as a result of precisely this (poor thermal contact). Twice: once in New Zealand and once in the UK. We bought the remaining product, the engineering designs, and the contents of five filing cabinets, and took on the remaining staff of two for 5k from the receiver.
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Old 14th May 2019, 11:24 pm   #51
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I'm not sure what the way forward is, other than to stick with rotary machines...

Andy
Ah, the quest to replace the rotating machine with an electronic equivalent.

I faced exactly this issue wanting to test the simple alternator & dynamo regulators. The PWM in these results from the fact that the regulator becomes enclosed in a feedback loop with the electromagnetic properties of the particular rotating machine, including the hysteresis and delays that occur at the output when the field voltage is manipulated.

So outside this environment, on a bench, only simple threshold tests can be performed on the regulator module, you cannot know its operating frequency. This is one of the many reasons why modern versions went to fixed frequency pwm, there were other reasons such as needing LRC (load response control).

In the end I designed and built an "alternator and dynamo emulator" to test regulators fully without the need for the rotating machine. As far as I know it remains the only one published anywhere. Apparently there was one in the USA, but there was nothing technical published on its operating theory.

The article is here:

http://nebula.wsimg.com/69e6a3e418e2...&alloworigin=1
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Old 15th May 2019, 6:12 am   #52
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"Ah, the quest to replace the rotating machine with an electronic equivalent." A few modern Class D PA amps and a spot sig gen? Modern amps can kick out over 1kw all day long at high efficiency, 400hz sinewave in the front, bish bash bosh jobs a good un no?

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Old 15th May 2019, 8:46 am   #53
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"Ah, the quest to replace the rotating machine with an electronic equivalent." A few modern Class D PA amps and a spot sig gen? Modern amps can kick out over 1kw all day long at high efficiency, 400hz sinewave in the front, bish bash bosh jobs a good un no?

Andy.
Yes I think the idea is basically good. They would require hefty switch-mode power supplies at least as elaborate and possibly more expensive than the amp modules themselves and some additional support circuitry to regulate and protect them from overload. It probably would be the fastest way to get there.
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Old 15th May 2019, 10:09 am   #54
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Default Re: Power MOSFETs

Second fastest to buying a commercial AC variable frequency power supply. Worth looking around just in case one turns up at a good price.

David
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Old 15th May 2019, 11:55 pm   #55
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I don't really understand your objection. Every well designed power supply unit, analog or switching type, samples its own output and feeds it back to the drive circuitry to make sure its voltage is regulated and stays stable under varying loads and many have overvoltage and some undervoltage and many current overload protection too. These are features expected in every well designed supply. Really, these features should be added if you feed a sine wave source into an amplifier module to derive the power.

Perhaps this might not apply if you are a power grid generator (I assume that is what CEGB is) sending power down a street to a remote location where is not possible or even useful to sense the voltage easily, at least at the source you can still sense the current.
Hi guys, I'm afraid you've missed my point about regulating the voltage supplied to the load, whether the load be your kitchen lights or some power conversion system. If you expect the CEGB to control the brightness of your lights by varying their supplied voltage, everyone's going to be in trouble when you go to bed and turn off... etc. etc. but this is what was being suggested in earlier posts.

For the power conversion case, let's assume you have a 5V DC regulated supply on the bench for some TTL logic. Trying to maintain the output voltage by controlling the input voltage with a Variac would be daft (over-voltage events?). What you should be trying to do is maintain a constant supply voltage against load changes: this needs a constant voltage, or low output impedance supply. The feedback should be internal to the supply system, and a class-B amplifier (or class-D) has its own NFB loop to achieve this. You set the 1600Hz source amplitude and the amplifier gain to give the required output voltage and rely on the amplifier's (internal) feedback maintain the output voltage. Any competent audio amplifier will do this.

The original aircraft supply would try to do the same, though it would be unlikely to regulate it so fast or accurately as the amplifier.
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Old 16th May 2019, 2:02 am   #56
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The feedback should be internal to the supply system, and a class-B amplifier (or class-D) has its own NFB loop to achieve this. You set the 1600Hz source amplitude and the amplifier gain to give the required output voltage and rely on the amplifier's (internal) feedback maintain the output voltage.
The amount of NFB built into audio amplifiers is nowhere near enough to stabilize the output voltage under widely varying loads compared to the feedback around a power supply circuit with a synthesized low Z output. In the audio amp case its useful to reduce distortion, not regulate output voltage. So I still think if an audio amp is deployed as a power supply device, it requires additional feedback to give it (synthesize) the stiff low Z output it requires for voltage regulation, or likely, its output voltage will be too load dependent.Though it is true that hefty semiconductor based power amps can have an intrinsic very low Z output, which is obviously helpful.

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Old 16th May 2019, 6:44 am   #57
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The audio amplifier approach has linear, fast acting feedback whose bandwidth is going to be greater than the frequency of the signal - 1600Hz.

The aircraft supply rectified and filtered its AC output voltage and then tried (with limited loop gain) to keep that result constant. If we think of fluctuations of current demand on the AC output in the frequency domain, we wind up with the concept of sidebands of an impedance function. The effective output impedance can be very low at small offsets from the 'carrier' frequency, and then grows rapidly away from it. In the time domain this means a regulator that allows large voltage variations short-term, but takes a number of cycles to catch up with things and to counter the droop or surge.

For a resonant converter these concepts cover the design of the resonant network, determining its minimum Q requirement and placing bounds on the permissible fluctuations of loads.

From a point of view of the design maths, a walloping great audio amplifier is the easy way out . I speak from experience!. Maybe harder still was having to keep explaining it to people who hadn't been involved enough to have had to understand the difference.

David
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Old 16th May 2019, 9:48 am   #58
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Second fastest to buying a commercial AC variable frequency power supply. Worth looking around just in case one turns up at a good price.

David
Something I've been doing for a number of years, with no result! I must be looking in the wrong place, can anybody suggest a likely source?

Andy
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Old 16th May 2019, 11:34 am   #59
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Stewart of Reading (sounds like a hair salon, but really a decent supplier of second hand test gear)

Telford Electronics, the Old Officer's Mess, Hoo farm, Telford Shropshire. Don't really put much on a website, might be worth asking.

Not just power supplies as such but also big amplifiers for the purpose - used to use one made by Elgar. Also have used Crown/Amcron DC300 amplifiers. made in the days when a watt was a watt and could be output continuously for days. Not to be confused with some of the modern offerings like the Samson 300 watters at work (blown up and piled in engineering)

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Old 17th May 2019, 12:25 am   #60
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I think what we are discussing here is the Damping Factor of an audio amp. The one I have offered is spec'd as "@1kHz into 8 ohms >400" which implies an output impedance <0.02ohm
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