I wound a toroid to use as a HS gate drive transformer for a 100W two switch flyback. I'm using the typical DC restore technique AC couple the transformer.
Here's my schematic;
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I'm using a PIC and a FET driver to simulate the extremes of the PWM controller duty cycle and drive level and it works well with the exception of Burst Mode. Burst mode is when the controller modulates the original gate signal at light load for anyone who doesn't know.
A picture is worth a thousand words.
Here's my gate waveform at 90kHz 70% duty. A little sloping but acceptable.
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Now here's where the fun begins I'm modulating the 90kHz to simulate burst mode.
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Here's a zoom of one of the burst.
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My primary and secondary are 1:1 ; inductance of 1.5mH.
Somewhere in the past I posted A DC restorer for transformer drive of MOSFET's... on the Hubble.
Surf for it. ...Jim Thompson
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Looks like you haven't wound your toroid transformer in bifilar fashion. That is essential for apps like this, leakage inductance must be minimized, else this rings like crazy.
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Regards, Joerg
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That is the one thing i hate about those stupid "controllers" that supposedly are good for DC-DC conversion. Why simulate? Toss the verdammdt controller and use the PIC directly; then you can have the program be as dirty or clean (or both) as you want... ..Namely slow non-giga-amp startup current spikes!
For really excellent fast pulse transfer, use coax cable - use the shield for the primary and the center for the (HV) secondary. It is a bit messy, but the shield can be split a number of times for input one-turn equivalent via paralleling...
I'm using 0.1uf caps and a 4.7k gate resistor the diode is just a
1n4148.
The burst mode frequency isn't specified in the data sheet so your right I'm probably going to have to dump it. The controller is the NCP1217D 100kHz.
The only reason I wanted to keep the feature is the supply may not have a load on it all the time or a small load. The data sheet doesn't say what the minimum duty is and I don't want the OVP kicking on and latching the controller off when lightly loaded.
I guess I'll test the controller for minimum duty and ensure there is a minimum load for it.
Joerg do you know how to take screen shots that show the measurement screen ? I'm assuming you can do it because they show pictures of screen shots in the manual with the measurement window.
Yep its wound Bifilar. Its also 5 turns past the minimum to avoid saturating for its worst case Duty and Drive level and I checked it using a current transformer.
Its wound tight and all the way around the toroid.
I have read through some decent DC/DC conversion app notes using PIC's and for LVDC I might try one down the road. This is an offline flyback for 90-140VAC in 100W out. It would get a little messy using one for that.
The bridge rectifier is beefy and more then capable of handling the turn on current surge the fuse and line filter help to reduce it. I see your point though you can deal with a lot of things in software. Adjusting the frequency (PFM) to suit the load demands, soft start and less dissipative current sense etc.
The only thing is PFM usually isn't recommended for line powered converters because it's difficult to design a line filter when the converter can operate over a wide frequency range. Soft start can be implemented on the secondary side with a couple of components I've done it several times before you have to tweak it but it works.
I like PIC's and use them a lot but they have their places the controller in a line powered converter isn't one of them.
Yea I thought of that later. I have been known for a few brain farts though;-).
The problem is the burst frequency isn't specified and it can vary. They don't even give a frequency range just some scope shots at various loads showing different frequencies. On semis usually pretty good about data sheets but this ones pretty sparse on some important stuff.
It is a nice feature though if your supply is going to be powered with a small or no load at least it wont pump up past the nominal voltage.
Ahm, didn't you just do that? Not sure what you mean here. Mostly I use their Freewave program but on the road also a USB stick. You can turn the menu off with the little gray round button. If you want a totally clean screen with just the plot, or an Excel-capable file you can also do that via Freewave. Also from the bare scope, if you hit "save all" it stores both formats on the USB stick.
But something is ringing. How are you measuring this? Are the caps sized right? Your schematic is only partial so I can't see what happens south of the FET's source. If the source is flying up and down that's a challenge. This scope isn't that great for differential measurements at high sample rates.
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Regards, Joerg
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True PWM is next to impossible with a regular uC. That is because it has a finite granularity of no better than one master clock cycle. At contemporary frequencies that's not very good. For example, my last design runs above half a megahertz. Try that with a PIC :-)
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Optocouplers aren't very well suited to drive FETs. They can't muscle a gate capacitance around within tens of nanoseconds and at a couple of amps. Transformers are the better choice.
Size the caps right, and maybe optimize the transformer coupling some more (twisting the wires before winding etc. unless you did that already). Burp mode in a converter isn't that great with transformers but there aren't many alternatives. In ultrasound we do this a lot, transmit a sequence of fast power pulses across a transformer, followed by a major pause (to wait for echoes).
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Regards, Joerg
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There are some fast optocoupled mosfet gate drivers.
The home-brew way to do this is to use a cheap (ca $4) potted DC-DC converter to make a supply that floats on the fet source. Then use a fast logic-type optocoupler and a real gate driver chip up there.
The advantage of optos is DC coupling without duty-cycle problems. And not having to wind transformers.
I meant the side screen which shows measurements automatically like rise/fall, PP,AVG,RMS.....
Probably the only way to do it is with Freewave. The software doesn't work on my XP machine but it works on my W2K pro machine?
I have 0.1uf caps on the input and output. Tim's right the 1.5mH and
0.1uf caps are resonating with the burst frequency.
I'm not taking a differential measurement my probe tip is right at the gate of the FET and the ground clip is right at the source.
I don't think its possible to do this without a floating supply and using an opto to send the HS control signal to a driver. Even if I disable the burst mode the controller could still go into burst mode under fault condition like over current. To many chances something could go wrong.
I suppose I could use an old UC384X it has none of these fancy features and a 0.5 duty cycle clamp. Other then the unpredictability of the burst mode it does work well.
I was also planning on using synchronous rectification a shottky dissipates 4W but this FET would have less then 1W conduction losses. I thought I would try something different;-)
That's a lot of stuff for a 50W-100W converter :-)
Another option is to use a chip with proper bootstrap circuitry.
So far transformers have been good to me. The only problem is that off-the-shelf gate transformers are IMHO way overpriced and the reasonably priced ones from China are custom runs and have MOQs. Which for a product design doesn't really matter. It seems the usual COTS vendors don't really realize how much of their lunch is being eaten. Maybe time to invest in Rinminbi, if I just knew where ;-)
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Regards, Joerg
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If you don't turn off the menu then the side screen will be stored in the image file.
It does work on XP, it runs on a laptop on the lab bench here and that's an XP machine. However, MS screwed up backwards compatibility in .NET and you must load its version 1.1 environment. Freewave did not install correctly with version 2.0 but .NET 1.1 can coexist peacefully with 2.0.
It's really nice, I take measurements for clients and then send the results over the LAN into the client directory on the office server. Then head to the big PC to compile the reports. Ok, it doesn't transfer my cup of coffee along :-)
Oops ...
With a flying source that's a problem because the Instek connects the probe shield to PE. So you get ground loops and tons of ringing. Plus you neighbors listening to AM radio won't be very happy about that. You can get battery packs for many Instek models but it's very expensive. I never checked whether one can hack into this and connect one's own battery. Even then you'd at least need to wrap the probe through a nice big ferrite a few times.
Maybe at your frequency range you could try a diff measurement with two channels.
With properly size caps it should be possible.
Question: Since this is a flyback why is the source flopping about? What's it doing? Maybe post the whole schematic if it isn't confidential.
But you have to drive over 3500pF of gate capacitance. Make sure the Rdson versus gate capacitance deal is somewhat optimized for the chosen frequency and power level. Ideally the losses due to both effects would be about on par.
It's pretty brazen that they advertize this as a 130W device. I wonder what it's lifetime will be under that sort of load (in milliseconds, from turn-on to a sharp popping noise).
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Regards, Joerg
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