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xrk971 10th April 2019 09:11 PM

Were did you see an oscillation, or is that just an assumption of the parade of awfuls to come. I have tested a variant of the CFP cap Mx before and had no issues with oscillation.

xrk971 11th April 2019 02:53 AM

1 Attachment(s)
Here is what the board layout will look like. Note the exclusive use of through hole parts. The pass transistors have the option of on board underhung mounting or externally mounted via flying leads on 90deg Molex locking quick connects.

meanie 11th April 2019 04:19 AM

Subscribed !

Count me in GB

Mark Johnson 11th April 2019 04:51 AM

CFP has a widespread and universal reputation for unwanted oscillation. Take a look at the Tektronix power supplies that use CFP, they've got lots of oscillation countermeasures. Then ask yourself: is Tektronix the kind of company which bothers to expend board area and parts cost, to solve a non-problem?

Me, I've experienced CFP oscillation when I foolishly used (a) no ballast resistors; at the same time as (b) BoHunker large beefy output transistors with lots of phase shift (low fT), simultaneously with low current complementary devices in the CFP, having high fT. I don't know whether this combo oscillates every single time, or only every time somebody doesn't bother to prototype it in a debug and tweak period into their project schedule. No data on that.

Evaluate the odds, then take your chances and roll the dice. Try not to remember the junior engineer's lament, "There's never time to do it right but there's always time to do it over".

jwjarch 11th April 2019 07:30 AM

Shall I officially start the list? baswamin and meanie, feel free to edit. ;)

SLB Group Buy List
baswamin - 2 boards?
meanie - board(s)?
jwjarch - 2 boards

meanie 11th April 2019 10:03 AM

SLB Group Buy List
baswamin - 2 boards?
meanie - 2 boards
jwjarch - 2 boards

xrk971 11th April 2019 12:00 PM

Thanks for self organizing the interest list Jwjarch!

SemperFi 11th April 2019 12:29 PM

You should try a bunch of different MOSFETs. The gate drive on those monsters you've selected will create tons of HF hash. No gate resistor even! The driver/MOSFET circuit should be treated as a high frequency circuit (due to fast edges and high current spikes on gate drive)
This circuit is a great idea if you have low voltage supply and high current demand where the voltage drop over the didoes make a difference.
If you can make for a smooth turn on/off of the MOSFETs then it may be better for audio as well.

xrk971 11th April 2019 01:00 PM

1 Attachment(s)
Definitely will test the circuit to make sure it’s clean and not oscillating. For the SMT prototype, I found that it was exceptionally smooth (SLB) without artifacts at the MOSFET switch on/off. This probably has to do with how smart the controller is in timing the gate switching with the zero crossing of the mains input. The ripple after the cap multiplier is so smooth that it looks like a sine wave rather than a sawtooth.

For reference, here is data from an initial test that I did with the active bridge and CRC but no cap multiplier. You can see a sawtooth at 120Hz but there are no spikes ornringing artifacts. This was using a 25V 250VA trafo to make 3.8A continuous at 32v into an 8ohm resistive load. This was giving 390mV rms ripple.

The manufacturer’s recommended circuit doesn’t call for a gate stopper resistor even (perhaps they are internal to the chip). They are trying to make the package as easy to use as possible and then only required component aside from the said MOSFETs, is a 1uF bypass cap across the output pins.

SemperFi 11th April 2019 02:11 PM

1 Attachment(s)
Well there may be no issues at all, but fact is gate drive is full of HF hash. Unless there is some resistance in the chip and there may be. You may have high frequency EMI radiating from the MOSFETs, but the magnitude and frequency may be of no real issues. The rectified sines are nice, and zero volt switching is probably the reason as you say, but at zero volts on the drain the gate capacitance is at it's most, it is huge! But there is no miller, so perhaps not so bad after all. I would treat the gate drive as a HF circuit, but as you have experienced, there may be no real world problems to worry about. But why not experiment a little.
The gate drive send and return paths should be as tight as possible, and it doesn't hurt to have a resistor to slow things down. Those should be SMD, 1206 is easy enough to work with and small enough.
These are the loops I mean, see picture.

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