RDS is the resistance from Drain to Source in a MOSFET.
The lower the RDS, the less wasted power and heat while current flows through when used as a switch.
The lower the RDS, the less wasted power and heat while current flows through when used as a switch.
Oké, I see we have 2 storylines. One is general talk of the pros of using active rectification (Bucks Bunny, ... ), the other is the issue associated with using the LT4320 (Analog, Tombo, me, ...).
It is confusing sometimes.
It is confusing sometimes.
Hm, even at 8 us/div scale there is no triggering voltage ringing like at post #67. What is present is some light jumping up and down at the very beginning. Its visible at attached screenshot taken as animated gif with 101 frames. Current is 100 mA and I attribute this jumping to comparator hysteresis or uncertainty zone.
I have a 3.3 uF ceramic capacitor close to LT4320 +- pins as prasi recommended for his LT4320 PS board. First capacitor after bridge is 33 mF.
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This second picture seems off: the spike of the Vgs peaks later than the Vds, implying that the Vds is the culprit, or might be triggered from the corresponding polar opposite.
Does anybody else see this?
The gif does not show the trailing edge, the leading edge seems beyond reproach on the previous pics too.
Thank you!
This prompted me to take a look at my working amps with 4320 boards and voila, mystery solved 🙂
None of the boards inside amps exhibited any trace of ringing or misbehaviour. Similar transistors, slightly different board layout and a seemingly insignificant difference: a snubber across the input, which the board on my bench lacks.
The lack of snubbering may be innocuous in many cases but in conjunction with a particular power transformer and at low currents it led to ringing. Please don't ask me to explain it, i can't. The addition of 68n +100R cured the problem completely.
This prompted me to take a look at my working amps with 4320 boards and voila, mystery solved 🙂
None of the boards inside amps exhibited any trace of ringing or misbehaviour. Similar transistors, slightly different board layout and a seemingly insignificant difference: a snubber across the input, which the board on my bench lacks.
The lack of snubbering may be innocuous in many cases but in conjunction with a particular power transformer and at low currents it led to ringing. Please don't ask me to explain it, i can't. The addition of 68n +100R cured the problem completely.
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Excellent! Now we all know what to do if ringing is present.
It must be a particular transformer effect as I was measuring PS board with no snubbers
It must be a particular transformer effect as I was measuring PS board with no snubbers
Transformer snubbing, both theory and practice, is discussed here on the Forums
Mark, great to see you have interest in this.
Perhaps you may have a suggestion regarding the 1170s halfwave rectifier. It gets a very clean Vgs voltage but Vds has severe ringing, audible too 🙂 Thanks to a resonance with the transformer inductance the peak voltage goes silly high. For some reason this only happens at lowish (<200mA) currents. No amount of snubbing has any effect upon this. Neither do caps across DS. Series resistance along the AC line fixes it but at a substantial cost in voltage. Artificially loading the output also fixes it but at the cost of heat and ripple.
Here is what the DS voltage looks like. The resonant frequency seems in the order of 10kHz. K is cathode, GND is anode.
Perhaps you may have a suggestion regarding the 1170s halfwave rectifier. It gets a very clean Vgs voltage but Vds has severe ringing, audible too 🙂 Thanks to a resonance with the transformer inductance the peak voltage goes silly high. For some reason this only happens at lowish (<200mA) currents. No amount of snubbing has any effect upon this. Neither do caps across DS. Series resistance along the AC line fixes it but at a substantial cost in voltage. Artificially loading the output also fixes it but at the cost of heat and ripple.
Here is what the DS voltage looks like. The resonant frequency seems in the order of 10kHz. K is cathode, GND is anode.
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Perhaps it makes more sense to address this from the side of the gate by slowing things down a bit. Maybe a large gate stopper.
I see the datasheet mentions on page 13 specifics of the Turn On Phase, ringing etc.
You sure the MOT configured correctly?
And are you sure about the layout? Figure 20 uses Direct fet's to minimize the path, those are very hard to use as a DIY-er. At these speeds almost every millimeter counts.
Does it ring on turning-on or turning-off? I can't see any snubber in the schematic.
The MOT adjustment is geared towards the frequency range this device is normally used at, max MOT is 3.6uS so it makes little if any difference at 50Hz. And the gate drive looks perfect. My layout is generally close to the app note but the mosfet is through hole. Not sure if this is critical, as i have no plans for 500kHz operation.
Looks like this has turned into a Quasimodo clone 🙂 High voltage, high dcr transformers are naturally immune but what i don't understand is the current dependence. Nothing much changes at the gate drive, yet the ringing disappears.
Interesting.
In your test setup, it looks like you're using a dual secondary in series.
For the sake of testing, what if you try using one secondary (or as is) and tightly twist the wires to your 4320 board with no snubber. Also tightly twist the wires to the 10uF.
Would it still ring?
What values are in the snubber? I would try more aggressive snubbing as it is hard to overdo it. E.g. 470nF + 82R