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| Lowering SMPS dissipation |
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| dzseki:
When Q20 gives up it conducts through D-S, obviously taking the current sense resistors as well, not much damage other than that. The FET is located in the bottom of the cage where the board slides in, so you can't access with a scope probe to check things while working, adjusting the gate drive has to be done "blindly", but I may try anyway. For that I should record at least the heatsink temperatures before/after to have any meaningfull output from the experiment, other than possible EMI problems. Regarding the SiHP FET it has almost double Gate capacitance than the IRF730 so wouldn't that inherently slow down the switching speed further with the given drive resistors (let's consider the one with the 220Ohm in it), which would also ease the EMI, but at the expense of possible increased dissipation? I also consider the possibility of using custom heatsink, like ones with nice fins you can find on DDR chips on video cards for example, with fins aligned to the air flow directon, and only a "translator" element has to be made between the transistor's Drain and the plane of the heatsink, which would be perpendicular. |
| xavier60:
--- Quote from: dzseki on December 02, 2018, 11:50:44 am ---When Q20 gives up it conducts through D-S, obviously taking the current sense resistors as well, not much damage other than that. The FET is located in the bottom of the cage where the board slides in, so you can't access with a scope probe to check things while working, adjusting the gate drive has to be done "blindly", but I may try anyway. For that I should record at least the heatsink temperatures before/after to have any meaningfull output from the experiment, other than possible EMI problems. Regarding the SiHP FET it has almost double Gate capacitance than the IRF730 so wouldn't that inherently slow down the switching speed further with the given drive resistors (let's consider the one with the 220Ohm in it), which would also ease the EMI, but at the expense of possible increased dissipation? I also consider the possibility of using custom heatsink, like ones with nice fins you can find on DDR chips on video cards for example, with fins aligned to the air flow directon, and only a "translator" element has to be made between the transistor's Drain and the plane of the heatsink, which would be perpendicular. --- End quote --- I hoped that R97 would blow first. The messy aspect is that damaged parts can get missed, for example, I see gate resistors go high resistance. The power supply will work for a while then fail again. I was comparing to the IRF740. Gate to Drain charge has the most effect on switching speed. It also adds directly to the losses as the MOSFET has to discharge itself every time it turns on. I'm not familiar with the DDR heatsinks. The drive mod with the 220Ω might not increases losses much at all depending on the operating mode of the supply, which is unknown and it's likely very reliable with the original fan speed. |
| NiHaoMike:
What about use the lowest gate resistance that doesn't ring to minimize switching losses and then add filtering to solve EMi problems? |
| xavier60:
Although there are many unknowns, looks like the control IC is configured to limit the MOSFET current to about 2A peak. Assuming DCM and 50% duty cycle, worst case MOSFET RMS current is 0.6A. And because it's running from 85V rather than 340V that I'm used to, the switching losses would be lower unless this power supply is running at an unusually high frequency. C50 should be checked, just in case. Does R90 really go to where it's shown? |
| xavier60:
Rt and Ct to me read 7.5K and 1nF on pins 18 and 16 of the TDA4718A. When I try to apply these values to the VCO frequency chart on page 15 of the TDA4718A data sheet, the result is literally off the chart, very high. Can someone confirm? |
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