| Products > Test Equipment |
| New DL24EW and DL150W load testers from Atorch. |
| << < (5/5) |
| Filippo52:
Spotted Dick The resistors are in series with the mosfet, that's all you need. With this the voltage across is "exactly" the current flowing through the mosfet", you don't need to know anything else about the PCB rails. The current is that anyway. Yes, the 0.25 resistors are those on the left side, where there is the connector from which current and voltage enter and on the side where there is also the double schottky diode against polarity reversal. I agree with you that with good dissipation this object can do 150W and also little more. But be careful when you change heatsinks. There are no insulators so the bolts and screws holding the heatsinks are live, there is positive voltage above them. The bad news is that these bolts force the PCB and the washers overlap or pass less than a millimeter from tiny tracks that carry the negative from the mosfet's Source towards the operational that drives the mosfet's gate. This is particularly serious for the mosfet which is close to the double schottky diode; if I tightened the screw too much or badly, you blow up the track and the mosfet obviously crashes, leaving you without driving the gate. You were right to point out that those mosfets are not for DC and if you look carefully at the SOA graph, in addition to not having the curve for DC, which you obtained in the drawing like me imagining ... it is measured with the case at 25° C and this is the most serious thing, much more serious...when will the case ever stay only at 25°C? If you look at linear mosfet the SOA graph is with case at 75°C that is the real situation in a linear application like DL24) So the amps and the power they can dissipate is fortunate, but from practical use I have seen that a single IRPF264 mosfet if original holds, well dissipated, even 80 w for long times (I don't know if it only lasts a few months... we'll see) ... so in four if well dissipated the bottleneck of everything becomes the double schottky diode especially if you use high currents. It should be replaced or eliminated. The manufacturer in the brand new DL24E-W model has replaced it with an automobile fuse and a very large diode. This way you don't heat anything and you also don't lose the 0.8V of the diode, which also allows you to discharge single-element Ni-Mh batteries. If you get the polarity wrong, hoping the diode is fast, it blows the fuse and saves everything, but you're not sure. |
| SpottedDick:
Hi Filippo52, The figures I gave where for the case temp at 100°C, I de-rated and calculated what the max they could do was :) That's the reason I think the smaller heatsinks may be fine. By using more MOSFETs at a higher temperature, they can safely stay within SOA and reduce the heatsink size due to the higher delta. You never mentioned what kind of temperatures you where seeing? For the resistor issue, is there any chance you can take a picture of the board from above, so maybe I can draw on it to explain better what I am trying to say? Ciao! |
| Filippo52:
Spotted Dick while waiting for the photos, I ask you if you have made any calculations on what temperature those small heatsinks must reach to dissipate 150W. Perhaps the 100 °C that you hypothesized is not enough and in any case in a completely open apparatus do you know the danger represented by 4 small dissipators at 100 °C? and do you still know how much the average life of a mosfet is reduced at those temperatures? If you want a reliable device and not a dangerous object, you have to limit yourself to case temperatures of 70 °C, look at the temperatures used by linear mosfets in the SOA. Then you and I invented the SOA with the DC work curve; we can't build anything else on it: it's like putting a weight on a sand castle. A greeting |
| thm_w:
https://www.eevblog.com/forum/testgear/atorch-dl24p-(or-pcb)-vs-dl24mp-function-costant-voltage/ https://www.eevblog.com/forum/testgear/atorch-electronic-load-dl24mp-e-dl24e-w/ You can post this stuff here or in the other Atorch thread, I don't see the need for new threads. https://www.eevblog.com/forum/testgear/cheezeball-dc-load-dl24p-pump-or-dump/ |
| Filippo52:
ok sorry, my mistake I did tried to move the message but receive this information "You cannot delete your own topics in this board. Check to make sure this topic wasn't just moved to another board." |
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