Help in reduce circuit losses

GH Crash said:

I'm looking at the DMN2040 data sheet and trying to determine if there might be a better FET for the job.

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You are looking for the on-resistance value in the data sheet (example below):
That allows you to calculate the drain-source voltage drop for a given drain-source current.

My feeling are pretty tough. A few negative comments will not even register. What does register, like an earthquake, is any comment that could be taken as derogatory. It is easy for those with knowledge to speak to a less knowledgeable person in a way that the less knowledgeable feels slighted. Also, it should be remembered that all the standard electronics measuring tools may not be available to non-electronics person. Short curt answers or questions often lead misunderstandings. Maybe we all should all watch how we say things.

Post #28 doesn't seem to address measuring loss across the switch. The attachment to the original post shows a switch. FYI; Switch resistance, voltage loss across the switch was determined and accounted for.

If I have said that I will follow, or have followed, the direction given, why is assumed that I haven't?
If I say that I know nothing, then indicate that I know a little about one thing, is that sufficient reason to assume that I’m completely knowledgeable in the subject.
If I say resistance and someone else says voltage drop, isn't it likely that we are talking about the same thing but each using a language, or term, that is familiar to them.

I apologize to everyone for getting off subject. I really do know very little about electronics, and it is sometimes frustrating when thing seem to get buried down in details that I don't understand.


I said that we had 0.2v unaccounted for. By that I meant that the voltage loss in a particular area was higher than expected, or calculated. I also said that we were checking the ATTiny85 coding. That in itself indicates that it appears the unaccounted voltage loss may be the FET not being fully on, duty cycle less than 100%.

Thanks danadak, So that number isn't of real importance in this application?


Again, I apologize.

Thanks danadak, So that number isn't of real importance in this application?

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Used as a simple switch I would think not of concern.


Regards, Dana.

crutschow said:

You are looking for the on-resistance value in the data sheet (example below):
That allows you to calculate the drain-source voltage drop for a given drain-source current.

View attachment 147183

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Thanks. I suspected that was important. I was wondering that since this FET is being used in a PWM application if other factors, like turn-on and turn-off times might also be important.

GH Crash said:

being used in a PWM application if other factors, like turn-on and turn-off times might also be important.

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Yes, but usually of secondary importance, except for high PWM frequencies above 20kHz or so.
The rise and fall times are mostly determined by how fast the gate drive can charge and discharge the large MOSFET gate capacitance (total gate charge).
So for a given gate-drive current, a MOSFET with a lower gate capacitance would be faster.

GH Crash said:

Any suggestions on how to reduce the resistance of the attached circuit.

A supercapacitor (EDLC) is used to power a small motor. There is an ATTiny85 chip in the circuit that is used as a timer and a PWM controller for the motor. The problem that I'm loosing about half a volt between the capacitor's voltage and the voltage across the motor's leads. I need to minimize losses in the circuit as much as possible.

Can you suggest means to reduce voltage loses to the motor?

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I am not reading this thread, but you could check the datasheet of the capacitor and check for series resistance?

Lightium said:

I am not reading this thread, but you could check the datasheet of the capacitor and check for series resistance?

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I can, but realize that the capacitor is not a part of the circuit we are discussing, as different capacitors are used at different times. Also, series resistance is capacitor specific, that is different capacitors (different capacitance and/or different brands) have different resistances.

Hello everyone, I read this post and I've several observations in the analysis of the schematic and the PCB.
The choice of the type of Mosfet used seems inadequate to me, why a channel P rather than a N? If we look in more detail may see that there is a permanent circulation of current via the motor and the construction diode of the mosfet, little by little the super capacitor is unloaded even if the FET is "off". The use of an N-FET eliminates this unnecessary consumption (blocking diode).
Regarding the on/off switch, why not benefit from the Reset pin of the Atiny85, the Pin 1 of the UC maintained at ground via a switch places the UC in standby mode and in fact consumes only very little, a few uA in standbye mode, that eliminate another serial resistor on power traces.
Then C4 utility? We have a 10F super-cap the 100nf seems illusory.
C5 is already in parallel with the 667PF MOSFET caps, but why not if PWM pulses are slow.
The time constant R4/C3 seems huge (without the UC code impossible to know what PB4 does?)
Another concern in the design of the PCB, the traces seems to be of small section for power and ground, why not put a ground plane eliminating the connections layer-1/Layer-2 and allowing to have more important width track section?
Hope this help.

Hi Vidalv,

First off, I appreciate your comments very much.

The drawing is incorrect. The MOSFET used in the drawing should have been an N-channel to accurately match the real circuit. My fault for not catching that before posting the drawing.

The switch was placed in the main line to completely isolate the capacitor when not in use.
Agree, C4 is extraneous. (Original circuit testing was done with a bench top power supply.) The C4 traces still exist on the circuit board, but the capacitor is not installed.

"C5 is already in parallel with the 667PF MOSFET caps, but why not if PWM pulses are slow." I don't understand. Could you elaborate?

Someone else pointed out the issue with R4/C3. The original 22k R4 has been replaced with a 100 ohm resistor. Do you agree with that change? What values would you suggest for R4 and C3?

Strictly out of curiosity, how did you determine seemed too small? (I ask solely to increase my knowledge.) On the actual circuit board, the traces from the capacitor to the motor connections are 0.5mm in width, all others are 0.25mm. Does this sound adequate?

The idea of a ground plate was considered but decided against as one of the goals of the project was to design as small and light a circuit as possible. It was felt that a copper ground plate would be significantly heavier than separate power traces.

Your complete circuit analysis has been most helpful. Please continue to contribute to this forum thread.

GH Crash said:

It was felt that a copper ground plate would be significantly heavier than separate power traces.

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Highly doubt that the weight of a copper ground plane would be a significant percentage of the total system weight.