Film resistor vs Carbon resistor.

[spec]

Several posts ago, you asked me to validate what experience I had.

I can't find that- can you quote John?

 

[spec]

1/R?

https://dictionary.reference.com/browse/conductivity

Hi Keep,

I'm having to duck bullets left right and centre here on ETO. Can you explain what you post means? I have looked at it upside down sideways and back to front, and I just can't figure it.

chuck

 

[gary350]

I have learned with my experiment's I can run 10 amps through the mosfet and it stays room temperature for hours. If I increased the load so current goes up 1 amp each time there is a time where the mosfet started to become warm. As the load increases the mosfet gets hotter and hotter. Each time the load increases by 1 amp it takes several minutes for the temperature to increase and stabilize. At about 120 degrees F the mosfet can run like that for hours. When the mosfet temperature gets up in the 140 degree range even if the amp load remains the same the mosfet continues getting hotter and hotter and soon it will burn up. There is a place where the mosfet heat is not able to transfer to the heat sink fast enough to keep the mosfet cool.

I know transistor cases use to be epoxy, I'm not sure if they still are or if they are some type plastic these days. Either way the case material has a very low heat transfer rate compared to the tab. The case actually has an insulating effect that holds in the heat, the tab is hotter than the case. If a mosfet sets for 3 minutes the case will soon warm up to the same temperature as the tab and heat sink.

Heat sinks have hot spots. A hot mosfet cools faster when attached to a thick metal heat sink. Thin meter heat sinks get hot at the mosfets area because the heat can not transfer away from the mosfet fast enough. A 3 lbs solid block of aluminum makes a very good fast acting heat sink compared to a 10 time larger aluminum heat sink with cooling fins with fan. For high duty cycle you must have cooling fins, the best of both worlds is to attach a large block of aluminum to a large surface area heat sink with fins with fan. This is good for experimenting but not necessary for the correct mosfet amp load.

I have learned the mosfet I am experimenting with can handle 37 amp but not 38 amps. I can turn the amps up slow or fast the mosfet still can not do 38 amps even from a cold start with a mosfet temperature of 70 degrees F. I have decided the mosfet maximum current limit much be 37 amps but it can not do 37 amps for more than about 1 minute 30 seconds. After reading the link above I now understand what is happening. There is a happy amp load the mosfet likes before is starts over heating and I need to find that amp load and stay below that limit. If the amp load turnes out to be 15 amps then I should be able to put enough mosfets in parallel so they all run at about 12 amps then it should be able to run all day and never over heat the mosfet or the heat sink. Also if I stay in the safe zone there is no need to have .1 ohm current limiting resistors on pin 3.

 

[spec]

I have learned with my experiment's I can run 10 amps through the mosfet and it stays room temperature for hours. If I increased the load so current goes up 1 amp each time there is a time where the mosfet started to become warm. As the load increases the mosfet gets hotter and hotter. Easy time the load increases by 1 amp it takes several minutes for the temperature to increase and stabilize. At about 120 degrees F the mosfet can run like that for hours. When the mosfet temperature gets up in the 140 degree range even if the amp load remains the same the mosfet continues getting hotter and hotter and soon it will burn up. There is a place where the mosfet heat is not able to transfer to the heat sink fast enough to keep the mosfet cool.

That is right gary. That's more or less what I have been saying all along. If you look at my second circuit you will see that that it handles a total of 50A. I assumed that figure as an educated guess because you would not answer my requests for the maximum current the motor takes. Rather rather than carp on about a silly resistor in your model motor circuit, I took a positive approach. It was the only way forward. The circuit uses 5 MOSFETS, thus each MOSFET takes a maximum of 10A , because of a well-designed current sharing approach.

I know transistors cases use to be epoxy, I'm not sure if they still are or if they are some type plastic these days. Either way the case material has a very low heat transfer rate compared to the tab. The case actually has an insulating effect that holds in the heat, the tab is hotter than the case. If a mosfet sets for 3 minutes the case will soon warm up to the same temperature as the tab and heat sink.

The TO-220 case was only originally designed for medium power dissipation and is totally unsuited to your application. Would you think of moving to TO-3 or TO-247 - be much better. I already stated that early on in this thread, or the associated thread on this subject. Just because the data sheet says 55A, does not mean that that can be used in a practical circuit. Early on, you said that you were surprised that such thin pins could take 55A- with good reason.

Heat sinks have hot spots. A hot mosfet cools faster when attached to a thick metal heat sink. Thin meter heat sinks get hot at the mosfets area because the heat can not transfer away from the mosfet fast enough. A 3 lbs solid block of aluminum makes a very good fast acting heat sink compared to a 10 time larger aluminum heat sink with cooling fins with fan. For high duty cycle you must have cooling fins, the best of both worlds is to attach a large block of aluminum to a large surface area heat sink with fins with fan. This is good for experimenting but not necessary for the correct mosfet amp load.

What you say is the right information but it is mixed up. A heatsink needs good conductivity (thick) near the MOSFET and a large surface area presented to the air to get the heat from the heatsink into the air by convection to cool it.

If you want to see about heatsink design and characteristics, see my post 507 @ https://www.electro-tech-online.com/threads/transistor-equivalent.146091/page-26#post-1242633

I have learned the mosfet I am experimenting with can handle 37 amp but not 38 amps. I can turn the amps up slow or fast the mosfet still can not do 38 amps even from a cold start with a mosfet temperature of 70 degrees F. I have decided the mosfet maximum current limit much be 37 amps but it can not do 37 amps for more than about 1 minute 30 seconds. After reading the link above I now understand what is happening. There is a happy amp load the mosfet likes before is starts over heating and I need to find that amp load and stay below that limit. If the amp load turnes out to be 15 amps then I should be able to put enough mosfets in parallel so they all run at about 12 amps then it should be able to run all day and never over heat the mosfet or the heat sink.

That is all correct. But you will not find the happy amp by just blowing MOSFETs. What the MOSFET can and cannot do should be calculated from the data sheet. If there hadn't been all the flack and messing about on this post you would have had a design by now which you could start building.

Also if I stay in the safe zone there is no need to have .1 ohm current limiting resistors on pin 3.

You have that the wrong way around. The source resistors are ESSENTIAL to ensure that each transistor only takes a fifth of the total load current. The source resistors are what makes that happen! All this to-and-frowing about source resistors or not, isn't worth a bean. It's just a distraction, and a waste of time, time that could be spent on optimising the design. The source resistors only amount to a length of wire or store bought 2.5W parts. Your cabling and battery source resitance, especially when partly discharged, would add more resistance in the circuit! The source resistors will do no practical harm to your motor controller, in fact they will do a lot of good.

It would be a different matter if someone experienced in this type of circuit were doing the layout design, build, and development testing. In that case the source resistors could probably be omitted, but even so, for initial testing, I would leave them in. I don't mean to be unkind but, from your posts, I get the impression that you are inexperienced in this area, so I would suggest that the source resistors are a wise pecaution. In fact, when you first test the circuit I would advise increasing the source resistors and just taking a light current from the circuit, say 10A total. Once the circuit is reliably working at that current, you can then take it in steps to the maximum current.

If it turns out that the source resistors are not needed, and therory says they will not, simply take them out- no big deal, but you will have to be exact with the wiring and layout in the gate and and source circuits. In practice, every MOSFET, has a source resistor that is caused by the resistance of the wire and connections in the case and the external wiring. Note that very low risistance values are involved, but because of the high currents, these lead to significant voltage drops.

When this post started, switch mode operation was not mentioned, so the MOSFETs would be operating in the linear mode. When in that mode MOSFETS definately do need source resistors due to the varying gate characteristics and the adverse temperature coificient.

Just to let you know, the design and build of your controller is straight-forward and similar circuits have been built many times before, but a logical, measured, and odjective approach is the only way to get a good motor controller. So far, you have been chasing your tail and have achieved nothing. If you are not careful, you will still be knee-deep in dead MOSFETs a year from now!

​

 

[KeepItSimpleStupid]

Hi Keep,

I'm having to duck bullets left right and centre here on ETO. Can you explain what you post means? I have looked at it upside down sidways and back to front and I just can't figure it.

chuck

Conductivity is in units of Siemens and is defined as S=1/R or the reciprocal of resistance.

So, their might be an ambiguity between "conducts more" and "conductivity".

Quote
https://en.wikipedia.org/wiki/Thermal_runaway

Bipolar junction transistors (BJTs)

Leakage current increases significantly in bipolar transistors (especially germanium-based bipolar transistors) as they increase in temperature. Depending on the design of the circuit, this increase in leakage current can increase the current flowing through a transistor and thus the power dissipation, causing a further increase in collector-to-emitter leakage current. This is frequently seen in a push–pull stage of a class AB amplifier. If the pull-up and pull-down transistors are biased to have minimal crossover distortion at room temperature, and the biasing is not temperature-compensated, then as the temperature rises both transistors will be increasingly biased on, causing current and power to further increase, and eventually destroying one or both devices.

Different mechanisms may be at play in a BJT vs a MOSFET.

Just dotting i's and crossing t's. (the lower case i's)

Measured the properties of way too many as deposited silicon films. Particulary conductance vs temperature. Conductanc(1/R) is ALWAYS higher (e.g. 1e-9 S) at room temperature. Goal was to measure activation energy, conductance at room temp and high temp (200 C) ) e.g. 1e-6 S ( 1E+6 ohms)

I can't remember actual numbers.

 

[spec]

Conductivity is in units of Siemens and is defined as S=1/R or the reciprocal of resistance.

So, their might be an ambiguity between "conducts more" and "conductivity".

Quote
https://en.wikipedia.org/wiki/Thermal_runaway

Bipolar junction transistors (BJTs)

Leakage current increases significantly in bipolar transistors (especially germanium-based bipolar transistors) as they increase in temperature. Depending on the design of the circuit, this increase in leakage current can increase the current flowing through a transistor and thus the power dissipation, causing a further increase in collector-to-emitter leakage current. This is frequently seen in a push–pull stage of a class AB amplifier. If the pull-up and pull-down transistors are biased to have minimal crossover distortion at room temperature, and the biasing is not temperature-compensated, then as the temperature rises both transistors will be increasingly biased on, causing current and power to further increase, and eventually destroying one or both devices.

Different mechanisms may be at play in a BJT vs a MOSFET.

Just dotting i's and crossing t's. (the lower case i's)

Measured the properties of way too many as deposited silicon films. Particulary conductance vs temperature. Conductanc(1/R) is ALWAYS higher (e.g. 1e-9 S) at room temperature. Goal was to measure activation energy, conductance at room temp and high temp (200 C) ) e.g. 1e-6 S ( 1E+6 ohms)

I can't remember actual numbers.

I know all that!

 

[KeepItSimpleStupid]

You might,. For the uneducated conductivity (1/R) and resistivity (R) may mean the same thing. They aren't.

 

[spec]

OK Keep, but you didn't make that clear.

 

[KeepItSimpleStupid]

Sometimes I'm on another wavelength. Sometimes(like today) I have a headache. I won't call this one a migraine. Dialog resolves differences.

 

[spec]

Your a kindly person Keep. One thing to bear in mind is that my brain is shrinking as I age so ... k..e..e..p it s..i..m..p..l...e.