Help with calculating inductance of dummy load

[trilerian]

For quite awhile now I have been using a dummy load made up of 5x 0r33 100W resistors (aluminum cased, wirewound).  So the total resistance of the load is 66mΩ.  The load is driven by mosfets, and the load is used to discharge batteries.  What never occurred to me when I designed this, was that the power resistors I am using are wirewound resistors, therefore they will have an inductance.  Unfortunately, I only have a cheap LCR meter and really can't get a good reading on the inductance of the resistors.  Measuring 1, is says 2µH, but measuring 5 in parallel gives the same measurement.  My understanding is inductive loads are like resistive loads in parallel, so the inductance should go down.  Based on that, I say my meter is reading correctly. 

This is the first circuit I designed using mosfets, and like I said, I took no precautions for inductive kickback.  That said, there have been no issues thus far regarding damage to the mosfets, but that doesn't necessarily mean there will not be.

I am using 4x IPP013N04NF2S (https://www.mouser.com/datasheet/2/196/Infineon_IPP013N04NF2S_DataSheet_v01_00_EN-3011955.pdf) in parallel to do the switching.  The pwm frequency is 490Hz.  The mosfets are heatsinked and I have an NTC measuring the temp at the junction of the tab and the heatsink.  The temp gets up to about 45°C, so the mosfets are not running too hot. 

I would like some help figuring out all the parameters I need to decide what I need in my future version of this.  I've done a fair amount of reading and have read that some may use TVS diodes, some just say a flyback, some say maybe just a resistor (I'm using 5 in parallel).  But regardless, I need to figure out how to calculate what I need.

Thanks

[calzap]

Wire-wound resistors typically have a very low inductance (nH range) because winding layers are in alternate directions in most of them.  Data sheets often omit any mention of inductance.  If you’re using no-name resistors, anything could happen.  If you have a digital scope, you should be able to capture any inductive kickback.  Non-wire-wound resistors do exist in the resistance and power range you’re using.  Check data sheets on possible derating of wattage based on ambient temperature.  It’s good practice to provide cooling (heat sink or fan) for power resistors if they will be operated continuously at even ½ their rated wattage.

Mike

[trilerian]

Wire-wound resistors typically have a very low inductance (nH range) because winding layers are in alternate directions in most of them.  Data sheets often omit any mention of inductance.  If you’re using no-name resistors, anything could happen.  If you have a digital scope, you should be able to capture any inductive kickback.  Non-wire-wound resistors do exist in the resistance and power range you’re using.  Check data sheets on possible derating of wattage based on ambient temperature.  It’s good practice to provide cooling (heat sink or fan) for power resistors if they will be operated continuously at even ½ their rated wattage.

Mike

I do have a digital scope that I can use.  Would I just be adding the scope probe to the side of the load connected to the drain? If that is the case then the scope does capture a spike on the rising edge of about 32V.  But I would think kickback would be on the falling edge?

As to the resistors, they are no name, but the manufacture gave me the data sheet, but no inductance value.  I tried to use a signal generator and my scope, but well, these resistors being very low resistance only give about a 50mV signal when loaded on the signal generator.   

And yes, the resistor bank is cooled with a heatsink and a couple of 60x60x25mm fans.  I have calculated for the derating of the resistors vs heat.

[Kleinstein]

The low resistance is a good thing when it comes to measuring the inductance. The low voltage drop from the generator is not a problem, it is still enough to get a trigger. There are different ways to get an idea of the inductance. One would be using a low inductance resistor of the 50 Ohms directly in series. resistors of some 50 ohm usually have relatively good with the parasitic inductance and capacitance about balancing. One can than measure the phase between the voltage at just the 0.33 ohm and the 0.33 plus 50 ohms for a few relatively high frequencies (e.g. 1 MHz or 10 MHz).

The other option would be to directly measure the final circit and check for negative effects of inductance (e.g. ringing, overshoot). The rather large MOSFETs may result in slow switching, so that the inductance may not be that much of an issue.

With low cost low values wire wound resistors I would not expect alternating winding direction and more like only 1 layer.

For the LCR meter one may have to do a dummy measurement with a short with largely identical cable positions. Cables may have more inductance and it's not just the cable length, but loop area that matters - so a function of the shape, not just the length or cable type.

[trilerian]

Here are 2 pics.  Pic one a scope signal of the gate.  Pic 2 is the scope signal of one of the mosfet drain, note here the measurement is a max of 48V, but this measurement is taken right at the drain pin of the mosfet, compared to the earlier measurement taken at the drain side of the resistor bank.  So the 12" of 10AWG stranded wire is also causing a little spike, and that wire is not looped. 

So the question, is this bad, and if so, how do I fix it?

Thanks

[Xena E]

Any spike on the falling edge of the square wave will be clamped by the mosfets 'body' diode. The inductive kick will be positive as the mosfet switches off, if they are N channel and the resistors connected to the drain(?)

That positive going spike may be a phantom, but otherwise clamp it below the mosfet Vds with a Z diode, or snubber network if it bothers you.

If you want to measure the inductance of those resistors connect them in series, (keeping them apart!), and divide the result by the quantity.

[trilerian]

The low resistance is a good thing when it comes to measuring the inductance. The low voltage drop from the generator is not a problem, it is still enough to get a trigger. There are different ways to get an idea of the inductance. One would be using a low inductance resistor of the 50 Ohms directly in series. resistors of some 50 ohm usually have relatively good with the parasitic inductance and capacitance about balancing. One can than measure the phase between the voltage at just the 0.33 ohm and the 0.33 plus 50 ohms for a few relatively high frequencies (e.g. 1 MHz or 10 MHz).

The other option would be to directly measure the final circit and check for negative effects of inductance (e.g. ringing, overshoot). The rather large MOSFETs may result in slow switching, so that the inductance may not be that much of an issue.

With low cost low values wire wound resistors I would not expect alternating winding direction and more like only 1 layer.

For the LCR meter one may have to do a dummy measurement with a short with largely identical cable positions. Cables may have more inductance and it's not just the cable length, but loop area that matters - so a function of the shape, not just the length or cable type.

I found instructions on https://daycounter.com/Articles/How-To-Measure-Inductance.phtml and used my scope and function generator to get an approximate value of 340nH. 

Any spike on the falling edge of the square wave will be clamped by the mosfets 'body' diode. The inductive kick will be positive as the mosfet switches off, if they are N channel and the resistors connected to the drain(?)

That positive going spike may be a phantom, but otherwise clamp it below the mosfet Vds with a Z diode, or snubber network if it bothers you.

If you want to measure the inductance of those resistors connect them in series, (keeping them apart!), and divide the result by the quantity.

Right, so if there is an inductive load it should happen right after the falling edge?  That would make sense to me.  But you are saying the spike on the rising edge is just noise?  It doesn't bother me if it doesn't hurt the components.  If it could, then I want to fix it.

[Xena E]

In the conjunction that the circuit is how I assume, (have you posted a diagram of the arrangement?):

You say that you have taken the second scope picture from the drain of the Mosfets, if this is ground referenced to the source then that trace falling edge is when the Mosfet turns on... that will not cause an appreciable inductive spike.

In any case: It is when the current in the inductance is turned off the spike will be generated.

How is the circuit arranged? Can you do a sketch?

[trilerian]

In the conjunction that the circuit is how I assume, (have you posted a diagram of the arrangement?):

You say that you have taken the second scope picture from the drain of the Mosfets, if this is ground referenced to the source then that trace falling edge is when the Mosfet turns on... that will not cause an appreciable inductive spike.

In any case: It is when the current in the inductance is turned off the spike will be generated.

How is the circuit arranged? Can you do a sketch?

Ok, yes, these are N channel fets.  Drain to the resistor bank and source is gnd referenced.  So the falling edge is when the mosfet turns on, I had that backwards.  Rising edge it turns off and there is that spike.  So I do need to do something about it.  For a DC motor you would add a flyback diode.  This seems like the easiest option.  How do I calculate the value I would need for that?

Thanks