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
Hi,
You should really show the schematic when you ask questions about a circuit of any type. For example, we have no idea how you are driving the MOSFETs. That's an inportant bit of information that affects not only the rise and fall times but the degree of inductive kickback.
Anyway, the spike is caused by ANY inductance in series with the load, including the wiring to the MOSFETs, and that also includes the load itself.
The ringing is usually caused by the inductance in the source lead of a MOSFET, which includes the connection from the internal die to the pin of the case of the MOSFET package. What happens is the inductance in the source lead causes the gate to rise and fall several times as the MOSFET tries to switch. That's because the source terminal is never perfectly connected to the driver common in any MOSFET switching circuit no matter how well it is made. However, getting the source terminal connected to the driver common is paramount to reducing this ringing effect.
The spike problem is usually best done with a snubber connected as close to the MOSFET as possible. If you use a diode to +Vcc you have to be careful that the +Vcc line does not shoot up higher when the spike occurs. The diode conducts when the spike appears, and the energy from the spike causes +Vcc to rise. That means that energy has to be absorbed by something which could just be a capacitor of the right value, or a resistor across the +Vcc to ground leads close to the MOSFET along with some capacitor. In any case, the energy dissipated by the +Vcc line must at least be equal to the spike energy or else the voltage of the +Vcc line will keep rising with each new spike generation.
I asked about the drive circuit because it is very possible to mitigate the spike and the ringing with a small modification to the driver circuit. Spikes are caused by fast rising wave fronts, so if you slow that down a little (think step change versus ramped change) then the spike comes down. The tradeoff is the MOSFET gets a little warmer, but in this case it's not going to be a problem as long as you don't go too far with the ramping effect.
So to summarize:
1. Ringing is caused mostly by the connection from the source to the driver common. The shorter the better.
2. The spike is caused by the fast wave front. Slowing it down means less spike and could reduce ringing also.
3. If a diode is used to reduce spike, make sure the +Vcc line voltage does not go up when the spike occurs with a simple scope shot. It may be hard to reduce the inductance needed to connect this diode depending on terminal distances.
4. A snubber could be used for reducing the spike also and may work better than anything because it can be placed closer to the MOSFET drain and source terminals than any other solution.
The nice thing is, whatever you do you can just test with reduced load. That will give you a nondestructive test.