Hi All,

I am using for ex: IRFML8244TRPBF as a MOSFET and trying to figure maximum on time and required off time (duty cycle) when pulsing 20A into a load. Would anyone be able to provide examples or direction on calculating these values?

Thanks,
Kevin

 

The SOA (safe operating area) graph in the datasheet will tell you how long of a pulse it can handle for a given current level. The offtime is related to the RMS power which must be less than the rated DC power of the device.

 

The SOA (safe operating area) graph in the datasheet will tell you how long of a pulse it can handle for a given current level. The offtime is related to the RMS power which must be less than the rated DC power of the device.

Based on my understanding then if my drain current is 20A, and Rds of 0.024ohms then Vds = 0.48V. That chart tells me I can be on at least 10ms?
Thanks,
Kevin

 

If you can keep it cool. As it's a tiny device, it's going to warm up quickly, and as it does so, Rds(on) will increase by about 30%. That will make it dissipate more power, and warm up even faster. (see fig. 4 in the datasheet)
Also, make sure you have plenty of gate voltage - at least 12V. If you are trying to switch it with 5V, you are heading for trouble.

 

If you can keep it cool. As it's a tiny device, it's going to warm up quickly, and as it does so, Rds(on) will increase by about 30%. That will make it dissipate more power, and warm up even faster. (see fig. 4 in the datasheet)
Also, make sure you have plenty of gate voltage - at least 12V. If you are trying to switch it with 5V, you are heading for trouble.

So realistically if I'm pulsing at 20A, Rds = 0.024ohms, Vds = 0.48V my total power at that moment is 9.6W. If I were to use the Pd max of 0.8W at 70C, having a duty cycle of ~8% would be my max?

On your comment about if my gate voltage being less then 12V. Are you saying that based on Figure 1 & 2? I can see that anything below 5V my drain to source voltage will be much higher meaning more power dissipation.

Thanks!

 

Looking at the datasheet Thermal Response Fig. 11. Could someone shed some light on what Pdm is?

 

Looking at the datasheet Thermal Response Fig. 11. Could someone shed some light on what Pdm is?

I think it can only be the power dissipated.

 

Data sheets often iclude a description of the on time for the max current rating, and usually that is a very short pulse, just a few milliseconds. So use the constant current rating if the on time will be much more than a very few milliseconds. AND be sure to use whatever heat sinking is described because heat is the enemy.

 

I don’t think I’d use a SOT23 device for 20A no matter how short the pulse might be.

 

Thanks for the replies everyone. Essentially I'm making a circuit to reverse the polarity of an Alnico magnet. The pulse will actually only be 50us max every 10ms and based on the thermal response figure I should never notice the MOSFET heating much at all.

 

The MOSFETS absolute Rds max at 150C is 0.0372ohms. When using the formula from figure 11, using duty cycle of 0.01 I get the following.

Pm = (20A * 0.0372ohm) * 20A
Pm = 14.88

Tj = Pm * Zthja + Ta
Tj = 14.88 * 1.5 + 25
Tj = 47.32C