MOSFET H-Bridge Thermal Calculations  - Page 1
Forum section: electronics / beginners

author: XenonFlash54
author_rank: Contributor
timestamp: July 03, 2015, 11:33:26 am
content: I wonder if you guys could check my thermal calculations for an DRV8839 MOSFET H-bridge to see if I did them right (the datasheet ishere). I'm really new to doing this type of stuff, and I don't want to release the magic smoke!So first I calculated the power dissipation in one of the H-Bridge's halves using the formula on page 10. The formula is: PTOT=RDS(ON)x (IOUT(RMS))2For RDS(ON), they said to add the RDS(ON)values for the high side and the low side of the H-Bridge. So 330m+330m=660m.And for IOUT(RMS)they said to multiply the maximum current by 0.7 - I am expecting about 1.6A max, so 0.7x1.6A=1.12.So I put those values in the formula:PTOT=0.66 x (1.12)2=0.66 x 1.2544=0.827904So the power dissipation is about 830mW per H-Bridge half. I'm going to be using both of the half's (on the same motor) so the total power dissipated in the device will be 1.66W (2 x 830mW), right? (Or would it only be 830mW because only two of the four MOSFETs will be on at once?)Assuming that it's 1.66W, then with the RøJAof 50.4 C/W, I get 50.4 x 1.66=83.664. Then Plus the 25 C ambient temperature, the junction temperature will be about 108 C. Which is below the maximum operating junction temperature, so it should work fine - right?Thanks a ton Guys!!Dan

author: rs20
author_rank: Super Contributor
timestamp: July 03, 2015, 12:01:04 pm
content: That datasheet is incredibly vague -- I suspect the 330m figure is the total figure (i.e., they've already added it up for you). The use of the "+" symbol everywhere is just stupid. But anyway, you might not need to go 330m+330m=660m.I also can't tell where the "IOUT(RMS)= 0.7x the full-scale output current setting" comes from. Isn't it possible that you will have one coil on 100% of the time when the stepper is stationary? Where does your "I am expecting about 1.6A max" come from; and how does it connect with the full-scale output current setting?Quote from page 10: "Note that RDS(ON) increases with temperature, so as the device heats, the power dissipation increases." But there's no graphs or numbers anywhere to help you quantify that. But if you're expecting to be at 100 degreesbeforefactoring that in, you might have a bad time.In short, I've hardly ever seen such a useless datasheet.Also, the datasheet says that the thermal resistance value you used is tested according to JESD51-7, which is titled "high effective thermal conductivity test board for leaded surface mount packages". So if you're not using a high effective thermal conductivity test board (or a PCB laid out and constructed similarly), then that figure won't be relevant for you.

author: rs20
author_rank: Super Contributor
timestamp: July 03, 2015, 12:07:26 pm
content: Specifically, JESD51-7 specifies a four-layer board with 2 buried power/thermal planes, which lowers thermal resistance by 20 to 40% (i.e., increase by 25% to 67% to compensate, but only if your board looks like JESD51-3, with 2oz copper traces).Somewhat useful reference on all the JESD51 things:https://www.intersil.com/content/dam/Intersil/documents/tb37/tb379.pdf

author: XenonFlash54
author_rank: Contributor
timestamp: July 03, 2015, 12:19:55 pm
content: Yeah, thatisa pretty crummy datasheet. I didn't even think that the 330m figure might be the total resistance!As for the current, I'm actuality going to be driving a single motor with the device. The stall current of the motor is 1.6A, and there is a good chance that the motor will stall in my application.Do you know of any better H-Bridges that would work? I have been looking for days, and this is the first one I have come upon that will meet my requirements. The supply for the motor will be in the 6.4-8.4V range, and the controller logic is operating at 3.3V (but I can use level shifting if necessary).Thanks for the help, rs20!DanEdit: Another thing, I would like to be able to use PWM on the motors. Would putting a PWM signal on the enable pin be a bad idea?

author: XenonFlash54
author_rank: Contributor
timestamp: July 03, 2015, 09:07:16 pm
content: Never mind, I think I got it figured out.One more question though: Should I calculate the full power dissipation as the sum of four of the MOSFETs power dissipation, or as the sum of just two of them? I would think two, since only two of the MOSFET's will be on at once.ThanksDan

author: rs20
author_rank: Super Contributor
timestamp: July 04, 2015, 04:44:42 am
content: Just two, I'd say. But of course, you may be getting two mosfets just by going 330m * I2, since the 330m might include two mosfets.

author: XenonFlash54
author_rank: Contributor
timestamp: July 04, 2015, 11:50:21 am
content: Okay, Thanks a ton rs20!