DRV8825EVM

Hi Mario,

It is difficult to create a practical guide because there are many variables to deal with. The variables include size of the pcb heatsink, location of the device on the pcb, surround heat sources, thickness of the copper, vias, solder coverage on the thermal pad, and ambient temperature.

The thermal information in the datasheet is based on a standard pcb. This provides an equation to calculate the temperature assuming the pcb has the dimensions and only the DRV8825 installed.

At one time there was some additional information on line, but I cannot locate it. Once I find it, I will point you to it.

Thank Rick.
But about the evalutation board: what's his limit about current and temperature ?
Mario.

Hi Mario,

Please refer to the thermal calculator link ( LINK ). This link allows you to select the DRV8825 and evaluate the effect of changing copper weight and coverage area. Also, there are additional guides and articles to the right.

Using the calculator and the formula P=4 x Rdson x Iout, if i have 2A i would have a Tj = 280°C at maximum area ?? I don't understand.
Therefore how can it work at 2,5A ?
Mario.

Hi Mario,

What Rdson are you using for your calculation? Also are you using Irms? At max current in one winding, the other winding is off.

Also, please note the copper thickness is 1 oz and 1/2 oz in the calculator. Increasing the size of the thermal plane or the thickness of the copper will help.

It may take up to two weeks before I can answer your EVM question. Is the 1.7A mentioned full scale current or rms current?

I'm using the equation 5 of datasheet and 1,7A is the full scale current.There are 2 motor windings.rdson=0.3.

Mario.

Thanks,

So using the equation P = 4 * Rdson * (Irms squared) = 4 *.3 * ((1.7 *.707) squared) = 1.73 W

Using the pcb thermal calculator with a conservative 1.9 W (1.7W + 10%) and 25C ambient shows the junction temperature at 125 with 1oz copper and 8 sq inches of thermal plane. Does this make sense? 

1.7A is the current in full step mode. therefore i have 3,468 w dissipation power.
Mario.

Hi Mario,

Thanks for the clarification. You are correct. You have 3.468 W. This is the top end of the device operation. The device may become marginal depending on how long the motor is run.

I will provide information regarding the EVM temperature as soon as possible. The EVM is approximately 80 sq inches of 2 oz copper.

May I suggest you consider the DRV8711 as an alternative. This device is a stepper gate driver, allowing you to connect lower Rdson FETs.

I read in datasheet " The DRV8825 is capable of driving up to 2.5 A of current from each output (25°C proper heatsink)".Therefore is pratically not possible ? How can do the EVM ? Whit the thermal calculator already at 25°C i have a junction temperature over 160°C....
I don't understand the limit.
Mario.

Hi Mario,

Sorry for the delay getting back to you. As mentioned, the EVM has approximately 80 sq inches of 2 oz copper while the thermal calculator uses 8 sq inches of 1 oz copper (effectively 20 times the thermal mass).

Please refer the LINK , which shows how to calculate the junction temperature in the lab.

Using this link and running the DRV8825EVM at 12V and 2.5A for 10 minutes, the case temperature was measured at 62.6C using an IR camera.

Using the estimate junction temperature formula from the link, the junction temp is calculated to be:

62.6C + (0.2C/W x 3.99W) = 63.4C  -- This uses the max RDSon of 320mOhm and 2.5A full scale current to calculate the power

I hope this helps explain the difference.

Hi Rick.

When you say 80 sq do you refer at  total area layers ?  the board, if i'm not wrong, is 5"x5" = 25"  therefore the total is for 4 layers 100" ? The current 2.5 A is the peak of sine wave (microstepping) or a continue current ? 25° C  ambient ?

I would have in my device (i still haven't done it) a 1.5A rms (32 microstepping) for max 30 seconds (1 minute or more pause) and 4 layer  pcb of 70mm x 180mm .

Thanks

Mario.

Hi Mario,

Sorry, I failed to mention the board was at 25C ambient.

The board is 5"x5" and 4 layers (total 100 sq in), but the total ground plane on the board is smaller totaling approximately 80 sq in. The 2.5 current measured is the peak of the sine wave, RMS current 1.77A.

So the temperature rise from ambient was ~40C.

Assuming you use 2 oz copper on your board and flood the ground plane, you should see similar results. Also it helps that you are using 1.5A RMS, which reduces the power by ~30%.