Part Number: OPA549
I'm plannig using this power op amp as a power supply for a DC motor (it drives a water pump).
I'm briefly describing the circuitry (I tried using webench but I'm getting some issue, anyway is quite simple); I'll use the nomenclature you can find on page 10 of the datasheet: non inverting configuration, I have a Vin=0-5 VDC signal in the input (this signal comes from a low pass filter that straighten a PWM 490Hz signal), I'll power up the amplifier with V(+)=+29 and V(-)=-29 V. in order to get an (ideal) 0-24VDC in the output, I selected R2=3.9 kOhm and R1=1 kOhm (first question, is that a nice choice? I calculated around 5 mA on this resistors when Vout=24V). I also limited the current with a Rcl=3.3 kOhm (I got a nice 6.9 Amps max).I had to use the 29V supply beacause this is not a rail to rail op amp, so the maximum offset from V(+) is 5V (correct?).
Then I got into the calculation to know the Power Dissipation of the op amp ( Itried to take as a reference the SBOA022.pdf, from TI): I don't know the internal Resistance of my load varying the voltage, so to get the power dissipation I multiplied the Current with the difference between V Output and V(+):PD=Io*(V(+)-Vo).I got a peak of 39 Watts (!) with Vo=17V.
Now, it seems that I need heat sinking. I would like to keep the device at least under 85C, since my project invole Hydrogen, and I don't want to kill anyone (and myself).
Can you suggest some hardware or something else? Maybe a little of explanation on how to calculate the amount of heat sinking "power" required and which characteristic look for. I can use a fan to cool down the electronics, and I thought (hope) that I can use those aluminium heat sinker chips, glued with thermal glue.
Thankyou for the help, I'm almost new to electronics, beeing and aerospace engineer does not help a lot
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In reply to kai klaas69:
It's more or less linear, it's better that I'll show you a raw picture of Amps drawing vs voltage. Sorry, it's very raw
sorry, I'm messing up a bit
The information regarding the motor output current vs motor voltage is very useful in determining the worst-case power dissipation for the OPA549. The worst-case point is when Vo is 19 V, and Io is 4 Amps; that results in a power dissipation Pd equal to 40 Watts. All of the other points produce a Pd from 30, to almost 40 watts. Therefore, 40 Watts is what the thermal system must be capable of dissipating while keep in the OPA549 junction temperatures (Tj) below 150 °C. Keep in mind that 40 Watts is a lot of power to dissipate from such a small package area.
The equation presented at the top of OPA549 datasheet Pg. 11, column 2 is useful in determining the maximum thermal resistance of the heat-sink:
θHA = [(TJ – TA)/PD] – θJC – θCH
Applying practical values as described in the paragraph associated with the equation:
θHA = [(125 °C – 25 °C)/40W] – 1.4 °C/W – 0.5 °C/W
θHA = 0.6 °C/W
Unfortunately, that is a very low thermal resistance value and would necessitate a very large, expensive heat-sink. This application really calls for a low thermal resistance heat-sink, with a high CFM fan blowing across it; especially because you want to try and keep the OPA549 body at 85 °C or less. An example of the improvement in the thermal resistance of the heat-sink/fan combination, over a heat-sink alone can be seen in Fig. 6 of this Design World magazine article:
I do think that if you try to employ just a heat-sink alone to remove the dissipated heat that it may be difficult to attain the 85 °C maximum device temperature you would like to achieve.
Precision Amplifiers Applications Engineering
In reply to Thomas Kuehl:
Thankyou Thomas, very usefull and complete reply. TI forum is really usefull.
Do you think, thanks to your experience, for example, that this heat sink ( http://de.farnell.com/abl-heatsinks/bga-pp-030/heat-sink-bga-push-pin-7-c-w/dp/2084421 ) might be enough with its 7C/W couple with a fan (2.5 Watts). I know it's nearly impossible to answer without numerical data, I just would like having more infos about "forced convection" situation.
From what I read on the usefull article you posted, should not be enough and I should go for a 2 C/W heat sink + fan
I think I should think on a different way to power my pump.
In reply to Giorgio Nicola:
Kai has good advice; I agree with using a heat-sink having a thermal resistance (θHA) of less than 2 °C/W. Unfortunately, because of the numerous physical variables involved in the application your thermal system development will require some lab work. Certainly, the optimum fan size and speed, heat-sink effectiveness, and physical location of these devices will require some experimentation to achieve effective heat removal.
Kai's idea to use a switch-mode power supply has merit, but unfortunately I am unable to assist you with those devices. Talking with my colleagues who have experience with switch-mode devices they indicate that this would likely require a HV buck DC-DC switching supply that has a track pin, or soft start pin, that can override the internal reference.
I suggest you contact TI's experts in switch-mode product area on their E2E forum. I am not certain of the exact forum, but I would start with:
If you explain what it is you are attempting to accomplish as you did with us, they may be able to assist you directly or move you to the correct forum.
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