DRV101 PWM solenoid driver

[drkidd22]

Hello,

I'm building a circuit to control a solenoid. I'm almost done, but the one issue I'm having is calculating the power dissipation. The datasheet states that the power dissipation is the product of the output current times voltage across the conducting output transistor times duty cycle.

My circuit is pretty similar to the one in figure 13 of the data sheet from the DRV101.
My duty cycle is set for 48% and delay is 100ms.
So is the voltage across the the output transistor the saturation voltage they are talking about? if so how do I measure this to calculate power dissipation?
Any advice will be appreciated.

https://www.electro-tech-online.com/custompdfs/2011/12/drv101.pdf

 

[user_88]

If you want some degree of accuracy in your measurement, you should use an oscilloscope.
You would be able to actually see the collector to emitter voltage waveform of the output transistor.
A scope image will allow you to establish an average value for the transistor voltage while it is turned on, and to verity the duty cycle and period.
To measure the current, you could insert a low value resistor or shunt in the output current path and get a reasonably accurate idea of the actual current waveform.

These two values, voltage and current, will provide sufficient data to find the output power.

It does not seem likely that you will obtain accurate measurements by using only a voltmeter or an ammeter.

 

[drkidd22]

I have used an oscilloscope and verified duty cycle and output current. Output current is 300mA when the device enter pwm mode.
When I hook up the scope probe between pin 6 and ground to pin 4 I get the waveform and pwm signal, but the thing is I'm not sure how to look for the saturation voltage, where or how should I hook up the scope probe?

The datasheet says it should be about maybe 300mV, but I have not been able to measure this.

 

[user_88]

If you have the voltage waveform between pin 6 and ground, the Vce saturation voltage is just the minimum voltage ...indicating that the transistor is fully turned on.
According to the data sheet, the saturation voltage should be no more than 0.3 volts, @ 0.1 Amps. ... that particular case being without a heat sink.

It looks like you might want to determine the heat sink area which would keep the die junction temperature below a reasonable temperature ... maybe 125 C.

Equation 1 on page 13 might be useful.
... Also take a look at fig. 12 ... the larger the heat sink area, the more power dissipation that is allowed.

 

[drkidd22]

Here are the waveforms, one shows the complete pwm and the other is the same but changed the amplitude, so is my Vce about 356mV?

 

[user_88]

Yes ... it looks like the saturation voltage is a little over 300 mV.
However, it looks like this value exceeds the maximum allowable number that is given in the data sheet ... for the case with no heat sink.

To be able to use this component ... for something like a solenoid coil ... you should see if you can affix a section of copper printed circuit board to it, to act as a heat sink.

Another interesting option would be to connect a resistor and an LED to pin 7. Pin 7 is described as a FLAG which goes low when the part gets excessively hot.

 

[drkidd22]

Ok thanks.
It is higher than 300mV because I'm running it at 300mA, not 100mA so I guess that should be expected.

As for power dissipation according to the datasheet It should be the voltage across output transistor (I assume this would be Vce = 356mV correct?) times the duty cycle (I'm at 48%) times the current (I'm running 300mA) so the power dissipation would be 51mW??

Look at page 12. So I figured I should be ok without a heat sink since the max power that can be dissipated by the IC without a heat sink would be: (125C - 30C)/(65C/W)=1.46W

 

[user_88]

You might be right ... not absolutely requiring a heat sink for your application.
However, it you eventually encounter thermal latch-up ... try the pcb heat sink ... The data sheet says it is directly soldered to the back of the chip.

If you try the LED flag ... note that the FLAG pin can only accommodate 2 mA.

 

[drkidd22]

Ok thanks for your help. I will try it as it is. In case I require more power I will solder the IC to a pad on the PCB.

Appreciate your help.

 

[user_88]

@drkidd22:
... Just curious here...
What is the reason for using the DRV101? Is your application battery powered and you want to hold a solenoid on for an extended period of time? ... Conserving the battery capacity?

 

[drkidd22]

not battery powered, and yes the solenoid might be held open for a long time.

 

[user_88]

I see ... hope everything works well.