Linear Fet motor controller shut off problem

Thread Starter

dlatch

Joined May 15, 2016
91
This high side linear PFET motor controller, with brake, works pretty well. However the LM358 does not fully shut off the main fets. Fet2 is the work around: It opens the output channel during brake mode to prevent shoot through.

I have tried a RR op amp (LMC 6482 IN) but it too does not return all the way to VCC at zero input.

I have tried a pull up resistor with the 358 and found the same off gate voltage (VCC minus about 1.3) The SUP90 PFETS have a -1 threshold.

(Maybe a pull up resistor would work with a RR op amp?)

MAIN QUESTION: Can I use an analog switch IC to allow the Feedback leg to select a reference voltage at shut off to send a RR op amp all the way up?

Is the DG419 an appropriate switch for this? (I cannot get it to simply switch like a relay. I don't understand the VL input)

I welcome all suggestions and comments on this circuit; and how best to achieve shut off so I can eliminate the series FET #2.

(those are hall effect switches on the left for the speed control)
 

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crutschow

Joined Mar 14, 2008
34,717
I have tried a pull up resistor with the 358 and found the same off gate voltage (VCC minus about 1.3)
Connect two diodes (1N4848 or similar) in series with the 358 output (cathode to output) and add a 2kΩ resistor from the MOSFET gate to source.
MAIN QUESTION: Can I use an analog switch IC to allow the Feedback leg to select a reference voltage at shut off to send a RR op amp all the way up?
Not sure what you mean. :confused:
 

Thread Starter

dlatch

Joined May 15, 2016
91
Thank you crutschow. Never mind the switch IC for now. If a PU resistor can work that's better.

Before I try it, let me press for details. I'm using 4 PFETs in parallel with a gate resistor for each. (my diagram only shows one) To be clear, the diodes and pull up resister can go before the gate resistors?

Also I don't see 1n48 48 listed. Similar? for this very low current circuit can I use any 1n48? Can you be more specific?

I amended the drawing and attached it below. Do I have your suggestion correct on the drawing attached below?
 

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crutschow

Joined Mar 14, 2008
34,717
To be clear, the diodes and pull up resister can go before the gate resistors?
The diodes go at the LM358 output and the pull-up resistor goes directly between the gate and source.
The gate resistor is between the gate and the diodes.
Also I don't see 1n48 48 listed.
Sorry, that was a typo.
It should have been 1N4148, or 1N914, or just about any small silicon junction diode.

Note that your circuit is likely to oscillate because of the added gain in the feedback loop provided by the P-MOSFET.
It would likely be better to use an N-MOSFET there and use feedback from the source (source-follower) so there's no added gain in the loop.
 

Thread Starter

dlatch

Joined May 15, 2016
91
I hope to get to the proto board later today. Help me understand how it works... with the diodes in place, the op amp can still pull the gates down for throttle up but, how does it go the other way? As you throttle down I assumed the op amp was still in play to raise the gates?

Regarding oscillation: If there was run-away oscillation it wouldn't work, correct? I do see lots of hash at the gates and FB loop. is that oscillation?

Overall design: The motor is grounded. So I am stuck with the high side layout. I would love to see a similar circuit with NFETS and a high side gate driver. I am committed to the Hall Effect wiper as the front end. Another design constraint is that any and all power must come from the one battery. A booster circuit is fine, as long as it all starts and works when the outboard battery is connected.
 

crutschow

Joined Mar 14, 2008
34,717
I hope to get to the proto board later today. Help me understand how it works... with the diodes in place, the op amp can still pull the gates down for throttle up but, how does it go the other way? As you throttle down I assumed the op amp was still in play to raise the gates?
As the op amp voltage rises, the gate-source resistor pulls the voltage that is 2 diode-drops higher than the op amp voltage.
Regarding oscillation: If there was run-away oscillation it wouldn't work, correct? I do see lots of hash at the gates and FB loop. is that oscillation?
Depending upon the type of oscillation it could appear to work.
What does the "hash" look like?
Overall design: The motor is grounded. So I am stuck with the high side layout. I would love to see a similar circuit with NFETS and a high side gate driver.
Does the output voltage need to go all the way to the supply voltage?
 

Thread Starter

dlatch

Joined May 15, 2016
91
I will enjoy testing this. I want to simplify the circuit. I hope to eliminate the series Pfet.

Does the output need full supply voltage? Yes, It is a motor being raced so it needs the full supplied power.

What does the hash look like? I am beginner on the scope. basically the hash frequency varies with motor speed. I assume the 358 is very busy back and forth modulating the gate voltage as it yields a seemingly linear relationship between the hall wiper and the output. The motor is brushed and draws several amps so....hash abounds?. Can I bypass some of it?

It only needs a small gain since the hall wiper cannot reach full vcc like a pot could. The hall wiper can go to about vcc minus 1.

Is there a better way to get the gain? Note I am boosting the bottom of the input divider.
 

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crutschow

Joined Mar 14, 2008
34,717
Are you concerned about battery life?
If so, you should consider a PWM circuit instead of linear.

You can use N-MOSFETs if you put the motor on the plus side of the battery instead of the minus side, and use an N-MOSFET low-side switch.

If you tell me what the inputs are (voltage and characteristics) and exactly what the circuit is supposed to do I could likely help you better.
 
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