help me in designing a bldc driver..!!

Thread Starter

naren_iisc

Joined Jan 13, 2015
21
hello guys,
I am building a BLDC driver to drive my 24V, 300Watt BLDC hub motor. I am using Allegro's A3931KJPT IC to do so. But the problem is motor is running fine for some time (fine upto 24V and 4Amp). I already burned 3 ICs. Still not able to find the fault. Please help me.
I am attaching a snapshot of my ckt. if there is any fault in the circuit please let me know.
Ckt_dia.PNG
 

Alec_t

Joined Sep 17, 2013
11,277
I see you have several IC pins floating (not counting the NC pins). Is that allowed ? (I haven't checked the datasheet). Most ICs need a defined voltage at unused pins to prevent erratic behaviour.
 

Thread Starter

naren_iisc

Joined Jan 13, 2015
21
Hi Alec_t !! Thanks for ur reply.

The thing is the J6 connector is connected to the micro controller. J1 is directly connected to the three phases of the motor. J2 connector is used to interface Hall sensor output from the motor.
In my case logic inputs,
ESF=1;
COAST=1;
MODE=0;
PWM FREQUENCY IS 42KHz
DIR=1;
BRAKE=1;

I used Rt=47K and Ct=680nF, hence according to formula mentioned in the datasheet it gives the frequency of 32-33KHz

and one thing i messed in the ckt dia is Sa,Sb,Sc pins are connected to the A,B and C phases of the motor as per the datasheet.
I am attaching the datasheet to this reply. Please check and help me in solving the problem.
 

Attachments

Alec_t

Joined Sep 17, 2013
11,277
I can't see the recommended decoupling caps for VBB? Also, RDEAD is floating. Shouldn't the RDEAD pin have a resistor to ground, or else be connected to V5?
 

Thread Starter

naren_iisc

Joined Jan 13, 2015
21
Sorry. . I didn't mentioned here. . But I connected a resistor of 47k between RDEAD and ground and even the decoupling cap of 100nf between Vbb and ground.
 

Alec_t

Joined Sep 17, 2013
11,277
Can you post a revised schematic showing exactly what is connected where, on all the IC pins? Otherwise, we're shooting in the dark :(.
 

Thread Starter

naren_iisc

Joined Jan 13, 2015
21
hi,
I am attaching revised schematic. It has all the connections as per the board I am working on.
MOSFETs I am using are csd18532kcs.

ckt.PNG
 
Last edited:

Alec_t

Joined Sep 17, 2013
11,277
I see you have a 50k pot connected to REF pin 20. If its wiper were inadvertently set much above the mid-point you would not comply with the datasheet spec which says "RREF should have a value between 20 kΩ and 200 kΩ". Apart from that I can't see any obvious problem. Have you followed the pcb layout recommendations in the datasheet? Are all electrolytics low ESR types?
 

Thread Starter

naren_iisc

Joined Jan 13, 2015
21
Thank you sir for your reply. . I will try with changing that pot.
And yes all the capacitances I used are of low ESR only
 

Alec_t

Joined Sep 17, 2013
11,277
I will try with changing that pot.
You don't need to change it; just put 22k in series with the wiper. There's no guarantee, though, that too low a resistance here was what caused your IC to fry. I suspect board layout may have played a part.
 

Thread Starter

naren_iisc

Joined Jan 13, 2015
21
Now I separated RREF and VDSTH from V5. Removed NPN transistor and shorted V5BD and V5 (as suggested in datasheet). Now the board is fine. Motor is able to run. But if I increase the duty cycle of external PWM beyond 65% it stops after a initial jerk. At 65% duty it is running well (current =0.4Amp and Voltage=20V).
My logic inputs are as MODE=1, ESF=1, DIR=0, BRAKE=1,COAST=1

According to the formula given in the datasheet my internal frequency is 29.75KHz. (t(blank)=0.86usec, t(dead)=0.794usec)
VDSTH I adjusted to 2V and voltage at RREF=2.22V (Rsense=10mOhm so from the formula given in the datasheet the Vrref comes to 2.22V for 10A)

should I upload the layout file also..?? I have .max file for layout.
 

Thread Starter

naren_iisc

Joined Jan 13, 2015
21
.max file can not be uploaded here. I tried.
It is designed in Orcad.
Anyone here using Orcad..?? Please give me your mail-id so that I can mail you that file
 

Alec_t

Joined Sep 17, 2013
11,277
.... or you can change the file extension to an acceptable one (e.g. .txt) to upload it, then anyone with Orcad can change it back to .max.

BTW, is BRAKE=1 COAST=1 a valid logic combination? You seem to be commanding the motor simultaneously to coast (freewheel) and stop.
Does the motor have a snubber or other spike suppresssion arrangement fitted?
 
Last edited:

Thread Starter

naren_iisc

Joined Jan 13, 2015
21
Hi
I am attaching the screen shots of TOP side and BOTTOM side of the board.

Here the left side 4 pin connector has connections as, H1, H2 H3 (Hall sensor signal from the motor) and GND. while right side 3 pin connector is motor phases A,B,C respectively.
In this layout I missed connection from Sx pin to motor phase. But later I connected wires externally.

Please tell me if there is any mistake with the layout
top_side.PNG bottom_side.PNG
 

Alec_t

Joined Sep 17, 2013
11,277
I'm no expert on layout but it looks like your FET gate resistors are a long way from the gate pins. That is a recipe for spurious oscillation and hence over-heating of the FETS. Perceived wisdom has it that those resistors should be as close to the pins as possible, i.e only a few mm away at most.
 
hi,
I am attaching revised schematic. It has all the connections as per the board I am working on.
MOSFETs I am using are csd18532kcs.

View attachment 78760
Hi,

I can see multiple potential issues with your schematic, when compared to Allegro's reference design.
- Add 3 x 1000uF low esr caps across the supply as close to each mosfets pair as possible.
- Increase the boost caps value to 220n - they might not be large enough to fully charge to gate of each high-side mosfet.
- Add an extra resistor (100k) between the gate and source of each mosfet, to bias if off.
- The bulk capacitor on Vreg should be 10uF.
- Add some 1nF caps between vdsth and gnd, and vref and gnd
- Try increasing each gate resistor from 15R to 33R to reduce peak gate currents sourced by the A3931
- Try increasing the deadtime resistor to 100k.
- Try reducing the PWM frequency. A large motor (huge inductance) doesn't like high switching frequencies.
- CSP and CSN should be filtered, not directly connected to the shunt.

Btw, from what I've discovered with this chip, if you want a motor with maximum torque at low rpm and also want it to run at high speed, you are going to have to consider changing the mode input dynamically.

Let me know how you go?

Mark
 

Thread Starter

naren_iisc

Joined Jan 13, 2015
21
Thanks a lot Mark for ur reply. I am using external PWM to control speed and disabled internal pwm by connecting Rtct pin to AGND. Still when the duty cycle is increased beyond 60% (irrespective of external pwm frequency) motor stops.

Can u suggest me typical pwm frequency. . ??
My motor has time constant of 2msec
 
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