Zener Diode Schematic Check - Page 1
Forum section: electronics / beginners

author: bizkits
author_rank: Contributor
timestamp: November 22, 2020, 04:17:15 pm
content: Hi, I'm recently designing a PCB to drive multiple 24V actuators which can move bidirection.I use the DRV8876 and even though it has internal diodes to protect the component, I thought it would be good to add some zener diodes at the actuator side to clamp any voltage spikes.However, I am not sure if my placement is correct. My actuator runs on either 24V+ and 24V-.I chose the LM3Z24VT1G because it has a Vz of 24V but I am not sure if it's correct.Appreciate any comments or suggestions!Thanks

author: Vovk_Z
author_rank: Super Contributor
timestamp: November 22, 2020, 04:45:10 pm
content: What is the rated current of a motor? It seems to me 0,2 W rated Zener is too weak to be a good clamp.And 24V rated voltage is too close to the power supply voltage (it may vary).

author: Manul
author_rank: Super Contributor
timestamp: November 22, 2020, 05:01:43 pm
content: If you really want additional clamping, then clamp to PGND and VM (+24V) with some schottky diodes. Because strictly speaking, the idea is NOT to limit actuator voltage, idea is to not allow driver OUT1 or OUT2 to go below PGND or above VM.

author: bizkits
author_rank: Contributor
timestamp: November 23, 2020, 01:09:19 am
content: Quote from: Vovk_Z on November 22, 2020, 04:45:10 pmThe rated current is 1A max, but it usually draws about 300mA.What voltage rating would you recommend?

author: bizkits
author_rank: Contributor
timestamp: November 23, 2020, 01:11:08 am
content: Quote from: Manul on November 22, 2020, 05:01:43 pmI sort of get what you are saying. However, when the actuator moves from a stationary position, won't there be a potential spike in voltage from OUT1 and OUT2?

author: Manul
author_rank: Super Contributor
timestamp: November 23, 2020, 03:24:56 am
content: Hopefully I did no mistake, cause I'm sleepy. Take a look. Here is a typical H-bridge and inductor (for eg. motor). If your driver just turns off all mosfets, current is still going inside inductor, because of its inertion. This creates a voltage spike and it could be both polarities. It finds its way through body diodes of mosfets and through the power supply. If you add additional diodes (I do not say that you necessarily need them), you make it easier for mosfets. But in any case this current kind of regenerates into your power supply. That also means, that you need enough bulk capacitance.Zener clamping would be practicaly very bad, because zeners are not so accurate or steep-curved to clamp effectively exactly at power supply voltage. Also, what will happen if power supply will go up to lets say 24.5V? Your zener will start conducting in normal operation. Or if supply will be 23.5V then maybe zener will never conduct at all and be useless, because the voltage will just clamp through mosfets. There is just a very narrow window, where they can be made to work (or not at all, because not enough steep, too much zener resistance). So as you see, it is not good for such application.With clamping to power supply, maximum spike on the motor will always be supply voltage + 2 diode drops. No matter what is supply voltage.

author: Vovk_Z
author_rank: Super Contributor
timestamp: November 23, 2020, 09:02:37 pm
content: Quote from: bizkits on November 23, 2020, 01:09:19 am- a design with diodes (usual or Shottky, proposed above) seems much better then.I see that this IC has intrinsic body diodes, so it is better to use an additional of a Shottky type but not usual (to they can help).

author: bizkits
author_rank: Contributor
timestamp: November 24, 2020, 01:20:20 am
content: Quote from: Manul on November 23, 2020, 03:24:56 amThank you for the drawings, it helped a lot. I see clearly how the current runs through the circuit even when the H-Bridge is turned off.I see that adding a zener is not good for this application, but because the outputs could be in different polarity, I thought only a zener diode could clamp the voltage spikes.In this case, if clamp the voltage at VM and PGND, can I say that it'll protect the rest of my circuit?

author: bizkits
author_rank: Contributor
timestamp: November 24, 2020, 01:21:27 am
content: Quote from: Vovk_Z on November 23, 2020, 09:02:37 pmWhere should I place the diodes? I was thinking at the VM and PGND of each motor driver.Or do I only need 1?

author: Manul
author_rank: Super Contributor
timestamp: November 24, 2020, 09:50:50 am
content: Quote from: bizkits on November 24, 2020, 01:20:20 amYes, because the important thing is to keep OUT1 and OUT2 in the voltage range between PGND and VM. By clamping any out of range voltage with diodes, you achieve this. Note also, that same technique is used to protect inputs of IC's (they are called ESD diodes) and the idea is to clamp anything out of range (negative or above supply volatge).Your drivers contain mosfets which have body diodes, and I they should be rated to pass the required current. Additional diodes would just make everything more robust.

author: bizkits
author_rank: Contributor
timestamp: November 24, 2020, 11:57:37 am
content: Quote from: Manul on November 24, 2020, 09:50:50 amThank you for explaining in detail. I'm still considering whether to add additional diodes (schottky diodes).I came across this section in the datasheet (Attached the snippet in this post) that talks about bulk capacitance.I'm using 100uF now, most probably going to increase it to 200uF.