# Electronic – How to tackle regenerative current in the H-bridge

currentmotor

I am using 2 of the following H-Bridge to control 2 x 24V 350W electric scooter DC motors. The power is from 2 x 12V 12AH SLA batteries in series.

EDIT: These are the entire schematic and PCB photos:

The PWM signal controlling this H-Bridge is in locked-antiphase mode at 40KHz.

This H-Bridge works great except that only when the motor brakes and changes direction SWIFTLY, this will make the microcontroller hang oweing to electrical ripples/noise.

I am new to H-Bridges and I am unable to see how regenerative current from the motor (generated when the motor slows down and changes direction) is circulated around the circuit. Would it be the cause of my problem? How to tackle it?

A friend of mine who designs electronic circuits suggests using a 0.1uF capacitor in series with a high power resistor (5W) and connecting both between the motor terminals, in order to absorb the power generated by the motor whenever it works as a power generator. Has anyone done this before?

I am grateful to any help from you!

Ah, I forgot to put in the schematic, but C6 and C9 are 1000uF each. The half bridge driver is IRS2184S. If you want to see the schematic clearer, please do a "Save image as". Thanks in advance.

EDIT 17 Nov 2014 Based on Andy aka's & JonRB's answers:

Thank you both so much for your invaluable advice. I believe you are poiting me in the right direction now!

I plan to get this circuit board redesigned, fabricated and tested with updated result, and these are the changes I am going to make:

• Since a 4 layer PCB is not possible here, I intend to use a DIP version of the micro (pitch=2.54mm) so that I can make a full earth plane around the micro
• The earth plane of the signal part will be connected directly to Battery (-) rather than to that of the power side, so that return current of the power side goes directly to the battery rather than polluting the signal side

I would like to hear your comments on the following if possible:

• If I make the power side and signal side into 2 separate PCBs connected by some wires, with GND of each connected directly to Battery (-), will this be even cleaner? Would you recommend it?
• @RonRB: is the braking circuit a must (to make the board run without hanging/data corruption)? I guess we can live without it?

As a self-taught electronics enthusiast, I have been pulling my hair on this problem for a month now, hopefully I will have it resolved thanks to your help.

The issue here seems to stem from poor layout more than anything. This has been covered by Andy aka, but to compliment what was written BUT more from a powerCore point of view (rather than a sensitive digital point of view, where a solid GND plane & decoupling is pretty much mandatory...)

The H-Bridge is powered from the 24V directly & returns via GND.

The control is powered from 5V, 3v3 ... & returns via GND, the same GND.This doesn't have to be an issue UNTIL you see the route the return current of the H-Bridge takes... Some of it will start polluting the digital section, especially the bridge on the far right (topside view), Especially considering that bank of via's top left (topside view)

Digi & Power can share the same 0V But you have to be conscious of the return currents.

Three things I would do

1. I would clearly separate the Power 0V and the Control 0V and have a single startpoint near the connector
2. I would introduce a CM choke between the 24V:GND to produce a 24V_CORE:GND_CORE. this will help block any HF going back onto the sensitive section & equally a similar arrangement from 24V:GND to produce a 24V_CON:GND_CON. Equally consider setting up a Pi filter to help with EMI.
3. Deal with the regenerative energy.

W.R.T. #3 what is your load inertia? what are your rotor inertia's? what is the deceleration rates?

These three things in conjunction with \$\frac{1}{2}J\omega^2\$ will tell you how much energy you need to pull out of the system. All this energy will be dumped into your DClink (those nice little... 10uF capacitance). THUS with \$\frac{1}{2}CV^2\$ you know how much your DClink will rise.

Then you can decide what todo

1. reduce your current limit - slows the decel --> exchange of power
2. reduce your rotor acceleration limit - slows the decel --> exchange of power
3. reduce your current/speed loop gains - slows the decel --> exchange of power
4. Introduce a braking circuit

Braking circuit (edit)

During deceleration of the machines rotor+load inertia, you will be pulling energy out of the mechanical system into the electrical system.

Right now you have some MOSFETS that (for argument sake) are switching in 50nS. They are switching 24V ==> 480V/us

When you start to actively brake/regen two things occur

1. At higher speeds you will be initially be switching higher voltages: say +10V due to BackEMF & -24V due to inverted H-bridge ==> 680V/us
2. There will be an exchange of energy: Mech --> Elec

Hopefully it is #1 you are being susceptible & the layout would hint at that. Split 0V plane plus a CM choke & Caps (creating a Pi filter) going to the control should significantly mitigate this - Make sure the starpoint is big enough to cater for the stator current. Equally ensure referencing considerations w.r.t. IRS2184 FET driver (and Pin#3 or Pin#5 depending on if you have 8 or 14pin variant)

#2 is a bit different and is only really considered IF you are unable to regen onto the supply. You have a battery so you should be permitted to. The only complication will come from the leads that connect the battery to your Powercore.

Say you are using 32AWG, 10m worth & untwisted... This will have a lot of stray inductance & will essentially decouple your powerCore from your supply. Depending on specifics of your system (coming back to inertias and such) you may find your local capacitance is more than enough to absorb this energy with only a slight change in voltage. Worst-case however is you have a lot of energy to remove from the mechanical system & your local DClink rises significantly.

The stray inductance in your power harness now is a hindrance and your local voltage could rise to dangerous levels. In these instances IF you cannot reduce the inductance or you cannot regen, you would deploy a brake circuit which would dissipate that mechanical energy as heat in the electrical system - a waste.

A simple comparator circuit that measures the actual DClink (ie at the Hbridge NOT the 24V coming in ). A simple FET+Resistor+Freewheel dide (as the resistor will be inductive...) to then chop the DClink between the voltages of say... 35V:28V.

My gut feeling is you are susceptible to #1 (the difference in dv/dt & your present layout) but for completeness above is a quick overview of resistive regen.