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This project was created in order to provide protection for my battery projects. I didn’t really trust the cheap low cost protection boards you find online so I decided to try and create my own.
The protection board I’ve designed has bidirectional current detection (can detect charging and discharging currents) which in turn means you can set independent OCP( over-current protection) for discharging current and charging current.
One thing to note before hand is that the current maximum current that can flow is about 5A due to lack of heatsink on the powerFETS. Adding heatsink is necessary to allow more current
- Over current protection with output latch open when tripped
- Under voltage protection for the whole battery pack
- Theoretically 45-50A pass-through current ( will require heatsink)
In the next sections I will discuss in more detail this project.
Here are some more technical details of this project
|Minimum input voltage (V)||10|
|Maximum input voltage (V)||34|
|Maximum current (A)||50A*|
|Quiescent current (mA)||**|
|Undervoltage protection||Yes (adjustable)|
|Overcurrent protection||Yes (adjustable)|
|Short circuit protection||Yes***|
*Maximum current passing through this board will depend on sizing and adding a heatsink to the power mosfets
**Quiescent current has not been measured yet but it is below 1mA during normal operation with a 4S battery. This will change slightly when using more cells in series
***Theoretical the supervisor IC will latch when the overcurrent protection section detects its threshold. This will need testing to confirm
SCHEMATIC AND CIRCUIT DETAILS
Here in this section I will discuss the project in more detail by talking about the different sections used to create this whole circuit.
Lets first discuss the over-current/short-circuit protection sections of the circuit:
Here we have the heart of the over-current detection. We have the MAX4081FASA which is a bidirectional current sense IC that can handle a max VCC of 76V which is more than enough for our applications. The FASA has an internal gain of 5V/V coupled with a 10 milliohm sense resistor value ( I have two 20 milliohm in parallel for size reduction) this will allow the sense current of up to 50A.
REF1A is tied to 5V reference IC and REF1B to ground. With this we have a bias point of 2.5V meaning that when no current is passing the voltage on the OUTPUT pin 5 is 2.5V. When current is discharging into a load the output will rise up to 5V being the 50A max. When current is flowing the other direction to charge the batteries then it will go below 2.5V to 0V being 50A max.
In order to both detect charging and discharging current since I will be using this for a battery pack, I had to create a circuit that detects both an upper limit and lower limit. Hence I went with a window comparator to detect charge and discharge values.
Here is where you can adjust your own over-current detection points for charging and discharging and they can be set independently from each other. The idea here is when the output from the max IC is within the window of the voltage values you set from the voltage divider values on pin 3 and pin 6, the output of the opamp will be high. As soon as the output value goes above or below those set points, it will pull the output of the op-amp low and trigger the supervisor IC.
The supervisor IC is very important because this is what controls the power mosfets allowing current to pass through. This one is different from normal supervisor ICs because when this triggers, the output is latched low and this pulls the power mosfets high causing them to stop current flow. The only way to clear the fault condition is to pull the clear pin 1 to 5V. What I have done is add the option for a push button on the board directly or pads to run the clear pin trigger somewhere else.
Here is a simple equation to calculate the discharge current detection resistor values on PIN3 of the op amp:
(0.05 V/A * X) + 2.5V = VdetectHigher ; X = current trigger desired
For charging current detection here is the equation for the lower voltage detection of the op amp PIN 6
(0.05 V/A * X) – 2.5V = Vdetectlower; X = current trigger desired
Here is the power mosfets controlled by the supervisor IC. When no fault condition is triggered then the supervisor IC pulls Q4 to high causing the 10k resistor to be tied to ground. This then creates a voltage difference between the gate and source for the powerFETS allowing current to flow. When an overcurrent fault condition is triggered the supervisor IC is then pulled low and Q4 stops conducting which in turns unties the 10k resistor from ground and makes the gate and source the same potential.
Now the under-voltage protection scheme works by using a different supervisor IC measuring the input voltage. When the voltage falls below the threshold then the output pulls low and stops current flow in the powerFET
Here you see this circuit in action. A voltage divider is used to bring down the input voltage of the pack to a range the supervisor can handle. I have defined some voltage divider resistor values for different size battery packs in which it will trigger when the threshold is hit. I only used one MOSFET for this because this will stop discharging current but will allow charging current to flow via the body diode until the input voltage is above the threshold and then turn on the mosfet.
Here you have to take some precautions not to kill the mosfet when an event triggers an under-voltage event and you try to charge the pack.
When an event happens, start charging the pack with 0.5A-1A until there is about 1V above the under-voltage value. Then maximum current charging can be applied. This part of the circuit will need more tweaking in the future
CONCLUSION / IMPROVEMENTS
Hopefully this project will help someone out. I created this to help protect my batteries and to allow the flexibility to adjust as much values as I can and have as much control of my protection circuitry as possible. Of course this is not perfect and I am still working on improvements.
- Figure out a way to protect Under-voltage powerFET from getting damaged when charging through the body diode initially until the supervisor IC allows current to flow through the FET and not the body diode
- Use DPAK powerFETS instead of IPAK due to cost. Size increase is negligible or non-existent
- Create a heatsink to provide maximum current discharge through mosfets. Currently the maximum current discharge without heatsink is roughly 5A without getting too hot. Will do testing after its completed.
- Perform thorough testing and post results.
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