Author Topic: High-Power Current Limiter for battery charging  (Read 2275 times)

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Offline MooshooMattTopic starter

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High-Power Current Limiter for battery charging
« on: June 05, 2020, 07:32:14 pm »
Hello EEVblog forum! First post here. My name is Matt and I am a rising senior in university studying EE. I am working on a project and was hoping for some guidance from more knowledgeable people before I commit to this idea that I have.

So I have a Li-Ion battery pack with a nominal voltage around 30 V and an off-the-shelf power supply. The battery has a maximum charge current of 10 A. I have chosen a power supply that can deliver 10.8 A before going into overcurrent protection (hiccup mode). (Before you ask, I have already worked out the details of the charging assuming proper current. There are provisions for preventing overcharging, etc)

I want to design a circuit around this power supply that limits the current output of the supply when the load tries to draw more than 10 A, but otherwise leaves the operation affected. Minimal voltage drop is acceptable (let's say around 0.2V max when operating under 10 A).

My first idea was to use a current sense amplifier and a feedback loop to accomplish this, kinda like this:


In an ideal world this would work as is but as soon as I start replacing ideal parts with models, it becomes wildly unstable. The system oscillates between an overcurrent condition and turning completely off. Either this solution is impractical or my implementation is simply not stable as is. I am not familiar with how to stabilize feedback loops like this.

How do you guys think I should proceed? Any guidance would be greatly appreciated!

Best,

Matt
« Last Edit: June 05, 2020, 07:34:13 pm by MooshooMatt »
 

Offline temperance

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Re: High-Power Current Limiter for battery charging
« Reply #1 on: June 05, 2020, 07:59:41 pm »
Hi,

-you cannot simulate a battery charger with a resistor as a load. The load should be a voltage source with a resistor in series equal to the battery internal resistance.

-The voltage dependent voltage source has infinite bandwidth while the real amplifier will have a limited bandwidth. Because of this, the phase around the loop might exceed 180° while the loop gain is still greater than 1.

-Did you consider the power loss in the linear current source? A switching step down regulator might be a better option.
 

Offline MooshooMattTopic starter

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Re: High-Power Current Limiter for battery charging
« Reply #2 on: June 05, 2020, 08:24:09 pm »
Hi, thanks for the reply.

1) Ok that makes sense, although I don't think it would make much difference in the simulation performance... (Edit: just checked, it does not make a difference)

2) Yes I understand that. The voltage-dependent voltage source is just there to show what my idea is. That would be replaced by a high-side current sense amplifier in the long run. The frequency response of that amplifier simply introduces more stability problems which is exactly what I am hoping to fix...

3) Yes, I have considered the power dissipation in the PFET. During normal operation power dissipation should be minimal. Worst case power dissipation would be in the tens of watts, which is manageable. I fail to see how a switching converter would help me here. I am trying to limit current, with as little voltage drop as possible.

Matt
 


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