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Please Help Fix the Charge Controller

by Chris - June 21st, 2012.
Filed under: Charge Controller.

As I announced on the mailing list, I’ve been working with Jameco to develop a Free Charge Controller Development Kit (free as in freedom, not free beer). The design has been built and tested, but I’ve run into a ‘bug’ that I just can’t seem to squash. It is my hope that the community can help me get to bottom of this issue. If you have some ideas, please post them in the comments below or on the mailing list.


Free Charge Controller – Version 4.03

Here is the most up to date schematic of the free charge controller. The schematic is based on Tim Nolan’s origonal design, but modified to use Jameco parts:

MPPT charge controller

Schematic for Version 4.03 of the Free Charge Controller

If this is the first time you’re seeing the schematic, then you should read Tim Nolan’s hardware description to understand the theory of operation.


The Problem

The image below is an isolated section of the schematic showing the H-bridge driver, composed of IC2, M2, and M3. M1 is can be ignored as it is permanently wired ‘on’ in this test scenario. The image also shows the ‘load’, which is an inductor in a buck configuration. The problem I’m experiencing is a droop on the gate voltage of the the high-side MOSFET (M2) which seems to be load dependent.

Buck Converter composed of half H-bridge and Inductor in a buck configuration.

IC2, the MOSFET driver, is responsible for generating a a drive signal to the high MOSFET (M2) and low MOSFET (M3). The signal should look like this:

This is what the voltage on the gates should look like.

Instead, the voltage on the gate of the high-side MOSFET (M2) looks like this:

Voltage on the high-side MOSFET (M2). Blue is what it looks like, red it what it should look like.

Here are additional voltage signals and analysis that I captured using a combination of my oscilloscope and Matlab:

Voltage signals captured with oscilloscope

Differential voltage signals calculated from the voltage signals captures above.


Experimentation Results

What makes this voltage droop so odd, is that it goes away when I replace the ‘load’ (the inductor/buck converter circuitry isolated by the grey dashed line in the second picture) with a 150 ohm resistor. Additionally, when I leave the ‘load’ in place and attach a resistor at the output, the droop gets worse (shifts to the left / decreases duty cycle).

To further complicate the matter, the droop shouldn’t even exist in the first place. Since IC2 is driving a MOSFET gate, there shouldn’t be any steady state load on the chip – since the resistance through a MOSFET gate is infinite. Another way to state this is that once the capacitance of the MOSFET gate has been charged, there should be zero (or near zero) load to the MOSFET driver chip. In that sense, the droop towards the end of the square wave doesn’t make any sense. It’s as if the chip is getting tired, but it’s completely unloaded.

One thing that may stick out to the observer is the diode tying the gates of M1 and M2 together. The 470K resistor (R1) however, makes the current flow very small. I verified that the circuit created by M1, R1, and D1 does not effect circuit behavior by disconnecting it from the circuit. I get the exact same droop behavior on the gate of M2 weather or not these components are connected.

Please share your thoughts in the comments section below or on the mailing list!

2 Responses to Please Help Fix the Charge Controller

  1. Have you tried replacing C2 with a larger cap? Say 1 Micro farad? I haven’t tried it myself yet but I am currently working on a very similar schematic and I’m using a 1 micro farad cap.

  2. Also, a few other things to point out here, when the low side fet (M3) switches on its going to short the output to ground and short the battery through the inductor, I would recommend placing a diode between the inductor and the output cap/battery. I have my circuit up and running and it seems to work a bit better that way, I also left M1 out of the circuit. You might also want to increase C3 to 1 micro farad, but unlike my previous comment C2 should be fine. I also left out C2 and R5 for testing. Hope this helps!

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