I am trying to get the Boost Start-Up Circuit used in the AN2121 (Batt-EV charge controller) to work on the AN2241 (SM72445 Evaluation Module). I copied the circuit and haven't had any luck in getting it to work.
Something strange. Because the SM72445 EVM doesn't have this circuit incorporated, I had to build it on a project board. I therefore had to solder a wire onto the board to access the point. The funny thing, which I found out through trial and error, is that having this wire connected affects the output voltage when in panel mode using the H-Bridge FETS. This means that the output voltage is 28v when the input is 31v. When I desolder the wire, I get close to 31v out. Why I mention this is because this wire should connect to the Boost Start-Up Circuit, which doesn't work. I thought I would mention it as background.
The reason I am trying to use this circuit is that I am aiming to connect the output to a 48v battery bank. I would like the SM72445 EVM to enter MPPT mode so it can boost the input voltage to charge the battery before the input exceeds the battery voltage.
At the moment it stays in panel mode, using the H-Bridge FETS. They are switching in this mode. I didn't expect this to be the case. I expected that the two high side FETS would just stay on to achieve connection between input and output.
So the situation is, when I input 31v (maximum voltage of my variable PSU) the output rises to about the same. When I solder a wire onto he AIIN point (whether connected to the Boost Start-Up Circuit or not) it affects the output and pulls it down a bit. I don't believe this to be ideal.
When I activate the Boost Start-UP Circuit, the output does't rise as expected. So the module just sits there with 31v on the input and some voltage between 28-31v ( wire soldered or not) on the output. Not enough to create a flow of current through the external blocking diode to the battery bank.
If I connect a 33 ohm resister to the output (with battery still connected) resulting in a flow of current. The output voltage starts to rises and when it starts to charge the battery (monitored with a clamp meter). I can disconnect it. Once in this mode I can vary the input voltage across a wide range and the converter copes with it fine. the problem is all about getting the converter into MPPT mode when the input is less than the battery voltage.
The Boost Start-Up Circuit is supposed to do two things. Confuse the SM72445 into believing that 1) input voltage is high and 2) that as the pulse width of the converter is increased the input current measured appears to increase. The intention behind this is that eventually the output voltage will exceed the battery voltage resulting in a flow of current. The circuit can then be disabled because there is a flow of current. A prerequisite to MPPT start. Should the flow of current drop away. The circuit can be re-engaged.
Any suggestions
