High Efficiency Step down converter

[techie]

I need to make a simple 200Vdc to 24Vdc step down converter at 1000watts for use in solar power application. The application demands a controlled current output (instead of voltage control) depending on the available power from solar panels. Since the application is for home use, cost is important and I want to salvage some parts from an old power supply. I want to use the ferrite transformer of an old 1500w 220v-24V power supply, controlling the PWM of input powerh to control the output current. Can it be done ? Any suggestions.

 

[FvM]

You should specify what "a controlled current output (instead of voltage control) depending on the available power" exactly means. Is it a MPPT algorithm?

Besides the control algorithm for the step down converter, the input and output voltage and current range must be known to decide about the switcher topology, e.g. buck, transformer forward or flyback converter.

 

[techie]

Right now, mppt is not a requirement. I just want to harness a reasonable amount of power. The problem is that the panels are located far away (multi-storey buildings top) about 100m away and I can only use a series combination which gives about 230Vdc on open circuit. I want to charge my 24V battery bank with it. The charge controllers available in the market only handle upto 150Vdc Voc so I thought if I could make a converter myself. HEre is the data
Input : 230Vdc on open circuit, 145Vdc at max power, Current about 8amps on short, about 7amp at max power.
Batteries, 24V, 400AH

 

[WimRFP]

Besides the core, what other components do you have as this will have influence on your design? I am thinking of half bridge or full bridge driver IC's , voltage/current rating of mosfets, IGBTs, rectifiers, small cores (for pulse/driver transformers), etc.

You will need some form of power tracking (for example input voltage controlled maximum charge current). When, for example, the batteries accept 6A charging current, but the solar panel is not able to provide the power, the DC input voltage will collapse.

I would go for an isolated converter. From a safety point of view, you may expect several kV overvoltage transients because of the long vertical run and the system is partly outdoor (on the roof). Besides this, you should think about what happens when you get a full lightning strike on the roof or panel.

Without component limitations, I would go for a full bridge design with mosfets, as this doesn't require low leakage inductance and ripple current is relatively low. To reduce switching loss, you may consider zero voltage switching. If you can't get sufficient current control range, you could use burst mode control to get almost zero output current.

If you are familiar with phase shift between left and right half bridge, you can use that to increase the control range without losing zero voltage switching.

If you have very limited hardware, and are well into electronics and magnetics, you may consider a self oscillating solution in combination with burst mode control.