multiple output voltage for buck converter

Hi,

I am thinking of having a charger which can charge different battery voltages, for example 12V, 24V, 36V, and 48V. The charging process uses PWM signal to turn gate of MOSFET ON/OFF to deliver average power to the battery. In this case, I would need 4 different buck converter.

I was thinking, is it possible to design one buck converter which able to charge 4 different battery system. For example, 230V, 50-60Hz, supply is rectified to 120VDC and use it as input for the buck converter. by varying the PWM duty cycle, the average output voltage required can be achieved. is this possible?

another question is, what is the most efficient duty cycle for PWM signal? must it be always below 50% for most efficient result?

Regards,

Mike

It's possible, just vary the feedback network so the output voltage changes.

The rectified voltage from the 230V mains is 325VDC.

Building an SMPS which works directly off the mains is very dangerous if not done correctly.

kimimaro_kiddo said:

another question is, what is the most efficient duty cycle for PWM signal? must it be always below 50% for most efficient result?

Click to expand...

The most efficient duty-cycle is not necessarily less than 50%. The duty-cycle generally has only a small effect on efficiency and the optimum duty-cycle will vary with the design.

kimimaro_kiddo said:

another question is, what is the most efficient duty cycle for PWM signal? must it be always below 50% for most efficient result?

Click to expand...

You don't have much choice of duty cycle in a buck converter. It is approximately Vout/Vin. (This is not exact, as it assumes no losses in any component.)

If your input is 120VDC, then the PWM will be approximately 10%, 20%, 30% and 40%, for outputs of 12V, 24V, 36V, and 48V.

If you actually have 325VDC then the PWM will be approximately 4%, 8%, 11%, and 15%.

I should take a moment for two cautions:
(1) Applying mains directly to the battery without isolation is incredibly dangerous.
(2) It's very difficult to make an efficient buck converter with Out/In ratios more than about 10:1.

I wouldn't recommend a buck for something like this.

I'd advise using a forward or push-pull converter, which requires a transformer.

If the input voltage of 120V is isolated from the mains by double insulation, then this does make it a lot safer. 120VC still poses a shock risk though so all connections need to be insulated from the user. I'd also advise adding adding some overvoltage protection so it shuts down if the output voltage exceeds say 65V, 60V would be ideal but you might need to charge to 58V for a 48V battery so 60V might be too close to avoid it falsly triggering.