Requirements of 3.3V and 5V for powering chips. There is the need for 15V and -15V for the analogue outputs for oscilloscope viewing (very low current/power though). Finally, for IGBT driving there is the need for 15V and 0V (or -15V?) (high current pulses – can a charge pump be used?).
- A single 24V supply with all other voltages derived.
- A single 15V supply with all other voltages derived.
- A ±15V supply with all other voltages derived.
I like the first and third options.
Which of these needs to be isolated?
Other voltages can be derived with:
- Linear voltage regulator – very cheap, smooth output, but inefficient when the voltage difference is high (the device essentially acts as a variable resistance). The power consumption is (Vin – Vout) x Iout. With a large voltage drop or large current, the power loss is large and a heat sink must be used. Going from 5V to 3.3V requires a low dropout (LDO) regulator. This places extra load on the 5V supply. Alternatively, the 3.3V can be derived directly from the higher 15/24V supply.
- Buck converter – more efficient at higher power levels.
- Isolated DC-DC converter when necessary.
Let’s calculate the voltage required, and the current draw, of all the different components to work out which supply to use: