Now that I've got my specs figured out, it's time to start some high level design. This will allow me to get the layout of the power supply set before diving into the small details. Hopefully this will make the design process more efficient.
One of the biggest things that will affect this high-level design is one particular design specification. That is, the call for a switching knock-down stage. The reason I chose to include this is efficiency. Many lab power supplies I've seen out there have one thing in common: Many of them use linear regulators like the LM7805 or LM317. These are good devices, but they all have very low efficiency, especially when the dropout voltage is high. Enter switching regulators. Switching regulators can have very high efficiency (upwards of 95%) which allows for higher current handling, and less heat dissipation. However, they have a drawback. Switching regulators typically have more noise on their outputs. They may be OK for some circuitry, but this inherent noise will not do for the lab power supply I intend to build.
To get the best of both worlds, I plan to use both types of regulators in my design. The switching regulator will take care of most of the voltage dropout first, while leaving about 2-3 volts for the non-switching (a.k.a. linear) portion to drop second. This will reduce power dissipated in the non-switching section of the power supply, which has numerous advantages, including (hopefully) eliminating the need for a noisy fan, as I'd like to make this thing as small, quiet, and cool as possible. This would definitely not be possible without the switching section in front.
Now, it's time to make some initial part choices:
- Linear Output transistor: P-Channel MOSFET IRF9540
- Switching Regulator: LM2679-ADJ
- Switching Regulator Inductor: Digikey# 553-1121-ND
- Switching Regulator Capacitor: Digikey# P15372CT-ND
- Current Sensor: ACS712
This should help lay the groundwork of the power supply. Next we'll look at putting in some control circuitry, including op-amps and so on...