Power Supplies, Batteries and Power Management
Domain 7: Power Supplies, Batteries and Power Management
This domain tests linear and switch-mode supplies, rectification and filtering, regulation, battery chemistry, charging, and protection. Calculations on dissipation, ripple, capacity, and runtime are common.
Core rules
- Linear supply: transformer, rectifier, filter capacitor, regulator. A 78xx regulator needs input headroom (about 2–3 V dropout) and a heatsink if dissipation is high.
- SMPS: higher efficiency, smaller size, but EMI and ripple must be controlled. Never probe the primary side without an isolation transformer and suitable probe.
- Ripple is the AC variation on a DC output; filter capacitance and load current set its size.
- Battery capacity in Ah = current × time. Runtime ≈ capacity × usable depth of discharge / load current, allowing for efficiency and ageing.
- Li-ion charging is CC/CV, typically 4.2 V per cell; lead-acid uses float/boost stages. A BMS protects against overcharge, over-discharge, and short circuit.
Worked example
A 7805 regulator is fed 12 V and supplies 500 mA. Dissipation = (12 − 5) × 0.5 = 3.5 W. The TO-220 package without a heatsink may have a junction-to-ambient thermal resistance around 50 °C/W, giving a 175 °C rise, so it will overheat. A heatsink with 5 °C/W plus a 5 °C/W junction-to-case gives about 10 °C/W, a 35 °C rise, which is workable. Check the input remains above about 7.5 V at low line.
Traps
- Ignoring dropout voltage and losing regulation.
- Reversing battery polarity or charging a Li-ion cell without a BMS.
- Using a linear regulator at high current and forgetting heatsink calculations.
- Comparing Ah ratings across different voltages without converting to Wh.
- Assuming a 12 V lead-acid battery is 12.0 V at rest; it is about 12.6–12.8 V full.