Constant Current Power Supplies, Variable Linear Power supplies & Switchmode Variable Power Supplies Justin Kai EE 136 Project Prof. Zhou

Constant-Current Power Supplies

Important Parameters Nominal operating voltage rage Compliance voltage (upper limit) Current limit

Nominal Operating Mode

Voltage Over-loaded Mode

Linear Variable Power Supply (Char.) Low noise level (micro-volt. Range) Fast full recovery time High power dissipation (low efficiency)

Basic Drive Circuit “Piggyback”

Basic Operation Considerations Low voltage, High current output Q2=off  Volt. drop in R1 (high power dissipation) High voltage, High current Q2=on  short R1 (low power dissipation)

Power Loss Chart

Switchmode Variable Power Supply SMV power supply are small, light & efficiency will be maintained near 70% One SMV can replace up to Three Variable Linear power supplies SMV power supplies are very noisy  need special kind of filters are required

Requirements of SMV Power Supply Noise filtering must be optimum. Conventional filters cannot obtain the requirements in high accuracy applications Faraday screen for switching sections and filters are essential, need metal or screened cabinets  minimize the EMI problems Operating low output voltages demand very narrow power pulses Basic supply with Fly-Back Converter transformer can be considered a constant energy source

Block Diagram of SMV Power Supply

Comparison (VL & SMV) Advantages: Low noise levels Fast full recovery Disadvantages: High power dissipation Large, heavy & inefficient Expensive heat sinks & transistors are required Advantages: small, light & efficient 1 SMV can replace 3 VL power supplies Disadvantages: SMV power supplies are very noisy  need special kind of filters are required