AN954: Off-Line Capacitive Power Supply

* From page 10 of AN954:

Figure 10 shows a capacitive power supply with several UL considerations designed in. A fuse is added to protect the circuit during an over-current condition. Adding R2 in parallel with C1 creates a filter that will attenuate EMI from traveling back onto the line. A varistor, or MOV, provides transient protection.

Advantages of Capacitive Power Supply:

• Significantly smaller than a transformer-based power supply.

• More cost effective than a transformer-based or switcher-based power supply.

• Power supply is more efficient than a resistive transformerless power supply.

Disadvantages of Capacitive Power Supply:

• Not isolated from the AC line voltage which introduces safety issues.

• Higher cost than a resistive power supply.

* Efficiency calculated as (4.1V x 9mA)/(120V x 16mA).