Electronic – Two voltage regulators in series vs in parallel

voltage-regulator

What is a better idea – to connect two voltage regulators in series or in parallel? I don't need much current (max 300-400mA). I need both voltages. Transformer output is about 9V. U2 gives 1A max and U3 800mA max.

series
parallel

Best Answer

The important difference here is where the power is dissipated. For both circuits it can easily be calculated:

Parallel:

  • \$P_{U2}= ( V_{IN} - V_{OUT} ) × I_{OUT} = ( 9 - 5 ) × 1 = 4 \text{W}\$
  • \$P_{U3}= ( V_{IN} - V_{OUT} ) × I_{OUT} = ( 9 - 3.3 ) × 0.8A = 4.6 \text{W}\$
  • Total power = 8.6W

Series configuration

  • \$P_{U2} = ( V_{IN} - V_{OUT} ) × I_{OUT} = ( 9 - 5 ) × ( 1 + 0.8 ) = 7.2\text{W}\$
  • \$P_{U3} = ( V_{IN} - V_{OUT} ) × I_{OUT} = ( 5 - 3.3 ) × 0.8A = 1.4W\$
  • Total power = 8.6W

The question is where can you dissipate these amounts of power most comfortably, which regulator. The higher the dissipated power, the bigger heat sink required. For both solutions, the total dissipated power is identical.

Notice that for the series configuration your 5V regulator must be able to do almost 2A, whereas in the parallel configuration both regulators have to cope with "only" about 1A.

Related Topic