Linear Power Supplies.

A half-wave rectifying circuit using an input sine wave gives an average d.c. value of

V=Vpeak

I=Ipeak

Vrms=Vpeak/2

Irms=Ipeak/2

A full-wave rectifying circuit using an input sine wave gives

V=2Vpeak

I=2Ipeak

Vrms=Vpeak/1.414

Irms=Ipeak/1.414

Smoothing capacitors with very large values draw very large peak current through rectifying diodes, which can destroy them.

If we approximate the current pulses that charge the capacitor as a rectangle with a height Icharge and width tcharge, and the discharge current as a rectangle with height Idc and width tdischarge, we have:

Qcharge=Qdischarge

Icharge x tcharge = Idc x tdischarge

or

Icharge = Idc x (tdischarge\tcharge)

So Icharge can be many times the load current. This current is called the peak repetitive forward current IFM(rep) or repetitive surge current.

Often a surge limiting resistor is inserted between the bridge rectifier and smoothing capacitor. The value of this resistor is equal to the peak input voltage divided by the surge current value of the diodes.

Ripple factor, r = rms ripple current / load current

or r = rms ripple voltage / load voltage

For a smoothing cap the ripple is approximately triangular with an rms value of

Vrms = Vripple(p-p) / (2 * (3^0.5))

Sometimes the ripple factor is expressed as a percentage

r = (rms ripple voltage / dc average voltage) x 100%

Standards Relating to Power Supplies

British Standards:

  • BS2204 - Transformers
  • BS5850 - Transformers

IEC Standards:

  • IEC380 - smps
  • IEC435 - smps

VDE Standards:

  • VDE0730 - power supplies
  • VDE0840 - power supplies

Foldback or 'Re-entrant' Current Limiting

Power Supplies are usually designed to limit output current. This prevents damage to the supply, and can help to prevent damage to the load. If the reason for excess current is due to a fault in the load, limiting the current can sometimes help to prevent further damage to the load. The safest method of limiting current is known as 'foldback': not only does the current limit, but it is 'turned down' to a low, safe level. The diagram below illustrates this: diagram (a) shows simple current limiting, whilst diagram (b) shows foldback current limiting. Less power is dissipated in the supply during short circuit conditions in the foldback configuration.

Foldback Current Limiting

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