Series regulator efficiency; bandgap-reference regulator

Why a transistor series regulator has low efficiency

In a series regulator, the pass transistor is in series with the load and carries the whole load current IL. It acts like a variable resistor that drops the extra input voltage:

• Power delivered to load: Po = Vo IL
• Power drawn from the source: Pi = Vi (IL + Iq), where Iq is the small bias/control current
• Power wasted in the pass transistor: PD = (Vi − Vo) IL

η = Po/Pi ≈ Vo/Vi

So if Vi = 12 V and Vo = 5 V, the efficiency is at most 5/12 = 42 %; the rest, 7 V × IL, is lost as heat in the transistor. The efficiency is poor when Vi is much larger than Vo, or when Vo is adjusted to a low value. The wasted heat also needs a heat sink. The transistor also works as a linear (class A like) element all the time, so the power loss continues at every load.

(Switching regulators avoid this by using the transistor as a switch.)

Voltage regulator using a bandgap reference

 Vin o----+--------[ Q1 pass ]--------+----o Vo
          |             ^             |
          |             |            [R1]
          |        +----+----+        |
          |        | Error   |<------+--- feedback
          +--------|  amp    |        |
          |        +----+----+       [R2]
          |             ^             |
          |      +------+------+      GND
          +------| Bandgap Vref|
                 |   ~1.25 V   |
                 +-------------+

1. Bandgap reference supplies a stable Vref ≈ 1.25 V, independent of temperature and Vin. It is formed by adding VBE (negative temperature coefficient, about −2 mV/°C) to a PTAT voltage K·VT (about +0.086 mV/°C × K). With K ≈ 23, the drifts cancel.
2. Feedback divider R1, R2 takes a sample of the output: Vf = Vo R2/(R1 + R2).
3. Error amplifier compares Vf with Vref and drives the base of the pass transistor Q1.
4. If Vo rises (higher Vin or lighter load), Vf rises above Vref, the error amplifier reduces Q1 drive, and Vo falls back. If Vo falls, the opposite happens.
5. In steady state Vf = Vref, so

Vo = Vref (1 + R1/R2)

For example, Vref = 1.25 V, R1 = 3 R2 gives Vo = 5 V. The output is stable against temperature, line and load changes, as the reference does not depend on them. The LM317 and most IC regulators work on this principle.