Series regulator stability factor and overload protection

A series regulator keeps the output dc voltage constant when the input voltage or the load changes. The control element (pass transistor) is in series with the load, and an error amplifier compares a sample of the output with a Zener reference.

Circuit diagram

 Vi o----+------------------------+
         |                        | C
        [R]                       |
         |                  B     |
         +-----------+------------Q1  (series pass)
                     |            |
                     | C          | E
                     Q2  B---+    +------+-------+-----o Vo
                     E       |           |       |
                     |       |          [R1]    [RL]
                   (Dz)      |           |       |
                     |       +-----------+      GND
                    GND                  |
                                        [R2]
                                         |
                                        GND

Connections: Q1 collector to Vi, emitter to Vo (emitter-follower pass element). R, from Vi, feeds the base of Q1 and the collector of Q2. The emitter of Q2 is held at Vz by the Zener Dz, which is biased by Q2's emitter current. The base of Q2 is connected to the mid-point of the divider R1-R2 across Vo.

Operation

If Vo tends to rise, the base of Q2 rises, so Q2 conducts more and diverts current from the base of Q1. Q1 conducts less, and Vo falls back. The opposite occurs when Vo falls.

Output voltage

VB2 = Vo R2/(R1 + R2) = Vz + VBE2

Vo = (Vz + VBE2)(1 + R1/R2)

Voltage stability factor (Sv)

The stability factor Sv = ΔVo/ΔVi (at constant load) tells how much the output changes for a change in input.

Vo depends only on Vz and VBE2. When Vi rises by ΔVi, the base of Q1 stays at about Vo + VBE1, so the current in R rises by about ΔVi/R. This extra current flows through Q2 into the Zener, and through the Zener dynamic resistance rz it changes Vz by

ΔVz ≈ ΔVi · rz/(R + rz) (≈ rz ΔVi/R, since rz ≪ R)

Taking VBE2 as constant (it changes by only about 2 mV/°C with temperature):

ΔVo = (1 + R1/R2) ΔVz

Sv = ΔVo/ΔVi = (1 + R1/R2) · rz/(R + rz)

Because rz is small (a few ohms to tens of ohms) compared with R (hundreds of ohms to kΩ), Sv is very small. For example, rz = 10 Ω, R = 1 kΩ and R1 = R2 give Sv = 2 × 10/1010 = 0.0198, that is about 2 % (20 mV per volt of input change). The regulation is improved by feeding the Zener from the regulated output, or by using a constant-current source in place of R.

For the simple Zener-follower series regulator (Vo = Vz − VBE): Sv = ΔVo/ΔVi ≈ rz/(R + rz).

Overload protection circuit

A series regulator with current limiting protects the pass transistor and the load from damage in the event of an overload or short circuit. A sense resistor Rsc and a transistor Q2 limit the maximum output current. (This Q2 is the limiting transistor, separate from the error amplifier above.)

Circuit diagram

 Vi o----+-----------------------+
         |                       | C
        [R]                      |
         |                 B     |
         +---------+-------------Q1  (series pass)
         |         |             |
 (error amplifier  |             | E
  / Zener drive    |             +-----+---[Rsc]---+-----+----o Vo
  to Q1 base)      |                   |           |     |
                   | C                 |           |    [RL]
                   Q2  B---------------+           |     |
                   E                               |    GND
                   |                               |
                   +-------------------------------+

Connections: Q1 is the pass transistor (collector to Vi). Its emitter goes through the small sense resistor Rsc to the output Vo. The base of Q2 is connected to the Q1 emitter side of Rsc, and the emitter of Q2 to the output side, so VBE2 = IL Rsc. The collector of Q2 is connected to the base of Q1. Q1 is driven by the regulator (error amplifier or Zener) in the usual way.

Working

1. Normal load: IL Rsc < VBE2(on) ≈ 0.6 V, so Q2 is OFF and has no effect. Q1 is controlled by the regulator and keeps Vo constant.
2. Overload or short circuit: IL rises. When IL Rsc reaches about 0.6 V, Q2 turns ON.
3. Q2 now takes (shunts) the base current away from Q1 (Q2's collector is connected to the base of Q1). Q1 conducts less, so IL cannot rise further.
4. Thus the output current is clamped to

IL(max) = VBE2 / Rsc ≈ 0.6 V / Rsc

5. In a short circuit the output falls and the current stays at IL(max), so the power dissipation of Q1 is limited. This is called constant-current limiting.

Example: For IL(max) = 0.5 A: Rsc = 0.6/0.5 = 1.2 Ω.

A disadvantage is the drop IL·Rsc in the output path (about 0.6 V at full current), which reduces the efficiency. Foldback limiting reduces the current further as the output voltage falls, giving lower dissipation in a short circuit.