Switching Power Supply Fundamentals/The Linear Regulation Problem

Regulation

As far as power supples go, no other power supply has as good of regulation as a linear power supply. Unfortunately that regulation is at the cost of efficiency. most linear regulators can only get 68% efficiency some very low dropout linear regulators can get up to 83% efficiency, but are not resilient against large ripples on the input voltage with only .6 v dropout.

Efficiency Calculation

(Power out / Power in) * 100 = % efficiency
Where does the extra power go?
It's dissipated as heat.

Linear Regulation

Linear regulation is the method of regulating the output voltage by dissipating the excess energy as heat.

Zener Diode

The Zener Diode regulator drops excess voltage across a power resistor R2. Current is drawn by the Zener diode in reverse bias for all input voltage above the Zener voltage. Below the Zener voltage this regulator will fail to maintain regulation.
This regulation method is mostly limited by heat output of R2 and current limit of the Zener diode(diodes if multiple are in parallel).

part Vdrop Amps Watts of heat R1(load) 5.178V 517.8mA 2.681W R2 6.822V 682.2mA 4.6539W D1(regulator) 5.178V 164.4mA 851.2mW

LM78xx, LM317, LM79xx

The 3-pin series of linear regulators is a integrated package that regulates the output voltage to a fixed distance to its reference or ground. The regulator acts as a voltage controlled resistor changing its value to make the output match the target voltage constantly. The regulator its self dissipates all the heat so massive heat sinks are required for large currents because, Watts of heat = Voltage dropped * Amps of flow.

part Vdrop Amps Watts of heat R1(load) 5V 500mA 2.5W U1(regulator)in<>out 7Vdrop 505.2mA 3.535W U1 quiescent current(gnd pin) 5mA

Quiescent current

Quiescent current or quiescent wattage is the current flow or power required to operate a regulator regardless of how much power is flowing through the regulator. It's most important in low current applications that still demand high efficiency.

Inefficiency and Heaters

efficiency% - 100 = inefficiency%
70% - 100 = 30%
wattage out * inefficiency% = watts of heat dissipated
500W output * .3(30% inefficiency) = 150W of heat
Average soldering iron is 30W and reaches 700F, without cooling devices, what temp would that same soldering iron reach at 150W!
Not bad in winter but in summer inefficiency will also cost in air conditioning as well as wasted power.

Not all bad

There are many applications where even a inefficient linear regulator is desirable. Any time these are important

• Extremely precise voltage regulation
  • voltage or current reference for calibrating test equipment or powering critical sections of same test equipment.
• Extremely small layout & low current demands from the load.
• Extremely stable voltage supply for references or operating analog to digital converters.
• To power the Quiescent requirements of a high efficiency / high power switching power supply that runs off of mains or other voltages beyond its VCC in limits.