Power Supply Design

1. Power Supply Design

2. Power Supplies • For most electronic devices it is necessary to provide a stable source of DC power. Batteries often serve this function but they are limited in the amount of energy available (see Chapter 14 - a 9 v alkaline will last a day at 100 mA, a car battery 2 hours at 1 A). • One of the simplest and most useful devices you can design is a linear voltage source. We already have many of the components: in lab 3: Diodes we saw the full wave bridge, transformer and filter capacitor. We now need a regulator circuit. • There are two common ways of doing this (if the circuit does not involve an operational amplifier (next week’s topic) • Pass transistor with voltage reference • integrated circuit with internal pass transistor (“three terminal regulator”)

3. Unregulated DC supply We already know how to make an unregulated DC supply. The first component is a full wave bridge, which can be purchased as a package. (http:// www.irf.com/product-info/bridges/articles.html) I There are two equations to remember: Then the choice of capacitors depends on the level of ripple tolerated and the current drawn. V0

4. Unregulated DC (2) One reason that we still need a regulator is the variations in the input AC. This can be in the form or surges or sags. A two year study by Bell Laboratories showed that most locations will experience approximately 25 power line disturbances/year, many sagging below 96 volts. This is a result of power loading, and you probably have noticed light bulbs flicker or dim. A two year study by IEEE members Martzloff and Hahn completed in 1970 shows that surges and impulse voltage spikes can occur as frequently as twice per hour in a typical residence with peak values of 1500V to 2500V. All these variations in voltage (they are all somewhat smoothed by the capacitor) result in changes in the output voltage. Another application of a regulator is for a battery-powered circuit, which varies with load and with time.

5. RC R Unregulated Vin Regulated Vout Better Zener diode voltage regulator The voltage output Vout is VZ - VBE ~ VZ - 0.7. Remember that VZ depends on the current through the zener. However since the base current is small compared to the current through the zener, which depends only on Vin the output is well controlled. The choice of resistor R is dependent an the current we want in the zener and the current in the supply. If the supply needs 1 A of collector (and emitter) current through the pass transistor, this means the base current is about 10 mA. Then the zener current should be greater than this so that changes in the output current don’t change the regulated voltage much. Also for a 1 W zener the voltage is normally specified at ~20-50 mA (for a 0.5 W zener it is 10’s of mAs)

6. RC R Unregulated Vin Regulated Vout Better Zener diode voltage regulator (2) The resistor RC is for current limiting in the event that the output is shorted (you have probably done this in lab at least once already). If this happens then it is not regulated anymore and the full Vin falls across both R and RC (since the emitter and base are nearly grounded). A guideline for selection of this resistor: make it so the voltage drop across RC is less than the voltage drop across R when the maximum current is flowing. So if the maximum output current is 1 A and the unregulated voltage is at most 5 V greater than the regulated output RC ~ 4.3 V/1 A = 4W. This keeps the BJT from saturation and if there is a short on the output current is less than Vin/RC.

7. The previous design shows that a regulator is just a power dissipating element, the pass transistor, that is adjusted by an input voltage so that the voltage drop across it is controlled to fix the output voltage. This is a fairly simple design, but it is sensitive to changes in temperature. In addition you are limited to the values and precision of zener diodes available. Three-terminal regulators An alternative is a three-terminal regulator. An example are the 7800 series fixed positive voltage regulators, which come as a three pin package and can be connected with a few optional capacitors to make a regulated supply.

8. Three-terminal regulators (2) How do they work? For a standard three-terminal there are 20-30 transistors internally as well as a voltage reference, often a zener diode! The output stage is a Darlington pair. The output voltage is sampled and compared to the reference. If there is an error a correction is applied to the base of the Darlington to make the output correct. An important consideration in the use of this is that there is a minimum voltage difference between Vin and Vout called the dropout voltage. This is typically ~2 V. The reason for this is that the Darlington is kept far from saturation which slows its response to changes. From the diagram you can see that it is 2 VBE + VCE(PNP).

9. Adjustable 3-terminal regulators There are only a handful of voltage that fixed regulators are made for. In addition if you want to change the voltage you need to rebuild the supply. An alternative is an adjustable supply where you change a potentiometer (variable resistor) to get a voltage out. The value of VREF for a LM117-series is 1.25 V. The current through the adjustment connection IADJ is less than 100 mA. A typical value for R1 is 240 W. The actual value of the output voltage is normally tweaked with the adjustable resistor since there are variations from chip-to-chip of the actual values of VREF and IADJ.

10. 317 pinout Adjustable 3-terminal regulators (2) The rest of the parameters that you need for design are listed in the datasheets for the regulator. Typically a designer uses a reference such as Horowitz and Hill to help with selection once the performance criteria are chosen. More recently manufacturers provide web info that helps with design selection in their product line (see www.national.com). Ultimately in either case the actual circuit is based on material in the datasheets. If you have not done so already, download and read the LM317 data sheet.