Electronic Troubleshooting Chapter 9 Regulated Power Supplies

Overview Unregulated power supplies Output voltages vary with loads- Higher loads – lower voltages Designs of many circuits assume stable power supplies for proper operation e.g., test equipment, digital circuits Types of Regulated power supplies Covered Zener Diode Regulators Series Regulators Adjustable Voltage Regulator Current Limiters

Regulated Power Supplies Overview Types of Regulated power supplies Covered Troubleshooting Series Regulators Single Chip Regulators Switching Regulators Other Switching Regulator Modes

Zener Diode Regulators Characteristics One of the simplest types of regulated power supplies However, Inefficient in High current applications When Load currents are low high currents flow through the zener Requires a minimum unregulated voltage Zener diode must always be in reverse bias and conducting for regulation Zener characteristics are critical to the regulator operation Zener Diode Characteristics Acts like a normal diode when forward biased Current increases rapidly when V Forward exceeds 0.7V

Zener Diode Regulators Zener Diode Characteristics Reverse Biased characteristics Due to doping of the semiconductor material when the doped for reverse biased voltage (aka, Zener Voltage – V Z ) is exceeded the zener diode conducts The heavier the zener conducts the greater the voltage drop across the power supplies internal resistance Represented as R Series (R S ) » Thevenin Equivalent resistance

Zener Diode Regulators Zener Diode Characteristics Reverse Biased characteristics Zener Circuit Operation Voltage across R S = E –V Z Source current is split between the zener and the load resistor » I S = > I Z and I L If the load decreases (R L increases) » I L decreases » I Z increases enough to keep the voltage across it constant If the load increases » I L increases » I Z decreases See Example Problem 9-1 on page 228

Zener Diode Regulators Zener Diode Power Supply The zenrer circuit is feed by a simple unregulated PS V P is much higher than the zener voltage The head room allows for regulated output over a range of different loads Sample shown at the right V O = Zener voltage V S(ave) =4V w/2V of ripple The regulator eliminates ripple on the output until the load gets to large

Zener Diode Regulators Zener Diode Power Supply Sample shown at the right Continue - load gets to large The ripple voltage across C 1 increases and V S(ave) decreases and the I S isn’t large enough for the load and to maintain a few milliamps through the zener, the Zener stops conducting – you have ripple on the output » Limit before ripple shows in the out put depends upon zener rating

Zener Diode Regulators Zener Diode Power Supply Sample shown at the right Continue - load gets to large If the increases more the zener will regulate even less of the out put Example Problem 9-1, page 228 Replacements Power rating is critical Replace with equal or higher rating Typical ratings range from 1/4W to 10 W or higher Power dissipated by a zener

Series Regulators Characteristics More efficient than a Zener regulated PS The Pass transistor is placed in series with the load and unregulated PS Acts as a variable resistor that is adjusted to maintain V O the same Operation As long as the unregulated voltage (V CC ) is greater than V B the output voltage will be regulated Circuit is a basic emitter follower Output voltage = V B - V BE

Series Regulators Operation Conventional representation The transistor is called the Pass Transistor The Pass transistor must not go into saturation In saturation all regulation stops and the output is a scalar representation of the input Unlike the zener regulator, when load currents are low the regulators power dissipation decreases More efficient operation

Series Regulators Operation Conventional representation Pass Transistor Power Formulas Equation in textbook is wrong Example Problem 9-2 on page 232

Series Regulators Operation Conventional Representation Pass Transistor Power Formulas Equation in textbook is wrong Example Problem 9-2 on page 232

Series Regulators Operation Real Circuit Figure 9-7 on page 233 Note the rating on the zener and the measured base voltage on the transistor Zener diodes have tolerances such as 5%, 10% or 20% Regulation isn’t perfect Changes in load cause I B changes which result in V BE changes In the case of the graph to the right VO would change by 0.1 V

Series Regulators Operation Calculation of Percent Regulation For V Out Example problem 9-3 on page 234

Adjustable Voltage Regulator Characteristics Adds components for better regulation and some for adjustment of the output Better Regulation Thus the output stays at a more constant level and higher percent regulation From the given condition and voltages Load increases, V A tends to decrease V B decreases, Q2 conducts less V D goes higher and then V A goes higher

Adjustable Voltage Regulator Characteristics Output level adjustment

Current Limiters Characteristics Provides a means to protect the PS from excessive loads or shorted outputs. Added components – Q3 and R SC R SC s sized so that normal operating currents will develop much less than 0.7V across it and Q3 is off If I L is large enough Q3 turns on Point D is tied to point A Q1 conducts much less Example Problem on 236

Troubleshooting Series Regulators Characteristics Significant difficulty due to the interaction of many of the components Best approach may be isolate some parts of the circuit and test Sample walk through Assume: V O is abnormal Adjusting RX doesn’t fix the problem Follow suggested flow chart on page 238

Single Chip Regulators Characteristics Internal circuitry is at least as sophisticated as Current limiting circuit cover before Typical packages

Single Chip Regulators Characteristics Typical part numbers 78XX and 340XX - the XX are replaced by the rated voltage Check data sheets for rated currents, min/max input voltages Typical Configuration

Single Chip Regulators Single Chip Regulated Adjustable PS Example Problem 9-5 on page 240

Single Chip Regulators When current demands exceed a signal chip You may find separate regulators on multiple circuit cards in a multi-card systems Outputs of multiple regulator should never be connected – Check Specs Provide an parallel higher current path See the circuit on the next slide or Figure 9-18 on page 241 of the textbook Operation On startup Q1 is off and the regulator starts delivering power Example Problem 9-5 on page 240

Single Chip Regulators When current demands exceed a signal chip Provide an parallel higher current path Operation Current through R2 biasing the base of Q2 (Note R2 is sized to match the emitter-base resistor) and Q2 turns on V R1 = 0.7V Example Problem 9-6 on page 241

Switching Regulators Why use Series regulated PS still can consume substantial power just operating Work Example Problem 9-7 on page 242 Characteristics of Switching Regulators Pass transistor isn’t always on It is switched on/off at a high rate to keep the output voltage at a desired value The power delivered by the Pass transistor depends upon the average value of the pulses that result from the on/off switching The pulses look like a rectangular digital waveform with varying duty cycles – aka, Pulse width modulation See Figure 9-19 on page 243 of the textbook

Switching Regulators Characteristics of Switching Regulators Filtering Circuit for Switching PS Switch subs for the Pass Transistor The inductor/choke is critical to operation Act to keeps current through the load constant The Cap helps smooth out ripple voltages– the larger the better Closed switch Current flows through S, L, & R L Counter emf (voltage) developed across L to prevent load current from changing too rapidly

Switching Regulators Characteristics of Switching Regulators Complete PS Sampler tests the output voltage Compare/Control Compares the sample to a reference Changes the pulse width of the pulses out of the Pass Transistor » The greater the difference between the sample and reference the greater the pulse width The complete sampling, comparing, and control circuitry is available in a monolithic IC – e.g., SG 1524 (Pulse Width Modulation)

Switching Regulators Characteristics of Switching Regulators Complete PS See the example circuit on page 245 that uses the SG 1524 for PWM It shows a simplified view of the IC’s circuitry » 5V reference, comparator, Sawtooth oscillator, error amplifier, and other components » Note the OSC typically operate at 5-100kHz Vs comes from a pot Rx Difference Amp feed by Vs and the reference voltage V 3 multiplies the difference between them by R F /R 1.

Switching Regulators Characteristics of Switching Regulators Complete PS See the example circuit on page 245 that uses the SG 1524 for PWM The amplified error signal feeds the + input of the comparator and the the sawtooth OSC feeds the - input » When more positive than the sawtooth OSC output Q 2 and Q 1 are turned on. Else Q 2 and Q 1 are off » Note: Additional not shown control circuits prevent the voltage on the base of Q1 at zero if the Ramp voltage is less than +!V Q 1 is switched on/off with longer/shorter duty cycles, as needed to maintain a constant output voltage (See Fig 9-23 on page 246) » Thus the pulse width is directly related to the magnitude of the differences between the sampled output voltage and reference » During conduction Q 1 is in saturation, very low voltage drop, thus low power loss, In cut-off zero current flows and zero power

Switching Regulators Characteristics of Switching Regulators Sample Waveforms for a Pulse Width Switching PS

Switching Regulators Characteristics of Switching Regulators The Fairchild µA78S40 is similar but not a pulse width It outputs fixed width pulses when the comparator indicates a need for more energy in the output filtering circuit Otherwise – NO PULSES Sample circuit using the IC and a simplified view of its internal circuitry – at the bottom of page 248 Internal timing signals at the top of Page 248 When the sampled output Vs falls below V ref the And-Gate is enabled and Q outputs a square wave, else it is off Which turns Q 1 and Q 2 on/off until Vs is greater than V ref Typical output signals at the top of page 249 Oscillator output compared to output voltages under light, medium and heavy loads

Other Switching Regulators Key Factors Three basic types of switching PS Step down Vo less than the input » Just covered - previous section Step Up Vo greater than the input Inverting Vo opposite polarity than the input

Other Switching Regulators Step-Up Switched PS With the switch closed A significant current is established in the inductor When the switch opens A cemf voltage develops across the inductor The cemp adds to E and charges the output Cap Cap maintains the voltage on the load when the switch closes again

Other Switching Regulators Inverting Switched PS With the switch closed A significant current is established in the inductor Diode ids reversedbiased When the switch opens A cemf voltage develops across the inductor The cemp charges the output Cap Cap maintains the voltage on the load when the switch closes again

Inverting Switcher