Switching Power Supply Regulators

Transcription

1 Switching Power Supply Regulators Recall the linear power supply concept: A linear control element is series with the unregulated DC is used, with feedback, to maintain constant output voltage. For the linear power supply regulator: The output voltage is always lower than the input voltage The control element acts as a variable resistance to keep the output voltage constant under changing load. The control element is always dissipating power, so this causes loss in efficiency a characteristic of this kind of regulator. Here is an alternative approach to the regulator problem a Switching Regulator: 1

2 Switching regulators operate by rapidly switching the pass transistor between two efficient operating states: 1. Cutoff where there is a high voltage across the pass transistor, but no current flow. 2. Saturation where there is a high current through the pass transistor, but a very small voltage drop. Characteristics of Switching Power Supplies: Since the control element is either on or off, there is little power dissipation. Can generate output voltages higher than the unregulated input. Can generate output voltages of opposite in polarity to the input. Can be smaller and lighter weight than a linear PS regulator. Problem: NOISY and Complex to build Many applications require both types to be used. For example, a switching regulator may provide the initial regulation, then a linear regulator may provide post-regulation for a noise sensitive part of the circuit, such as a sensor interface or A/D converter. Here is a comparison of linear versus switching power supplies 2

3 The switching power supply is an improvement (in many ways) over the linear power supply. Here is the concept: Switching Power Supply Block Diagram Here are the essential waveforms that describe the operation of this system. load current Switching and Control Waveforms 3

4 Before going further, let s review how inductors charge and discharge. recall: di t I VL L L dt T Switched Inductor Action There are three types of switching power supplies: 1. Step down regulator sometimes called a buck regulator. 2. Step up regulator sometimes called a boost regulator. 3. Inverting regulator sometimes called a flyback regulator. In Project #2 we will focus on buck and boost regulator design. 4

5 Here s a closer look at the control element, plus capacitor and inductor, showing how these are configured for the three types of regulators: Make sure you understand the principles described by these diagrams Diode D1 is often called the catch diode. It is usually a Schottky-barrier diode, having low forward voltage drop and fast switching speed. 5

6 Step-Down Switching Regulator t off Let s analyze this circuit and learn how it works: 1. The filter averages the voltage square waves which are created by switching Vin on and off. The filter averages these waveforms to produce a dc voltage. t t V V V on on o in in T ton toff duty cycle 2. When Q1 switches ON, the inductor charging current is: V V V V L L L in sat o IL t t Vsat is the voltage dropped across Q1 when it is saturated. 3. When Q1 switches OFF, the inductor current is: V V L o D1 IL IL( peak) t 6 See and on the next page

7 A closer look at the waveforms in this system: V Voltage across switch Q1 V in D1 V sat V in Voltage across Diode D1 V in V sat V in Switch Current Q1 I in I V D1 I pk C( Avg ) Diode D1 Current I pk I D 1( Avg ) Inductor Current I I I I 2 I pk pk o C( Avg) D1( Avg) t off t on T Capacitor Current I pk I pk 2 I pk 2 Capacitor Ripple Voltage V V o o V pk V o V pk V ripple( p p) t 2 2 on t off 7

8 Here s how to configure the switching regulator for step-up and inverting operation. 8

9 Now look at the components of the control circuitry 9

10 Tying it all together, we have the following step-down switcher The control circuitry can be implemented using individual devices or you can make use of an integrate controller such as the 74S40, as shown below. However, we will not use a device such as this. Instead we will implement the functions using a 723 and a dual 555 timer. 10

11 Here is a schematic of the way we will implement a switching power supply for Project #2. 11

12 It is critically important that you have a complete understanding of how this circuit operates before you try to build it. You must also understand how the 723 and 555/556 devices function. Now let s look at some design considerations. There are three primary subcomponents to this project. They are: 1. The control element and charging circuitry This schematic shows the TIP-42 used as the control element. You are not constrained to using a PNP device here. You can also investigate the use of the TIP-41, which is an NPN transistor. You also might want to try using a power field-effect device. Try the IRF511. IRF-511 Can the 556 provide sufficient drive current to use this kind of implementation? TIP-41 What is necessary for the 556 to drive this device? Is it much easier to use than a BJT? 12

13 2. The oscillator and pulse width modulator C B Pulse-Width Modulator (bi-stable mutivibrator) A Oscillator (astable multivibrator) (20 khz 100 khz) 13

14 3. The voltage sensor and error amplifier Here we have the familiar 723 you should understand all about this device at this point. B Error voltage output to the PWM Output voltage is sensed here In this application, the 723 is operating in a circuit that has an output voltage that is lower than the internal reference voltage. Recall this diagram from Project #1: maybe you should use one of these? 14

15 Design Steps for the Step Down Switching Regulator Here are some suggested steps to designing the output circuitry for the switching regulator. These are to be used as guidelines to get you in the ballpark. Use these in conjunction with the document entitled Step Down Switching Regulator Operation & Design. Don t hesitate to experiment there is plenty of latitude in the design. Step #1 State your givens VIN DC input voltage V OUT DC output voltage I Maximum load current OUT max V D1 Forward voltage of catch diode V sat Saturation voltage of switching transistor V Peak-to-peak ripple (~0.5%) ripple(p-p) Step #2 Determine the peak output current I L( pk ) 2 I OUT Step #3 Determine the ratio of on/off time for the switching transistor ton VOUT VD1 t V V V off IN sat OUT Step #4 Select a switching frequency Select a frequency within the following range: 30,000 Hz f 60,000 Hz and note that: f t on 1 t off 15

16 Step #5 Calculate the value for the switching inductor L min V V V I IN sat OUT pk ( switch) t on Step #6 Calculate the value for the switching capacitor C O I t t pk ( switch) on off 8V ripple( p p) Step #7 Determine the regulator efficiency VOUT VIN Vsat VD 1 VOUT VD 1 VIN 16

17 The 555 Integrated Circuit Timer Operates from a wide range of power supplies (see H & H p. 289) Timing intervals of several minutes. Frequencies as high as a few MHz Remember how the comparator functions:

18 Summary of Operation: Three 5 k resistors are configured in series to divide Vcc into thirds. The junctions of these resistors are tied to comparators. This serves as a constant reference voltage that is only dependent upon Vcc. To force the output of the 555 low, the voltage on the Threshold input must exceed 2/3 Vcc. This also turns the discharge transistor on. To force the output of the timer high, the voltage on the Trigger input must fall below 1/3 Vcc. This also turns the discharge transistor off. The diagram on the previous page shows the 555 connected in freerunning (astable) mode. We can design the 555 to operate at a specific free-running rate by choosing Ra, Rb and C. Let s examine how this is accomplished. You might recall that: t V t A 1 e RC defines the charging of an general series RC circuit. 1. Time to charge 0 2 V 3 CC V t A V 2 V CC A 3 CC R R R B t 2 RC VCC V CC 1e 3 t 1 RC e so t 1.09 RC 3 18

19 2. Time to charge 0 1 V 3 CC 1 VCC V CC 1e 3 t RC t 0.405RC 3. Time to charge 1 V 2 CC V 3 3 CC t 1.09RC0.405RC 0.69RC high t 0.69 R R C high A B The next thing we need to do is look at the discharge behavior of the RC circuit. Note that the discharge of an RC circuit is defined as: V t Ae RC t 4. Time to discharge 2 1 VCC V 3 3 CC 1 2 VCC VCC e 3 3 t RC R R B But here, because of the discharge path. t low 0.69R C B 19

20 5. Finally, the period of the output signal is: T t t high low 0.69 R R C0.69R C A B B 0.69 R 2R C A B also f T R 2R C A B 20

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22 The inductor will be hand-wound using the Ferroxcube 2213PA500-3C8. These are ferrite inductor cores that are snapped around a plastic bobbin on which is wound the coil. Since the wire is wound on a bobbin rather than around the ferrite material itself, it is easy to change bobbins and experiment with the various inductances produced by different numbers of turns and kinds of wire. Two features of this core: Winding the Inductor This core is a manganese-zinc ferrite substance with medium permeability and low losses. It is designed specifically for high flux-density applications such as power supplies. 22

23 The inductance of a coil can be determined from the following equation: L N 2 A r o where: L = Inductance in Henries N = number of turns A = Cross sectional area of coil (m 2 ) r = relative permeability of core material = permeability of air (1.26 x 10-8 ) H/m l = avg. coil length (m) You can use this information and the data in the Ferroxcube data sheet to determine the number of turns needs for a given inductance. A shortcut is to use the following formula: N L10 A L 9 where: N = number of turns L = desired inductance in henries A L = millihenries/1000 turns (~ 500 for the 3C8 core) To produce a 275 microhenry inductor, approximately 23 turns are required. The result is not exact, so you will have to experiment with it. 23

24 Tips for success: 1. Understand how step-down switching regulators work. Refer to H&H (Sect. 6.19) and the reference material posted on the JEDL web site. 2. Understand how the 555 functions and how to select components to make it function predictably as an astable and monostable circuit. The 556 is just two 555 s in one package. There is reference information on using the 555 posted on the web site. 3. Understand how the 723 operates in the application. It is used differently than in the previous project. 4. Read the note entitled Step-Down Switching Regulator Operation and Design. This is your primary source of information for this project. 5. Break the circuit into three parts: Error amplifier (723), oscillator and PWM (556), and the output circuitry. Allow one person to focus on one of these so that there is sufficient effort from all team members. 6. Breadboard parts of the circuit just to get familiar with it. In the process you should learn to wind and test the inductor. Play around with it and understand what happens when you change the peripheral components. 7. Implement a Spice simulation of the pass transistor, diode, inductor and output capacitor. The purpose of this is to see how to bias the transistor and to make sure it switches properly. Also this is a check on the inductor 8. Construct the completed circuit and test it. When testing the circuit, always keep a load on the circuit it doesn t function well without a load. Try to keep a minimum load of about 200 ma on the circuit when testing. 24