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Table of Contents 4.0 SMPS Design Examples 4.1 Generic SMPS ICs 4.2 LM78S40 Universal Switching Regulator 4.3 MAX641 Step-Up Switching Regulator Assignment Questions For Further Research An Introduction to Power Supplies i _____ Notes _____ 4.0 SMPS Design Examples Objectives • Identify various types of switch mode converters. • Determine the approximate current waveforms in SMPS circuits. • 4.1 Generic SMPS ICs Of these components, only the switch and diode can be integrated; the inductor and capacitor are external components. In high power applications, even the switch and diode are discrete. It may therefore seem pointless to integrate a switching regulator. However, this is not the case. The real trick is to control the switch. Switching converters can achieve a power conversion efficiency of 70 – 90%. However, they also create electro-magnetic interference (EMI), which can have adverse effects on other nearby electronics. Low-loss ferrite materials, high permeability magnetic shielding, and smaller semi-conductors can reduce EMI. Bipolar switching transistors, with a gain-bandwidth product in excess of 4 MHz are often used as the principle switching element. To minimize adverse effects from inductive kickback, Schottky or fast-recovery diodes are also used. Buck Switching Regulator An Introduction to Power Supplies 4-1 Switch Mode Power Supplies _____ Notes _____ Boost Switching Regulator Most switching regulators consist of: Switching transistor Diode clamp LC filter Control logic It is possible to design switching power supplies to operate directly from the hydro input. This eliminates the need for a transformer, rectification, and prefiltering. A switching regulator IC contains the four basic components found in a linear regulator, but adds an oscillator and some control logic in order to control the transistor switch (control element). Generic Switching Regulator IC There are several different control methods that can be used to control the conduction in the series control element: PWM – pulse width modulation: the frequency is held constant and the ON time is varied. This is the most common technique used. PFM – pulse frequency modulation: the on or the off time is held constant and the frequency is varied. 4-2 An Introduction to Power Supplies Switch Mode Power Supplies _____ Notes _____ PBM – pulse burst modulation: the oscillator frequency and duty cycle is held constant and oscillator cycles are gated on or off. This technique is used in simple converters below 10 watts. As the switching frequency increases, the size of the magnetic components decreases and the switching losses increase. If the switching frequency is in the audio range, it is possible for the coil windings to vibrate, thus creating an annoying singing tone. This is a common phenomenon in TV sets, where the flyback transformer operates at 15.75 KHz. The sampling circuit generally consists of a simple voltage divider. Under normal operating conditions, it produces an output equal to the built-in reference voltage. An op amp is used to compare the sampled and reference voltages. This creates a difference signal that used to control the series pass device (switch). A similar technique is used in linear regulators, the principle difference is that the series device is operated at the extremes of its load line instead of the linear region. This subtle difference is what gives switching converters their high power conversion efficiency. 4.2 LM78S40 Universal Switching Regulator LM78S40 Datasheet by National Semiconductor uA78S40 Datasheet by OnSemi AN-711 LM78S40 Applications by National Semiconductor Many of the functional blocks in this circuit are disconnected. This gives the designer a great deal of design flexibility. If required, the transistor switch can be used directly in low power applications, or it can be used to drive a high power series pass element. An Introduction to Power Supplies 4-3 Switch Mode Power Supplies _____ Notes _____ Each SMPS IC has its own design peculiarities. In some cases, the restriction placed on the designer may preclude the use of any SMPS IC and a completely discrete circuit design must be considered. This situation however, is beyond the scope of this presentation. Transistor Driver and Switch The 78S40 uses a Darlington pair in the switching arrangement. The collectors of both transistors are brought out to external pins. This allows them to be connected together as is the standard configuration, or an external resistor can be placed in the driver collector to control the switch saturation current. Oscillator The 78S40 chip is designed to operate within a switching frequency range of 100 Hz to 100 KHz. Increasing the switching frequency increases the electromagnetic radiation and PCB layout problems but decreases the size of the inductor. Most designs based on this IC have an operating frequency of 20 - 30 KHz. The oscillator, the charge/discharge ratio is preset to approximately 6:1. The overall switching duty cycle can be varied from approximately 17% to 50% by means of two feedback loops. The switching frequency and duty cycle are controlled by current and voltage feedback. This will at times make triggering an oscilloscope to the switching waveforms somewhat problematic. As the load increases, the switching frequency tends to increases. Reference Voltage The 78S40 has an internal 1.245 volt temperature compensated, band-gap voltage reference which is available at an external pin. This reference voltage should be bypassed by a 0.1fd capacitor to ground to help insure stability. Current Feedback The current feedback circuit modifies the switch ON time. A current sensing resistor RSC generates a voltage proportional to the switching current. When this potential exceeds approximately 0.3 volts, the oscillator (and hence switch) is turned OFF. This control mechanism takes priority over voltage feedback. Voltage Feedback The voltage feedback loop, consisting of the voltage divider and comparator controls the switch ON time. If the output voltage is too low, the ON time is increased. 4-4 An Introduction to Power Supplies Switch Mode Power Supplies _____ Notes _____ Current and Voltage Limitations The internal Darlington transistor switch can handle a maximum peak current of 1.5 amps during the t on period, and a maximum of 40 volts during the t off period. An external transistor switch is needed if the design requires either more current or a higher input voltage. Voltage Sensing Resistors The voltage divider at the output represents a minimum load. The voltage at the junction of the two resistors must equal the 1.245 reference voltage when one resistor is attached to ground and the other to the output. Efficiency The efficiency of a well-designed power converter can be in excess of 90%. The output voltage divide constitutes a minimum load on the switching converter and therefore reduces the efficiency. The current drawn by the divider can be as low as 100 a, but is more typically in the region of 1 ma. This may not be significant with high load currents, but it becomes more dominant as load current decreases. The saturation voltage of the Darlington transistor can be as high as 1.3 volts. This decreases efficiency as load current and t on increase. The internal Darlington transistor has a switching speed of 300 – 500 nSec. During this time, the transistor is neither ON nor OFF, and therefore dissipates power. Any current sensing resistor in series will also dissipate power. In the following formulas: Vin = Nominal input voltage Vout = Desired output voltage (determined by the ratio R2/R1) Vsat = ON voltage drop across the switching element VD = Forward voltage drop across the flyback diode. I out = Desired output current. I s = Voltage divider sampling current (~ 1 ma) Vripple = Desired peak-to-peak ripple voltage. RSC = Short circuit current sensing resistor. An Introduction to Power Supplies 4-5 Switch Mode Power Supplies _____ Notes _____ NOTE: When breadboarding these circuits: It may be necessary to reduce RSC to 0 . Always keep the circuit leads as short as possible. Always use a large decoupling capacitor at the circuit input. Spread Sheet Design Buck (Step down) Circuit Simplified Circuit Buck Design Formulas Formula R Vout Vref 1 2 R1 I pk 2 I out(max) Comment The internal reference voltage is Vref = 1.245 volts. I pk is the peak inductor current. I out (max) is the maximum output RSC 0.33 I pk t on Vout VD t off Vin Vsat Vout load current. The value for the current sensing resistor is a given, not derived, formula. ton and toff > 10 s ton + toff < 50 s V VD t off L out I pk I pk L t off Vout VD 4-6 An Introduction to Power Supplies Switch Mode Power Supplies _____ Notes _____ CT 45 105 toff CO Timing capacitor I pk t on t off Minimum output filter capacitor 8 Vripple Vin Vsat VD Vout V in Vout VD I pk Vout VD I in( ave)(max load) 2 Vin Vsat VD Converter efficiency. Input current. Buck Design Example (AN711) Vin = 25 volts Vout = 10 volts Iout (max) = 500 ma Vripple < 1% Step 1 - Calculate the peak current I pk 2 I out(max) = 1 amp Step 2 - Calculate the current sense resistance RSC 0.33 = 0.33 I pk Step 3 - Calculate the ton/toff ratio t on Vout VD 10 1.25 0.8 t off Vin Vsat Vout 25 1.1 10 Step 4 - Calculate the values for ton and toff Since t on 0.8 t off and t on t off 50 sec Let t off 22 sec then t on 18 sec Step 5 - Calculate the timing capacitor CT CT 45 105 toff 45 105 22 106 0.01fd An Introduction to Power Supplies 4-7 Switch Mode Power Supplies _____ Notes _____ Step 6 – Determine the inductor size V VD L out I pk t off 10 1.25 22 10 6 250H 1 Step 7 – Determine the minimum storage capacitor size CO I pk t on t off 8 Vripple 1 18 10 6 22 10 6 50fd 8 0.1 Step 8 – Determine the feedback resistor sizes R2 R1 Vref Is 1.245 1.25 K 110 3 Vout Vref Is (Select 1.3 K) 10 1.245 8.76 K 110 3 (use a 10 K potentiometer) Final Circuit Note: the above schematic is missing the reference and input bypass capacitors. 4-8 An Introduction to Power Supplies Switch Mode Power Supplies _____ Notes _____ Basic Waveforms Boost (Step Up) Circuit Simplified Circuit An Introduction to Power Supplies 4-9 Switch Mode Power Supplies _____ Notes _____ Boost Design Formulas I pk Formula R Vout 1.25 1 2 R1 V VD Vsat 2 I out(max) out Vin Vsat RSC 0.33 I pk t on Vout VD Vin t off Vin Vsat Comment The internal reference voltage is 1.245 volts. I pk is the peak inductor current. I out (max) is the maximum output load current. The value for the current sensing resistor is a given, not derived, formula. ton and toff > 10 s ton + toff < 50 s V V D Vin t off L out I pk I pk L t off Vout VD Vin CT 45 105 toff CO I I out t off 2 pk Timing capacitor Minimum output filter capacitor 2 I pkVripple Vin Vsat Vout Vin Vout VD Vsat I pk I in( ave)(max load) 2 Vin Vref R3 IB Converter efficiency. Input current. Driver collector resistor IB I pk , assume 20 Boost Design Example (AN711) Vin = 5 volts Vout = 15 volts Iout (max) = 150 ma Vripple < 1% Step 1 - Calculate the peak current V VD Vsat 15 1.25 0.45 2 0.15 I pk 2I out(max) out 1 amp 5 0.45 Vin Vsat 4 - 10 An Introduction to Power Supplies Switch Mode Power Supplies _____ Notes _____ Step 2 - Calculate the current sense resistance RSC 0.33 = 0.33 I pk Step 3 - Calculate the ton/toff ratio t on Vout VD Vin 15 1.25 5 2.5 t off Vin Vsat 5 0.45 Step 4 - Calculate the values for ton and toff Since t on 2.5 t off and t on t off 50 sec Let t off 10 sec then t on 25 sec Step 5 - Calculate the timing capacitor CT CT 45 105 toff 45 105 10 106 4.5 nfd Step 6 – Determine the minimum inductor size V VD Vin t off 15 1.25 5 10 10 6 100 H L out I pk 1 Step 7 – Determine the minimum storage capacitor size CO I I out t off 2 pk 2 I pkVripple 1 0.152 10 10 6 2 1 0.15 24 fd Step 8 – Determine the feedback resistor sizes Let I s 1 ma R2 R1 Vref Is 1.245 1.25 K 110 3 Vout Vref Is (Select 1.3 K) 15 1.245 13.8 K 110 3 An Introduction to Power Supplies (use a 25 K potentiometer) 4 - 11 Switch Mode Power Supplies _____ Notes _____ Step 9 – Determine the base drive resistor size (if desired) R3 Vin Vref and IB R3 IB I pk , assume 20 5 1.245 72 1 20 Final Circuit Note: the above schematic is missing the reference and input bypass capacitors. 4 - 12 An Introduction to Power Supplies Switch Mode Power Supplies _____ Notes _____ Basic Waveforms Inverter Circuit In some designs (such as the one below) the switch current is extremely large. As a result, an external switching transistor may be required. Simplified Circuit An Introduction to Power Supplies 4 - 13 Switch Mode Power Supplies _____ Notes _____ Inverter Design Formulas I pk Formula R Vout 1.25 1 2 R1 Vin Vout VD Vsat 2 I out(max) Vin Vsat RSC Comment The internal reference voltage is 1.245 volts I pk is the peak inductor current. I out (max) is the maximum output load current. The value for the current sensing resistor is a given, not derived, formula. ton and toff > 10 s ton + toff < 50 s 0.33 I pk t on Vout VD t off Vin Vsat Vout V D t off L I pk I pk L t off Vout VD CT 45 105 toff CO I Timing capacitor I out t off Minimum output filter capacitor 2 pk 2 I pkVripple Converter efficiency. Vout Vout VD I in( ave)(max load ) I pk Vout VD 2 Vin Vout Vsat VD Input current. Inverter Design Example (AN711) Vin = 12 volts Vout = -15 volts Iout (max) = 500 ma Vripple < 1% Step 1 - Calculate the peak current Vin Vout VD Vsat I pk 2 I out(max) Vin Vsat 4 - 14 An Introduction to Power Supplies Switch Mode Power Supplies _____ Notes _____ 12 15 1.25 2 2.6 I pk 2 0.5 12 2 Step 2 - Calculate the current sense resistance RSC 0.33 = 0.13 I pk Step 3 - Calculate the ton/toff ratio t on Vout VD 15 1.25 1.625 t off Vin Vsat 12 2 Step 4 - Calculate the values for ton and toff Since t on 1.6 t off and t on t off 50 sec Let t off 10 sec then t on 16 sec Step 5 - Calculate the timing capacitor CT CT 45 105 toff 45 105 10 106 4.5 nfd Step 6 – Determine the inductor size V VD L out I pk 15 1.25 t off 10 10 6 70 H 2.6 Step 7 – Determine the minimum storage capacitor size CO I I out t off 2 pk 2 I pkVripple 2.6 0.52 10 10 6 2 2.6 0.15 60 fd Step 8 – Determine the feedback resistor sizes R2 Vref Is 1.245 1.25 K 110 3 An Introduction to Power Supplies (Select 1.3 K) 4 - 15 Switch Mode Power Supplies _____ Notes _____ Vout Vref R1 Is 16 1.245 17.5 K 1 10 3 (use a 25 K potentiometer) Step 9 – Determine the switching transistor bias resistors Select 100 R3 300 R4 where Vin Vsat VT VBE I pk VT 0.3 v (current sense threshold voltage) hFE (of the external transistor) 4 R4 12 1.3 0.3 0.7 720 2.6 190 Final Circuit Note: the above schematic is missing the reference and input bypass capacitors. 4 - 16 An Introduction to Power Supplies Switch Mode Power Supplies _____ Notes _____ Basic Waveforms An Introduction to Power Supplies 4 - 17 Switch Mode Power Supplies _____ Notes _____ 4.3 MAX641 Step-Up Switching Regulator MAX641 Datasheet MAX641 Block Diagram This particular IC requires the designer to only select the value of the inductor. Boost Design Example Vin 5v 10% Vout 15v VD 0.4v 0.25v Vsw 0.75v f osc 50 KHz 20% 8Sec t on 12Sec 15ma I load 450ma To determine the value of the inductor, it is necessary to make two calculations, one for a maximum size and the other for minimum. Case 1 Maximum L I pk I pk Vout VD Vin 4 I load Vin Vsw 15 0.4 4.5 4 15ma 174ma 4.5 0.75 L 4 - 18 Vin Vsw t on I pk An Introduction to Power Supplies Switch Mode Power Supplies _____ Notes _____ L 4.5 0.75 8Sec 172 H 174ma Case 2 Minimum L From the datasheet Ipk(max) = 450ma. L 5.5 0.25 12Sec 140H 450ma A value of 160 H would be a reasonable compromise. An Introduction to Power Supplies 4 - 19 Switch Mode Power Supplies _____ Notes _____ Assignment Questions On-Line Test Quick Quiz 1. Composition Questions To answer these questions, it will be necessary to do some research. 1. Analytical Questions 1. Create a spreadsheet to implement the design of a Buck Regulator based on the LM78S40. 2. Create a spreadsheet to implement the design of an Inverter based on the LM78S40. 3. Create a spreadsheet to implement the design of a Boost Regulator based on the LM78S40. 4. 4 - 20 An Introduction to Power Supplies Switch Mode Power Supplies _____ Notes _____ For Further Research An Introduction to Power Supplies 4 - 21