Implementing a Step-Down DC Voltage Regulator (SwitchingMode Power Supplies)

Transcription

1 Implementing a StepDown DC Voltage Regulator (SwitchingMode Power Supplies) Omar X. Avelar, Omar de la Mora & Diego I. Romero POWER ELECTRONICS (ESI 01A) Instituto Tecnológico y de Estudios Superiores de Occidente () Departamento de Electrónica, Sistemas e Informática (DESI) 1. OBJECTIVES To try and check the behavior of stepdown, stepup or voltage inverters. Evaluate some integrated circuits in the market. Measure the efficiency of a switching power supply and assure the performance is better than a linear power supply. 4. EXTRA INFORMATION Reference diagrams for the MC REQUIREMENTS Output voltage in the range of ± 3.3 to ±45 V. The output current must be greater than A. The final design must be built in a PCB. 3. MATERIALS MC34167 Integrated circuit. 5.0 A StepUp/Down/Inverting Switching Regulator. Fig. 1: StepDown application. (MC34167). Internal switch information. Pin { 1. Voltage Feedback Input.. Switch Output. 3. Ground. 4. Input Voltage/VCC. 5. Compensation/Standby. 68 kω Resistor. 6.8 kω Resistor. 189 µh toroidal coil µf electrolytic capacitor. 100 nf capacitor. 330 µf. SR560 Schottky diode (5A). Fig. : Switch Output Source Saturation versus Source Current. 1

2 The compensation network regulates our PWM timing and thus controls the duty cycle internally. 5. DESIGN We start by defining our desired converter parameters. Voltage Input Range 1036 V Voltage Output (Vo) 5V Output Ripple ( Vo) Maximum Output Current (max) 5.0 A Maximum Load Impedance 00 Ω We will use a buck converter, the design equations provided by the manufacturer are shown in the following table. Calculation Step Down V out V F Fig. 3: Compensation network closeup. f osc Fig. 4: Timing Diagram. 1 Duty Cycle f osc I L avg I out I pk As the compensation voltage changes the pulse width can also be controlled. I L avg (switch) L V {ripple(pp)} IL IL 1 8f osc C 0 V ref V out IL ESR R 1 R1 Table 1: Design Equations Courtesy of ON Semiconductor. Notes: 1. Vsat Switch Output source saturation voltage (Check Fig. ).. VF Output rectifier forward voltage drop. Typical value for 1N58 Schottky barrier rectifier is 0.35 V. 3. Duty cycle is calculated at the minimum operating input voltage and must not exceed the guaranteed minimum DC(max) specification of 0.9. Following note 3, we calculate the duty cycle at the maximum operating voltage, V out =V in D 5 D= = then D= f osc and since the typical oscillator frequency is 7 khz, =1.736 µs. Now to calculate the inductor value if we want operate in continuous mode at all times. L 1 D RT L µh khz we chose an inductor value of inductor ripple of: I L=, therefore L=190 µh which yields to maximum = ma L

3 6. IMPLEMENTATION 7. CONCLUSIONS & COMPARATIVE 6.A. SCHEMATIC Based on the MC34167 reference designs. DC power supplies can be broadly classified as linear and switching power supplies. A linear power supply is the oldest and simplest type of power supply. The output voltage is regulated by dropping the extra input voltage across a series transistor (therefore, also referred to as a series regulator). They have very small output ripple, theoretically zero noise, large holdup time (typically 1 ms), and fast response. Linear power supplies have the following disadvantages: very low efficiency, electrical isolation can only be on 60Hz ac side, larger volume and weight, and, in general, only a single output possible. However, they are still used in very small regulated power supplies and in some special applications (e.g., magnet power supplies). Three terminal linear regulator integrated circuits (ICs) are readily available (e.g., MA7815 has +15V, 1A output), are easy to use, and have builtin load shortcircuit protection. Switching power supplies use power semiconductor switches in the on and off switching states resulting in high efficiency, small size, and light weight. With the availability of fast switching devices, HF magnetics and capacitors, and highspeed control ICs, switching power supplies have become very popular. They can be further classified as pulsewidthmodulated (PWM) converters and resonant converters... [ ] [1] Richard Dorf. Summing some elemental characteristics from both power supplies we can now check the following table. Fig. 5: Switching Power Supply Regulator. Notes: 1. Capacitor units are in µf.. The inductor value is 190 µh. Parameter Linear PS Switching PS Efficiency Size and Weight Larger Smaller Fast Switching Devices 6.B. PCB LAYOUT Fig. 6: Single sided printed circuit board (top view). 3

4 Linear regulators can only provide the same current than that at the input, while typical switchedmode power supplies are in general more efficient than a linear power supply, this results in cooler operating modes. Simply put the efficiency of a linear regulator is given by: = V out V in 8. EXPERIENCES AND PROBLEMS We experienced quite a few problems while implementing the circuit, overall the regulator chip was replaced 3 times. 8.A. FIRST ATTEMPT We started with a working regulated 5 V at the output that could have anywhere between 10 to 3 V at the input. We could only get around 400 ma curren the load and the regulated voltage dropped as it demanded more curren lower impedance loads. Which as explained earlier means ily drops the excess voltage. Ignoring the current consumed within a linear regulator, the power consumed within the chip itself can be ignored therefore efficiency depends merely on the input voltage for a constant output voltage. 8.B. SECOND ATTEMPT This excess power is wasted as heat which would suck the battery out of portable devices for example []. The Efficiency of a linear regulator looks better as the relationship between voltages from the input and output reduces. Built now onto a printed circuit board (Pag. 3) it had a regulated output of 5 V and a maximum current of nearly the same ( ma). In this prototype phase, the MC34167 stopped working for no apparent reason (at least apparent to us). While switching converters turnon and off to control the voltage, efficiencies of such devices are typically in the range of 90% independent to the input and output voltages. We blew two MC34167 chips during this phase. Taking note on an example from a white paper by Aivaka Portable Regulators: If a battery with 1000mAh (Milliamp Hour) capacity and an average voltage of 3V were used to power a 100mA load at 1V, the battery would last approximately 30 hours with a switching regulator. If a linear regulator were used in the same system, the battery would last less than 10 hours. This would be a 3X improvement in operating time and 3X reduction in generated heat and power loss. Reducing the heat in a portable device would in turn help to prolong the battery life as well since the excess heat will often degrade the performance of the battery. [] 8.C. THIRD ATTEMPT We replaced the MC34167 with our spare one from the first order and replaced the inductor value with a smaller one to increase the current ripple. The result of this was an increase at the maximum curren the load of 1.83 A approximately. This third version worked while keeping the input voltage between 1018 V, but as we reached 0 V at the input, the MC34167 stopped working, we believe the cause of the nonfunctional MC34167 is a high current in the feedback compensation circuit. 8.D. FINAL ATTEMPT As we ran out of MC34167 integrated circuits (lowbudget students on final projects season) we had to borrow the MC34167 chip from another team to take the following screenshots while using a similar circuit to the third attempt (8.C). By choosing the right power supply for the application can increase the battery life, reduce heat and achieve more efficient solutions in electronic appliances. On the other hand implementation and cost can be important factors; enough to decide if the application requires a switchedmode power supply over a linear regulator. 4

5 9. SCREENSHOTS Then we lowered the load downto Ω. Fig. 7: Output load. Fig. 11: New load. Fig. 8: Input voltage (right). Fig. 13: Output with regulated Fig. 1:current Input voltage (right). voltage. 10. REFERENCES Fig. 10: Output current: left. Output voltage: right [ 1 ] The Electrical Engineering Handbook, Ed. Richard C. Dorf. Boca Raton: CRC Press LLC, 000. [] Linear or LDO Regulators & StepDown Switching Regulators, Aivaka, 007. Fig. 9: Inductor ripple with an 8 Ω load. 5