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Introduction to DC-DC Conversion

Introduction to DC-DC Conversion

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Introduction to DC-DC Conversion

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  1. Introduction to DC-DC Conversion EE174 – SJSU Tan Nguyen

  2. OBJECTIVES • Introduction of DC-DC Converter • Types of DC-DC Converters • Linear regulator (LR) • Switching mode power supply (SMPS) • Advantages and Disadvantages

  3. Introduction • DC to DC converters are important portable electronic devices used whenever we want to change DC electrical power efficiently from one voltage level to another. • A power converter generates output voltage and current for the load from a given input power source. • Depending on the specific application, either a linear regulator (LR) or a switching mode power supply (SMPS) solution to be chosen.

  4. Typical applications of DC-DC converter • Car battery 12V must be stepped down to 3-5V DC voltage to run DVD/CD player • Laptop computers or cellular phone battery voltage must be stepped down to run several sub-circuits, each with its own voltage level requirement different from that supplied by the battery. • Single cell 1.5 V DC must be stepped up to 5V operate an electronic circuitry. • A 6V or 9V DC must be stepped up to 500V DC or more, to provide an insulation testing voltage. • A 12V DC must be stepped up to +/-40V or so, to run a car hifi amplifier circuitry. • A 12V DC must be stepped up to 650V DC or so, as part of a DC-AC sinewave inverter.

  5. LINEAR REGULATORS How a Linear Regulator Works In an embedded system, a 12V bus rail is available from the front-end power supply. On the system board, a 3.3V voltage is needed to power an operational amplifier (op amp). The simplest approach to generate the 3.3V is to use a resistor divider from the 12V bus, as shown in Figure 1. Does it work well? The answer is usually no. The op amp’s VCC pin current may vary under different operating conditions. If a fixed resistor divider is used, the IC VCC voltage varies with load.

  6. LINEAR REGULATORS (cont.) As a result, the resistors divider cannot provide a regulated 3.3V to the op amp to ensure its proper operation. Therefore, a dedicated voltage regulation loop is needed. As shown in Figure 2, the feedback loop needs to adjust the top resistor R1 value to dynamically regulate the 3.3V on VCC.

  7. LINEAR REGULATORS (cont.) This kind of variable resistor can be implemented with a linear regulator, as shown in Figure 3. A linear regulator operates a bipolar or field effect power transistor (FET) in its linear mode. So the transistor works as a variable resistor in series with the output load. The linear regulator circuit keeps the output voltage Vcc at constant and well regulated.

  8. LINEAR REGULATORS (cont.) • The linear regulator is a DC-DC converter to provide a constant voltage output without using switching components. • The linear regulator is very popular in many applications for its low cost, low noise and simple to use. • It was the basis for the power supply industry until switching mode power supplies became prevalent after the 1960s. • Power management suppliers have developed many integrated linear regulators.

  9. LINEAR REGULATORS (cont.) A typical integrated linear regulator needs only VIN, VOUT, FB and optional GND pins. Figure 4 shows a typical 3-pin linear regulator, it only needs an input capacitor, output capacitor and two feedback resistors to set the output voltage.

  10. LINEAR REGULATORS DRAWBACK • A major drawback of using linear regulators can be the excessive power dissipation of its series transistor Q1 operating in a linear mode. • Since all the load current must pass through the series transistor, its power dissipation is PLoss = (VIN – VO) •IO. • The efficiency of a linear regulator can be estimated by:

  11. LINEAR REGULATORS DRAWBACK Figure 5 shows that the maximum efficiency of the linear regulator is proportional to the VO/VIN ratio. In example where input is 12V and output is 3.3V  linear regulator efficiency 27.5%. The 72.5% of the input power is just wasted and generates heat in the regulator. This means that the transistor must have the thermal capability to handle its power/ heat dissipation at worst case at maximum VIN and full load. So the size of the linear regulator and its heat sink may be large, especially when VO is much less than VIN.

  12. LINEAR REGULATORS DRAWBACK • The linear regulator can be very efficient only if VO is close to VIN. • The linear regulator (LR) has another limitation, which is the minimum voltage difference between VIN and VO. The transistor in the LR must be operated in its linear mode. So it requires a certain minimum voltage drop across the collector to emitter of a bipolar transistor or drain to source of a FET. When VOis too close to VIN, the LR may be unable to regulate output voltage anymore. • The linear regulators that can work with low headroom (VIN – VO) are called low dropout regulators (LDOs). • The linear regulator or an LDO can only provide step-down DC/DC conversion.

  13. LINEAR REGULATORS APPLICATIONS There are many applications in which linear regulators provide superior solutions to switching supplies: 1. Simple/low cost solutions. Linear regulator or LDO solutions are simple and easy to use, especially for low power applications with low output current where thermal stress is not critical. No external power inductor is required. 2. Low noise/low ripple applications. For noise-sensitive applications, such as communication and radio devices, minimizing the supply noise is very critical. 3. Fast transient applications. The linear regulator feedback loop is usually internal, so no external compensation is required. 4. Low dropout applications. For applications where output voltage is close to the input voltage, LDOs may be more efficient than an SMPS.

  14. References: Linear Tecnology - Application Note 140