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SMPS Buck Converter Design Example Part 1 of 2

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SMPS Buck Converter Design Example Part 1 of 2.jpg

This example converts a 12 volt power source to an output of 5 volts with a 2 amp load. Switching frequency is selected at 400 kHz. The current repo will be limited to 30% of maximum load. Here is a schematic for the Buck converter for which we will select component values. This example either a P-channel or N-channel MOSFET may be used. The choice will be based on the cost and complexity issues. Starting with the basic equation for current flow through an inductor, V = DI/DT. We rearrange the terms to calculate L so that V = DT/DI. For the design example, the calculated inductor value is 12 uh. From a catalogue we can select a 12 uh 3 amp conductor that has a resistance of .037 ohms. The power dissipated due to copper losses is I of the load squared times the ESR. Here it is .15 watts. Note the benefit of information on core loss characteristics are often difficult to find. The voltage ripple across the output capacitor is the sum of the ripple voltages due to the effective series resistance. The voltage sag due to load current that must be supplied by the capacitor as the inductor is discharged and the voltage ripple due to the capacitors effective series inductance. VSL specification is usually not specified by the capacitor vendor.

For this example, we will assume that the ESL value is zero. As switching frequencies increase. The ESL specification will become more important. Equation shown here shows that we are solving an equation with multiple unknowns. ESR, C, and ESL. A reasonable approach is to remove terms that are not significant and then make a reasonable estimate of the most important parameter that you can control. That is the ESR value. The capacitor ESR value was selected from a vendor’s catalogue of SMPS rated capacitors. Given a ripple current and target output voltage ripple, an ESR value of .030 ohms as selected from a list of capacitors rated for 0.6 amp ripple of current.

Now we will calculate the required capacitance of the output capacitor given the desired output voltage ripple defined as 15 millivolts. The term of the equation denominator shows that the capacitor ESR rating is more important than the capacitance value. If the ESR is too large, the voltage due to the ripple current will equal or exceed the target output voltage ripple. You will have to divide by zero issue indicating then an infinite output capacitance is required. If a reasonable ESR is selected, then the actual capacitance value is reasonable and an electrolytic capacitor with the required ESR has a capacitance of 1200 uf which easily meets the minimum requirements.

There are specialty polymer electrolytic capacitors with 47 uf and an ESR of. .025 ohm that are much smaller but cost more. The estimated power dissipation in the output capacitor is .01 watt. Estimate the maximum ESR value your application can tolerate due to output voltage ripple requirements. Make sure the capacitor is rated for the ripple current. If operating in high switching frequencies greater than 1 megahertz, contact the manufacturer to determine the ESL specifications for the capacitors you are considering using.

When considering ESL, also include ESL of the PCB traces that interconnect the capacitors to the other components. Rarely is the capacitor’s capacitance value an issue when operating at moderate frequencies of less than 100 kHz. Exotic capacitors such as special field electrolytics, large ceramics, or film capacitors are useful in space limited applications. But these advanced capacitors feature at extremely low ESR for their small size. But their small size applies very limited capacitors. Limited capacitance and advanced capacitors may create issues of system stability and voltage droop.

The worst thing is ripple current occurs when a duty cycle is 50% and the worst case ripple current on the input of the buck converter is about one half of the load rate current. Like the output capacitor, the input capacitor selection is primarily dictated by the ESR requirement needed to meet voltage ripple requirements. Usually the input voltage ripple requirement is not as stringent as the output voltage ripple requirement. In this example, the maximum input voltage ripple is defined as 200 millibles. The input ripple current reading for the input capacitors may be the most important criteria for selecting the input capacitors. Often the input ripple current will exceed the output ripple current.

A 16 volt 470 uf electrolytic capacitor that meets the ESR ripple current requirements is chosen for this example. The estimated power dissipation in the input capacitor is 5 ripple square times ESR which equals .12 of a watt. The diodes average current is equal to the load current times the portion of the time the diode is conducting. The time diode is on is one minus the duty cycle. The maximum reverse voltage on the diode is VN which is 12 volts in this example. The current voltage ratings are low enough that a small Schottky diode can be used for this application. By using a Schottky diode, switching losses are negligible. The afford voltage drop for the selected diode is about 0.4 volts at the peak current of 2 amps. The estimated diode power dissipation is 0.47 watts.

To simplify the gate drive circuitry for the MOSFET, a P-Channel device was selected. An N-channel device would require a gate drive circuit that incorporates a method to drive the gate voltage above the source. Cost of loble translator and a charge pump will outweigh the savings using an N-channel device versus using a P-channel device. A 20 volt MOSFET was not selected because the available devices in the catalogue had maximum gate to source voltage ratings of only 12 volts. With a 12 volt input voltage the applied gate volt might exceed the device specifications. If a 20 volt MOSFET was used it would be a good design practice to incorporate a voltage clamp in a peak driver circuit. Here a 30 volt device was selected on the basis of the 20 volt gate to source specification. The device current rating is more than necessary. But the low RDS on specification minimizes temperature rise. Most small surface mount packages have thermal resistance of about 50 degrees Celsius per watt. With a calculated power dissipation of 0.3 watts, the MOSFET should experience a temperature rise of only 15 degrees C.

This buck converter design example has a calculated efficiency of 90.5 %. The diode losses represent almost one half of the total losses. If the diode’s ford voltage drop can be lowered the converters efficiency can be raised. This buck converter design example is called asynchronous buck converter because the diode commutation switching is independent of the MOSFET switching.

Watch & Read: http://dev.emcelettronica.com/smps-buck-converter-design-example-part-2-2

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