Sometime we need step up the input voltage to be in higher level voltage. A linear power supply can not do this, but switching power supply can do this. Even more its better efficiency. BOOST DC DC converter is a common name for step up Switch Mode Power Supply (SMPS). You can read more from Switch Mode Power Supply (SMPS); an efficient and a tricky power supply article.

Fig. BASIC SWITCHING CONVERTER

I had made this device to fulfill my final task about Solar Home System Battery charger using Optimum Power Track Control Instrument. My device here is only a DC DC converter block. The switching/ PWM generator and sensor interface (voltage divider, R shunt/I sense etc) devices are not included. I just gave terminal inputs for those. I used this device to evaluate and analyze SMPS.

For switching device you can use a function generator to generate square wave signal that performing PWM. Or you can use AT89SXX Microcontroller to generate PWM. You may read and learn from Program the Microcontroller AT89S using C language to Generate Pulse Width Modulation. For a low cost PWM generator you can use a single PWM control IC  e.g. TL494, TL598, SGx524 etc. Usually, those PWM control ICs have a voltage comparator for error input to control the output voltage, so you may not build the voltage comparator.

To drive MOSFET (Drive_in) using AT89S, you must use a MOSFET Driver IC because the PWM that generate by AT89S is only in TTL and deliver small current. It is not suitable to drive a large capacitive load as a MOSFET with high slew rate. I used TC427, but you may use another MOSFET driver e.g. MC34152, MAX4420, LM2725, SN75372 etc.

In switching power supply power stages, the function of inductors is to store energy. The energy is stored in their magnetic field due to the flow of current. Thus, qualitatively, the function of an inductor is usually to attempt to maintain a constant current or, equivalently, to limit the rate of change of current flow.

The minimum value of inductor to maintain CCM can be determined by the following procedure. First, define I _O(Crit) as the minimum output current to maintain CCM, normally referred to as the critical current. This value is shown in Figure below :

Since we are working toward a minimum value for the inductor, it is more straightforward to perform the derivation using the inductor current. The minimum average inductor current to maintain continuous conduction mode is given by:

Using the inductor value just calculated ensures CCM operation for output load currents above the critical current level, I _O(crit). Always try to use a low EMI inductor with a ferrite type closed core. It would be toroid or encased E coreinductors. For building the inductor you may find from Designing Inductor for DC DC Converter.

In switching power supply power stages, the function of output capacitance is to store energy. The energy is stored in its electric field due to the voltage applied. Thus, qualitatively, the function of a capacitor is to attempt to maintain a constant voltage.

The output capacitance for a boost power stage is generally selected to limit output voltage ripple to the level required by the specification. The series impedance of the capacitor and the power stage output current determine the output voltage ripple. The three elements of the capacitor that contribute to its impedance (and output voltage ripple) are equivalent series resistance (ESR), equivalent series inductance (ESL), and capacitance (C). The following discussion gives guidelines for output capacitor selection.

For discontinuous inductor current mode operation, to determine the amount of capacitance needed, the following equation is used assuming all the output voltage ripple is due to the capacitor’s capacitance.

In many practical designs, to get the required ESR, a capacitor with much more capacitance than is needed must be selected.

For continuous inductor current mode operation and assuming there is enough capacitance such that the ripple due to the capacitance can be ignored, the ESR needed to limit the ripple to Δ V _O V peak-to-peak is:

Ripple current flowing through a capacitor’s ESR causes power dissipation in the capacitor. This power dissipation causes a temperature increase internal to the capacitor. Excessive temperature can seriously shorten the expected life of a capacitor. Capacitors have ripple current ratings that are dependent on ambient temperature and should not be exceeded. Referring to Figure 3, the output capacitor ripple current is the output diode current, I _CR1 , minus the output current, I _O . The RMS value of the ripple current flowing in the output capacitance (continuous inductor current mode operation) is given by:

ESL can be a problem by causing ringing in the low megahertz region but can be controlled by choosing low ESL capacitors, limiting lead length (PCB and capacitor), and replacing one large device with several smaller ones connected in parallel. Three capacitor technologies: low-impedance aluminum, organic semiconductor, and solid tantalum are suitable for low-cost commercial applications. Low-impedance aluminum electrolytics are the lowest cost and offer high capacitance in small packages, but ESR is higher than the other two.

Organic semiconductor electrolytics, have become very popular for the power-supply industry in recent years. These capacitors offer the best of both worlds—a low ESR that is stable over the temperature range, and high capacitance in a small package. Most of the OS–CON units are supplied in lead-mounted radial packages; surface-mount devices are available, but much of the size and performance advantage is sacrificed. Solid-tantalum chip capacitors are probably the best choice if a surface-mounted device is an absolute must. Products such as the AVX TPS family and the Sprague 593D family were developed for power-supply applications. These products offer a low ESR that is relatively stable over the temperature range, high ripple-current capability, low ESL, surge-current testing, and a high ratio of capacitance to volume.

The output diode conducts when the power switch turns off and provides a path for the inductor current. Important criteria for selecting the rectifier include: fast switching, breakdown voltage, current rating, low forward-voltage drop to minimize power dissipation, and appropriate packaging. Unless the application justifies the expense and complexity of a synchronous rectifier, the best solution for low-voltage outputs is usually a Schottky rectifier. The breakdown voltage must be greater than the maximum output voltage, and some margin should be added for transients and spikes. The current rating should be at least two times the maximum power stage output current (normally the current rating will be much higher than the output current because power and junction temperature limitations dominate the device selection).

The voltage drop across the diode in a conducting state is primarily responsible for the losses in the diode. The power dissipated by the diode can be calculated as the product of the forward voltage and the output load current. The switching losses which occur at the transitions from conducting to non conducting states are very small compared to conduction losses and are usually ignored.

In switching power supply power stages, the function of the power switch is to control the flow of energy from the input power source to the output voltage. In a boost power stage, the power switch  connects the input to the output filter when the switch is turned on and disconnects when the switch is off. The power switch must conduct the current in the inductor while on and block the full output voltage when off. Also, the power switch must change from one state to the other quickly in order to avoid excessive power dissipation during the switching transition.

Other than selecting p-channel versus n-channel, other parameters to consider while selecting the appropriate MOSFET are the maximum drain-to-source breakdown voltage, V _(BR)DSS , and the maximum drain current, I _D(Max) . The MOSFET selected should have a V _(BR)DSS rating greater than the maximum output voltage, and some margin should be added for transients and spikes. The MOSFET selected should also have an I _D(Max) rating of at least two times the maximum inductor current. However, many times the junction temperature is the limiting factor, so the MOSFET junction temperature should also be calculated to make sure that it is not exceeded.

At unibase frequency at 200kHz, darlington transistor can be used with minimum bandwidth at 1 Mhz. For example 2N6836 with switching frequency maximum at 10 Mhz/BDW42 with frequency maximum 4 Mhz.