Sometime we need step down the input voltage to be in lower level voltage. BUCK DC DC converter is a common name for step down 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.

A buck power stage can be designed to operate in continuous mode for load currents above a certain level usually 5% to 10% of full load. Usually, the input voltage range, the output voltage and load current are defined by the power stage specification. This leaves the inductor value as the design parameter to maintain continuous conduction mode. The minimum value of inductor to maintain continuous conduction mode can be determined by the following procedure. First, define I _O(crit) as the minimum current to maintain continuous conduction mode, normally referred to as the critical current. This value is shown in :

Second, calculate L such that the above relationship is satisfied. To solve the above equation, either relationship, D _{I L (+)} or D _{I L (–)} may be used for D _{I L} . Note also that either relationship for D _{I L} is independent of the output current level. Here, D _{I L (–)} is used. The worst case condition (giving the largest L min ) is at maximum input voltage because this gives the maximum D _{I L} .Now, substituting and solving for L min :

Using the inductor value just calculated will guarantee continuous conduction mode operation for output load currents above the critical current level, I _O(crit).

The value of output capacitance of a Buck power stage is generally selected to limit output voltage ripple to the level required by the specification. Since the ripple current in the output inductor is usually already determined, the series impedance of the capacitor primarily determines 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 gives guidelines for output capacitor selection.

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

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.

BUG

Actually I had make 2 BUCK Converters with different types of power switch. The first one is using an N-channel MOSFET and the second one is using a P-channel MOSFET. I had a problem to drive the MOSFET, when I was using an N-channel MOSFET (I used IRF630 the cheapest one) it would be problem when duty cycle is above 85%. MOSFET tend not work in proper, the output voltage will saturate and can not respond the input voltage alteration.   And when I was using a P-channel (IRF9630) and drive it with duty cycle below 10% the problem above would happen.

So I have to see and check the input voltage and the output voltage of the system. If the input and output voltage ratio is => 2:1 I use a P-channel MOSFET, and if the input and output voltage ratio is =< 2:1 I use an N-channel MOSFET.

I think it is quite difficult to drive MOSFET in a floating condition. If you have any idea and solution about this problem please tell me.