The resistors were calculated using the following formula, which is given in the data sheet:. Low values would mean that the converter will be less efficient when light loads are connected. However, using this option, the converter would be less susceptible to noise. High values would have the opposite effect, as can be seen in the graph below. An additional consideration is that choosing resistance values for the divider that are outside of the recommended range as specified in the data sheet can result in undesirable effects, such as a change in the output voltage due to feedback leakage current, as shown below.
As this design is intended for use with higher loads, the feedback resistors that were considered are low values. So now that we have selected the R1 resistor, this just leaves us needing to find the value of the R2 resistor using the equation below. A feedforward capacitor with a value of nF has been added to increase the overall stability of the feedback circuit:. The MIC controller has an adjustable frequency, which means we can set the required operation switching frequency using external discrete components.
This is typical for many analog components: a voltage divider is used to drop down some voltage to a specific value, and this voltage is read into the buck voltage controller. The switching frequency can be adjusted between kHz and kHz by changing the resistor divider on the FREQ input: The frequency can be calculated using the following formula:.
Selecting a higher switching frequency will reduce the required output capacitance and inductance values and reduce the actual size of output capacitors and inductors. But at the same time, the higher the frequency, the lower the efficiency becomes. For this design, a frequency of kHz was chosen as we have sufficient available space for component placement, and we do not want to significantly reduce efficiency.
As we now have the switching frequency we want and the corresponding R20 resistor value, we can calculate the one resistor that is left, which is R19, using the equation below:. As we are designing a switching voltage controller that can supply 6 A of output current, we do not want to exceed that limit by much more. This is because it could damage our controller, the voltage source, or even the load. The current limit is set by a resistor, the value of which can be calculated using the following formula from the data sheet.
The current limit that was chosen is 7 A because we do not want to exceed the output current by too much, but at the same time, we do want our power supply to be able to generate the required 6 A of current and not to turn off if there are any small rises in output current caused by transient current spikes. In addition, a parallel capacitor was connected to increase the accuracy of the current sensing circuit and provide more stable protection as it filters the switching node ringing during the off-time prevents the current limit function from false activation.
The capacitor value should be chosen so that the RC time constant would be much less than the minimum off-time of the IC. Alternatively, a calculator can be chosen for this task. There is plenty of online calculators available, so do not be shy to use them as it can save a lot of time and sometimes prevents mistakes.
A small capacitor of 10 pF was chosen, and the time constant of the RC circuit was 0. Also, this IC includes hiccup mode short circuit protection. This type of short circuit protection is very reliable because it shuts down the regulator IC if a short circuit is detected. When the short circuit condition is removed, the IC recovers and starts working again.
Firstly, these are parameters which should be considered:. However, using a MOSFET with a Vgs that is too low can result in false switching, which can adversely affect the switching power supply output.
The higher the on-resistance is, the greater the voltage drop will be between the source and drain, and consequently, a larger voltage drop will result in the need for more heat dissipation. By taking the on-resistance and the maximum current we are using, we can calculate the maximum power dissipation for the MOSFETs.
Minimum and maximum duty cycle values were chosen to allow calculation of the maximum power dissipation that can occur at the rated specifications of the designed power supply. The other characteristic is the total gate charge. The gate drive circuit supplies the gate charge for the MIC From the formula, we can see that the lower the total gate charge is, the better this will be as it will require less current to turn the MOSFET on and off, and therefore increase the efficiency.
The total power requirements for the gate drive can be calculated by using the following formula:. As we are designing a power supply with a maximum input voltage of 14 V, it is not difficult to find MOSFETs that can withstand such voltages.
This circuit has a diode and a capacitor connected in series:. It is cheap and relatively compact. Also, this particular diode was recommended in the design of the MIC evaluation board. The capacitor is typically nF. In common with the MOSFET selection, the inductor selection for high current switching power supplies is critically important.
First of all, we need to calculate the required inductance for our switching supply using the following equation:. Note that this ignores the voltage drop across the MOSFETs, which is generally small enough to ignore at the voltages we're looking at here. Now that we have calculated the inductor value of 7. An inductor with this specific value that was also rated for the required high current was not found in the product library.
This inductor has a current rating of 7. It is essential that these inductor parameters are higher than the peak current that we want to draw from our power supply. Input and output capacitors are also critical components for any switching power supply, as they provide noise filtering and the bulk capacitance for energy sourcing and decreasing the voltage ripple.
The input and output capacitor values have been selected from the data sheet recommended design. It was seen that our requirements were quite similar to those that have been used in this design. The only difference is that instead of the 6 A output current, a value of 10 A has been used:.
However, instead of using one big uF capacitor, two 68 uF SMD electrolytic capacitors were used for the buck capacitance. Also, three 2. Nevertheless, for the selection of the input capacitors, it is not only the value of capacitance that is important, but other parameters are also very important, including:. Then we need to calculate the power which will be dissipated through the capacitor, which is calculated as follows:.
To calculate the dissipated power, we first need to determine the total ESR for the input capacitors. However, the ESR value for the selected ceramic capacitors was not clearly defined. So, the impedance graph from the relevant capacitor page was taken, and the resonance frequency was chosen so that we can see the minimal impedance value, which is the DC resistance or ESR value of the capacitor.
The total ESR value of the whole network can now be calculated using the parallel resistance formula, or alternatively, you can calculate this using an online parallel resistor calculator. For the voltage rating, we must choose the right type of capacitors. If you decide to use tantalum capacitors, you need at least double the voltage rating than the maximum input voltage. The only difference is that instead of using an expensive uF ceramic capacitor, two cheaper 47 uF capacitors were used.
Now that we know the total ESR of the output capacitors, we can calculate the output ripple voltage, the RMS output current, and the maximum power dissipated by these capacitors using the following equations. The values of the required capacitor and resistor can be calculated using the formula from the datasheet; however, in this case, the recommended nF ceramic capacitor and As this design is for a standalone power supply, these functions are not required and are put in the appropriate default state by pulling high the pins in the circuit using pull-up resistors.
Now we can finally see the entire schematic of the designed DC-DC buck controller power supply using our calculated values:. Now that the circuit design has been completed, it is good practice to simulate the circuit whenever possible. This free demo version has some design and simulation functionality restrictions, but it works great for this design. After finding and downloading the pre-made MIC DC-DC buck controller design, changes can then be made corresponding to our designed schematic, as shown below.
As we can see from the simulation, the IC has a soft start function that enables the circuit to start operating after a delay of 4 ms by holding the Enable pin Green LOW for this time. Simulating the feedback pin voltage is a good way to select the best feedforward capacitor value as the data sheet suggests a pretty wide range of values from 1 nF to nF. Choosing the right value is very important as it directly influences the output voltage ripple.
Performing circuit simulation is a great way to try out your designed circuit and discover what power supply parameters influence the components. I made sure my graduate engineer fully understood the implications of modifying values as part of the teaching exercise.
Also, by observing the effect due to the absence of one or more recommended components, you can find how much they affect the operation. Output current: up to 3A; 4. Conversion efficiency: up Output Voltage :DC 24V 3. Output Current Output PowerW Max 5. Auto start voltage will be pulled down to 7V or less, and engine will at high speed when the voltage up to 15V or higher. It is hard to work for 12V electrical equipment, this automatic buck boost module can solve this problem,.
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