In our previous experiment, we designed the 5V converter of AP65111AWU-7 SMD IC. In this experiment, we will explore a new IC to convert 12V to 5V DC and then compare the performance curves of both ICs.
We will use an LM2576 adjustable switching regulator, a Buck regulator with a fixed frequency of 52 kHz. The IC can provide a maximum output current of 3A.
Note: For detailed information about the DC-DC to Buck Converter and how it works, see our previous series on SMPS.
Basic design
Fig.1: Block diagram shows the operation of a switched mode regulator.
Basic principle
IC switching mode works based on the principle of power conversion and switching mechanism. They try to maintain equal input and output powers, but due to component losses between the circuits, the efficiency of these ICs does not reach 100%. However, they are more efficient than linear ones, which dissipate a lot of energy in terms of heat.
We can also design a switching mode regulator without using any discrete component switching mode IC as we designed in our previous series.
Components
Below are the pieces we used in our design.
Circuit Diagram
Fig. 2: Circuit Diagram of 12V to 5V Buck Converter Using LM2576 IC
Building the circuit
The LM2576 internally has a switching mechanism and error amplifier, which maintains the output voltage. Additionally, we want the following external components for your design.
Inductor and diode
These are the basic needs of the circuit; the job of the inductor is to store energy in the form of a magnetic field and on the other hand, the diode acts as a switch that completes the switching cycle.
Inductor selection
The chart below can be used to find the suitable inductor for our application. This graph is in the IC datasheet, which needs output current and E*T (V.us) to calculate the inductor value.
Figure 3: LM2576(HV)-ADJ
Step 1: The following equation can be used to calculate the above values
E*T = (Vin-Vout)*(Vout/Vin)*(1000/F(in kHz)) V. us
Desired parameters
Vout = 5V, Vin = 12V, F = 52kHz
After putting in all the values, we get
E*T = 55.7 V.us
Step 2: Selecting output current = 2A
Step 3: For an output current of 2A and ET value of 56 Vus, we can see the intercepted part in the graph and choose the inductor. For our application, we get a 100uH inductor from the graph, and the rated current of the inductor should be 1.15 times the output load current.
This results in L = 100uH/2.3A
Input/output capacitors
The input and output capacitor helps in filtering any high voltage spikes. Furthermore, the output capacitor plays a vital role in the off period of the switching cycle and provides sufficient power for the regulated output voltage.
Feedback Network
For the desired output voltage, we need a resistor divider network that will feed the output voltage to the feedback pin. The internal error amplifier transmits this signal and maintains a regulated voltage at the output.
Selecting feedback network
The following equation calculates the value of the resistor divider
V out =V reference (1+R 1 /R 2 )
V reference = 1.23 V, IC reference voltage
For V out = 5 V
We can take,
R1 = 4.7K and R2 = 1.5k
Practical observation
V on Input voltage = 12V, V off (no load) = 5.07V
Fig. 4: Efficiency vs. Efficiency Curve Current Output
Fig. 5: Load regulation curve for output voltage versus output current
Thermal management
Each component has its operating temperature range. To maintain the temperature below its operational limits, heat sinks and cooling fans are used, and sometimes TEC (thermoelectric cooler) is also used.
See our article “Power Supply Thermal Management” to learn about the parameters for selecting a heatsink or cooling fan.
Precautions
- A capacitor must be connected between the IN pin and ground to keep the DC input voltage regulated.
- The circuit capacitor must have a higher rated voltage than the input supply voltage. Otherwise, the capacitor will start to leak current due to excess voltage on its plates and will explode.
- Make sure all capacitors are discharged before working on a DC power supply.
- The rated current of the inductor must be 1.15 times greater than the output current.
- Do not provide an input voltage higher than the operating input voltage range of the IC.
- Do not short the output terminals; This will reverse the current flow in the IC and the IC will become faulty.
- Also, do not short the input terminals; This will generate a large current in the circuit and the circuit components will become faulty.
- Frequency effect
The higher the frequency of the IC, the greater the switching losses, which decreases efficiency. But the high switching frequency reduces the size of the energy storage element and improves the transient response.
PCB Design Guidelines
- Keep the energy strokes thick and short.
- Place the input and output capacitor as close as the input and output pin of the IC.
- Minimize the path length of the inductor and diode.
- Keep voltage and switching nodes away from each other.
Fig. 6: PCB design layout
Fig. 7: View of the PCB
Comparison between LM2576 and AP65111AWU
We made a 12V to 5V Buck Converter in our previous projects with AP65111AWU (SMD). As availability, we take an IC in the package through the application IC in the SMT package. Now it's time to compare the two so that you can select the best IC according to the application needs.
