Chapter 08 - MPLAB® Mindi™ Analog Simulator - Peak Current Mode Control Buck-Boost Converters
This chapter demonstrates the functionality and the performance of Microchip’s monolithic buck DC-DC devices used in a buck-boost system, the MPLAB® Mindi™ analog simulator tool will be used in several examples.
8.1 Prerequisites
8.2 Buck-Boost Converter Experiments
The goal of these case studies is to understand the impact of input voltage and load current on the overall converter performance. There are applications in which a simple switching converter will be able to output a constant voltage (in this example 12 VDC) while the input voltage is either below, close to, or above the required output.
The proposed examples to analyze include a typical MCP16301 step-down (buck) converter application with the addition of a logic-level NMOS transistor, a gate driver, an extra Schottky diode, and few passives. For simulation with the MPLAB Mindi analog simulator, the integrated gate driver (from the following schematic used for the ADM00399 Evaluation Board) was replaced by a pair of bipolar transistors in totem pole configuration.
Figure 1
8.3 Case Study: MCP16301 Used as Buck-Boost Regulator
The goal of this section is to understand and analyze the MCP16301 in a buck-boost topology. Performing separate simulations for each input voltage to verify output voltage regulation is a tedious and unnecessary task if we consider the capability of MPLAB® Mindi™ to sweep certain parameters on the fly. Efficient use of the simulator can reduce the effort to only a couple of simulations during the preliminary application design.
8.3.1 MCP16301 Buck-Boost Simulation
Open the (MCP16301/H) Buck example startup schematic from Power Management > Switching Regulators > MCP16301. The next steps modify this standard buck topology schematic to the buck-boost configuration as seen in Figure 2.
Figure 2
Remove VEN supply, and tie EN to VIN.
Set RLOAD to the minimum output current (80 Ω in this case).
Increase the inductor’s inductance to 47 uH.
Place a Zener diode (BZX84-7V5) in series with the bootstrap diode.
Place a FDMA3028N NMOS Power FET, Q1 in Figure 2.
Place the NPN (Q2N2222) and PNP (MMBT2907) bipolar transistors to be used as a totem pole gate driver. For the NPN, click on Search under the Place > Semiconductors > NPN menu. Similarly, find the PNP.
Place two identical 2 kΩ resistors as divider for the gate driver.
Copy and paste the Schottky diode (B140) as a rectifier for the buck-boost output.
Edit RTOP according to the desired output voltage (140 kΩ for a 12V output).
Place a voltage probe on VIN and set it to display using a separate grid and graph (named OUTPUT).
Alter the VIN source to step from 5V to 30V with a 50 ms rise time and no delay.
Place a “Voltage Controlled Current Source with Limiter” (U2 in Figure 2) setting the Gain to 20m, the minimum output to , and the maximum output to 13.
Run a transient analysis with a Stop time of 50 ms.
Stack all curves to view the results. Compare them to the similar graphs in the “MCP16301 High Voltage Buck-Boost Demo Board User’s Guide”.
Figure 3
The input voltage sweeps from 5V to 30V while simultaneously increasing the load current from 250 mA to 750 mA. Throughout the sweep, the output voltage remains in regulation.
8.3.2 Testing the Application Output Regulation When powered by a Car Battery Supply and With a Stepping Load
First we must change the input voltage range according to the car environment, as shown in Figure 4.
Figure 4
The battery internal resistance is not included in this simulation. If you know the value of the battery internal resistance, you must add it on the schematic in series with the input voltage supply!
Delete the controlled current source, U2, and replace it with a current source.
Figure 5
Run the simulation and stack the three plots for VIN, VOUT and IOUT. Zoom as needed.
Figure 6
These plots demonstrate that for this input range, a load variation from 150 mA to 500 mA keeps the output overshoot and undershoots in an acceptable region of less than five percent. As expected, in the 500 mA load region the output ripple is a bit higher.
8.3.3 Additional Exercises
Observe the inductor current ripple and maximum values by adding an inline probe.
Figure 7