It is possible to build a synchronous buck and boost power supply with 100 percent analog control for output regulation using the same microcontroller. In both cases, the implementation has the advantage of not using processor power, thus freeing up the kernel for more complex firmware. Also the analog loop has a much faster response to input voltage and load variations, making it useful in many applications.
The microcontroller in question is Microchip's PIC16F753. The buck and boost converters need the same set of peripherals: a complementary output generator; comparator; operational amplifier; 9-bit A/D converter; fixed voltage reference; slope compensation module; and PWM capture and compare module. Peripherals should be connected internally by firmware, thus reducing the number of external pins needed.
circuit diagrams
The input operating range for the buck converter is 8 to 16V DC. The figures for the output are 5V DC, 2A and 10W. The code size is 105 words, the RAM is 0byte, the available code size is 1943 words, and the available RAM is 128 byte. The measured efficiency at 2A is 94 percent.
Fig. 1 shows a block diagram of the synchronous step-down power supply. The output voltage should be regulated by peak current mode control and comparison with the reference voltage through the error operational amplifier (OPA). The result can then be entered into the peak current comparator. The internal current compensation module subtracts a software programmable ramp from the error amplifier output before the peak current comparator. The PWM capture and compare module provides a fixed frequency, fixed duty cycle control signal, and the output of the peak current comparator is selected as a second (level-based) source for the falling edge of the complementary output generator. (complementary output generator, COG).
The boost converter works in the same way and its block diagram can be seen in Fig. 2. However there are some differences in the specifications. In this case, the input voltage range is 3 to 5V DC. The outputs are the same, as is the size of the RAM. The code size is 99 words and the available code size is 1949 words. The efficiency at 2A is 87 percent.
How do they work
After configuring and interconnecting the peripherals, the control loop will work on its own, without the need for processor time. Peak current control techniques need slope compensation for duty cycles greater than 50 percent to prevent oscillation. For lower duty cycles, slope compensation will also help stabilize the control loop if the current shunt is small. The PIC16F753 has an internal slope compensation module that can be used to subtract a programmable ramp from the error amplifier output before it is fed into the peak current comparator.
For synchronous switching power supplies a short dead time is required for the transistor control signals to avoid current tripping. The COG can generate this signal from the oscillator frequency or from an analog delay chain. The delay chain allows the user to set the dead time with a resolution of 5ns, which is more suitable for small transistors. In this application the dead time was set to 30ns.
In the buck topology, the inductor current is equal to the load current. In order to be able to measure the peak current of the inductor by means of a derivation on the low potential side (low-side) it is necessary to add some modifications. Normally the shunt sees the filtered output current that is not usable by the peak current control technique. By connecting the output capacitors to ground through the shunt, ESR increases but the resulting waveform closely approximates the inductor current waveform. The drawback of this method is its slightly lower efficiency, but a tap on the high-side usually requires additional circuitry (current mirror or specialized IC), which adds to the cost. For the buck topology, the inductor current is equal to the input current. The inductor peak current was measured directly on a resistor placed between the transistor source and ground.
Input and output
Output current limiting is not built into the control loop and for such a function a second selected comparator should be used as the COG auto shutdown source. The output of the error amplifier is the inductor peak current limit, so keeping this low using a resistive divider helps if inrush current problems and catastrophic short circuits occur. The drawback of this approach is that the system gain is reduced and it responds more slowly to transients.
The pin out of the op amp is the same as the pin in of the slope compensation module, so the two peripherals can be used together without adding external connectivity. If a resistive divider is used to limit the output voltage of the op amp, it must be connected externally to the input pin of the fixed voltage reference (FVR) buffer. The input voltage to the boost converter should be connected to the microcontroller via a small boost diode at the output. In this way, when the output voltage increases, it will feed the microcontroller and the MOSFET driver.
This is more efficient because higher VGS will improve RDS(ON) and the range below 4,5V is problematic for most power transistors. This makes the FVR the only stable reference available and the circuit requires some changes to ensure that the loop reference voltage is always independent of the source or output voltage. Since the control loop reference voltage is derived from the D/A converter, this peripheral also needs a stable reference. The 1,2V FVR has been selected as the reference for the D/A converter to meet these requirements.
The boost topology provides a clear DC path between power supply and output via the inductor and rectifier diode, even if the switching transistor is blocked.
The current limiting loop can only prevent overcurrent until the switching frequency goes to zero. Catastrophic short circuits can occur beyond this point if a protection switch is not added. A second transistor can be placed on the low potential side of the output to disconnect the load in the event of a short circuit. For comparator-based short-circuit protection, the reference must be stable over the entire input operating range. Since the output current shunt voltage is generally too small to use directly with the 1,2V FVR, you need to connect to the outside through the FVR buffer and then through a resistive divider to get the reference voltage. desired for the comparator. By using the FVR buffer in this way, the op amp output must be used directly with the slope compensation module, without adding another divider.
It takes no processor time but also uses more pins and peripherals. For A/D converter based short-circuit protection, current shunt voltage and FVR are read in the firmware. The FVR voltage is needed to calculate VDD (when it is less than 5V), which in this case is the reference voltage of the A/D converter. While it doesn't use the added comparator, I/O pins, and external resistors, it will require some program space and processor time.
The drive has to be compensated for a given load and stability has to be verified for the whole range of operating conditions. Compared to using a specialized PWM controller chip, performance is similar but a PIC microcontroller adds flexibility. In addition, the analog control loop works autonomously, so the microcontroller core is completely free to execute user algorithms, measure power supply parameters or transmit relevant information.
Applications
The analog control loop makes the power supply fast enough for dynamic loads and input voltage variations. For current controlled loads such as LEDs or thermoelectric cells, voltage feedback can be replaced by average current feedback. The power supply can also be used for applications that require voltage and current control, such as CC and CV battery chargers.
The PIC16F753 D/A converter has 9-bit resolution, which translates to a minimum voltage step of 20mV with a half output divider for the buck converter and 50mV with a fifth output divider for the up converter.
The application requires an op amp, a comparator, and a D/A converter. The output of the D/A converter can be connected internally to the op amp, thus saving one pin. The CCP module generates a fixed frequency, fixed duty cycle signal for the COG. Depending on the option chosen by the user to limit the output of the OPA, the resistive divider has to be connected externally to the input of the FVR buffer. If the resistive divider is not used, only one pin is used instead of two. In this case, the output of the op amp, which is the same as the input of the slope compensation module, is configured as an analog pin and should not be used for any other purpose.
The input-only digital pin can be used as a button or for a similar function. During runtime, the programming data I/O pin and the other two pins are freed for user-specific functions.
Alternate Boost Converter
A digitally controlled step-up power supply can also be realized using the PIC12F1501, which features good efficiency at small loads, as well as hardware overvoltage protection, and uses a small number of components. The necessary peripherals are two channels of 10-bit A/D conversion, an FVR, comparator, numerically controlled oscillator, and complementary waveform generator. Peripherals are internally connected by firmware, thus reducing the number of external pins to three. Its block diagram can be seen in Fig. 3.
The output voltage and current are regulated with a proportional control loop. The output values are read by two A/D conversion channels and the control signal is adjusted accordingly. The numerically controlled oscillator uses fixed-time, variable-frequency pulses to adjust the frequency modulation on the duty cycle pulse.
Conclusion
Using Microchip's microcontrollers to create buck and boost converters, thus saving processing power for other tasks, has been described. All three examples shown only need a small set of peripherals to achieve their goals.
