Understanding the Boost Converter Operation
A boost converter, a fundamental circuit in power electronics, elevates a low input voltage to a higher output voltage. The circuit comprises key components: an inductor (L1), a capacitor, a diode, and a switch (transistor). These components are controlled by a pulse width modulated (PWM) signal, dictating the on and off states of the switch. Each switch cycle consists of a period (T) encompassing the switch’s on-time (t1) and off-time (T – t1). During the on-time, illustrated in the lower diagram of Figure 1, the switch is closed, connecting the input voltage (Ue) across the inductor (L1). If the input supply (Ue) has sufficiently low impedance, it induces a linearly rising current (IL) through the coil, storing growing energy in the magnetic field.
Energy Transfer Mechanism
Upon the switch opening, the collapsing magnetic field of the coil generates a reverse voltage across the coil. This induced voltage is combined with the supply voltage, creating a forward current flow through the diode. In this phase, energy is stored in the capacitor. Essentially, the energy stored in the magnetic field of the coil, primarily contained within the ferrite core, is transferred through the diode to the capacitor during the switch-off stage. For those interested in delving deeper into this topic, a comprehensive introduction is available for further exploration.
Configuring the Step-Up Converter Circuit
The circuit in Figure 2 illustrates the configuration of the step-up converter using components L1, D1, C8, and MOSFET T1. An Atmel ATmega8-16PU microcontroller, programmed with specific firmware, generates PWM signals to control MOSFET T1. The PWM signal, emanating from pin PB1, operates at a frequency of 66 kHz, utilizing the internal fast-PWM mode. The output voltage is regulated by adjusting the mark/space ratio of the PWM waveform. To achieve this, the microcontroller needs to sense the output voltage level. This feedback occurs through a voltage divider network consisting of R6, R7, and P2. The preset adjustment is essential because the data sheet specifies a reference voltage level ranging from 2.3 to 2.9 V.
P2 allows for calibration, ensuring accuracy. If resistor values or the 43 kΩ value of R7 pose challenges, firmware adjustments can compensate. During setup, an accurate DVM can be used to measure the output voltage, and the preset can be tweaked until it matches the DVM reading. The microcontroller’s built-in A/D converter provides a resolution of 10 bits. Utilizing a voltage divider network of 47 kΩ (R7+P2) and 2.7 kΩ (R6), the setup achieves a measuring resolution of 46 mV. Consequently, the voltage reading displayed on the 2×16 character LCD changes in increments of 0.04 V or 0.05 V.
Implementing Current Limiting
Step-up converters employing this topology lack built-in current limiting features. To prevent overload, shunt resistor R5 is included in the ground output pin. The voltage drop across R5 is measured by a second A/D input of the controller. The firmware adjusts the mark/space ratio of the converter switching waveform to decrease output current before the converter enters a discontinuous mode.
Suppression of RF Noise and Operational Control
Networks formed by C10, C11, and R8 are designed to suppress RF noise on the analog A/D inputs. The LCD screen displays operational parameters, such as output voltage and current, through menu selection. The circuit integrates three push buttons: S1 resets the microcontroller, while S2 and S3 enable incrementing/decrementing control of the output voltage. Simultaneously pressing S2 and S3 activates the current limit mode, allowing these buttons to adjust the current limit setting. After a period of inactivity, the display reverts to showing the voltage. LED D3 signals the presence of input voltage; if it goes out, fuse F1 may have blown, indicating excessive current draw or a fault in the external power supply. D2 illuminates when the current limiter is active.
Putting it all together
The reset button S1 is fitted directly to the board because it should only be necessary to access it occasionally. Push buttons S2 and S3 are connected by flying leads to the pads on the PCB: Up, GND and Down. The buttons can be either two PCB mounted push buttons fitted to a small square of perf board or the larger panel mounted type fitted directly on the front face of the enclosure. The two LEDs should, of course also be mounted where they can be seen. Preset P1 provides contrast adjustment of the LCD module. Jumper JP2 enables the LCD backlight and can be replaced by a switch if required.
Cooling the MOSFETs and Attention to Capacitors
To maintain a proper operating temperature, a small finned heat sink is utilized for the MOSFETs. A heat sink with a thermal resistance of 21 K/W suffices for output currents up to 1 A. When using a standard radial-leaded or TO220 outline version of diode D1, it’s essential to check the corresponding datasheet for the correct polarity of the TO220 outline. Special attention must be given to electrolytic capacitors C7 and particularly C8 used in the switching circuit. Given the switching frequency of 66 kHz, it’s crucial to employ low-loss type capacitors specified in the parts list.
Standard electrolytic capacitors are unsuitable for this application. During lab testing, the LCD display board was fitted with a pin header strip and directly plugged into a box header strip on the PCB. However, when the unit is enclosed in a project box or another form of an enclosure, mounting the display underneath the PCB might be more practical. It’s advisable to place a small insulating material square between the display and the PCB to prevent potential short circuits, typically a thin sheet of Pertinax or Paxoline.
Microcontroller Programming and Switching Point Stability
An ISP connector (K5) is standard for microcontroller programming. The controller needs power via the 7805 voltage regulator IC2 during programming. Pin 2 of K5 supplies the programming adapter (e.g., AVRISP mkII) and determines the microcontroller’s supply voltage (3.3 V or 5 V). During programming, the controller’s outputs are undefined, so a jumper (JP1) is included in the connection to the MOSFET’s gate. It must be removed during programming to keep the MOSFET off. Leaving the jumper in place can cause the MOSFET to turn on through PB1, leading to a short circuit and blowing the fuse.
R3 pulls the gate to ground when JP1 is removed, ensuring it stays off. It’s crucial to remove this jumper before programming. R2 addresses switching point instability caused by the high gate capacitance of T1. The gate capacitance introduces a delay when the MOSFET switches on or off, resulting in increased power dissipation in T1. A higher switching current could remove the gate charge faster, leading to a quicker MOSFET switching time and reduced heat dissipation. The current supplied by a standard ATmega output is limited to about 30 mA, making it a relatively weak current source. The maximum output voltage level is restricted by the voltage rating of D1 and T1. Consider replacing these components to enhance the circuit specification. As it stands, the circuit serves as a basic step-up converter demonstration and encourages further improvements.
Source Code and Current Handling
The current version of the source code is written in BASCOM-AVR and is freely available on the Elektor website. Currently, it features a basic charge pump regulator, leaving ample room for enhancements. Key improvements encompass the implementation of a genuine lead-acid battery charger with multiple charge phases. The input current is approximately 3.5 times higher than the output current, necessitating a 5 A slow-blow fuse at the input.
During testing on two different lead-acid batteries, an interesting observation was made. For instance, with a current setting of 0.2 C (relatively high for a Lead-Gel battery), the battery voltage dropped rapidly after reaching the maximum voltage setting, prompting the charger to switch off. Consequently, careful monitoring of the charging process and understanding when to terminate the cycle is vital when using this software.
User Interface and Charge Phases
The existing software lacks a sophisticated user interface offering various battery charging methods. The project’s primary goal is to showcase how such a charger can be created. Due to its open design and software, it offers readers an opportunity to modify and implement their enhancements. Here’s a suggestion for a complete recharge cycle of a lead-acid (Gel) battery, which ideally comprises two to four phases. A partially discharged battery undergoes the ‘bulk-phase’ with a constant current (0.1 to 0.2 C is reasonable) until a terminal voltage of 2.4 V per cell is achieved (the firmware’s current form reaches up to this point, indicating about 80% charge).
Subsequently, the voltage is limited to the final terminal voltage, while the charge current is monitored until it drops below one-tenth of its maximum value, constituting the ‘absorption-phase.’ This phase nearly fully charges the battery to around 98%. In the final ‘float-phase,’ the terminal voltage is reduced to 2.23 V per cell, allowing the battery to stay connected without gas production.
Exploring Further Phases and Collaboration
You might wonder about the fourth phase. It’s necessary only when the battery is deeply discharged (below 1.75 V per cell). In this state, a small trickle charge restores the battery until it reaches its lower voltage threshold. If you’ve been working on software enhancements for this design or have made progress but reached a roadblock, consider visiting our project page to share your experiences and collaborate with others.