LCD Display Inverter

This article will show how to design a circuit to regulate the current on a high-power light or motor. The device operates with an MCU, which ensures that the electrical load is driven with a PWM signal. The switching elements are represented by SiC MOSFETs.

One of the best techniques for changing the brightness of a high-powered light bulb or motor is pulse-width modulation (PWM). In automotive Electronic systems, control units have used PWM commands to control and manage various actuators for some time. For example, the brightness of diesel pressure regulators, electric fans and headlamps is managed using PWM signals. By driving the load with a periodic signal, the efficiency of the circuit is very high, and all the power generated can be transferred to the load, that is, the losses are almost zero. By using SiC MOSFETs as switching elements, the overall efficiency will be higher.

equipment
The circuit to be discussed in this article is a simple DC power regulator that can withstand a strong 24V load. Obviously, the voltage can be changed by adjusting the characteristics of the PCB. It can be used to change the brightness of lamps or to speed up or slow down DC motors. Logic operations are performed by the MCU. The adjustment operation of the power supply is managed by two buttons. The magnitude of the duty cycle is monitored by an LED diode.

PWM signal
A PWM signal is a square wave with a variable “duty cycle” (Figure 1), which can be used to control the power drawn by an electrical load (in this case, an actuator or motor) by modulating the duty cycle. PWM signals are characterized by a fixed frequency and variable duty cycle. “Duty cycle” is the ratio of the time the square wave is “high” to the period T, where “T” is the inverse of the frequency: T=1/f. E.g:

The square wave corresponding to 50% duty cycle remains high for 50% of the time and low for the remaining 50%;

A square wave corresponding to a 10% duty cycle remains high for 10% of the time and low for the remaining 90% of the time;

The square wave corresponding to 90% duty cycle remains high for 90% of the time and low for the remaining 10%;

The signal corresponding to 100% duty cycle is always high;

The signal corresponding to 0% duty cycle is always low.

To be more clear, if the last two cases above are considered, a duty cycle equal to 0% indicates zero pulse duration (no signal in effect), while a value close to 100% indicates maximum signal transmission, i.e. the device under control Get full, constant power.

Figure 1: PWM signals and their effect on the load.

block diagram
Figure 2 shows the block diagram of the system. The MCU manages logical operations and receives commands from operators. It can also generate a PWM (low power) signal to drive the pre-driver. The latter amplifies the current signal and passes it to the driver, which in turn controls the load.

Figure 2: System block diagram.

Electrical schematic
The wiring diagram can be seen in Figure 3. The system is powered by approximately 30V. It is then stepped down to 5V by three voltage regulators (7824, 7812 and 7805) for the MCU logic. This technology limits heat compared to just using the 7805. The GP0 port of the PIC 12F675 drives an LED diode and acts as a monitor for the PWM signal. The GP1 port controls a pre-driver consisting of an IRL540 power MOSFET – this is especially useful for applications using an MCU, since the energy supplied to the “gate” is very low at this time. The “drain” terminal of the first MOSFET drives the second SiC MOSFET, switching the current across the load (resistive or inductive). Two fast diodes eliminate overvoltages from inductive loads. It is also possible not to use them as SiC MOSFETs are well protected, but it is better to consider using them. If resistive loads are used, they can be removed from the circuit. The two normally open buttons are connected to the GP4 and GP5 ports of the MCU through corresponding pull-down resistors, which are guaranteed to be low if they are not pressed.

Figure 3: Electrical schematic.

electronic Component
The electronic components of the circuit are listed below. They are not in short supply and can be easily found in the market. Figure 4 shows the pinout of the various components.

resistance:
R1: 330Ω

R2: 10kΩ

R3: 10kΩ

R4: 100Ω

R5: 10kΩ

R6: 47kΩ

R7: 220Ω, 5W

capacitance:
C1: 100nF

C2: 100nF

C3: 100nF

C4: 100nF

C5: 100nF

C6: 100nF

C7: 1,000µF electrolytic capacitor

semiconductor
D1: Red LED, 5mm circumference

D2: fast recovery diode RFN5TF8S

D3: fast recovery diode RFN5TF8S

Q1: MOSFET SiC UF3C065080T3S

Q2: MOSFET IRL540 (not IRF540)

Miscellaneous:
U1: PIC12F675_P MCU

U2: LM7812CT voltage regulator

U3: 7805 Voltage Regulator

U4: LM7824CT voltage regulator

F1: Fuse, 40A

J1: terminal block

J2: terminal block

S1: Normally open button

S2: Normally open button

Figure 4: Component pinout.

PCB
To make a prototype, it is necessary to design the PCB with the traces shown in Figure 5. Even though it is very simple, we strongly recommend using photolithography for more reliable and professional results. Once the base is prepared, it needs to be drilled with a 0.8mm or 1mm drill bit corresponding to the pads, increasing the precision of the pads associated with the integrated circuit. To increase the thickness of the traces for better heat dissipation, tin can be melted over them.

Figure 5: PCB.

components
Now you can start soldering the components (Figure 6). Start with low-profile components such as resistors, capacitors, and sockets, and then proceed to larger components such as terminal blocks, LED diodes, MOSFETs, fuses, and electrolytic capacitors. Particular attention should be paid to polarized components. Use a small soldering iron with a power of about 30W when soldering, taking care not to overheat electronic components that cannot withstand excessive heat. Finally, attention needs to be paid to the pinout of the IC and its socket.

Figure 6: Component placement and 3D view of the circuit.

firmware
This article ends with a source listing (.BAS)—written in BASIC using the GCB (Great Cow Basic) compiler—and an executable (.HEX). After the initial configuration of the fuses and I/O ports, an infinite loop is entered, checking the logical state of the two buttons. When the first button is pressed, the duty cycle decreases; when the second button is pressed, the duty cycle increases. The percentages of duty cycle are 10%, 30%, 50%, 70% and 90%. Of course, other values ​​can also be added according to the program specification. Due to the low speed of the PIC’s internal clock (4MHz), the timing of the wait states cannot be parameterized by variables. Instead, dedicated subroutines with different percentage duty cycles have been created. In this case, the frequency of the PWM signal generated by the firmware is about 2kHz. Waiting for pauses can be parameterized and code optimized using a faster PIC. Low frequency PWM may produce audible cues on inductive loads. However, this problem does not exist on resistive loads.

circuit simulation
It is interesting to observe the behavior of the circuit at the switching point and to study the work of SiC MOSFETs. Figure 7 shows the PWM signal waveforms at 50% duty cycle for the following points:

PWM signal on GPIO1 port of MCU

PWM signal on drain of MOSFET IRL540

PWM signal on drain of SiC MOSFET UF3C065080T3S

Figure 7: PWM signal waveforms at different points.

Figure 8 shows the waveforms of the PWM signal at the output of the MCU at various duty cycle percentages (10%, 30%, 50%, 70%, 90%).

Figure 8: Waveform plots at different duty cycle percentages.

circuit efficiency
In terms of power transfer, the efficiency is very high when using SiC MOSFETs. This efficiency is usually considered good, but unfortunately, the presence of pre-drivers can make it less. Figure 9 shows a graph of the overall efficiency of the circuit, depending on the load applied to the output. To improve circuit efficiency, try slightly increasing the value of the MOSFET IRL540 drain resistor R7 to ensure that the SiC MOSFET turns on without problems.

Figure 9: Circuit efficiency versus applied load.

It is interesting to measure the RDS(on) value of a SiC MOSFET directly from the various operating points of the circuit during the turn-on of the element. According to Ohm’s law, we have:

Figure 10 confirms the values ​​given in the official data sheet.

Figure 10: Measurement of RDS(on) values ​​for SiC MOSFETs.

UF3C065080T3S SiC MOSFET
UnitedSiC’s cascode products combine its high-performance G3 SiC JFET with a cascode-optimized MOSFET to produce the only standard gate-drive SiC device on the market today. This series not only has extremely low gate charge, but also has the best reverse recovery characteristics of any device of similar rating. When used with the recommended RC snubber, these devices are ideal for switching inductive loads, and they are also ideal for any application requiring standard gate drive. Features include:

Typical RDS(on) is 80mΩ

The maximum working temperature is 175℃

Excellent reverse recovery characteristics

low gate charge

Low inherent capacitance

ESD Protection, HBM Class 2

Its typical applications are:
Electric vehicle charging

PV Inverter

switching power supply

Power Factor Correction Module

motor driven

Induction heating

Thanks to the accompanying SPICE files, SiC MOSFETs can be used with the most important electronics simulation programs.

Summarize

PWM control allows for better qualitative performance of electric actuators such as motors and lights. Although the brightness can be changed at will, the quality of the light is better. Engine torque is high even at low revs. The circuits presented in this article are primarily intended for guidance and provide a foundation for further research in this area. It is useful to be familiar with PWM. Clearly, designers can make improvements in power and efficiency. However, it is recommended not to move the supplied power to the maximum to avoid overheating the circuit.