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Oscillator with PLL, half-bridge output and overcurrent protection
One of the most popular modern circuitry solutions for class D amplifiers is the half-bridge circuit. To control the output transistors of such a circuit, specialized drivers of the series IR2104 - IR2109 - IR2184 and others like them. With all the advantages of these devices, they do not have oscillation circuits, phase-locked loop (PLL) and protection. Such solutions are needed for resonant voltage converters such as DC-DC converters, Tesla transformer generators and devices for generating high reactive power.
The circuit proposed in this work stably generates and maintains the current phase necessary for resonance in a wide frequency range and a large scatter of the parameters of the output circuits. The circuit also has adjustable overcurrent protection. For example, with the data of the driving circuit indicated on the diagram, it can capture and hold the frequency in the range of 8-25 kHz and regulate the protection striking current in the range of 1-10 A. At the same time, the indicated ranges can be easily changed to any other by simply replacing the master RC-circuit. The supply voltage can vary from 24 to 50 V, which, with appropriate replacement of the supply resistors, can also have a different range (Fig. 1).
Fig.1. Schematic diagram of a half-bridge driver with a generator, PLL and current protection
The heart of the circuit is the DD1 chip, which contains a PLL generator 74HC4046. At its 4th pin, it generates rectangular pulses, which are fed to the input of the DA1 driver. But before that, the microcircuit captures the frequency and phase of the output current pulses and thus tunes the entire circuit to resonance. This is done using a current transformer TA1, which generates voltage pulses at its output proportional to the current flowing through it. These pulses are fed to the input of the analog comparator DA3.2, and from it to the input of the phase comparator, which compares these pulses with those that it generates itself. At its output, a signal is generated proportional to the phase difference of the two compared pulses, which pass through the low-pass filter R8C6 and correct the frequency and phase of the reference oscillator. When capturing the frequency and phase, the microcircuit signals a positive potential at the FC output, and hence the glow of the VD6 LED. The initial frequency of the generator in this microcircuit is set by the capacitor C3, which, by the way, can be proportionally changed for other ranges of the circuit. The frequency and phase locking are corrected by the trimming resistors R7, R12.
The received pulses are amplified by the driver of the lower and upper arms of DA1 and the output transistor switches VT1 and VT2. The drain of the latter includes two low-resistance resistances R3-R4, at the upper terminal (according to the diagram) of which a positive voltage is formed, proportional to the current passing through them. When a certain value adjusted by R5 is exceeded, the analog comparator DA3.1 is triggered and locks the SD control input of the DA1 driver, after which the output transistors are turned off for a while, determined by the R18C13 chain, and through the transistor connected here according to the emitter follower circuit, VT3, the VD7 LED lights up. This is how the circuit is protected by the output current, which protects the output transistors and the driver itself.
In half-bridge circuits, between the output of the control microcircuit and the gates of the output transistors, the use of a chain of parallel-connected diode and resistor (R1VD1 and R2VD2 according to the scheme) is shown. The resistor limits the reverse current, which necessarily occurs here due to the Miller effect [1], and thereby protects the DA1 microcircuit from overloads and latching [2], and the diode reduces the closing time of the field-effect transistor, which actually contributes to its less heating.
The two internal supply voltages necessary for the circuit are obtained using powerful resistances R16-R17 and a zener diode ZD1 (+12V), a voltage stabilizer DA2 (+5V). R16-R17 are selected for the circuit supply voltage - 48V, and can be proportionally changed for other values. It is only necessary to add that if the supply voltage is more than 100V, then instead of these resistances it is more rational to use a low-power DC-DC converter for 12V.
The proposed circuit design (Fig. 1) is a development of a well-behaved proven analog of a spark interrupter with a PLL.
Fig.2. Various options for enabling the GG1 generator
The standard connection of the circuit in Figure 1 is shown in Figure 2a, where it is conventionally designated GG1. The resonant capacitance Cr and the inductance Lr are connected to it. For example, for a highly efficient DC-DC converter, the circuit in Figure 2b is suitable, where the resistive load Rn is connected directly in parallel with the inductor. You can do the other way around: swap Lr and Cr, and connect the load in parallel with the resonant capacitor. If the device requires galvanic isolation between the power supply and the output, then a coil with a secondary winding is used: a resonant transformer (Fig. 2c).
Element base
List of circuit elements. Valid substitutions are given in parentheses:
  • DA1 - half-bridge driver IR2104 or IR2109. You can use the driver IR2184, but remember that it has a slightly different pinout;
  • DA2 - 5 volt voltage regulator L7805;
  • DA3 - two analog comparators in one package LM393P;
  • DD1 - PLL generator microcircuit 74HC4046;
  • VT1-VT2 - mosfet transistors. At supply voltages up to 80V, circuits work fine IRFP4568 (IRFP260), more than 80V - better suited 47N60 or similar;
  • VT3 - p-n-p transistor S8550;
  • VD1-VD5 - diodes UF4007;
  • VD6-VD7 - LEDs green and red glow color;
  • ZD1 - unidirectional suppressor 1.5KE12A;
  • TA1 - current transformer ACST-002. For other brands - you need to select R14, R15 and C7;
  • R3-R4 - 0.1 Ohm resistance and 5 W power;
  • R15-R17 - 2 W resistors;
  • C7, C10, C14 - polypropylene capacitors with an operating voltage of 250-400V;
The circuit must be connected to the load as shown in Figure 2. Setting up the circuit should start with a relatively low supply voltage, for example, 24V. In this case, the trimming resistance R5 must be brought to the extreme right (according to the diagram) position. By rotating the trimmers R7 and R12, it is necessary to obtain stable sinusoidal oscillations on the load. After that, you can increase the supply voltage to the operating value, and with the resistance R5 set the protection operation current, for example, by reducing the load resistance Rn.
The scheme presented here works very stably and reliably, but can you remove some elements from it and simplify it? In the next part of this note, we will offer just such a circuit design of a generator with a PLL, assembled on just one half-bridge driver microcircuit, the protection of which will be built on several other principles.
Materials used
  1. Wikipedia. Miller Effect
  2. Power electronics. Drivers for controlling power elements. [Website]