Analysis of PRT self-excitation oscillation mode VRC soft switching power conversion technology

In the switching power supply circuit, the excitation and driving methods of the resonant type conversion switch element are defined into two categories, namely, the method with a dedicated excitation and driving circuit is called external excitation and driving; the excitation and driving method realized by the transformer feedback circuit is called self-excitation and driving. Here, the self-excitation oscillation method voltage resonance type soft switching power supply technology composed of the orthogonal transformer PRT feedback circuit is described.

1 Control technology of orthogonal transformer

For the control technology of self-excited resonant converter, the orthogonal transformer PRT using various ferrite cores is particularly important. Figure 1 shows the PRT structure, inductance characteristics and circuit graphic symbols. Among them, Figure 1 (a) is the old single-mouth ferrite core PRT; Figure 1 (b) is the new double-mouth ferrite core PRT; Figure 1 (c) is the PRT circuit symbol. After comparing their shapes and inductance characteristics, it is known that the magnetic path length of the new double-mouth PRT is longer than that of the old single-mouth type, and the magnetic resistance is increased. Due to the changes in the inductance Ln of the main coil N and the DC control current Ic of the control coil Nc, the Ln change amplitude and linear range of the new double-mouth type are expanded.

In Figure 2, the magnetic flux generated when the DC current Ic flows through the control coil Nc is φc, and the magnetic flux generated when the AC current I1 flows through the main coil N1 or N2 is φ1. If the direction of the arrow in Figure 2(a) is positive, the magnetic flux φc and φ1 on the magnetic circuits A and D are in opposite directions, and the magnetic flux is φ1-φc; while the magnetic flux φc and φ1 on the magnetic circuits B and C are in the same direction, and the magnetic flux is φ1+φc. The BH curve of the main coil N1 loaded on the magnetic circuits B and D in Figure 2(b) is equivalent to the hysteresis curve modulated by the change of Lc. Since the φ1 induced voltages loaded on the magnetic circuits A and B of the coil Nc cancel each other, no AC voltage is generated on Nc, so the current Ic signal of the PRT can be used as the magnetic flux on the control magnetic circuits B and D, and it can be used as a controllable inductance element to realize the control technology of the resonant converter. Figure 2(c) is the circuit symbol of this PRT.

2 Self-excited voltage resonance converter

When the switch element is disconnected, the voltage waveform applied to the switch element is a sinusoidal voltage generated during LC resonance, also known as voltage resonance. Various soft-switching power supplies can be constructed by combining a voltage resonance converter VRC circuit and a PRT. Commonly used self-excitation VRC control methods are as follows:

2.1 Parallel resonant frequency control method

Figure 3 is a switching power supply circuit of a single-tube self-excited oscillation VRC in parallel resonant frequency f0 control mode. Figure 3(a) is a circuit diagram, Figure 3(b) is a control characteristic diagram, and Figure 3(c) is an operating waveform diagram.

The structure of the PRT in Figure 3 (a) is shown in Figure 2. The coil N1 is connected in series with the inductor Ls of the pulse current converter PCC, and then connected in series with the parallel circuit (including the high-voltage BJT tube Q1 with VCBO>1200 V, the freewheeling diode D1, and the parallel resonant capacitor Cr). In addition, the full-wave rectifier coil N2 with a center tap is connected in parallel with the resonant capacitor Cs.

The self-excited oscillation circuit in the figure is composed of the following components and small circuits, such as the starting resistor Rs, the series resonant circuit (including the pulse current converter PCC with a coil of 1 turn, the current limiting resistor RB, the timing inductor LB, and the timing capacitor CB), and the parallel circuit (including the clamping diode DB and the base-emitter of Q1). It can be seen that the working waveform of this self-excited oscillation and driving circuit is a low-noise, sinusoidal waveform.

In addition, when RB is small, the switching frequency fS is determined by the series resonance value of LB and CB, as shown in formula (1):

In order to express the resonant frequency fo and output DC voltage Eo of the VRC circuit, after connecting the load resistor RL to the Eo terminal, the inductance values ​​of N1 and N2 are set to L1 and L2 respectively; the turns ratio is n=L1/L2; the voltage across the filter electrolytic capacitor Ci is Ei, then the derived analytical results of the equivalent circuit are fo and Eo. See formula (2), formula (3):

It can be seen from the formula that if fs is fixed and the variable inductance L1 of the PRT is controlled, the resonant frequency fo and the output voltage Eo can be controlled. Assuming fo>fs, ω=2πfs, as shown in Figure 3(b), according to the PRT control principle, if Ic is controlled, the value of the output voltage Eo can be stabilized.

When Q1 is turned off, the collector-emitter pulse voltage Vcp generated is the parallel resonant voltage of L1+L2 and Cr, and its peak value is 5 to 6 times that of Ei, but the switching loss when Q1 is momentarily off is small. When the load power Po="180" W, the AC input voltage VAC=220 V, and FS=50 kHz, the AC-DC power conversion efficiency can be obtained to be ηAC-DC=83%. From the working waveforms of the excitation current I1 of the PRT at the Ci end and the AC voltage V2 at both ends of Cs on the N2 side, it can be seen that it is basically close to a smooth sine wave, which can achieve low noise and meet practical purposes.

2.2 Resonant voltage pulse width control method

In Figure 3, the main coils N1 and N2 of the PRT are wound with 40 to 50 strands of φ100μm single wires, which not only ensure the insulation gap of the ferrite core, but also increase the volume. In order to reduce the volume of the circuit, it can be thought that if the inductance Ls of the PCC is controlled, Eo can also be controlled. Therefore, the PCC in Figure 3 is replaced with the PRT in Figure 1, and the VRC is formed by connecting the PRT in series on the primary side of the PIT, as shown in Figure 4. Figure 4(a) is a circuit diagram; Figure 4(b) is a working waveform diagram.

The principle of this circuit is that the primary side of PRT and PIT has a parallel resonant circuit of LR+L1 and Cr; the secondary side has a parallel resonant circuit of N2 inductor L2 and Cs. V1 and V2 in Figure 4 are two groups of parallel resonant pulse voltages. The current-driven transformer CDT is used to control the on-off operation of the switch tube Q1. Since the NR inductor LR of the PRT is controlled, the pulse width △T1 of the resonant circuit V1 can be controlled to achieve the purpose of stabilizing the output voltage E0. The voltage resonance waveform is shown in Figure 4(b). The working parameters in the figure are fs=110 kHz, the control range is T1=3~4.5 μs, the control width is △T1=1.5μs, and the power efficiency is ηAC-DC=83%.

In addition to the pulse width control VRC with the PRT connected to the primary side of the PIT in Figure 4, there is also a pulse width control VRC with the PRT connected to the secondary side of the PIT. The principle of this circuit is that the primary side of the PIT has L1 and Cr, and the secondary side N2 has two sets of parallel resonant circuits with inductors L2+LR and Cs. For the voltage regulation of Eo, the pulse width △T2 of the secondary side resonant voltage V2 can be controlled by controlling the NR inductor LR of the PRT. The typical operating parameters of the pulse width control VRC with the PRT connected to the secondary side of the PIT are fs=71.5 kHz, control range T2=7~12μs, and control width △T2=5μs.

The above two resonant voltage pulse width control circuits do not require the distance between the main coil NR, control coil NC and magnetic core of the PRT, so they can be miniaturized. In addition, the above VRC is the case of maximum load power Pomax ≥ 150 W. When the AC input voltage VAC = 220 V, in order to ensure the reliability of the switching element Q1, PIT and PRT, the input rectifier filter circuit is almost designed as a full-bridge rectifier.

Since the DC input voltage Ei supplied to the VRC circuit is high, along with VAC↑→Ei↑, the resonant current on the primary side of the transformer is ↓, and the voltage resonant pulse voltage Vcp on Q1 and Cr is ↑, and its Vcp can be as high as 1500 V or more. Therefore, Q1 and Cr should use components with a high voltage resistance greater than 1800 V, and the saturation voltage drop VCE(SAT), fall time tf and high-frequency characteristics of Q1 should also be limited. Therefore, the above circuit is improved to obtain the boost type composite voltage control mode VRC as shown in Figure 5.

2.3 Boost type composite voltage control method

Figure 5(a) shows a boost type composite voltage control mode VRC composed of the tertiary coil N3 of the PIT, the boost diode DB, the main winding tapped PRT (the main winding NR is divided into NR'' and NR" coils; NR'' is the boost control coil; NR" is the resonant voltage pulse amplitude control coil), and the filter electrolytic capacitor Ci. This is a composite voltage control mode VRC that can control the boost EB and the parallel resonant pulse voltage amplitude Vcp at the same time and achieve Eo stability using one set of control circuits.

Assuming that the forward conduction voltage of DB is VF, the total inductance of the PRT main winding NR is LR, and the inductance of the primary coil N1 of the PIT is L1, the boost voltage EB obtained from Ei and the primary measurement VRC is expressed as formula (4).

In the formula: Assume NR"+N3=1.2N1; variable inductor LR=0.2L1~1.2L1; EB is Ei~2Ei. By controlling the change of LR, a voltage change of 2 times the value of Ei can be obtained. When NR''=NR"=14T, the dynamic control range of LR is about 6 times. The working waveform of the load power Pomax is shown in Figure 5(b). For the change relationship between VAC and Pomax, the plot curve of Ei and EB is shown in Figure 5 (c). According to this control method, controlling EB can stabilize Eo. As VAC increases, the LR of the PRT is controlled to increase, so that the voltage resonance pulse peak Vcp on Q1 and Cr is fixed at about 700 V, so Q1 can use a low-voltage device with VCBO<900 V.

Typical circuit parameters: Pomax = 180 W, Pomin = 60 W, switching frequency is 100 kHz, Ci = 1 000 μF / 400 V, Ci'' = 1 000 μF / 250 V, Cr = 6 800 pF, C2 = 0.01 μF. When VAc = 220 V, the efficiency reaches ηAC-DC = 86%, which can basically achieve high efficiency and light and small structure. This VRC not only has high output power, small size and light weight, but also is a practical circuit with good control effect.

3 Conclusion

The comprehensive characteristics of this circuit are: high output power, Po>150 W; high power conversion efficiency, ηAC-DC>83%; wide allowable input voltage variation range, VAC=220 V (-20%～+10%), good control performance and wide application.

The self-excited oscillation soft-switching power conversion technology using an orthogonal transformer PRT has not only voltage resonance but also current resonance for the resonance mode. For the control mode of DC output voltage, there are parallel resonance frequency, resonance voltage pulse width, boost type, composite type and other control modes. However, for the current resonance type CRC (which is not in line with this topic and is omitted due to limited space), there are also control modes such as switching conversion frequency and series resonance frequency. They are all resonant soft-switching power conversion technologies based on the self-excited oscillation mode of controlling the PRT inductance to achieve automatic stable output voltage Eo.