Design and Application of Frequency Conversion Based on UC3846

The main features of UC3846 are: (1) automatic feedforward compensation; (2) Programmable pulse by pulse current limit; (3) Push pull configuration, automatic symmetry correction; (4) Parallel operation capability of modular power supply system; (5) Enhanced load response characteristics; (6) Wide common mode girth differential current detector; (7) Double pulse suppression; (8) 500mA double totem column output; (9) Low voltage latch; (10) Soft start; (11) Output shutdown protection; (12) The operating range is up to 500KHz. The following is its internal block diagram:

Pin 1 is the current level setting terminal; Pin 2 is the reference voltage output terminal; Pin 3 is the inverting input of the current detection amplifier; Pin 4 is the in-phase input of the current detection amplifier; Pin 5 is the in-phase input of the error amplifier; Pin 6 is the inverting input of the error amplifier; Pin 7 is error amplifier feedback compensation; Pin 8 is the external capacitance port of the oscillator; Pin 9 is the external resistance port of the oscillator; Pin 10 is the synchronization port; Pin 11 is the a output of PWM pulse; Foot 12 is the ground; Pin 13 is the collector power terminal; Pin 14 is the B output port of PWM pulse; Pin 15 is the control power input; Pin 16 is the closed port.

2. UC3846 frequency conversion circuit design and specific analysis

2.1. UC3846 widening principle

UC3846 is a current mode control chip, with a voltage and current double closed-loop control system, and the amplitude frequency characteristics change from double poles to single poles. Therefore, the gain bandwidth is high, the voltage stabilizing amplitude is, and has good frequency response characteristics. The 3-pin and 4-pin amplifiers of UC3846 are used as the current detection section to change the output totem column pulse width through G = vpin7 / vpin4. The high level is 1-pin voltage plus the built-in triode voltage of 0.7V, and the low level is about 0.7V, which is 0 to 1V. It is a constant value. Here, the output pulse width can be changed by changing. When G becomes larger, the output pulse width becomes narrower to achieve voltage stabilization. The voltage loop of UC3846 is a voltage negative feedback amplifier composed of 5 pins, 6 pins and 7 pins. Here, the PI regulator is composed of R13, R25, C2 and C6, of which pin 5 is the voltage setting terminal and UF is the voltage feedback terminal. When the feedback voltage is greater than the given voltage, the output pulse width becomes narrow to achieve voltage stabilization. Current loop and voltage loop complement each other.

2.2. Design of UC3846 variable width frequency circuit

Figure 2   Schematic diagram of frequency conversion circuit

The frequency conversion circuit is composed of D6, D7, R6, C12, zd3, R5, D5, Q3 and C11. When the pulse width of the voltage loop or current loop changes, the dead zone of the waveform passing through D6 and D7 gradually widens, the on time of Q2 increases, the charging time of C11 shortens and the output frequency increases. C12 plays the role of integration and accumulates dead time pulses. Zd3 can inhibit the spike. If is the resonant current rectifier input, and an RC integral circuit is formed through R15, R20, R14, R23 and C5 as the current detection amplifier in phase

The low level of the integrating circuit is provided by the dead band of the output totem column. When two totem poles have dead band, LM339 outputs zero level and clears C5 voltage to high level. LM339 outputs high level and C5 is charged. Thus, the to 4 feet are triangular waves. The inverting input of the transmission current detection amplifier is ground.

3. The frequency conversion circuit is applied to LLC resonant circuit

LLC resonant inductor has attracted much attention because of its advantages of high efficiency, wide range input and zero voltage switching. The schematic diagram of LLC resonant half bridge current is shown in Figure 3. The main electrical routes of the circuit are LR, LM and Cr to form LLC resonant circuit, in which there are two resonant points

LLC half bridge resonance achieves the effect of voltage stabilization by changing the frequency. Therefore, the frequency conversion circuit of UC3846 can be applied to the controller of LLC resonant power supply. When the frequency is higher than FR1, zero voltage switching can be realized, so the frequency is higher than FR1 in this design.

4. Design examples and conclusions

Taking a 48W resonant half bridge converter as an example. The input voltage is dc130v and the output voltage is 24V.

The following is the experimental waveform:

Figure 4   No load resonant current waveform

Figure 5   2A resonance current waveform

Since the input voltage of V2F must be DC, the resonant current waveform here passes through the rectifier bridge. When the load changes from no-load to 2a, the frequency changes from 50K to 42K. The working frequency is higher than FR1 to realize frequency modulation and voltage stabilization.

5. Conclusion

By analyzing the advantages of UC3846 current mode chip, the design of frequency modulation circuit is discussed. Experiments on LLC resonant converter prototype verify the effectiveness and practicability of the UC3846 frequency modulation circuit.