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 system; (5) enhancement Load response characteristics; (6) wide common mode range differential current detector; (7) double pulse suppression; (8) 500mA dual totem pole output; (9) low voltage latch; (10) soft start; (11) output Shutdown protection; (12) working range up to 500kHz. The following is its internal block diagram:
Figure 1 UC3846 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 terminal of the current detecting amplifier; pin 4 is the non-inverting input terminal of the current detecting amplifier; pin 5 is the non-inverting input terminal of the error amplifier; 6 is the inverting input of the error amplifier; pin 7 is the error amplifier feedback compensation; pin 8 is the external capacitor port of the oscillator; pin 9 is the external resistor port of the oscillator; pin 10 is the synchronous port; pin 11 is the PWM pulse A Output terminal; pin 12 is ground; pin 13 is the collector power terminal; pin 14 is the PWM output B output port; 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 principleThe UC3846 is a current-mode control chip with a dual-loop control system for voltage and current. The amplitude-frequency characteristic also changes from a double pole to a single pole. Therefore, the gain bandwidth is high, the voltage regulation range is high, and the frequency response characteristic is good. The 3-pin and 4-pin amplifiers of the UC3846 are used as current detection sections. The output totem pole pulse width is changed by G=Vpin7/Vpin4. The high level is the 1-pin voltage plus the built-in transistor voltage of 0.7V. 0.7V, 0 to 1V. It is a constant value, and you only need to change it to change the output pulse width. When G becomes larger, the output pulse width is narrowed to achieve voltage regulation. The voltage loop of the UC3846 is a voltage negative feedback amplifier consisting of 5 pins, 6 pins, and 7 pins. Here PI regulation consists of R13, R25, C2, C6, where 5 is the voltage reference terminal and UF is the voltage feedback terminal. When the feedback voltage is greater than a given voltage, the output pulse width is narrowed to achieve voltage regulation. The current loop and the voltage loop complement each other.
2.2, UC3846 widening to achieve frequency conversion circuit design
Figure 2 schematic diagram of the frequency conversion circuit
The frequency conversion circuit is composed of D6, D7, R6, C12, ZD3, R5, D5, Q3, C11. When the voltage loop or current loop produces a pulse width width change, the dead zone of the waveform after D6 and D7 is gradually widened, Q2 guide As the pass time increases, the C11 charging time is shortened and the output frequency is increased. Among them, C12 acts as an integral, accumulating dead zone pulses. ZD3 acts to suppress spikes. IF is the resonant current rectification input terminal, and an RC integration circuit is formed through R15, R20, R14, R23, C5 as the current detection amplifier in phase.
The low level of the integrating circuit is provided by the dead zone of the output totem pole. When two totem poles have a dead zone, the LM339 outputs a zero level, which clears the C5 voltage. When it is high, the LM339 output is high and C5 is charged. Thus, the four feet are triangular waves. The inverting input of the current sense amplifier is ground.
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