​switching power supply

Update date:2018-05-31 Source:MAXGE

switching power supply

1. PWM switching power supply integrated control IC-UC3842 working principle

How UC3842 works

The following figure shows the UC3842 internal block diagram and pin diagram. The UC3842 uses a fixed operating frequency and pulse width controllable modulation method. It has 8 pins in total. Each pin has the following functions: Pin 1 is the output end of the error amplifier, and the external RC capacitor is used to improve The gain and frequency characteristics of the error amplifier; Pin 2 is the feedback voltage input. This pin is compared to the 2.5V reference voltage at the non-inverting input of the error amplifier to generate an error voltage to control the pulse width. Pin 3 is the current sense input, when detected When the voltage exceeds 1V, the pulse width is reduced to make the power supply intermittently; the 4th pin is the timing terminal; the operating frequency of the internal oscillator is determined by the external resistor-capacitor time constant, f=1.8/(RT×CT); and the 5 pin is public ground. End; 6 pin push-pull output, internal totem pole type, rise and fall time is only 50ns drive capability is ± 1A; 7 pin is DC power supply end, with under-voltage lock, over-voltage lock function, chip power consumption is 15mW ; 8 feet for the 5V reference voltage output, with 50mA load capacity.

Internal Block Diagram

UC3842 Internal Block Diagram

UC3842 is a PWM switching power supply integrated controller with excellent performance, wide application, and simple structure. Since it has only one output terminal, it is mainly used as a switching power supply for acoustic control.

The UC3842 pin 7 is a voltage input and its starting voltage range is 16-34V. When the power is turned on, VCC<16V, the input voltage comparator comparator output is 0, no reference voltage is generated at this time, the circuit does not work; when Vcc>16V, the input voltage Schmidt comparator sends high level to 5V fern The regulator generates a 5V reference voltage, which on the one hand supplies the internal circuit of the pin, and on the other hand provides the reference voltage to the outside via pin 8. Once the Schmidt comparator is toggled high (after the chip starts to operate), Vcc can be varied from 10V to 34V without affecting the operating state of the circuit. When Vcc is lower than 10V, the Schmidt comparator is turned low again and the circuit stops operating.

When the reference regulator source has a 5V reference voltage output, the reference voltage detection logic comparator achieves a high level signal to the output circuit. At the same time, the oscillator will generate an oscillating signal of f=/Rt.Ct according to the external Rt and Ct parameters of the 4 pins. This signal is directly added to the input of the totem pole circuit and the other way is added to the PWM pulse width of the commercially available RS flip-flop. Set terminal, R terminal of RS type PWN pulse width modulator connects the current detection comparator output end. R terminal is the duty regulator control terminal. When R voltage rises, the Q terminal pulse is widened, and the width of the pulse sent from pin 6 is also widened (the duty cycle increases); when the voltage at terminal R decreases, the Q terminal pulse becomes narrower. At the same time, the pulse width of the 6-pin transmitter also becomes narrower (duty cycle decreases). UC3842 point timing as shown in the figure, only when the E point is high when there is signal output, and a, b points are all high, the d point is sent high, c point sent low, otherwise d The point is sent low, and point c is sent high. Pin 2 is generally connected to the output voltage sampling signal, also called the feedback signal. When the voltage of pin 2 rises, the voltage of pin 1 will decrease, and the voltage of pin R will also decrease. As a result, the pulse of pin 6 becomes narrower; otherwise, the pulse of pin 6 becomes wider. 3 feet for the current sensing end, usually in the power transistor source or emitter in series with a small resistance sampling resistor, the current through the switch tube into a voltage, and this voltage is introduced into the environment. When the load short-circuit or other causes the power tube current to increase, and the voltage on the sampling resistor exceeds 1V, the 6-pin stops the pulse output, so that the power tube can be effectively protected from damage.


2. 12V, 20W Switching DC Regulated Power Supply Circuit Consisting of TOP224P

The 12V and 20W switching DC stabilized power supply circuit formed by the TOP224P is shown in the figure. Two integrated circuits are used in the circuit: the TOP224P three-terminal monolithic switching power supply (IC1) and the PC817A linear optocoupler (IC2). After the UR and Cl rectification and filtering, the AC power source generates a DC high voltage Ui to supply the primary winding of the high-frequency transformer T. VDz1 and VD1 can clamp the spike voltage produced by leakage inductance to a safe value and can attenuate the ringing voltage. VDz1 adopts P6KE200 transient voltage suppressor with reverse breakdown voltage of 200V, and VDl uses 1A/600V UF4005 ultrafast recovery diode. The secondary winding voltage is rectified and filtered through V砬, C2, L1, and C3 to obtain 12V output voltage Uo. The Uo value is set by the sum of the VDz2 stable voltage Uz2, the forward voltage drop UF of the LED in the optocoupler, and the voltage drop across R1. Changing the turns ratio of the high frequency transformer and the voltage regulation value of VDz2 can also obtain other output voltage values. R2 and VDz2 V also provide a dummy load for the 12V output to improve load regulation at light loads. The feedback winding voltage is rectified and filtered by VD3 and C4 to supply the required bias voltage for the TOP224P. The current at the control terminal is adjusted by R2 and VDz2 to achieve a stable voltage target by changing the output duty cycle. The common mode choke L2 can reduce the common-mode leakage current generated by the high-voltage switching waveform at the D-side of the primary winding. C7 is a protection capacitor used to filter out the interference caused by the primary and secondary winding coupling capacitors. C6 reduces the differential mode leakage currents produced by the fundamental and harmonics of the primary winding current. C5 can not only filter out the spike current on the control terminal, but also determine the self-start frequency. It also compensates the control loop together with R1 and R3.


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