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# USB multi-output power supply --- circuit diagram 2 (BUCK discrete circuit)

2023-10-24【Archive】

**12V --> 5, 0.5A**

Considering protection of USB 5V output, a self-resetting fuse will be connected to USB 5V output later, but this fuse has a voltage drop, so it is necessary to make a BUCK circuit to output 5V voltage, which is built with discrete components 12V to 5V BUCK circuit.

**Back-volt-seconds balance calculation**

Since Boost volt-second balance has been deduced before, idea behind Buck circuit is actually same. Let's talk about basic idea behind Buck's output. The picture below shows topology of Buck circuit, let's analyze it together.

As shown in figure above, when switch is turned on, Vin charges inductor and supplies power to capacitor and load at same time; when switch is turned off, inductor discharges and inductor charges capacitor. At same time, it also provides power to load.

We know that reason why output voltage Vo can reach balance is because charging current of inductor = discharging current. Then we know relationship between inductor current and inductance:

When inductor is charging, Δt is equal to ton, when inductor is discharging, Δt is equal to toff.

Similarly, let's analyze voltage across inductor during ton and toff, respectively.

**Derivation of inductance formula**

**PWM generation circuit**

According to above formula, terminating resistance and capacitance are pre-calculated. By calculation, you can find out that lower threshold of triangular wave is 3.371 V, upper threshold is 7.014 V, and frequency of triangular wave is 59 kHz. Of course, due to error of resistance and capacitance, especially 20% capacitance error, actual measurement shall prevail. At same time, according to inductance calculated from above formula, select 100uH inductance. Also, this is when Io output reaches 0.5A * 0.5 = 0.25A and inductor is operating in BCM critical continuous mode. Since inductor enters BCM mode when Io=0.25A, ΔI/2=0.125A. Then when Io=0.5A exits, Ipk=Io+ΔI/2=0.625A (the formula will be detailed here later). Therefore, when choosing an inductor, we can choose an inductor with a slightly higher power of 100 μH 1A.

At same time, an undervoltage protection circuit must be provided. When it is lower than 7.8V, MOS lamp drive circuit will be cut off to protect MOS lamp. This is because MOS tube is turned off at a high level, so if high level is below threshold, it will not be able to turn off, which can easily cause damage.

The current loop has a self-locking function to prevent frequent switching due to overcurrent. The basic principle is that when an overcurrent occurs in circuit, voltage at both ends of selection resistor turns on transistor Q7, thus turning on Q8. Then in self-locking circuit on left, since C pole of Q8 is low, Q9 will be turned on; when Q9 is turned on, there will be current on base of Q10, so that Q10 will be turned on; enabling Q10 will enable Q8. The C pole of C pole is clamped, and at same time, DC level signal at output of comparator 6 is lowered, so that output of comparator is high and MOS tube is turned off. Only by turning power off and on again can circuit be reset and work normally.

Finally, figure below shows complete circuit of the 12-5V BUCK converter:

12V to 5V BUCK discrete circuit, because output voltage is only 5V, it can't be designed with N-MOS lamp + bootstrap capacitor, so we use P-MOS lamp here, but we need to pay attention to push Output logic pull, when push-pull output is high, this corresponds to P-MOS tube being turned off. The general idea is: use a comparator with hysteresis to create a triangular wave generator, then compare it to a DC level to generate a square wave, and use that square wave to drive a MOS lamp. be added in middle to improve running performance of MOS tube.

This BUCK circuit also has a precision voltage loop designed by TL431 to control output potential at 5V. adjust level of constant current 6 pins to realize adjustment. duty cycle and stabilize output voltage at 5V.

**How Precision TL431 works**

When VCC power supply is just installed, let TL431 reach its minimum operating current through R17.

Then, as output voltage gradually increases, when voltage rises to, for example, 5.002 V, potential after resistor divider is 2.501 V, that is, voltage difference at input of error amplifier is 0.001 V. Assuming that error gain of TL431 is 1000 times, then voltage difference is amplified to 1V, and this 1V is used to drive B pole of triode, so that CE triode turns on. Of course, there must be a change process between error amplifier from 0V to 1V, that is, since potential of output terminal of error amplifier changes between 0V and 1V, internal resistance CE of internal triode is 431Ω. is also gradually decreasing...

When internal resistance of internal triode CE 431 becomes smaller, it shares voltage with resistor R17, so that DC level of 6-pin comparator decreases and duty cycle decreases, so how to realize output voltage regulation.

Due to large gain of error amplifier, as long as output voltage is slightly higher than 5V, internal equivalent resistance of TL431 can be reduced to achieve regulation and stabilization at 5V. In addition, it should be noted here that minimum operating current of TL31 is 1 mA. When designing circuit, it is necessary to ensure that minimum operating current of TL431 is at least 1 mA.

C22 in LDO circuit plays role of differential regulation. If we assume that there is no such capacitor, then with a sharp change in load, for example, with a sharp decrease in output voltage, internal resistance of TL431 will also increase sharply, and DC level of comparator 6 will quickly increase, which will lead to overshoot. cycle increases too fast, and then output voltage will be too large. Repeating in this way, output voltage ripple will be very large. However, due to presence of a differential control capacitor C22 in circuit, with a sharp change in load, since voltage across capacitor cannot change suddenly, capacitor potential will have a slow slope, so change in internal resistance of TL431 will not be so sharp. They will play role of differential regulation. (Hardware PID will be discussed later)

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