Transistor voltage stabilizing circuit simulation analysis and design plan, you are worth collecting

1. Principle of voltage stabilizing circuit

Many scholars have many of their own habits when designing their own voltage stabilizing circuit simulation design solutions. R3 and Z1 form a simple voltage stabilizing circuit. When the voltage of VIN is higher than the voltage stabilizing value Vz of Z1, the negative electrode of Z1 The voltage is stabilized at Vz; the negative electrode of Z1 is connected to the base of the triode. The emitter E of the triode is the output, forming an emitter follower. That is to say, the voltage of E pole changes as the voltage of B changes. Between BE There is a voltage drop, generally about 0.7V for silicon tubes. Assuming that the B pole is 6.7V, then the E pole is 6.7-0.7=6V. In summary, VOUT=Vz-Vbe, you can choose a suitable voltage regulator tube based on VOUT.

2. Simulation analysis

The following is a simulation experiment conducted with Tina-TI. Z1 is a 5.1V voltage regulator tube, the current limiting resistor R1 is 1.25K, the input V1=30V, the simulation result VF1=4.36V, we found that this circuit can be used. Then someone will say that after the transistor is turned on, 30V will go directly from CE to VF1. Why is the output still 4.36V?

After the transistor is turned on, the voltage at point E will increase. When it rises to Vbe < 0.7V, the transistor will turn off, the IC current will drop sharply, and the voltage at point E will drop sharply. When the voltage at E drops to Vbe = 0.7V, at this time The transistor turns on again, and the voltage at point E rises. When Vbe <0.7V, the transistor turns off again. Such a cycle keeps the voltage at point E stable at 4.36V.

3. Determine the resistance of R1

The following is the characteristic curve of the Zener diode. Combined with the graph, the parameters can be better understood. The Breakdown area of ​​the Zener diode is Izt~Izm. Within this range, the voltage stabilization effect of the Zener diode is the best.

From the SPEC of the voltage regulator tube, we can see that the parameter Vz@Izt refers to the voltage stabilization value of the voltage regulator tube Vz=5.1V when Izt=20mA, which means that the current flowing through the voltage regulator tube is above 20mA. , the best voltage stabilization effect.

Looking at the simulation diagram above, ignoring the current flowing into the base of the transistor, we can calculate R1≤(30-5.1)/20mA=1.245K, but what is the lower limit of R1?

The lower limit of R1 depends on Izm. The voltage regulator tube has a parameter dissipation power Pd. The voltage regulator tube cannot exceed this value under normal working conditions. Because the voltage regulator value remains unchanged at 5.1V, the maximum current flowing through the voltage regulator tube is Izm=500. /5.1=98mA, so the minimum value of R1 is: (30V-5.1V)/98mA=0.25K

Simulate again, change R1 to 2K, the current applied to the voltage regulator tube decreases, VF2 becomes 5V, VF1 becomes 4.26V, the voltage stabilization effect is not as good as R1=1.25K, which verifies the above theory.

4. Power of resistor R1

According to the calculation of R1=1.25K and the voltage regulator tube breakdown current Izt=20mA, the power added to R1 is P=0.02*0.02*1250=0.5W, so the power of R1 needs to be more than 0.5W, otherwise the resistor may be damaged.