Zener Diode Regulator: Why Output Voltage Drops?

[wavingerwin]

My teacher was talking about power supply voltage regulators.

I cannot get why does a zener diode regulator's output voltage drops after the load current through it exceeds some critical current?

thanks in advance

 

[K^2]

Here is the Voltage-Current characteristic of a Zener diode.

Do you see how at very, very low currents, the voltage actually drops bellow threshold?

So let us see what happens in the simplest voltage regulator. Let's say some applied voltage V_a is applied across the Zener diode with threshold voltage V_Z and a resistor R₁ connected in series. In parallel with Zener diode, you connect your load resistance, R_L at regulated voltage V_Z.

So let's look at the currents. The load current is obviously given by this.

[tex]I_L = \frac{V_Z}{R_L}[/tex]

And the current across R₁ is this.

[tex]I_1 = \frac{V_a - V_Z}{R_1}[/tex]

And of course, the current flowing through R₁ then splits to go through R_L and the diode. So we can easily find the diode current, I_Z.

[tex]I_Z = I_1 - I_L = \frac{V_a - V_Z}{R_1} - \frac{V_Z}{R_L}[/tex]

If V_Z remained constant despite absolutely any changes, this value would become zero whenever

[tex]R_L = \frac{V_Z}{V_a - V_Z}R_1[/tex]

Clearly, load resistance can be that low, or even lower. If it is lower, the value of I_Z would become negative, which would mean the current is flowing against the potential, and that's nonsense.

In reality, of course, as seen in the diagram, V_Z begins to drop before you quite get to that point. In the limit where R_L goes to zero, V_Z also goes to zero.

If the regulator you are thinking of is more complex, you might have to do a bit more work to get this result, but the idea is the same. Find the current that flows through the Zener diode as a function of R_L, and you'll see when the regulator is not going to be very useful anymore.

 

[wavingerwin]

Thanks very much for the clear explanation, K^2!

I can see the where the formulas come from since you explanation fits my regulator well.

However, there is one thing I need to clarify: is the threshold voltage 17V in the diagram?
And you're saying that at low current e.g. 1mA in the diagram, voltage drops to 0.65V?

 

[Born2bwire]

The threshold voltage of interest here is the reverse breakdown voltage at around -17V. The idea being that once you setup a bias current that forces the zener diode into reverse breakdown voltage, then the voltage output is going to be fairly constant (-17 V) despite small variations in the bias current. So a simple AC-DC power supply can be made using a transformer to stepdown the AC voltage, rectifying the AC voltage to keep it always positive, passing it through a low pass filter to smooth out the rectified voltage a bit, and then using the voltage to create a bias current through a zener diode. The voltage output is taken off of the zener diode. There will still be a small amount of ripple voltage across the diode due to the variations in the bias current but it can be controlled via design to be below a certain threshold.

 

[pmacias]

I can explain why the output voltage of a zener diode regulator drops after the critical current is exceeded. A zener diode regulator is a type of voltage regulator that uses a zener diode to maintain a constant output voltage. The zener diode has a specific breakdown voltage, also known as the zener voltage, at which it starts conducting current in the reverse direction. This means that when the input voltage exceeds the zener voltage, the excess voltage is dropped across the zener diode, keeping the output voltage constant.

However, when the load current through the zener diode exceeds a critical value, the zener diode cannot handle the excess current and starts to heat up. This increase in temperature can cause the zener diode to change its characteristics, such as its breakdown voltage, leading to a decrease in the output voltage. This is known as the zener diode's thermal runaway effect, where the increase in temperature causes a decrease in the zener voltage, resulting in a drop in the output voltage.

Additionally, the zener diode also has a maximum power rating, which is the maximum amount of power it can handle before it gets damaged. When the load current exceeds this power rating, the zener diode may get damaged, leading to a decrease in its breakdown voltage and output voltage.

To prevent this drop in output voltage, it is important to ensure that the load current does not exceed the critical value and the zener diode's maximum power rating. This can be achieved by using a larger zener diode with a higher power rating or by using multiple zener diodes in parallel to share the load current. It is also important to consider the ambient temperature and provide proper heat dissipation for the zener diode to prevent thermal runaway.

In conclusion, the output voltage of a zener diode regulator drops after the critical current is exceeded due to the thermal runaway effect and the zener diode's maximum power rating. By understanding these factors and taking appropriate measures, we can ensure a stable and constant output voltage from the zener diode regulator.