Snubber Circuits, RC Snubber Circuits, Diode Snubbers, BJT Snubbers

Snubber Circuits

Protection of switching devices and circuits: Switching devices and circuit components may fail due to the following reasons.

1.        Overheating – thermal failure

2.        Overcurrent

3.        Overvoltage – usually happens during turn-off

4.      
Excessive        di / dt

5.      Excessive    dv / dt

6.   Switching loss – excessive switching loss is a major contributing factor of    overheating.

RC Snubber Circuits

RC snubber circuits are normally connected across a switching device to limit the di /dt an RC snubber circuit can be polarized or unpolarized. 

Figure 1. A forward polarized snubber circuit

Diode Snubbers

Snubbers are needed in diode circuits to minimize overvoltage’s. Overvoltage’s usually occur during turn-off process. Figure 4 shows a diode with a snubber circuit.

 

Figure 4. Diode, D with its snubber ciruit

BJT Snubbers

A Bipolar junction transistor (BJT) experiences high stresses at turn-on and turn- off when both its voltage and current are high simultaneously, thus causing a high instantaneous power dissipation. Transistors require turn-off, turn-on and overvoltage snubbers.

This can be confusing on first glance – so let’s dig into the theory of this a bit more.

We’ll focus on the secondary side of the circuit. If we replace the secondary transformer coils with a source voltage, we can simplify the circuit diagram of the half-wave rectifier as:

Now we don’t have the transformer part of the circuit distracting us.

For the positive half cycle of the AC source voltage, the equivalent circuit effectively becomes:

This is because the diode is forward biased, and is hence allowing current to pass through. So we have a closed circuit.

But for the negative half cycle of the AC source voltage, the equivalent circuit becomes:

Because the diode is now in reverse bias mode, no current is able to pass through it. As such, we now have an open circuit. Since current can not flow through to the load during this time, the output voltage is equal to zero.

This all happens very quickly – since an AC waveform will oscillate between positive and negative many times each second (depending on the frequency).

Here’s what the half wave rectifier waveform looks like on the input side (V_in), and what it looks like on the output side (V_out) after rectification (i.e. conversion from AC to DC):