What would be the disadvantage of using a thyristor instead of a mosfet in a DC chopper circuit?

Before answering this question, let’s show how an SCR may be used in a chopper circuit:

In green we have a DC power supply, a load, and $SC{R}_1$. When $SC{R}_1$ is triggered it will stay latched until the current drops to zero. This is a problem in a DC circuits; unlike an AC circuit there is no natural reversal of current.

What we need is a method to force the current in SCR_1 to zero and that is just what we shall do using the circuit in blue:

Step 0: Both SCR’s are off.

Step 1: $SC{R}_1$ is latched on completing the circuit for the load. $SC{R}_2$ is off.

Step 2: The capacitor charges via the charging resistor. Observe the polarity: one side of the capacitor is held near ground potential via $SC{R}_1$ while the resistor charges the capacitor to the main DC supply voltage.

Step 3: $SC{R}_2$ is triggered connecting the capacitor across $SC{R}_1$. This reverses the current releases $SC{R}_1$'s latch. We have effectively taken a low series resistance power supply and suddenly applied it with reverse polarity across $SC{R}_1$.

Step 4: The capacitor discharges.

Step 5: The capacitor attempts to charge via the load. Remember $SC{R}_2$ is still latched on so the “+” terminal of the capacitor is at ground potential. Also, remember, an SCR cannot turn off instantaneously as the majority carriers must be swept away.

Step 6: The latch for $SC{R}_2$ is released.

Step 7: Go to step 0.

Disadvantages:

• Slow: SCRs are know for their speed. Like a diode, the SCR takes time to turn on and turn off (sweeping of carriers). Also, in this particular circuit, the capacitor needs time to recharge. A failed commutation could cause additional delay or perhaps damage the load when it fails to turn off.

• Big components: $SC{R}_2$ is nearly as large as $SC{R}_1$ and the capacitor is also large.

• Noise - There are high currents associated with the the SCR trigger events.

Advantage:

• Tremendous current may be switched. MOSFET and even the largest IGBTs cant touch the 3000 plus Amp current rating of a thyristor.

• Cost: IGBT are getting better every day. Paralleling many individual modules together can produce impressive results. However, there is a break point where high powered thyristor circuits are more cost effective.

FUN FACT: A relatively new thyristor called the Gate Turn Off unit (GTO) allows turn off via the gate. While the circuitry is smaller than the forced commutation of the SCR, it still takes a punch of energy to turn off one of these Integrated gate-commutated thyristors. Here is a picture of the device that powers the world’s largest motors. Look at all of those capacitors, they are needed to turn off the GTO. Click the link below and check out section 12:

Photot: https://search-ext.abb.com/library/Download.aspx?DocumentID=Product%20brochure&LanguageCode=en&DocumentPartId=&Action=Launch