Shunt circuit clips large transients or regulates voltage

Diodes TL431

The circuit in Figure 1 is a high-power analog of the popular TL431 programmable shunt regulator. This two-terminal circuit is convenient when the drain of the P-channel FET is grounded, since it needs no isolation from a grounded heatsink.

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TL431-based shunt regulator or clipper.
Figure 1. TL431-based shunt regulator or clipper.

There is no mirror analog of the TL431 however, so when you have a positive ground, you need isolation between the drain and the grounded heatsink, degrading cooling performance.

The Design Idea presented in Figure 2 allows the use of a lower-cost N-channel MOSFET on a grounded heatsink. It also tolerates lower values of input voltage than Figure 1.

TL431-based shunt regulator or clipper using N-channel MOSFET.
Figure 2. TL431-based shunt regulator or clipper using N-channel MOSFET.

The circuit can be used as an adjustable clamp capable of high power dissipation, with precise level control and very sharp response. The ballast resistance RB can be replaced with a fuse if desired. Response time is not as fast as a Transil/Transzorb type of part, but a few microseconds is enough for the most applications.

The circuit can also serve as a high-current shunt regulator, though with stability caveats. The circuit is stable with a load capacitance below about 1 nF or above about 200 µF.

The circuit can be scaled up easily: just select a FET with a suitable drain current. The 75 A HUF75652G3 and 85 A IRF1010N have been used.

Zener diode D2 can be omitted if:

The clamp voltage is calculated using the same expression as for the TL431:

Absolute maximum ratings are also the same as for the TL431, except for current of course. The minimum input voltage is about 0.8 V higher than for the TL431 alone.

Materials on the topic

  1. Datasheet Diodes TL431
  2. Datasheet onsemi HUF75652G3
  3. Datasheet International Rectifier IRF1010N

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