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.
 |
|
| 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.
 |
|
| 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.
