Active clamp for semiconductor device

1. An active clamp circuit for avalanching and clamping voltage at a gate terminal of a first transistor connected to a power source, the active clamp circuit comprising:

a second transistor for turning ON the first transistor;

a third transistor having an epitaxial layer breakdown voltage less than that of the first transistor;

a resistor coupled between a node and source and gate terminals of the third transistor; and

an amplifier for comparing voltage on the resistor to a reference voltage and providing an output signal to control the second transistor, wherein, when the third transistor avalanches and the voltage across the resistor exceeds the reference voltage the output signal turns ON the second transistor thereby clamping a gate terminal of the first transistor,

wherein the active clamp circuit tracks the channel characteristic of the first transistor.

2. The active clamp circuit of claim 1 , wherein the first transistor is a DMOS MOSFET main power transistor, the second transistor is a bipolar transistor, and the third transistor is a MOSFET.

3. The active clamp circuit of claim 1 , wherein the active clamp circuit and first transistor are disposed on a wafer, and the third transistor is spaced at least one times the thickness of the wafer away from the first transistor to avalanche at a lower voltage during clamping.

4. An active clamp circuit for a first transistor having source, drain, and gate terminals, the drain terminal of the first transistor being connected to a power source, wherein, when the active clamp circuit avalanches, voltage at the gate terminal of the first transistor is clamped, the active clamp circuit comprising:

a second transistor having collector, emitter, and base terminals, where the collector terminal of the second transistor is connected to the drain terminal of the first transistor and the emitter terminal of the second transistor is connected to the gate terminal of the first transistor;

a third transistor, having source, drain, and gate terminals, the drain terminal of the third transistor is connected to the drain terminal of the first transistor; and

an amplifier having a first input terminal connected to the gate and source terminals of the third transistor and an output terminal connected to the base terminal of the second transistor and further having a second input terminal connected to a reference,

wherein the active clamp circuit tracks the channel characteristic of the first transistor.

5. The active clamp circuit of claim 4 , wherein the first transistor is a DMOS MOSFET main power transistor, the second transistor is a bipolar transistor, and the third transistor is a MOSFET.

6. The active clamp circuit of claim 4 , wherein the third transistor is spaced away from the first transistor to ensure that it operates at a lower temperature during clamping and at high temperatures.

7. The active clamp circuit of claim 4 , wherein the third transistor is disposed on a wafer with the first transistor and is spaced at least one times the thickness of the wafer away from the first transistor to avalanche at a lower voltage.

8. The active clamp circuit of claim 7 , wherein the third transistor runs cooler than the first transistor because of its spacing at least one thickness of the wafer away from the first transistor.

9. The active clamp circuit of claim 4 , wherein clamp voltage is set at just below the epitaxial layer breakdown voltage of the first transistor.

10. The active clamp circuit of claim 4 , further comprising a resistor connected between the source terminals of the first and third transistors.

11. The active clamp circuit of claim 10 , wherein the second transistor goes ON clamping the gate terminal of the first transistor when the third transistor avalanches and the voltage across the resistor exceeds the reference voltage.

12. The active clamp circuit of claim 11 , wherein breakdown voltage of the third transistor is less than that of the first transistor.