Power supply with digital control loop

1. An apparatus for switching a transistor, comprising:

a first current mirror providing i B =K 1 i 1 as a transistor base current, wherein the first current mirror is selectively driven by a current source

i

B

⁢

⁢

MAX

K

1

;

a second current mirror providing a feedback signal i 2 =K 2 i D to the first current mirror such that

i

1

+

i

2

=

i

B

⁢

⁢

MAX

K

1

,

wherein i D contributes to the transistor collector current, wherein i B =i BMAX −K 1 K 2 i D .

2. The apparatus of claim 1 wherein the transistor is a bipolar junction transistor.

3. The apparatus of claim 1 wherein K 1 >10 and K 2 >2.

4. The apparatus of claim 1 wherein K 1 ≈100 and K 2 ≈4.

5. The apparatus of claim 1 further comprising:

actuator switches wherein when a first actuator switch couples the current source to the first current mirror a second actuator switch decouples the transistor base from ground, wherein when the first actuator switch decouples the current source from the first current mirror a second actuator switch couples the transistor base to ground.

6. The apparatus of claim 5 wherein the first actuator switch is a field effect transistor of a first type, wherein the second actuator switch is a field effect transistor of a second type, wherein the first and second types are complementary.

7. The apparatus of claim 1 further comprising a variable clamp to establish a clamping point for the transistor, wherein i D flows through the clamp only when a collector-to-emitter voltage of the transistor falls below the clamping point.

8. The apparatus of claim 7 wherein the clamp further comprises a diode and a field effect transistor, wherein the clamping point is varied in accordance with a reference voltage applied to a gate of the field effect transistor.

9. An apparatus for switching a transistor comprising:

a first current mirror providing a base current i B =K 1 i 1 to a transistor from a current source

i

B

⁢

⁢

MAX

K

1

selectively coupled to the first current mirror;

a second current mirror from which the transistor draws a portion i D of its collector current when a collector-to-emitter voltage of the transistor falls below a clamping point, wherein the second current source provides a feedback signal i 2 =K 2 i D to the first current source, wherein

i

1

+

i

2

=

i

B

⁢

⁢

MAX

K

1

,

wherein i B =i BMAX −K 1 K 2 i D .