Slew-rate control apparatus and methods for a power transistor to reduce voltage transients during inductive flyback
Apparatus and methods that reduce the amount of conducted/radiated emissions from a power switch ( 200 ) when a transistor ( 210 ) is switched OFF are disclosed. In addition, apparatus and methods that reduce the slew rate in a power switch when the power switch is switched off are disclosed. The apparatus comprises a transistor ( 210 ) including an inductive load ( 230 ) coupled to the transistor, a plurality of current sources ( 222, 224 ) coupled to the gate of the transistor, and a clamp ( 250 ) coupled to either the gate and the drain of the transistor, or to the gate and to ground depending on the location of the inductive load, wherein the clamp comprises a resistive element ( 260 ) to increase the voltage of the clamp when current flows through the clamp, and wherein the increased voltage causes the apparatus to include a different slew rate.
1. An apparatus featuring inductive flyback slew rate modification, comprising:
a transistor including a gate, a source, and a drain;
an inductive load coupled to the drain;
a plurality of current sources coupled to the gate, wherein a first current source of the plurality of current sources is configurable for switching the transistor ON and a second current source of the plurality of current sources is configurable for switching the transistor OFF; and
a clamp coupled between the gate and the drain, wherein the clamp comprises a first resistive element configurable for increasing a voltage of the clamp in response to a current flow through the clamp, a first zener diode coupled in series with the first resistive element, and a second zener diode coupled in parallel with the first resistive element.
2. The apparatus of claim 1 , wherein the clamp is configured for a pre-determined voltage to decrease the slew rate of the apparatus.
3. The apparatus of claim 2 , wherein the pre-determined voltage is determined by at least one of a pre-determined voltage of the first zener diode, a pre-determined current of the first current source multiplied by a pre-determined resistance of the first resistive element, and a pre-determined voltage across the gate and source.
4. The apparatus of claim 3 , wherein the pre-determined resistance is determined by at least one of a desired voltage output of the transistor and a desired gate pull down current.
5. The apparatus of claim 1 , further comprising:
a third current source coupled to the gate and coupled in parallel with the first current source;
a first switch coupled between the third current source and the gate, wherein the first switch is configurable for connecting/disconnecting the third current source to the gate;
a fourth current source coupled to the gate and coupled in parallel with the second current source; and
a second switch coupled between the fourth current source, wherein the second switch is configurable for connecting/disconnecting the fourth current source to the gate.
6. The apparatus of claim 5 , wherein the clamp further comprises:
a second resistive element coupled in series with the first resistive element; and
a third switch coupled to the clamp, wherein a first side of the third switch is coupled on one side of the second resistive element and a second side of the third switch is coupled to an opposite side of the second resistive element, wherein responsive to the third switch being in an closed state, current is prohibited from flowing through the second resistive element and wherein responsive to the third switch being in a open state, current is enabled to flow through the second resistive element.
7. An apparatus featuring inductive flyback slew rate modification, comprising:
a transistor including a gate, a source, and a drain;
an inductive load coupled to the source;
a plurality of current sources coupled to the gate, wherein a first current source of the plurality of current sources is configurable for switching the transistor ON and a second current source of the plurality of current sources is configurable for switching the transistor OFF; and
a clamp coupled between the gate and ground, wherein the clamp comprises a first resistive element configurable for increasing a voltage of the clamp in response to a current flow through the clamp, a first zener diode coupled in series with the first resistive element, and a second zener diode coupled in parallel with the first resistive element.
8. The apparatus of claim 7 , wherein the clamp is configurable for a pre-determined voltage to decrease the slew rate of the apparatus.
9. The apparatus of claim 8 , wherein the pre-determined voltage is determined by at least one of a pre-determined voltage of the first zener diode, a pre-determined current of the first current source multiplied by a pre-determined resistance of the first resistive element, and a pre-determined voltage across the gate and source.
10. The apparatus of claim 9 , wherein the pre-determined resistance is determined by at least one of a desired voltage output of the transistor and a desired gate pull down current.
11. The apparatus of claim 7 , further comprising:
a third current source coupled to the gate and coupled in parallel with the first current source; and
a first switch coupled between the third current source and the gate, wherein the first switch is configurable for connecting/disconnecting the third current source to the gate.
12. The apparatus of claim 11 , wherein the clamp further comprises:
a second resistive element coupled in series with the first resistive element; and
a second switch coupled to the clamp, wherein a first side of the second switch is coupled on one side of the second resistive element and a second side of the second switch is coupled to an opposite side of the second resistive element, wherein responsive to the second switch being in an open state, current is prohibited from flowing through the second resistive element and wherein responsive to the second switch being in a closed state, current is enabled to flow through the second resistive element.
13. A method to reduce voltage transients during inductive flyback, the method comprising:
identifying a desired slew rate for a power transistor, the power transistor comprising:
a transistor and
at least one current source to provide an amount of input current to the transistor, an inductive load, and a clamp coupled to the transistor, wherein the clamp includes at least one zener diode coupled in series with at least one resistive element and at least one zener diode coupled in parallel with at least one resistive element; and
adjusting a voltage of the clamp to attain the desired slew rate.
14. The method of claim 13 , wherein:
adjusting the voltage comprises one of adding resistance to the clamp and subtracting resistance from the clamp.
15. The method of claim 13 , further comprising:
adjusting the amount of input current provided to the transistor.
16. The method of claim 15 , wherein adjusting the amount of input current comprises one of adding and subtracting input current from at least one additional current source.