Self-biasing the input controller in a power converter
A power converter having an integrated input bias self-supply function has an input switch, a controller, an output switch, and an output filter having an inductor. The controller includes control circuitry, a bias drive circuit, and a bias supply circuit. The bias supply circuit includes a bias transistor and a bias diode. The control circuitry and the bias drive circuit produce a bias drive signal indicative of the conductive states of the input and output switches. When the bias drive signal is asserted, the bias transistor is configured to be on and the bias diode is configured to be reverse biased and not conduct any forward current. When the bias drive signal is deasserted, the bias transistor is configured to be off.
1 . A controller for a power converter comprising:
a second drive circuit coupled to receive a feedback signal indicative of an output of the power converter;
a first drive circuit coupled to the second drive circuit and responsive to the feedback signal to produce a first drive signal;
a bias drive circuit coupled to the first drive circuit and responsive to the first drive signal to produce a bias drive signal;
a bias transistor coupled to the bias drive circuit to receive the bias drive signal; and
a bias diode having an anode of the bias diode coupled to a drain of the bias transistor and a cathode of the bias diode coupled to the bias drive circuit,
wherein when the bias drive signal is asserted, the bias transistor is turned on and the bias diode is reverse biased such that current is conducted through the bias transistor toward the output of the power converter, with substantially no current being conducted through the bias diode, and
wherein when the bias drive signal is deasserted, the bias transistor is turned off, causing the current that is otherwise conducted through the bias transistor to be diverted through the bias diode to charge a bias capacitor coupled to the bias diode.
2 . The controller as in claim 1 further comprising the bias capacitor coupled to the bias diode, wherein the bias capacitor is charged to store electrical energy to provide power to the bias drive circuit and to the first drive circuit.
3 . The controller as in claim 1 , the bias drive circuit comprising:
an inverter coupled to receive the first drive signal and to produce an inverted first drive signal;
a comparator coupled to a bias node between the bias diode and the bias capacitor, wherein the comparator asserts a bias current signal in response to a voltage at the bias node rising above a threshold voltage; and
a NAND gate coupled to the inverter and responsive to the inverted first drive signal and the bias current signal to produce the bias drive signal.
4 . The controller as in claim 1 , wherein the first drive circuit and the second drive circuit are included in a monolithic integrated circuit.
5 . The controller of claim 1 wherein the first drive circuit and the second drive circuit are included in a single integrated circuit package.
6 . The controller of claim 1 the first drive circuit and the bias drive circuit are included in a single integrated circuit package.
7 . The controller as in claim 1 , wherein:
the bias drive circuit and the bias capacitor are coupled to the cathode of the bias diode at a bias node, and
to produce the bias drive signal, the bias drive circuit is further responsive to a bias signal at the bias node.
8 . The controller as in claim 7 , wherein:
the bias signal is indicative of available charge at the bias capacitor, and
the bias drive circuit is configured to determine when to assert or deassert the bias drive signal based on the bias signal.
9 . The controller as in claim 1 , wherein the bias drive circuit is configured to deassert the bias drive signal only when the first drive signal is deasserted.
10 . The controller as in claim 9 , wherein:
the current that is diverted through the bias diode to charge the bias capacitor when the bias drive signal is deasserted is a freewheeling current, and
the freewheeling current is established when an input switch of the power converter is turned off in response to the first drive signal being deasserted.
11 . A buck converter comprising:
an input switch;
an output filter circuit coupled to the input switch and configured to produce a feedback signal indicative of an output voltage of the buck converter;
a bias capacitor coupled to the output filter circuit;
an output conductor circuit coupled to the output filter circuit; and
a controller coupled to the output filter circuit and to the output conductor circuit, the controller comprising,
a second drive circuit coupled to receive the feedback signal,
a first drive circuit coupled to the second drive circuit and responsive to the feedback signal to produce a first drive signal,
a bias drive circuit, coupled to the bias capacitor and to the first drive circuit, the bias drive circuit responsive to the first drive signal to produce a bias drive signal,
a bias transistor coupled to receive the bias drive signal, and
a bias diode having an anode of the bias diode coupled to a drain of the bias transistor and a cathode of the bias diode coupled to the bias drive circuit and the bias capacitor,
wherein when the bias drive signal is asserted the bias transistor is turned on and the bias diode is reverse biased to not conduct substantially any forward current, and when the bias drive signal is deasserted the bias transistor is turned off.
12 . The buck converter as in claim 11 , the bias drive circuit comprising:
an inverter coupled to receive the first drive signal and to produce an inverted first drive signal;
a comparator coupled to a bias node between the bias diode and the bias capacitor, wherein the comparator asserts a bias current signal in response to a voltage at the bias node rising above a threshold voltage; and
a NAND gate coupled to the inverter and responsive to the inverted first drive signal and the bias current signal to produce the bias drive signal.
13 . The buck converter as in claim 11 , the output conductor circuit comprising at least one diode.
14 . The buck converter as in claim 11 , wherein:
the second drive circuit is responsive to the feedback signal to produce a second drive signal, and
wherein the output conductor circuit comprises a switch responsive to the second drive signal to deliver current to one of the bias diode and the bias transistor.
15 . A forward converter comprising:
a first input switch and a second input switch;
an energy transfer element having a primary winding and a secondary winding, a first end of the primary winding coupled to the first input switch and a second end of the primary winding coupled to the second input switch;
a bias capacitor coupled to the first end of the primary winding;
a first clamp diode coupled from the second end of the primary winding to a source of the first input switch;
a second clamp diode coupled from a source of the second input switch to the first end of the primary winding;
a rectifier circuit coupled across a first end and a second end of the secondary winding;
an output filter circuit coupled to the rectifier circuit and configured to produce an output voltage signal; and
a controller coupled to the first input switch, the bias capacitor, and the second input switch, and responsive to an input voltage signal to produce a first drive signal, the controller comprising,
a second drive circuit coupled to the second end of the primary winding to receive a feedback signal indicative of the output voltage signal,
a first drive circuit coupled to the second drive circuit and responsive to the feedback signal to produce a first drive signal,
a bias drive circuit responsive to the first drive signal to produce a bias drive signal,
a bias transistor coupled to receive the bias drive signal, and
a bias diode having an anode coupled to the bias transistor and a cathode coupled to the bias drive circuit and to the bias capacitor,
wherein when the bias drive signal is asserted the bias transistor is turned on and the bias diode is reverse biased to not conduct substantially any forward current, and when the bias drive signal is deasserted the bias transistor is turned off.