Methods and Systems for Current Sensing in Power Converters
A system provides a current sensor for sensing an average output current of a switching power converter, which includes an energy storage element and a power converter switch coupled at a switching node, the power converter switch being arranged to selectively couple the energy storage element to a converter reference voltage, wherein the current sensor includes a pulse density modulator configured to generate a pulse density modulated signal, the pulse density modulated signal being dependent on an average current flowing through the power converter switch; and the current sensor is configured to sense the average output current of the switching power converter using the pulse modulated signal.
1 . A system comprising a current sensor for sensing an average output current of a switching power converter comprising an energy storage element and a power converter switch coupled at a switching node, the power converter switch being arranged to selectively couple the energy storage element to a converter reference voltage, wherein
the current sensor comprises a pulse density modulator configured to generate a pulse density modulated signal, the pulse density modulated signal being dependent on an average current flowing through the power converter switch; and
the current sensor is configured to sense the average output current of the switching power converter using the pulse density modulated signal.
2 . The system of claim 1 wherein sensing the average output current of the switching power converter using the pulse density modulated signal comprises counting a number of pulses in the pulse modulated signal.
3 . The system of claim 2 , wherein
the pulse density modulator comprises a differential circuit having a first input and a second input; and
the pulse density modulator is configured to generate the pulse density modulated signal based on a difference between a sensing signal and a compensating signal; wherein
the compensating signal comprises a difference between a voltage coupled to the first input and the converter reference voltage; and
the sensing signal comprises a difference between a voltage coupled to the second input and the converter reference voltage.
4 . The system of claim 3 , wherein the system comprises one or more return to zero switches, wherein
the one or more return to zero switches are controlled according to a duty cycle of the power converter switch; and
the one or more return to zero switches are configured to zero the difference between the sensing signal and the compensating signal when the power converter switch is open.
5 . The system of claim 3 , wherein
the pulse density modulator comprises a sensor switch, the sensor switch having an internal resistance which is dependent on an internal resistance of the power converter switch;
the pulse density modulator is configured to selectively provide a sensor current to the sensor switch; and
the current sensor is configured such that
the sensing signal is dependent on an average current flowing through the power converter switch; and
the compensating signal is dependent on an average current flowing through the sensor switch.
6 . The system of claim 5 , wherein
selectively providing a sensor current to the sensor switch comprises only providing a sensor current to the sensor switch when a pulse is generated by the pulse density modulator.
7 . The system of claim 6 , wherein the pulse density modulator comprises
a sensor current supply configured to provide the sensor current to the sensor switch; and
a DAC switch coupled to the pulse density modulated signal, the DAC switch being configured to selectively provide a path for the sensor current; wherein
selectively providing a path for the sensor current comprises providing a path for the sensor current only when a pulse is generated by the pulse density modulator.
8 . The system of claim 5 , wherein
the pulse density modulated signal is a signal configured to be either in a logic 0 state or in a logic 1 state;
the pulse density modulator is operated according to a clock signal: and
a pulse is any clock cycle in which the pulse density modulated signal is in the logic 1 state.
9 . The system of claim 1 , wherein
the pulse density modulator is a multi-bit pulse density modulator; and
the pulse density modulated signal is a signal configured to be either in a logic 0 state or in two or more non-zero states.
10 . The system of claim 8 , wherein the current sensor is configured to provide the number of pulses in the pulse modulated signal over a predetermined period of time.
11 . The system of claim 8 , wherein
the current sensor comprises a counter for counting the number of pulses in the pulse modulated signal;
the counter is operated according to the clock signal; and
the predetermined period of time comprises a predetermined number of clock cycles.
12 . The system of claim 10 wherein the sensor switch has a first terminal coupled to a compensating node and a second terminal coupled to the converter reference voltage at a converter reference node.
13 . The system of claim 12 , wherein the pulse density modulator comprises
one or more first coupling switches for selectively coupling the first input to the switching node; and
one or more second coupling switches for selectively coupling the second input to the converter reference node; wherein
selectively coupling the first input to the switching node comprises only coupling the first input to the switching node during a duty cycle of the power converter switch; and
selectively coupling the second input to the converter reference node comprises only coupling the second input to the converter reference node during a duty cycle of the power converter switch.
14 . The system of claim 3 wherein
when the power converter switch is open, the first input and the second input are both coupled to the first reference voltage.
15 . The system of claim 12 , wherein the pulse density modulator is configured such that
the compensating signal is dependent on a voltage difference between the compensating node and the converter reference node; and
the compensating signal is dependent on a duty cycle of the power converter switch;
wherein the number of pulses in the pulse modulated signal over the predetermined period of time provides a measure of the average output current of the switching power converter over the predetermined period of time.
16 . The system of claim 13 , wherein
the pulse density modulator comprises one or more first return to zero switches for selectively coupling the first input to the compensating node; and
selectively coupling the first input to the compensating node comprises only coupling the first input to the compensating node during a duty cycle of the power converter switch.
17 . The system of claim 12 , wherein the pulse density modulator is configured such that
the compensating signal is equal or approximately equal to a voltage difference between the compensating node and the converter reference node; and
the number of pulses in the pulse modulated signal over the predetermined period of time provides a measure of the average current flowing through the power converter switch over the predetermined period of time.
18 . The system of claim 10 , wherein the current sensor comprises a digital correction stage configured to compute the average output current of the switching power converter, wherein computing the average output current of the switching power converter comprises digitally multiplying the number of pulses in the pulse modulated signal over the predetermined period of time by a duty cycle of the power converter switch.
19 . The system of claim 12 , wherein the second input is coupled to the compensation node; and the current sensor comprises one or more third coupling switches for selectively coupling the first input to the switching node; wherein selectively coupling the first input to the switching node comprises
only coupling the first input to the switching node during a duty cycle of the power converter switch; and
coupling the first input to the converter reference node for the remaining time.
20 . The system of claim 12 , the system further comprises one or more second return to zero switches for selectively coupling the compensating node to the converter reference node, wherein selectively coupling the compensating node to the converter reference node comprises only coupling the compensating node to the converter reference node during a duty cycle of the power converter switch.
21 . The system of claim 13 , wherein
the switching power converter is a buck-boost converter comprising a boost low side switch; and
the current sensor comprises one or more third return to zero switches for selectively coupling the first input to the converter reference node; wherein selectively coupling the first input to the converter reference voltage comprise only coupling the second input to the converter reference node during a duty cycle of the boost low side switch.
22 . The system of claim 1 wherein the first pulse density modulator comprises a sigma delta modulator.
23 . The system of claim 1 comprising the switching power converter.
24 . A method for sensing an output current of a switching power converter comprising an energy storage element and a power converter switch coupled at a switching node, the power converter switch being arranged to selectively couple the energy storage element to a converter reference voltage, the method comprising
providing a current sensor comprising a pulse density modulator;
generating via the pulse density modulator a pulse density modulated signal, wherein the pulse density modulated signal is dependent on an average current flowing through the power converter switch; and
sensing the average output current of the switching power converter using the pulse density modulated signal.