BACKGROUND

1. Technical Field

The present disclosure relates to pulse width modulating (PWM) circuits, and particularly to a PWM circuit with a compensation unit.

2. Description of Related Art

Pulse width modulating (PWM) circuits generally include inductors and capacitors. In use, the inductors and the capacitors cooperatively form oscillator circuits. When activating currents are input to the oscillator circuits, pulse voltages are correspondingly generated in the oscillator circuits and output. Relevant parameters, such as peak values, widths, and periods of the pulse voltages can be determined by predetermining the inductances of the inductors and the capacitances of the capacitors. However, if the activating currents input to the oscillator circuits change quickly (e.g., at the moment of turning on/off the current sources for providing the activating currents), the inductances of the inductors may be changed. Thus, the parameters of the pulse voltages generated by the oscillator circuits may be correspondingly changed, which may adversely affect the quality of the pulse voltages and damage the PWM circuits.

Therefore, there is room for improvement within the art.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the present pulse width modulating (PWM) circuit can be better understood with reference to the following drawings. The components in the various drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the present PWM circuit. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the figures.

FIG. 1 is a circuit diagram of a PWM circuit, according to an exemplary embodiment.

FIG. 2 is a wave curve of a pulse voltage output by the PWM circuit shown in FIG. 1, using the compensation unit of the PWM circuit.

FIG. 3 is a wave curve of an pulse voltage output by the PWM circuit shown in FIG. 1, without using the compensation unit of the PWM circuit.

FIG. 4 is another wave curve of a pulse voltage output by the PWM circuit shown in FIG. 1, without using the compensation unit of the PWM circuit.

DETAILED DESCRIPTION

FIG. 1 shows a pulse width modulating (PWM) circuit 100, according to an exemplary embodiment. The PWM circuit 100 includes an activating module 10 and a pulse generating module 30. The pulse generating module 30 is electrically connected to the activating module 10. When the activating module 10 generates activating currents input to the pulse generating module 30, the pulse generating module 30 correspondingly generates pulse voltages having predetermined parameters, such as peak values, widths, and periods, etc.

The activating module 10 includes a current source 12, a switch 14, and a compensation unit 16. The current source 12, the compensation unit 16, and the pulse generating module 30 are all connected to the switch 14. The switch 14 has two electric connection statuses. In an activating status, the switch 14 connects the current source 12 with the pulse generating module 30. In a regulating status, the switch 14 connects the current source 12 with the compensation unit 16.

The compensation unit 16 includes a first amplifier A1, a second amplifier A2, a first resistor R1, a second resistor R2, and a capacitor compensation controller C0. The first amplifier A1 is a non-inverting amplifier and includes a non-inverting input connector A12, an inverting input connector A14, and an output connector A16. The second amplifier A2 is an inverting amplifier and includes a non-inverting input connector A22, an inverting input connector A24, and an output connector A26. The non-inverting input connector A12 is connected to the switch 14. The inverting input connector A14 is connected to the output connector A16. The first resistor R1 has one end connected to the output connector A16 and another end connected to the inverting input connector A24. The second resistor R2 has one end connected to the inverting input connector A24 and another end connected to the output connector A26. The non-inverting input connector A22 is grounded. The capacitor compensation controller C0 has one pole connected to the non-inverting input connector A12 and another pole connected to the output connector A26.

The pulse generating module 30 includes an inductor L0, a third resistor R3, a fourth resistor R4, a first capacitor C1, a second capacitor C2, a third capacitor C3, and a pulse output connector Vo. The third resistor R3 has one end connected to one end of the inductor L0 and another end connected to one pole of the first capacitor C1, and the fourth resistor R4 has one end connected to another end of the inductor L0 and another end connected to another pole of the first capacitor C1. Each of the second capacitor C2 and the third capacitor C3 has one pole connected to the end of the third resistor R3 that is connected to the first capacitor C1 and another pole grounded. Thus, the inductor L0, the first capacitor C1, the third resistor R3, and the fourth resistor R4 cooperatively form an oscillator circuit (not labeled). The pulse output connector Vo is also connected to the end of the third resistor R3 that is connected to the first capacitor C1. The switch 14 is connected to the end of the inductor L0 that is connected to the fourth resistor R4.

In use, the current source 12 generates an activating current I. The switch 14 is switched between the activating status and the regulating status back and forth in a predetermined frequency f, i.e., alternately connecting the current source 12 to the pulse generating module 30 and the compensation unit 16 in the frequency f. When the current resource 12 is connected to the pulse generating module 30, the activating current I is input to the pulse generating module 30 and generates pulse voltages in the oscillator circuit formed by the inductor L0, the first capacitor C1, the third resistor R3, and the fourth resistor R4. The pulse voltages are output through the pulse output connector Vo, and the second capacitor C2 and the third capacitor C3 can filter alternating portions mixed in the pulse voltages before the pulse voltages are output.

When the current resource 12 is connected to the compensation unit 16, the current I is input to the capacitor compensation controller C0 through the first amplifier A1 and the second amplifier A2. Thus, the capacitor compensation controller C0 detects the activating current I in the predetermined frequency f of switching the switch 14 between the two electric connection statuses. If the activating current I changes, the capacitor compensation controller C0 regulates the capacitance Cc thereof to regulate the change of the activating current I, such that the activating current I is kept unchanged when it is input to the pulse generating module 30.

Particularly, in Laplace domain, the impedance of the capacitor compensation controller C0 is represented as 1/SCc. If V represents the voltage on the non-inverting input connector A12, V_Arepresents the voltage on the output connector A26, R₁represents the impedance of the first resistor R1, and R₂represents the impedance of the second resistor R2, it can be inferred that:

$V_{A} = - \frac{R_{2}}{R_{1}} î¢ž V, V - V_{A} = \frac{I}{{SC}_{C î¢ž}}, and î¢ž î¢ž I = C_{C} î¢ž SV î¢ (1 + \frac{R_{2}}{R_{1}}) .$

Therefore, the current I can be regulated by regulating the capacitance Cc of the capacitor compensation controller C0. Referring to FIG. 2, when the activating current I is detected and kept unchanged by the compensation unit 16, the pulse generating module 30 activated by the current I can generate even pulse voltages output from the pulse output connector Vo.

On the contrary, if the compensation unit 16 is not used, the pulse voltages generated by the pulse generating module 30 may be adversely affected when the current I changes. Particularly, if Î”I represents the change of the activating current I, Vc represents the voltage on the first capacitor C1, C₁represents the capacitance of the first capacitor C1, Vout represents the voltage on the pulse output connector Vo, I_Lrepresents the current passing through the inductor L0, V_Lrepresents the voltage on the inductor L0 and the third resistor R3, L represents the inductance of the inductor L0, R₃represents the impedance of the third resistor R3, and R₄represents the impedance of the fourth resistor R4, it can be inferred that:

$L = \frac{(V - Vout) î¢ž f}{Î” î¢ž î¢ž I}, and$ $\begin{matrix} V_{C} = î¢ž V_{L} Ã— \frac{\frac{1}{{SC}_{1}}}{R_{4} + \frac{1}{{SC}_{1}}} \\ = î¢ž I_{L} î¢ (SL + R_{3}) Ã— \frac{1}{{SC}_{1} î¢ž R_{4} + 1} \\ = î¢ž I_{L} Ã— R_{3} Ã— \frac{\frac{SL}{R_{3 î¢ž}} + 1}{{SC}_{1} î¢ž R_{4} + 1} . \end{matrix}$

Therefore, the change of the activating current I (i.e., Î”I) can generate change of Vc when the compensation unit 16 is not used. According to principle of oscillator circuits, change of Vc can further generate change of Vout, which may adversely affect the quality of the pulse voltages output by the PWM circuit 100. Referring to FIG. 3 and FIG. 4, as known in experiments, without using the compensation unit 16, the pulse output connector Vo of the PWM circuit 100 outputs pulses as shown in FIG. 3 when the activating current I decreases, and outputs pulses as shown in FIG. 4 when the activating current I increases.

The present PWM circuit 100 uses the compensation unit 16 to regulate the capacitance Cc of the capacitor compensation controller C0 when the activating current I changes, thereby compensating the change of the activating current I, such that the activating current I is kept unchanged when it is input to the pulse generating module 30. In this way, despite the activating current I changing in use of the PWM circuit 100, the PWM circuit 100 can output even pulse voltages.

It is to be further understood that even though numerous characteristics and advantages of the present embodiments have been set forth in the foregoing description, together with details of structures and functions of various embodiments, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size, and arrangement of parts within the principles of the present invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.