Pulse width modulation current adjustment apparatus

The present invention provides a PWM current adjustment apparatus including a triangle wave generator (8) for generating a triangle wave voltage signal, a comparator (9), an FET (10), a power supply (14), a first resistor (11) and a second resistor (12). The triangle wave voltage signal generated by the triangle wave generator and a modulation signal provided by a modulation voltage source (13) are fed to the comparator, an output of the comparator is connected to a gate terminal of the FET, the power supply is connected to a source terminal of the FET through the first resistor, and a drain terminal of the FET outputs a driving current through the second resistor to a load.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relatives to a current adjustment apparatus, more particularly, to a Pulse Width Modulation (PWM) current adjustment apparatus.

2. Description of The Related Art

A current adjustment apparatus is a common component in a driver circuit for a light emitting diode (LED).FIG. 4represents a conventional current adjustment apparatus working in PWM (Pulse Width Modulation) mode. This current adjustment apparatus comprises a sawtooth wave generator1, a comparator2, a field effect transistor3(FET), a power supply7, and current limiting resistors4,5. One input of the comparator2is connected to an output of the sawtooth wave generator1, and the other input of the comparator2is connected to a modulation voltage source6. The output of the comparator2is connected to a gate terminal of the FET3. The current limiting resistor4is connected between the power supply7and a source terminal of the FET3, and the resistor5is connected to a drain terminal of the FET3.

Referring toFIG. 5, Vi1is a modulation voltage signal provided by the modulation voltage source6, Vr1is a sawtooth wave signal provided by the sawtooth wave generator1, and Vo1is a voltage applied to the gate terminal of the FET3. In use, the sawtooth wave signal Vr1is compared with the modulation voltage signal Vi1in the comparator2. The comparator2outputs a positive level signal when the modulation voltage signal Vi1is higher than the sawtooth wave signal Vr1, and the comparator2outputs a zero level signal when the modulation voltage signal Vi1is lower than the sawtooth wave signal Vr1. The output level signal Vo1is applied to the gate terminal of the FET3, and the FET3outputs a current Io1to a load (not shown) through the resistor5.

However, the conventional sawtooth wave generator1suffers from some disadvantages. Referring toFIG. 6, a circuit of the conventional sawtooth wave generator1is complex since it has two integral paths, i.e. a forward one D1-R3-C and a backward one C-R4-D2. Furthermore, considering a Fourier series expansion of a sawtooth wave signal:
V=(2/π)Vm[sin wt−(½)sin 2 wt+(⅓)sin 3 wt . . . +(−1)n−1/nsin (n) wt+ . . . ],
the Fourier series expansion reveals that a sawtooth wave signal V comprises both even harmonics and odd harmonics, and includes a considerable percentage of high frequency harmonics, which induces high frequency noise in the system and makes it difficult to built a high frequency sawtooth wave generator, and directly effects the stability of output current. In additional, to adjust the output driving current Io1in PWM mode, a frequency of the sawtooth wave signal Vr1must be 10 times higher than that of the modulation voltage signal Vi1in order to reduce unexpected harmonics in the output. A new current adjustment apparatus which solves these problems is desired.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a PWM current adjustment apparatus with a less complex circuitry.

Another object of the present invention is to provide a PWM current adjustment apparatus which reduces high frequency noise and increases system stability.

The present invention provides a PWM current adjustment apparatus comprising a triangle wave generator for generating a triangle voltage signal, a comparator, a FET, a power supply, a first resistor used as a current limiting resistor, and a second resistor. In operation, the triangle wave voltage signal generated by the triangle wave generator and a modulation signal provided by a modulation voltage source are fed to the comparator, an output of the comparator is connected to a gate terminal of the FET, the power supply is connected to a source terminal of the FET through the current limiting resistor, and a drain terminal of the FET outputs a driving current through the second resistor.

DETAILED DESCRIPTION OF THE INVENTION

Referring toFIGS. 1 and 2, a PWM current adjustment apparatus of the present invention comprises a triangle wave generator8, a comparator9, an FET10, a power supply14, and two current limiting resistors11,12. A triangle wave signal Vr2generated by the triangle wave signal generator8and a modulation signal Vi2provided by a modulation voltage source13are compared in the comparator9. The comparator9outputs a positive level signal when the modulation signal Vi2is higher than the triangle wave signal Vr2, and outputs a zero level signal when the modulation signal Vi2is lower than the triangle wave signal Vr2. An output signal Vo2from the comparator9is applied to the FET10, i.e. Vo2is an input voltage at the gate terminal of the FET10. The power supply14is connected to a source terminal of the FET10through one of the current limiting resistors11. The FET10outputs a driving current Io2from its drain terminal for driving a load (not shown) through the other current limiting resistor12.

The modulation signal Vi2changes slowly in a period T of the triangle wave signal Vr2, and if the modulation signal Vi2is higher than the triangle wave signal Vr2, the comparator9outputs a positive level signal and turns on the FET10, generating a certain output driving current Io2, which flows in the series loop of the power supply14, the FET10, the current limiting resistors11,12and the load. If the modulation signal Vi2is lower than the triangle wave signal Vr2, the comparator9outputs a zero level signal to turn off the FET10, and there is no output current. Furthermore, the FET10can be an N-channel enhancement-type FET, a P-channel enhancement-type FET, an N-channel depletion-type FET, a P-channel depletion-type FET or any other type of switching component.

FIG. 3shows a circuit diagram of the triangle wave generator8shown inFIG. 1. A front operational amplifier15, a front grounding resistor23, a first feedback resistor21, a second feedback resistor22and a current limiting resistor24in combination form a zero-crossing comparator (not labeled). A second operational amplifier16, a current limiting resistor18, a capacitor17and a back grounding resistor25together form an integrator (not labeled). An inverting input terminal of the front operational amplifier15connects to ground through the front grounding resistor23. A non-inverting input terminal of the front operational amplifier15connects to an output u1of the zero-crossing comparator through the first feedback resistor21, and connects to an output u0of the operational amplifier16through the second feedback resistor22. An anode terminal of a first zener diode19connects to the output u1of the zero-crossing comparator. A cathode terminal of the first zener diode19connects to a cathode terminal of a second zener diode20, and an anode terminal of the second zener diode20connects to ground.

In use, the output u1of the zero-crossing comparator is clamped to be a symmetric bipolar square wave by the zener diodes19,20. The integrator integrates the output u1of the zero crossing comparator, and outputs the desired triangle wave voltage signal u0, i.e., the signal Vr2shown inFIG. 2.

The circuit of the PWM current adjustment apparatus of the present invention is simple since it uses a single path. The Fourier series expansion for the symmetric triangle wave signal Vr2is:
V=(8/π2)Vm[sin wt−( 1/9)sin 3 wt+( 1/25)sin 5 wt+ . . . +(−1)n−1/(2n−1)2sin (2n−1)wt + . . . ].
Comparing this equation for the symmetric triangle wave signal Vr2with the Fourier series expansion of the prior art sawtooth wave signal Vr1, note that the equation for the signal used in the present invention only comprises odd harmonics, no even harmonics. A coefficient of an Nth order harmonic of the symmetric triangle wave is 8/[π2×(2N−1)2], which is much less in magnitude than that of a sawtooth wave signal, 2/(π×N). In other words, the symmetric triangle wave signal includes smaller magnitude harmonic components, and induces lower noise as a result. All of these significantly increase system stability. And, it is thus easier and less costly to build a triangle wave generator than a sawtooth wave generator for high frequency operation.

In additional, the PWM current adjustment apparatus of the present invention is not limited to using the triangle wave generator described above. Other, more precise generators can be used in the PWM current adjustment apparatus of the present invention. A more precise triangle wave generator may be more complex than the triangle wave generator described above, but compared to more precise sawtooth wave generators, the triangle wave generator is still simpler, because of simple, integral circuit employed. And, in cases where the modulation voltage source signal amplitude is out of a range between a maximum and a minimum of a triangle wave signal, the current adjustment apparatus can either just keep comparing the modulation signal and the triangle wave signal in the comparator, or an auxiliary circuit may be added to obtain a desired function, according to the application requirement.