JPS5831041B2 - pulse width modulation amplifier - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a pulse width modulation amplifier that has a simple configuration, high accuracy, and can effectively limit load current.
This is suitable for application to devices that utilize the average value of pulse width modulated voltages.

Figure 1 is a block diagram of a highly accurate pulse width modulation method.
A is an operational amplifier, and a comparator 1 compares voltage e2 and triangular wave voltage e3.

Figure 2 shows its operating waveform diagram, showing the comparator output voltage e
4 is in the state of 11111, and the voltage E41 is assumed to be the voltage E4o in the state of "'0".

If the average value of the voltage e4 is taken as a stone, it has a linear relationship with the voltage E1 due to the characteristics of the operational amplifier, and highly accurate pulse width modulation can be performed using the voltage E1.

The circuit of FIG. 3 can be considered to be equivalent to the addition of the outputs of two integrators of operational amplifiers A and A' as shown in FIG. 4, and operates in the same way as the circuit of FIG. 1.

When a power amplifier is configured using such a pulse width modulator, amplification with little distortion is possible, but it is necessary to protect the power amplifier and the load from overcurrent.

Conventionally, current limiting in this type of amplifier has been done by using a fuse to feed back the load current to the input signal, but this method effectively suppresses instantaneous overcurrent and returns to normal operation when the load current decreases. There were not enough limiting characteristics.

The present invention has been made in view of these points, and will be described below with reference to embodiments shown in the drawings.

FIG. 5 shows an embodiment of the present invention, which constitutes a pulse width modulation power amplifier that can perform highly accurate power amplification and can effectively limit current.

In Figure 5, resistors R11R2, R3, capacitor C
2C5 and the summing/integrating circuit consisting of the operational amplifier A, the input signal E1, the bias signal e3', and the comparator 1 are the same as those shown in FIG.

Further, 11, 1□ and 13 are all comparators, 2 and 4 are amplifiers, Trl and Tr2 are output switching transistors, and the transistors Trl and Tr2 are turned on alternately based on the output of the amplifier 2.

Dl and D2 are diodes that provide an active current path when the load is inductive, and R4 is a current detection resistor. Since this resistor R4 uses a very small resistance value, it has little effect on the output voltage. do not.

The DC power source Es is divided into two parts, and in the above equation (2), R41=ES, R40-ES.

The duty factor of the output square wave is becomes.

That is, the transistors Tr1 and Tr2 and the DC power supply E
Since s has a bridge configuration, in this circuit, when the utility function is high, the output voltage becomes O, and is positive.
It operates as a proportional amplifier in which the output power (1) is proportional to the input signal E1 over the negative range.

Note that when this circuit is used as an audio amplifier, the load is a speaker, and a frequency higher than an audible frequency is used for the bias signal e3'.

Next, the circuit operation of the embodiment shown in FIG. 5 will be explained.
In this circuit, the output current ■ is detected by the resistor R4, amplified by the amplifier 4, and then compared with the current setting values 11 and I2 by the comparators 12 and 13, and then by the AND circuit AND and the OR circuit OR.
It is combined with the output of comparator 1.

Here, when the current ■ is in the range of 12〈I〈I1, the output of the comparator 1□ becomes "1", the output of the comparator 13 becomes "0", and the output of the logical sum circuit OR and the logical product circuit AND becomes "1". In this case, the output of comparator 1 is equivalent to that connected directly to the input of amplifier 2, and the output voltage ■ is determined by the input signal E1 as shown in characteristic A in Figure 6, and the voltage of the load is determined only by the output. If the current characteristic is on the characteristic, the intersection point a on the characteristic is the operating point.

In this way, the output voltage (2) can be varied between the voltages Ed and -Ed by the input signal E1, and the current I is determined by the output voltage (2) and the load impedance.

That is, in this case, when the load impedance is constant, a power amplification operation proportional to the input signal E1 is performed.

However, in the above circuit, due to changes in load impedance, etc., this current I changes to the set value 1 in the positive direction or in the negative direction.
1. If the current is about to increase beyond I2, the current limiting operation is performed as follows.

First, the current limiting operation in the positive direction will be explained. This is performed by the operation of the AND circuit AND.

That is, assuming that the input signal E1 is fixed as shown in FIG. 7, if the load characteristics are normal, the operating point is a, and the circuit operates as a voltage amplifier.

Suppose that the impedance of the load decreases and becomes the characteristic of the motor.

If the AND circuit AND is not connected, the operating point is a'
However, the current I exceeds the set value 1 before the operating point reaches a', and a period of zero occurs in the output of the comparator 12.

The pulsations in the load current during one cycle of operation of the transistors Tr1 and Tr2 have a waveform that increases during the on-period of the transistor Trl and decreases during the off-period of the transistor Trl. The period always occurs during the "1" period of comparator 1.

Since the logical product circuit AND outputs the two inputs °"0" with priority, the duty factor of its output is smaller than the duty factor of the output of comparator 1□, and the voltage ■ is smaller than -EIR2/Rt. .

At this time, the output of the addition and integration circuit shifts to the positive direction, and the output of the comparator 11 is always '1', and the output of the AND circuit AN
The output of D is determined only by comparator 1□.

That is, at this time, in the circuit of FIG. 5, only the loop of resistor R4, amplifier 4, comparator 12, and amplifier 2 operates effectively, and transistors Tr1. On/off of Tr2 is determined, and constant current control with a set value of 11 is performed.

The output characteristic is the characteristic A in FIG. 7, and the operating point is the intersection point with '' in the characteristic.

Even if the load impedance further decreases, the operating point moves above the characteristic A, and the current ■ decreases, but the current ■ remains 11 (constant).

Conversely, when the load impedance becomes larger than ' due to the characteristics, the operating point moves above point b and the voltage ■ becomes -EIR2/
If the current becomes larger than R1, the comparator 11 starts operating and the output voltage is maintained at -EI R2/R1, so the current decreases from 11 and the output of the comparator 12 is always "1". It becomes unrelated to control and returns to the proportional amplification operation described above.

On the other hand, the current limiting operation in the negative direction is performed by the logical sum circuit OR.

As shown in Fig. 8, if the operating point C on the characteristic A tries to move to the C' point, the current I becomes more negative than the set value ■2, so the output of the comparator 11 is "011" (Tr2 is on). ), a period of “1” occurs in the output of the comparator 13,
Since the logical sum circuit OR has "1" priority, the voltage ■ is -EI
It is more positive than R2/R1.

At this time, the output of the summing and integrating circuit moves to the negative direction, the output of the comparator 11 is always "Oll", and the output is the resistor R4,
A constant current control operation of a set value I2 is performed in the loop of the amplifier 4, comparator 13, and amplifier 2, and at this time, the output characteristic becomes the characteristic shown in FIG. 8, and the operating point becomes d.

Thus, the setting value ■1. The output characteristic when current is limited by I2 is as shown in FIG.
The characteristic A and A are fixed by the current setting value.

As described above, according to the present invention, a pulse width modulation amplifier including a switching amplifier is constructed, and a current limiting function is combined and introduced inside the pulse width modulation amplifier using a logic circuit, thereby achieving highly accurate switching amplification. , effective current limiting can be achieved.

Note that the current limiting function limits the maximum value or minimum value of the load current, and can be provided individually or simultaneously.

Also, whether to use logical sum or logical product in the logic circuit to be combined for current limiting depends on whether each comparator has positive logic or negative logic, and whether the amplifier is a positive phase amplifier or a negative phase amplifier. It depends.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram of the pulse width modulation method, and Figure 2 is the circuit diagram of the pulse width modulation method.
Figure 3 is a circuit diagram of another pulse width modulation method, Figure 4 is a diagram explaining the operation of Figure 3, and Figure 5 is a circuit diagram of the case where the present invention is implemented in a switching amplifier. , FIGS. 6 to 9 are characteristic curve diagrams of the circuit of FIG. 5. In the figure, A, A': operational amplifier, 1: comparator, 2: output circuit, 4: amplifier, Es: power supply.

Claims (1)

[Claims] 1 The logic of the first comparator output that adds and integrates the human power signal and the output voltage and compares it with the bias signal to obtain a pulse width modulation signal, and the second comparator output that compares the load current with a set value. When a switched amplifier is driven by a product or logic output, and the magnitude of the load current of the amplifier is about to become larger than the set value in the positive or negative direction,
A pulse width modulation amplifier characterized in that the output characteristics of the amplifier are automatically switched from proportional amplification characteristics to constant current characteristics.