RU2409891C1 - Linear pulse width converter with two outputs on digital microchips - schmitt trigger and two inverters - Google Patents

Abstract

FIELD: electricity. ^ SUBSTANCE: invention refers to pulse width converters (PWC) on the basis of Schmitt triggers and can be used at the development of stable pulse power sources, in PWM (pulse width modulation) wires of direct current motors, in pulse computing systems and other devices of measurement equipment and automation. ^ EFFECT: enlarging the application area. ^ 4 cl, 6 dwg

Description

The invention relates to pulse technology, namely to pulse-width converters (PWM) or pulse-width modulators (PWM) based on Schmitt triggers, and can be used in the design of stable switching power supplies in PWM drives of DC motors, during pulse computing systems and other devices of measuring equipment and automation.

The purpose of the invention is the expansion of the functionality of circuits on Schmitt triggers, simplifying the design and improving the reliability of devices.

PWM or PWM (see Fig. 1) generates a sequence of amplitude-normalized pulses with a duty cycle γ, depending on the input voltage U _in . and equal to:

γ = T _and / T, 0 <γ <1,

where T _and is the pulse duration, T is the period of their succession.

SHIP transformation function defined as the ratio γ of the output quantity U to input _Rin.

The function of the linear Converter in the General case is described by the expression:

where γ ₀ is a certain initial value of the duty _cycle at U _in. = 0.

K is the steepness of the transformation function on the linear section, defined as the ratio: K = (Change γ) / (Change U _in ), [1 / B].

For an increasing conversion function, K> 0, for a decreasing K <0.

Known and used in the measuring technique of SHIP on operational amplifiers operating in the two-threshold comparator mode / 1 /, on digital microcircuits - Schmitt triggers, which are a two-threshold device. / 2, 3, 4 /.

The closest analogue of the proposed solution is voltage-controlled square wave (VCO) generators built on digital microcircuits - Schmitt triggers, the structure of which is similar to the proposed solution / 5, p. 81 /.

Our theoretical calculations and experimental results of such a VCO showed that these devices in a significant input voltage range have a linear characteristic inherent in pulse-width converters.

Thus, the application deals with the use of a well-known VCO device for a new purpose - as a NIP with a linear conversion function.

ShIP applied in the technique are generally divided into 2 types of operation:

- in the first type of converters, the change in the coefficient γ from U _in . occurs at a fixed operating frequency (the period T of the pulse sequence is fixed, and only the pulse duration T _{and is} subject to change);

- in the second type, both the duration T _{and the} period T change.

This invention relates to a second type of NIP.

1. The electrical circuit of the inventive device is presented in figure 2.

The NIP is connected to a power supply U _p , the input voltage U _I is applied to the first output of the input resistor 1, the second output of which is connected to the first output of the timing capacitor 2 (the second output of the capacitor is grounded), to the input of the trigger 4 and to the first output of the feedback resistor 3, the second output of which is connected to the output of trigger 4, and then to the input of inverter 5, the output of which is the first output of the circuit for condition K> 0, and to the input of inverter 6, the output of which is the second output of the circuit for condition K <0.

SHIP (on the example of its implementation on logic elements manufactured by CMOS technology) works as follows.

When a pulse is formed (the voltage at the output of the Schmitt trigger 4 is high and close to the supply voltage U _p ), the timing capacitor is charged with the current flowing from the trigger output through the resistor R and the current flowing under the influence of the input voltage U _in through the resistor R _у .

When the voltage at the trigger capacitor reaches the trigger threshold U _с > U _{, the} voltage at its output becomes close to zero, the current through the resistor R changes direction, and the capacitor discharge process begins (pause formation).

The capacitor is discharged until the voltage on it reaches the value of the lower threshold of the trigger U _с <U _н , while at the output of the trigger the voltage close to the supply voltage is established again. The pause ends, the capacitor charge process begins again, and therefore, the formation of the next pulse.

Thus, the converter generates rectangular pulses at its output, the width of which is determined by the ratio of currents, depending on the ratio of the resistances R _y , R, i.e. from R _y / R.

Inverters 5 and 6 are designed to form a normalized voltage at outputs 1, 2 and to exclude the influence of the load on the recharging process of the timing capacitor C, i.e. to work SHIP.

The NIP operates in the input voltage range from U _in.min to U _in.max :

Here U _n , U _{in the} lower and upper thresholds of the Schmitt trigger when the supply voltage U _p ,

R _y , R - resistance of the resistors of the Converter.

The fill factor γ of the converter within the operating range of the input voltage can be represented as:

Where

From relation (4) it can be seen that the dependence of the duty cycle γ on the input voltage is complex and nonlinear.

By calculations and experimental studies of the converter, function graphs are obtained for various ratios of the circuit parameters. In Fig. 3. a generalized graph is presented (initial curve 1) and its approximations by linear dependencies.

The nonlinearity of the function is manifested only near the boundaries of the input voltage range U _I ,, in the main part of the range the function is linear with high accuracy

with the exception of the boundary zones, near which limit values γ (0 or 1) are established.

The linear approximation of the original curve (Figure 3, line 2) corresponds to the function of the linear transducer defined by formula 2.

To evaluate the transformation function γ from above, when designing the NIP, we draw a line 0 connecting the boundary values of the sections of the converter working area, the slope K _{0 of} which is slightly higher than the slope K of the initial curve 1 in its linear section, and is determined by the ratio:

For example, the value of K ₀ (5) with a ratio of R _y / R = 1 is 0.125 1 / V, and the exact value of K obtained by formula (4) is 0.107 1 / V, that is, the overestimation of the estimated value of K is not more than 17 %, which is quite acceptable in preliminary calculations of the scheme. These values were obtained during experimental testing of a converter performed on a CMOS chip of type КР561ТЛ1 (four Schmitt triggers) for a supply voltage of U = 6 V, U _in = 3.7 V, U _n = 1.8 V, capacitor capacitance C = 4700 pF.

The input voltage range at which the linear dependence of γ on u in is satisfied _. is determined by the ratio of R _y / R. So, for the ratio R _y / R = 4, a linear dependence at output 2 is observed in the input signal range from 5.5 V to 9 V, with γ ₀ = 0.6, K = -0.05 1 / V; and for the ratio R _y / R = 1, the region of linear dependence narrows to the range from 2.25 V to 2.7 V, with γ ₀ = 0.75, K = -0.24 1 / V.

Note that the range of the linear region for the input signal with a decrease in the ratio R _y / R narrows with a simultaneous increase in the steepness of the transformation K.

Thus, dependence (5) is quite suitable for estimating the duty cycle of a linear NIS.

2. In a number of practical cases, the input voltage range should be shifted to the region of higher voltages. From relation (3) it is seen that for this it is necessary to simultaneously increase the values of the thresholds of the Schmitt trigger. However, its thresholds are fixed. This problem can be solved by means that do not affect the fixed parameters of the trigger.

Figure 4 shows the offset circuit of the input voltage range using an external voltage divider on resistors R ₁ , R ₂ .

Provided that R ₁ + R ₂ >> R, new values are substituted for the trigger thresholds u _in and U _n , respectively, U _in ¹ and U _n ¹ , namely:

To find new ranges of input voltage, we substitute values (6) into expression (3) and obtain:

Thus, the input voltage range is shifted to the region of positive values in proportion to the value (1 + R ₁ / R ₂ ) with a simultaneous increase in its width.

3. In some cases, it is required to change only one boundary of the input voltage in the linear section of the NIP operation. To do this, a 7-field-effect transistor with an induced gate is added to the converter circuit (see Fig. 4) (see Fig. 5). With a low input voltage, a logic 0 is applied to the Schmitt trigger input, a high voltage is generated at its output, transistor 7 open and the upper input threshold is determined by the ratio of R ₁ and R _{21 of the} divider, i.e., similarly (6):

For a high input voltage, a logical 1 is created at the input of the trigger, a low voltage is output, the transistor is locked, the divider is turned off, and the lower threshold is simply equal to the initial threshold of the Schmitt trigger itself. The resistance R ₁ does not affect the threshold, since CMOS circuits have a high input resistance (insulated gate), then:

U _n ¹ = U _n

If R ₁ >> R, then in formula (3), substitute U _{in the} value U _in (1 + R ₁ / R ₂₁ ). In this case, the upper boundary of U _{in, max} remains unchanged, and the lower one increases in (1 + R ₁ / R ₂₁ ) times, which changes the scope of the converter.

Thus, the new input voltage range is set within:

4. If it is necessary to change the boundaries of the input voltage range from two sides, you can use the SHIP scheme with two dividers and two keys (see figure 6).

A resistor of the second divider R ₂₂ and a key 8 are added to the circuit in Figure 5. With a low input voltage, a logic 0 is applied to the Schmitt trigger input, a high voltage is generated at the output, transistor 7 is open and the upper input threshold is determined by the ratio of R ₁ and R _{21 of the} divider, i.e., similarly (6):

For a high input voltage, a logical 1 is created at the input of the trigger, a low voltage is output, the transistor 7 is locked, and the transistor 8 is opened, connecting the voltage divider at the resistances R ₁ and R ₂₂ , which determines the lower threshold of the trigger:

Thus, the new input voltage range is set within:

Information sources

1. Yalyshev A.U. et al. Time-pulse blocks for converting electrical signals for differential pressure flow meters Sapphire-22. Sat scientific works. M .: NIITeplopribor, 1983.

2. Mironov A.A. Pulse Width Modulator. Utility Model Certificate RU 25231 U1, 2002.

3. Mironov A.A. Pulse Width Modulator. Certificate for utility model RU 21097 U1, 2001.

4. Mironov A.A. Pulse Width Modulator. Utility Model Certificate RU 34827 U1, 2003.

5. Braga Newton S.135 amateur radio devices on a single chip. Per. from English M .: DMK Press, 2007.

Claims (4)

1. Pulse-width converter (SHIP), assembled according to the scheme of a voltage-controlled generator (VCO) and containing a Schmitt trigger, two inverters, two resistors and one timing capacitor, and the input voltage U _I applied to the first output of the input resistor, the second output of which connected to the first output of the capacitor (the second output of the capacitor is grounded), to the input of the Schmitt trigger and to the first output of the feedback resistor, the second output of which is connected to the output of the trigger, and then to the input of the first inverter, the output of which is tsya first output circuit for converting a positive slope, and the input of the second inverter, whose output is the second output of the circuit for converting a negative slope,
characterized in that the well-known VCO scheme is used as the NIP, in which a linear dependence of the duty cycle of a sequence of rectangular pulses on the input voltage with a conversion coefficient K ₀ determined by the estimated ratio is realized:

in the range of input voltage:

where U _n , U _in - the lower and upper thresholds of the Schmitt trigger when the supply voltage U _p , R _y - input resistance, R - resistance in the feedback circuit of the Converter. 2. SHIP according to claim 1, characterized in that, in order to change the working zone of the converter by shifting the signals to the positive region of the input voltages and increasing the width of the zone of operation of the converter, an external voltage divider on 2 resistors is added to it, which is the output of the first the resistor is connected to the second terminal of the input resistor, the first terminal of the timing capacitor, the first terminal of the feedback resistor, while the trigger input is connected to the midpoint of the divider (second terminal of the second resistor of the divider azemlen), with modified input voltage range in accordance with the relations:

3. SHIP according to claim 2, characterized in that, in order to increase the minimum value of the input voltage of the converter while maintaining the maximum value, a key (a field effect transistor with an induced gate) is added to it, the drain of which is connected to the second output of the second resistor of the external voltage divider , the source is grounded, and the gate is connected to the output of the trigger, where the following input voltage limits are set:

4. SHIP according to claim 3, characterized in that, in order to change the working zone of the converter by shifting the signals to the positive region of both edges of the input voltage of the converter, an additional resistor is introduced into it, the first output of which is connected to the midpoint of the voltage divider, the second switch ( field-effect transistor with an induced gate), the drain of which is connected to the second output of the additional resistor, the source is grounded, and the gate is connected to the output of the second trigger, where the new input voltage range is set within:

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