JPH09135568A - Dc/dc converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To maintain a switching frequency at a constant value even if an input voltage or an output voltage is changed. SOLUTION: If a coil current IL is increased and exceeds a reference current Iref, a flip-flop 11 is reset and switches 5 and 12 are turned off. If the switch 5 is turned off, the coil current IL is attenuated. If the switch 12 is turned off, a current source 13 charges a capacitor C2 with a current Is proportional to an input voltage Vin and the potential of a point A is lowered. If the potential of a point A is lowered to an output voltage Vout, the flip-flop 11 is set and the switches 5 and 12 are turned on. If the switch 5 is turned on, the coil current IL is increased again. If the switch 12 is turned on, the potential of the point A is elevated to the input voltage Vin.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a DC / DC converter, and more particularly to a current mode DC / DC converter.

[0002]

2. Description of the Related Art A DC / DC converter is a device that converts a certain direct current input voltage into a different direct current voltage and outputs it, and is used for various purposes.

FIG. 5 is a diagram showing the structure of a conventional DC / DC converter. This DC / DC converter monitors the coil current IL and performs feedback control using the coil current IL to change the input voltage Vin to the output voltage V
produces out.

In FIG. 5, when the switch 5 is in the ON state, the coil current IL increases and becomes larger than the current consumed by the load, electric charges are accumulated in the capacitor C1 and the output voltage Vout rises. Comparator 1
Compares the coil current IL detected by the sensor 2 with the reference current Iref and transfers the comparison result to the monostable multivibrator 3.

The monostable multivibrator 3 is set with a time constant determined by the resistance value of the connected resistor and the capacitance of the capacitor, and the coil current IL is the reference current I.
When the comparison result indicating that the value becomes larger than ref is received from the comparator 1, a pulse having a pulse width of the time constant is output. The output of the monostable multivibrator 3 is a signal that controls the on / off state of the switch 5, and keeps the switch 5 off while the pulse is being output. That is, the coil current IL is the reference current Iref.
When it becomes larger than this, the switch 5 is turned off for the period of the time constant set in the monostable multivibrator 3.

When the switch 5 is turned off, the coil current IL decreases, and when it is smaller than the current consumed by the load, the electric charge is supplied from the capacitor C1 to the load,
The output voltage Vout decreases. When the time constant set in the monostable multivibrator 3 elapses, the switch 5 returns to the ON state, the coil current IL increases again, and the output voltage Vout increases.

By repeating the above operation, the output voltage Vout is stabilized at a predetermined value. The output voltage Vout fluctuates (ripples) by switching the switch 5 as described above, but the fluctuation width is negligible compared to the magnitude of the output voltage Vout,
It can be considered that the output voltage Vout is stable at a predetermined value.

This DC / DC converter has a set voltage V
Depending on the ref setting, the desired output voltage (eg 3V, 5
V etc.) can be generated. That is, the reference current Iref is the output of the amplifier 4, and the set voltage Vref and the output voltage Vout
Is output as a reference signal for feedback to make the output voltage Vout match with the set voltage Vref by feedback control using the reference current Iref. As described above, when the output voltage Vout is stable at a predetermined value, the reference current Iref can also be regarded as stable at a constant value.

FIG. 6 (a) is a diagram showing the coil current IL in the DC / DC converter of FIG. While the switch 5 is in the ON state, the coil current IL is (Vin-Vout) /
When the coil current IL reaches the reference current Iref, the switch 5 is turned off when the coil current IL reaches the reference current Iref. During the period in which the switch 5 is in the OFF state, the coil current IL decreases with a slope proportional to -Vout / L due to the influence of the counter electromotive force generated by the coil L. The DC / DC converter of FIG. 5 stabilizes the output voltage Vout at a predetermined value (set voltage Vref) by repeating this cycle. The repetition frequency of this cycle (hereinafter, switching frequency) is
It is about several tens of kHz to several hundreds of kHz.

The duty of the above cycle (the ratio of the time that the switch 5 is in the ON state to the time required for one cycle) can be calculated as follows. Here, the time required for one cycle is 1, the period in which the switch 5 is in the on state is D, and the period in the off state is (1-D).

When the output voltage Vout is stable at a predetermined value, time T1 to time T3 is one cycle, and time T
Since the coil current IL at 1 and the coil current IL at time T3 match, the increase in the coil current IL from time T1 to time T2 is the coil current from time T2 to time T3. The following equation is derived by utilizing the fact that it is equal to the decrease amount of IL. D · (Vin−Vout) / L = (1−D) · Vout / L Therefore, D = Vout / Vin (1) Formula As described above, when the output voltage Vout is generated from the input voltage Vin, the duty is D is the input voltage Vin and the output voltage Vou
It depends on the ratio with t.

[0012]

By the way, the input voltage V
in is, for example, when using a battery as a power source,
That value may change. That is, generally, when a fully charged battery is used, the output voltage of the battery gradually decreases with time. Also, in some DC / DC converters, the output voltage Vout is set to an arbitrary value according to the needs of the user (for example,
3V, 5V, etc.).

In this way, when the input voltage Vin changes or when the set value of the output voltage Vout changes,
As is clear from the above formula (1), the switch 5 is turned on /
The duty D representing the off state changes. That is, the ratio of the period in which the switch 5 is on and the period in which the switch 5 is off changes.

However, as described with reference to FIG. 5, in the conventional DC / DC converter, the period in which the switch 5 is turned off is fixed by using the monostable multivibrator 3 having a preset time constant. Was decided. Therefore, when the duty D changes due to the change of the input voltage Vin or the output voltage Vout, the switching frequency changes. How the switching frequency varies will be described with reference to FIG. Note that FIG. 6 (a)
6 (c), the scales on the time axis are the same.

FIG. 6 (b) is a diagram showing the coil current IL when the input voltage Vin is lower than in the state shown in FIG. 6 (a). However, in FIGS. 6A and 6B, the same output voltage Vout (for example, 5 V) is generated.

As described above, when the switch 5 is in the ON state, the coil current IL increases with a gradient proportional to (Vin-Vout) / L, and when the switch 5 is in the OFF state, -Vout / L.
It decreases with a slope proportional to. Therefore, when the input voltage Vin decreases, the gradient of the coil current IL increasing while the switch 5 is in the ON state becomes gentle, and the coil current IL becomes the reference current Iref as compared with the case of FIG. 6 (a). It takes longer to reach. On the other hand, the period during which the switch 5 is in the OFF state (indicated by TP in the figure) is fixedly determined by using the monostable multivibrator 3, so that the input voltage Vin
Does not change even if changes.

Therefore, when the input voltage Vin decreases,
The cycle for controlling the switch 5 becomes longer and the switching frequency becomes lower. FIG. 6C is a diagram showing the coil current IL when the output voltage Vout lower than that in the state shown in FIG. 6A is generated. Here, FIG. 6 (a) and FIG.
In FIG. 6C, the input voltages Vin are the same, and different output voltages Vout are generated from the input voltages Vin (for example, 5V is generated in FIG. 6A and 3V is generated in FIG. 6C). Indicate the case.

When the output voltage Vout is set low, the difference between the input voltage Vin and the output voltage Vout becomes large, so that the coil current IL increases steeply while the switch 5 is in the ON state, and the coil current IL becomes smaller. The time required to reach the reference current Iref is shortened. On the other hand, during the period in which the switch 5 is in the off state (indicated by TP in the figure), it does not change even if the output voltage Vout is set low.

Therefore, when the output voltage Vout is set low, the cycle for controlling the switch 5 becomes short and the switching frequency becomes high. If the switching frequency of the DC / DC converter fluctuates in this way, there are the following problems. That is, for example, by changing the switching frequency of the DC / DC converter,
If a certain frequency related to the operation of the device incorporating the DC / DC converter matches the switching frequency of the DC / DC converter (or a positive multiple), the device may malfunction. DC / DC
When a noise filter is provided to suppress radiated noise from the converter, when the switching frequency fluctuates, a configuration for suppressing noise in a wide band is required, which increases the cost.

In order to prevent the above problems, a DC / DC converter is desired in which the switching frequency is kept constant even when the input voltage Vin or the output voltage Vout changes.

An object of the present invention is to provide a DC / DC converter in which the switching frequency does not change even when the input voltage or the output voltage changes.

[0022]

The DC / DC converter of the present invention is premised on a configuration in which a coil current is changed by controlling a switching means to control an output voltage.

In the DC / DC converter according to any one of claims 1 to 4, an OFF time generating means for generating a time that is inversely proportional to the input voltage and proportional to the difference between the input voltage and the output voltage, and the coil current is equal to or larger than the reference current. The control means for turning off the switching means when the above condition is triggered, and for turning on the switching means when the time generated by the off-time generating means elapses after the switching means is turned off. Have.

The off-time generating means includes a current source for supplying a current proportional to an input voltage and a capacitor. When the capacitor is charged using the current source, the voltage across the capacitor is equal to the input voltage and the output voltage. By measuring the time required to change by the difference between and, a time is produced that is inversely proportional to the input voltage and proportional to the difference between the input voltage and the output voltage. At this time, the reference current is controlled so that the preset desired output voltage and the actual output voltage match.

In the above configuration, when the input voltage of the DC / DC converter changes or when the set value of the output voltage changes, the time generated by the off-time generating means is determined according to the change, and the switching means. The time when is turned off changes. At this time, the time during which the switching means is in the on state changes by feedback control of the output voltage and the coil current so as to cancel out the change in the time during which the switching means is in the off state. As a result, the sum of the time when the switching means is in the on state and the time when the switching means is in the off state becomes constant, and the switching frequency does not change.

In the DC / DC converter according to any one of claims 5 to 7, when the switching means is in the ON state, the same voltage as the input voltage is generated, and after the switching means transitions from the ON state to the OFF state, the switching means is activated. Control voltage generation means for changing the voltage generated in the on state at a rate proportional to the difference between the input voltage and the output voltage, and the switching means in the off state triggered by the coil current being equal to or greater than the reference current, The control means for turning on the switching means is triggered by the voltage generated by the control voltage generating means becoming equal to or lower than the output voltage.

The control voltage generating means includes a current source for supplying a current proportional to the input voltage, and a capacitor for applying the input voltage to the first terminal and connecting the second terminal to the current source. When the switching means is in the off state, the current source is used to change the amount of charge accumulated in the capacitor, and the voltage at the second terminal of the capacitor is used as the generated voltage.

In the above configuration, when the input voltage of the DC / DC converter changes or the set value of the output voltage changes, the voltage generated by the control voltage generating means is changed according to the change. The time required to decrease from the input voltage value to the output voltage value changes. Since this time is the time when the switching means is in the off state, when the input voltage or the output voltage of the DC / DC converter changes, the time when the switching means is in the off state changes according to the change. At this time, the time during which the switching means is in the on state changes by feedback control of the output voltage and the coil current so as to cancel out the change in the time during which the switching means is in the off state. Therefore, the sum of the time when the switching means is in the on state and the time when the switching means is in the off state is constant, and the switching frequency does not change.

[0029]

Embodiments of the present invention will be described below with reference to the drawings. DC / DC of this embodiment
The converter has a configuration in which the switching frequency does not change even when the input voltage or the output voltage changes. However, the change in the input voltage here does not mean a transient change that occurs in an extremely short time. For example, when the input voltage to the DC / DC converter is supplied using a battery, the passage of time At the same time, it is assumed that the output voltage of the battery will gradually decrease. Regarding the output voltage, it is not assumed that the output voltage changes abruptly due to a change in load current, but is output by, for example, the user setting the output voltage of the DC / DC converter to a predetermined value. It is assumed that the voltage changes.

FIG. 1 is a diagram showing the configuration of the DC / DC converter of the present invention. This DC / DC converter generates an output voltage Vout from an input voltage Vin. 1, the same reference numerals as those used in FIG. 5 indicate the same parts.

In FIG. 1, the output of the comparator 1 is
It is connected to the reset terminal of the flip-flop 11. The output of the flip-flop 11 controls the on / off state of the switch 5 and the switch 12.

A switch 12, a capacitor C2, a current source 13, and a comparator 14 are provided at the input part of the DC / DC converter. The input voltage Vin is applied to one terminal of the capacitor C2, and the current source 13 is applied to the other terminal.
Is connected. The current source 13 supplies a current proportional to the input voltage Vin. The switch 12 is a flip-flop 1
The output of 1 controls its on / off state.
When the switch 12 is on, both ends of the capacitor C2 are short-circuited, and the capacitor C2 is in a discharged state. The comparator 14 compares the output voltage Vout with the potential at the point A (terminal of the capacitor C2 on the side connected to the current source 13). The output of the comparator 14 is connected to the set terminal of the flip-flop 11.

FIG. 2 is a circuit diagram showing an example of the current source 13. In the current source 13 shown in the figure, the current Is can be expressed as follows. Is = Vin / R2 (K + 1) (2) The circuit that generates a current proportional to the input voltage Vin is
The configuration is not limited to that shown in FIG.
A current mirror circuit may be used.

Next, the operation of the DC / DC converter shown in FIG. 1 will be described. While the switch 5 is in the ON state, the coil current IL increases with a slope proportional to (Vin-Vout) / L. The comparator 1 compares the coil current IL detected by the sensor 2 with the reference current Iref, and outputs “H” when the coil current IL becomes equal to or higher than the reference current Iref.

Since the output of the comparator 1 is connected to the reset terminal of the flip-flop 11, when the coil current IL exceeds the reference current Iref, "H" is input to the reset terminal of the flip-flop 11 and the flip-flop 11 Is reset. Flip-flop 1
When 1 is reset, the switches 5 and 12 are turned off.

When the switch 5 is turned off, the coil current IL decreases with a slope proportional to -Vout / L. On the other hand, when the switch 12 is turned off, the capacitor C2 is charged by the current source 13.

In FIG. 1, the current Is generated by the current source 13 causes the positive charge to be extracted from the capacitor C2. However, focusing on the negative charge, the current source 13 accumulates the charge in the capacitor C2, which causes the capacitor C2 to accumulate. The voltage across C2 increases. In the description of the present embodiment, increasing the voltage across the capacitor C2 by accumulating electric charge in the capacitor C2 is called "charging", and short-circuiting both ends of the capacitor C2 makes the potential difference across the capacitor C2 zero. This is called "discharge".

As described above, when the switch 12 is turned off, negative charges are accumulated in the capacitor C2 (the capacitor C2 is charged by the current source 13), and A
The potential at the point decreases. Then, when the potential at the point A drops to the output voltage Vout, the comparator 14 outputs "H".
Is output.

Since the output of the comparator 14 is connected to the set terminal of the flip-flop 11, when the potential at the point A drops to the output voltage Vout, "H" is input to the set terminal of the flip-flop 11 and 11 is in the set state. When the flip-flop 11 is set, the switch 5 and the switch 12 are turned on. When the switch 5 returns to the ON state, the coil current IL increases again. When the switch 12 is turned on, both ends of the capacitor C2 are short-circuited and the potential at the point A matches the input voltage Vin.

By repeating the above operation, the output voltage Vout is stabilized at a predetermined value. The reference current Iref is the output of the amplifier 4 and is output as a reference signal for feedback for matching the set voltage Vref with the output voltage Vout. The feedback control using the reference current Iref causes the output voltage Vout to be output. To match the set voltage Vref.

FIG. 3 is a timing chart showing the coil current, the potential at point A, and the outputs of the two comparators in the DC / DC converter shown in FIG. In addition,
When the flip-flop 11 is in the set state, the switch 5 and the switch 12 are in the on state, and in the reset state, they are in the off state.

While the switch 5 and the switch 12 are in the ON state, the coil current IL increases, and when the coil current IL becomes equal to or higher than the reference current Iref at the time T1, the comparator 1 outputs a pulse. When the flip-flop 11 receives the pulse output from the comparator 1, the flip-flop 11 transitions to the reset state and turns off the switch 5 and the switch 12.

When the switch 5 is turned off, the coil current IL decreases. When the switch 12 is turned off, the capacitor C2 is charged. here,
The amount of charge accumulated in the capacitor C2 is Q, and the capacitor C is
If the capacitance of 2 is C, the elapsed time from time T1 is t, and the voltage across capacitor C2 is ΔV, then the relationship of Q = Is · t = C · ΔV holds, so the potential VA at point A is as follows. It is expressed by equation (3). VA = Vin-.DELTA.V = Vin-Is.t / C (3) As described above, the potential VA at the point A decreases with time according to the equation (3). Then, at time T2, when the potential VA at the point A drops to the output voltage Vout,
The comparator 14 outputs a pulse. When the flip-flop 11 receives the pulse output from the comparator 14, the flip-flop 11 transits to the set state and turns on the switch 5 and the switch 12.

When the switch 5 returns to the ON state, the coil current IL starts to increase again, and when the coil current IL becomes equal to or more than the reference current Iref at time T3, the cycle after time T1 is repeated. On the other hand, when the switch 12 returns to the ON state, the potential VA at the point A becomes equal to the input voltage Vin, and thereafter, the value is fixed to that value until the flip-flop 11 enters the reset state. Thus, this DC /
The DC converter repeats the operation from time T1 to time T3 as one cycle.

Next, it will be described that the repetition frequency of the above cycle (the switching frequency of the switch 5) does not depend on the input voltage Vin or the output voltage Vout. Time T1
, The potential VA at the point A coincides with the input voltage Vin, so the voltage across the capacitor C2 is zero. Also,
At time T2, the potential VA at point A is the output voltage Vout
Therefore, the voltage across the capacitor C2 at this time is the difference (Vin-Vou) between the input voltage Vin and the output voltage Vout.
expressed as t). The change in the voltage across the capacitor C2 during this period is caused by the charge accumulated in the capacitor C2 by the current Is from the time T1 to the time T2. Therefore, assuming that the time from time T1 to time T2 (the time during which the switch 5 and the switch 12 are in the off state) is Toff, the following relationship holds. Is * Toff / C = Vin-Vout Therefore, Toff = C (Vin-Vout) / Is is obtained. Here, the current Is generated by the current source 13
Is proportional to the input voltage Vin, so if Is = K1.Vin, the following equation (4) can be obtained. Toff = C (Vin−Vout) / (K1 · Vin) = K2 · (Vin−Vout) / Vin (4) where K2 = C / K1 is a constant. Thus, the time T during which the switch 5 and the switch 12 are in the off state
off has a value that is inversely proportional to the input voltage Vin and is proportional to the difference between the input voltage Vin and the output voltage Vout.

By the way, as shown in the above equation (1),
The duty D representing the ratio of the ON state of the switch 5 is D
= Vout / Vin, the switching cycle of the switch 5 (the period from the turn-on or turn-off of the switch 5 at a certain timing to the next turn-on or turn-off) is Ts: Toff = Ts (1-D) Become. Here, when the switching frequency is f, f = 1 / Ts = (1-D) / Toff = (1-Vout / Vin) / Toff Equation (5) Therefore, the above equations (4) and (5) When Toff is eliminated from the equation, the following relation is obtained. f = 1 / K2 Thus, the switching frequency becomes a constant that does not depend on the input voltage Vin or the output voltage Vout. When setting the switching frequency to a desired value, the capacitor C
2 or the input voltage Vin and the current Is when the current source 13 generates the current Is proportional to the input voltage Vin.
This is done by appropriately setting the proportionality constant between (a and b) (determined by the ratio of each resistance in the example shown in FIG. 2).

FIG. 4 shows the input voltage Vin or the output voltage Vou.
It is a figure which shows the coil current when t changes. FIG.
Based on (a). FIG. 4 (b) shows the coil current IL when the input voltage Vin is lower than that in the state shown in FIG. 4 (a).
FIG. However, in FIGS. 4A and 4B, it is assumed that the same output voltage Vout (for example, 5 V) is generated and the same load is connected.

As described above, since the switching frequency does not change even when the input voltage Vin decreases, the repetition cycle of the coil current IL is the same. Also,
It means that the coil current IL is (V
in-Vout) / L, the slope increases in proportion to, and in the OFF state, the slope decreases in proportion to -Vout / L. Therefore, when the input voltage Vin decreases, the slope of the coil current IL increasing becomes gentle according to the decrease of the input voltage Vin, but the slope when the coil current IL decreases does not change. Further, in FIG. 4 (b) and FIG. 4 (a), since the same output voltage is output and the same load is driven,
The average values (Iav) of the coil current IL are equal to each other.

The reference current Iref is automatically set to satisfy these conditions, and feedback control using the reference current Iref produces the same output voltage Vout as in FIG. 4 (a).

FIG. 4 (c) is a diagram showing the coil current IL when it is set so as to generate an output voltage Vout lower than that in the state shown in FIG. 4 (a). This setting is the setting voltage V
This is performed using ref, and for example, 5 V is generated as the output voltage Vout in FIG. 4 (a), and 3 V is generated in FIG. 4 (c). It is assumed that the input voltages Vin are the same as each other.

Even when the output voltage Vout is set low, the switching frequency does not change, so that the cycle of the coil current IL is the same. Also, the output voltage Vou
When t is set low, the difference between the input voltage Vin and the output voltage Vout becomes large, so that the coil current IL increases steeply according to the decrease in the output voltage Vout, and the coil current IL decreases. The inclination of becomes gentle. Then, the reference current Iref is automatically set so that the average value of the coil current IL matches the load current, and the desired output voltage Vout is generated by the feedback control using the reference current Iref.

[0052]

The switching frequency of the DC / DC converter does not fluctuate even when the input voltage of the DC / DC converter changes or the set value of the output voltage changes. Therefore, a device incorporating this DC / DC converter. Can be reduced, and radiation noise from the DC / DC converter can be suppressed easily.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of a DC / DC converter of the present invention.

FIG. 2 is a circuit diagram showing an example of a current source 13.

3 is a timing chart showing a coil current, a potential at a point A, and outputs of two comparators in the DC / DC converter shown in FIG.

FIG. 4 is a diagram showing a coil current when an input voltage or an output voltage changes in the DC / DC converter shown in FIG.

FIG. 5 is a diagram showing a configuration of a conventional DC / DC converter.

6 is a diagram showing a coil current in the DC / DC converter of FIG.

[Explanation of symbols]

 1, 14 Comparator 2 Sensor 4 Amplifier 5, 12 Switch 11 Flip-flop 13 Current source C1, C2 Capacitor L coil

Claims (7)

[Claims] 1. A DC / D for controlling an output voltage by changing a coil current by controlling a switching means.
In the C converter, an off-time generation means for generating a time proportional to a difference between an input voltage and an output voltage, and the switching means being turned off by using a coil current as a reference current or more as a trigger to turn off the switching means. A DC / DC converter comprising: control means for turning on the switching means when a time generated by the off-time generation means has elapsed after being turned off. 2. The DC / DC converter according to claim 1, wherein the time generated by the off-time generating means is proportional to the difference between the input voltage and the output voltage and inversely proportional to the input voltage. 3. The off-time generating means includes a current source and a capacitor for supplying a current proportional to an input voltage, and when the capacitor is charged by using the current source, the voltage across the capacitor is equal to the input voltage. The DC / DC converter according to claim 1 or 2, wherein the time is generated by measuring a time required to change by a difference from the output voltage. 4. The DC / DC converter according to claim 1, wherein the reference current is controlled so that a preset desired output voltage and an actual output voltage match. 5. A DC / D for controlling an output voltage by changing a coil current by controlling a switching means.
In the C converter, when the switching means is in the ON state, the same voltage as the input voltage is generated, and after the switching means transitions from the ON state to the OFF state, the voltage generated when the switching means is in the ON state is input. A control voltage generating means for changing the voltage in proportion to the difference between the voltage and the output voltage, and a voltage generated by the control voltage generating means for turning off the switching means triggered by the coil current becoming a reference current or more. And a control means for turning on the switching means when the voltage becomes equal to or lower than the output voltage as a trigger. 6. The control voltage generating means includes a current source for supplying a current proportional to an input voltage, and a capacitor for applying an input voltage to a first terminal and connecting a second terminal to the current source. 6. When the switching means is in the off state, the current source is used to change the amount of charge accumulated in the capacitor, and the voltage at the second terminal of the capacitor is used as the generated voltage. Described D
C / DC converter. 7. The DC / DC converter according to claim 5, wherein the reference current is controlled so that a preset desired output voltage and an actual output voltage match.