GB2123188A

GB2123188A - DC-DC Converter

Info

Publication number: GB2123188A
Application number: GB08218753A
Authority: GB
Inventors: David Nyman
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1982-06-29
Filing date: 1982-06-29
Publication date: 1984-01-25
Also published as: GB2123188B

Abstract

In a fly-back converter, a switching transistor (T1) has its base input controlled by a pulse-width modulator (PWM). Each pulse causes energy to be stored in the transformer (TF) which energy is delivered to the load (R1) via a diode (D) when the pulse ends. A feedback connection from the storage capacitor (C1) is used to control the pulse width modulator (PWM), and hence the output voltage. To ensure stability of operation, a small resistor (R4) is connected in series with the storage capacitor (C1), so that the feedback loop is predominantly inductive, at the highest operating frequency, since the impedance of the resistor (R4) swamps that of the capacitor (C1). This leads to improved stability due to the modified phase characteristic so obtained. <IMAGE>

Description

SPECIFICATION DC-DC converter This invention relates to a DC-to-DC converter, especially for use as the so-called programmable voltage source in a telephone line circuit. Such a source is used to supply the line current needed, and this is adjustable, i.e. programmable, to cater for a wide range of line loop resistances. However, it will be appreciated that the DC-to-DC converter to be described herein has other uses.

The main components of a conventional flyback converter are the transformer, switch, capacitor and diode, as can be seen from Figure la. In this circuit, the switch is formed by a transistor T1, which produces a pulsed current in the primary of the transformer TF1. On each pulse energy is stored in the transformer, which energy is released via diode D to the capacitor C1. Thus a smoothed direct voltage is developed across the load, represented by a resistor R1. The voltage output is controlled by a pulse width modulator PWM, clock pulse controlled as shown. To control the output characteristics, negative feedback is used, via the connection shown from the capacitor C1 via resistor R2 to the negative input of an operational amplifier A. Connected across this amplifier as shown is a further capacitor C2, the dominant lag capacitor.The output of the amplifier A is an error voltage VE which controls the pulse width modulator PWM. Thus the output is controlled.

Figure ibis the AC model of the basic converter shown in Figure la, and is valid where f [VE(ac)] is considerably less than the switching frequency. This converter model for a "no loss" system includes the original output capacitor C1 in series with a variable inductor, whose actual value is a function of the mark-space ratio, as defined by VE. Such a network can introduce 1800 phase shift. To ensure stability when connected in a feedback loop, the method usually used is to produce a dominant pole elsewhere within the loop, e.g. by C2. Unfortunately this tends to severely restrict the frequency response of the closed loop system.

An object of the invention is to provide a DC-to-DC converter which overcomes the above indicated difficulties in a simple manner.

According to the present invention there is provided a DC-to-DC converter, which includes a switching transistor driven by the input DC and transformer-coupled to a rectifier which supplies the output of the converter, an output capacitor in series with a resistor across said output control means connected to the base of the transistor so as to vary its on-off ratio, an amplifier via which the direct voltage input of the converter drives switching transistor, and a feedback connection from the converter output to the amplifier via which the output condition of the converter so influences the input condition of the amplifier as to maintain stability, whereby the series combination of the capacitor and the resistor ensures that the reactance at the converter output is predominantly inductive at high frequencies.

According to the present invention, there is also provided a DC-to-DC converter, which includes a switching transistor whose emitter-collector path is in series with the primary of a transformer, a rectifier connected between the secondary of the transfor mer and the output of the converter, an output capacitor in series with a resistor across the output of the converter, a pulse-width modulator connected to the base of the transistor so as to vary its on-off ratio, an amplifier connected to the direct voltage input of the converter and whose output drives the modulator, and a feedback connection from the converter output to the amplifier via which the output condition of the converter so influences the input condition of the amplifier as to maintain stability, wherein when the transistor is on its conduction causes energy to be stored in the transformer, which energy is released to the converter output when the transistor is off, so that the output voltage of the converter depends on the on-off ratio of the transistor, and wherein the series combination of the capacitor and the resistor ensures that the reactance at the converter output is predominantly inductive at high frequencies. An embodiment of the invention will now be described with reference to the accompanying drawings, in which Figure2a is a high speed DC-to-DC converter embodying the invention, and Figure 2b is an AC model for the conventer of Figure 2a.

Figure 3a is a Bode plot for the circuit of Figure 1 and Figure 3b is a Nyquist plot for the circuit of Figure 1.

Figure 4a is a Bode plot for the circuit of Figure 2 and Figure 4b is a Nyquist plot for the circuit of Figure 2.

In many respects the operation of the circuit of Figure 2 is similarto that of Figure 1, so we will concentrate the description on the points of difference. Similar references are used in the two Figures.

The basis of the novel method is to eliminate the need for a dominant lag in the feedback system by restoring the 1800 phase shift, introduced by the converter at higher frequencies, to 90". This is achieved by adding a small resistor R4 in series with the output capacitor C1. This resistor in one case had a value of 10 ohms. The amplifier A is a fast one, i.e.

one with a fast response to changes in its input parameters We now refer to Figure 2b, which is the AC model for the circuit of Figure 2a and is valued when f[VE(aC,] is considerably less than the switching frequency of the transistor T1. As the frequency increases, the reactance of L also increases and that of Cl reduces until a point is reached at which the resistance R4 due to the added resistor swamps the reactance of Cl. The model then reduces to an LR network whose maximum phase shift is 90".

The effect of the modification can be seen by reference to the open loop Bode and Nyquist plots for Figures 1 and 2 shown in Figures 3 and 4. Thus the Nyquist plot, Figure 3b, shows the response of a circuit such as that of Figure la, and it can be seen that encirclement of the point (-1,0) is avoided at the expense of greatly reduced bandwidth. The Nyquist plot for Figure 2a, shown in Figure 4b, shows that the point (-1,0) is avoided by a suitable margin.

The use of the resistor R4 in the manner described results in the converter being able to operate at a much higher open loop gain and bandwidth, giving a more accurate closed loop performance with excellent bandwidth. Although the introduction ofthe resistor R4 does increase the ripple content in the output of the converter, this is of little consequence as the ripple can be effectively filtered out since it is at a relatively high frequency.

Claims

1. A DC-to-DC converter, which includes a switching transistor driven by the input DC and transformer-coupled to a rectifier which supplies the output of the converter, an output capacitor in series with a resistor across said output control means connected to the base of the transistor so as to vary its on-off ratio, an amplifier via which the direct voltage input of the converter drives switching transistor, and a feedback connection from the converter output to the amplifier via which the output condition of the converter so influences the input condition of the amplifier as to maintain stability, whereby the series combination of the capacitor and the resistor ensures that the reactance at the converter output is predominantly inductive at high frequencies.

2. A DC-to-DC converter, which includes a switching transistor whose emitter-collector path is in series with the primary of a transformer, a rectifier connected between the secondary of the transformer and the output of the converter, an output capacitor in series with a resistor across the output of the converter, a pulse-width modulator connected to the base of the transistor so as to vary its on-off ratio, an amplifier connected to the direct voltage input of the converter and whose output drives the modulator, and a feedback connection from the converter output to the amplifier via which the output condition of the converter so influences the input condition of the amplifier as to maintain stability, wherein when the transistor is on its conduction causes energy to be stored in the transformer, which energy is released to the converter output when the transistor is off, so that the output voltage of the converter depends on the on-off ratio of the transistor, and wherein the series combination of the capacitor and the resistor ensures that the reactance at the converter output is predominantly inductive at high frequencies.

3. A DC-to-DC converter substantially as described with reference to Figures 2 and 4 of the accompanying drawings.