GB2153113A - DC-DC converter - Google Patents

Abstract

A DC-DC converter includes a circuit (30) for generating a bidirectional pulse waveform signal, and a converter transformer (T1), the transformed signal being rectified and filtered for providing a DC voltage output at load terminals (46,47). A transistor (50) is connected across the output terminals and is switched between conducting and non-conducting states by having a control electrode (53) connected to the transformer secondary (35). A voltage or current transformer (T2) is connected in circuit with the transistor (50) to provide a pulsed waveform in accordance with the switching of the transistor the magnitude of which is proportional to the output voltage. The waveform is peak detected (62, 63, 64) and provided as a feedback signal to adjust the duty cycle of the circuit (30) providing the bidirectional pulse waveform signal. The transistor (50) may be replaced by an FET, and the signal from transformer (T2) may be detected using a synchronous demodulator or operational amplifier circuit. <IMAGE>

Description

SPECIFICATION DC-DC converter The invention relates in general to DC-DC converters and, more particularly, to the accurate regulation thereof.

A DC-DC converter has the purpose of providing a constant DC voltage for providing power or operating potential to a load device.

The output of a chopper or pulse waveform generator is provided to the primary winding of a transformer, the secondary voltage thereof being rectified and filtered, and the output voltage being determined by the duty cycle of the pulse generator and the turns ratio of the transformer.

In order to provide for a constant DC output voltage, some sort of feedback arrangement is utilized to modify the duty cycle should the output voltage deviate from its desired value.

Normally, a portion of the output voltage is directly connected to the pulse generator for the feedback function of maintaining a constant output voltage. There are however many situations where the load side of the transformer must be isolated from the input side of the transformer for various reasons. These include the prevention of circulating currents which might destroy components, the prevention of unwanted fields which might pose health hazards, or for bilateral noise isolation by way of example.

In those situations where isolation between the output and input must be maintained, the feedback circuit is not a direct connection.

Isolated feedback arrangements include the use of optical couplers for transferring the feedback signal from the output side to the input side, or other circuits which require some additional power for their operation.

These circuit arrangements tend to be temperature sensitive and somewhat non-linear resulting in a less than accurate regulation.

The present invention has for its principal object to provide a converter of the type described wherein the output is isolated from the input by an arrangement which requires no external power, utilizes a minimum number of components, and is extremely linear.

Accordingly, there is provided apparatus which includes a circuit for generating a pulse waveform the duty cycle of which is variable.

The pulse waveform signal is applied to the primary winding of a transformer, the secondary winding of which is rectified and filtered for delivery to a load service via an output. A transistor means is connected across the output and is switched on and off in response to the pulse signal at the secondary winding.

Means electrically isolated from the transistor means and energized by its conduction and non-conduction is provided for generating a feedback signal indicative of the DC output voltage. The feedback signal is sampled or peak detected and is applied back to the circuit means to adjust the duty cycle thereof if the output voltage deviates from its desired value.

Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure I is a block diagram illustrating a typical DC-DC converter of the prior art; Figure 2 is a circuit diagram illustrating one embodiment of the present invention; Figure 3 illustrates a modification of the circuit of Fig. 2 providing for more precise feedback control; Figure 4 is a circuit diagram illustrating another embodiment of the invention; and Figure 5 illustrates still another embodiment of the invention.

The DC-DC converter of Fig. 1 includes a circuit means such as converter electronics 10 operable to generate a pulse waveform signal the duty cycle of which may be varied. The signal is applied to the primary winding of a transformer 1 2 for decreasing or increasing the voltage depending upon the transformer turns ratio. The secondary winding of the transformer is connected to rectifier circuit 1 4 which converts the bidirectional pulse waveform at the secondary winding to a unidirectional pulse waveform which is filtered by filter 1 6 and provided at output 1 8 as a constant DC voltage for delivery to a load.

A portion of the output voltage is sampled by a feedback arrangement 20 which provides a feedback signal to converter electronics 10 for adjusting the duty cycle if the output voltage deviates from a desired predetermined value. As previously mentioned, a direct feedback connection is unacceptable for those situations where electrical isolation is mandatory. An improved isolation feedback arrangement which obviates the disadvantages of prior art arrangements is illustrated in Fig. 2 to which reference is now made.

In Fig. 2, circuit means in the form of converter electronics 30 is operable to provide a bidirectional pulse waveform output signal.

Such circuits are well known to those skilled in the art and are commercially available items. Circuit 30 of Fig. 2 includes a feedback connection 31 for adjusting the duty cycle of the output waveform, and a ground or return connection 32.

The bidirectional pulse waveform signal is applied to primary winding 34 of transformer T1 having a center tapped secondary winding 35 the ends of which are connected to a rectifier circuit comprised of diodes 38 and 39 operable to convert the bidirectional pulse waveform at the secondary winding to a unidirectional pulse waveform which is filtered by filter components, inductor 42 and capacitor 43. The resulting unidirectional DC voltage is available at output terminals 46 and 47 for connection to a load device.

The isolated feedback arrangement includes a transistor means which in its simplest form comprises a bipolar transistor 50 having a first or emitter electrode 51, connected to output terminal 47, a second or collector electrode 52 and a third or control (base) electrode 53. The control electrode is connected to the transformer secondary winding 35 through resistor 54 chosen such that the transistor 50 switches from an off or nonconducting condition to a well saturated or conducting condition in accordance with the pulse waveform at the secondary winding.

Primary winding 58 of a transformer T2 is connected in the collector circuit of transistor 50 between collector electrode 52 and output terminal 46 and is energized by the on/off action of transistor 50 and the output voltage.

This operation produces a voltage at secondary winding 59 of transformer T2 and the magnitude of which is dependent upon the turns ratio of the transformer.

Diode 60 is provided in circuit between electrodes 51 and 53 in order to prevent damage to the base-emitter junction of transistor 50 during negative portions of the base drive waveform.

The magnitude of the voltage at the secondary winding 59 is sampled by a detector such as a peak detector, one example of which includes a resistor 62, a diode 63 and a capacitor 64. The peak detected feedback signal appearing at the junction of diode 63 and capacitor 64 is then applied to the converter electronics 30 at connection 31 to vary the duty cycle of its output signal should the output voltage at terminals 46 and 47 digress from a predetermined value. Resistor 66, connected in circuit between the feedback connection 31 and return connection 32, in conjunction with the input impedance at feedback connection 31, adjusts the decay constant of the peak detector.

The voltage drop across diode 63 is approximately 0.6 volts, varying approximately 0.25 volts over customary temperature extremes, and if the turns ratio of transformers T2 is made sufficiently high, the resulting voltage will be much greater than the voltage variation across diode 63 so as to minimize any non-linearity and eliminate the requirement for temperature compensation.

An even more precise control arrangement may be achieved with the arrangement of Fig.

3, which duplicates a portion of Fig. 2 and includes like reference numerals. In the arrangement of Fig. 3, however, an additional diode 63' is included in the return line and provided with bias current through resistor 68 connected to a source of potential V + with the voltage drop across diode 63' being added to the peak detector output and being equal to the voltage drop across diode 63 so as to compensate therefor.

Fig. 4 illustrates an alternate embodiment of the invention wherein the transistor takes the form of a field effect transistor (FET) as opposed to the bipolar transistor 50 of Fig. 2.

FET 70 includes first and second electrodes 71 and 72 connected across the output terminals 47, 46 and additionally includes a control electrode 73 directly connected to transformer secondary winding 35 so as to switch the FET between on and off states. An advantage of the FET is that no base current and accordingly no driving power is required.

By virtue of the FET connection and resistor 76, the current in the FET circuit, when it is conducting, is proportional to the output voltage. This current is sensed by current transformer 78 which also provides the electrical, isolation and the current transformer output is utilized to generate the feedback signal. This may be accomplished for example by the use of an operational amplifier operable to provide an output voltage proportional to the peak of the current provided by current transformer 78. Alternatively, and as simply illustrated in Fig. 4, a resistor 80 is connected in circuit for developing a voltage proportional to the current provided by the current transformer.The voltage, which is of a magnitude proportional to the output voltage and which is switched on and off by operation of the FET 70, is detected by the peak detector circuit including resistor 82, diode 83, and capacitor 84 operable to provide a feedback signal at connection 31, as previously described.

Fig. 5 is similar to Fig. 2 but illustrates an alternate detector which may be used in the feedback arrangement. The detector 86 is a synchronous demodulator which samples the voltage of secondary winding 59 when it is at its maximum and holds it until the next sampling . The converter electronics frequency is utilized, via connections 87, 88, so that the detector will operate in synchronism with the waveform coupled by feedback transformer T2. The use of the synchronous demodulator which is a well-known circuit commercially available in IC chip form, potentially allows for a wider bandwidth than the previously described detectors.

Accordingly, a DC-DC converter has been described which provides required isolation between the output and input sides of the converter transformer and provides for stabilized and accurate regulation of the output voltage with a minimal number of components requiring no auxiliary power source.

Claims (9)

1. A DC-DC converter comprising a transformer having primary and secondary windings; circuit means connected to said primary winding and operable to provide a pulse waveform signal thereto; a rectifier circuit connected to said secondary winding operable to provide a unidirectional pulse waveform signal in response to said signal applied to said primary winding; a converter output for connection to a load device; filter means operable to filter the output of said rectifier circuit and provide a nominally constant DC voltage to said output; transistor means having first, second and control electrodes, said first and second electrodes being connected to said output and said control electrode being connected to said secondary winding so as to cause conduction and non-conduction of said transistor means; means electrically isolated from said transistor means and energized by said conduction and non-conduction thereof for providing a feedback signal indication of DC output voltage; means for detecting said feedback signal; and means for applying said detected signal to said circuit- means for controlling the output pulse signal provided thereby.

2. A DC-DC converter according to claim 1, wherein said circuit means is of a type capable of varying the duty cycle of its output pulse waveform signal and said feedback signal is utilized to vary said duty cycle.

3. A DC-DC converter according to claim 1 or 2, including a second transformer having primary and secondary windings, the primary winding of said second transformer being connected in series with said first electrode, and said means for detecting being connected to the secondary winding of said second transformer.

4. A DC-DC converter according to claim 1, 2 or 3 wherein said transistor means is a bipolar transistor, the control electrode of said bipolar transistor being connected to said secondary winding of the first-mentioned transformer. through a resistor.

5. A DC-DC converter according to claim 1, 2, 3 or 4, wherein said means for detecting is a peak detector.

6. A DC-DC converter according to claim 1, 2, 3 or 4 wherein said means for detecting is a synchronous demodulator.

7. A DC-DC converter according to claim 1 or 2, wherein said transistor means is an FET having its control electrode directly connected to said secondary winding.

8. A DC-DC converter according to claim 7, including a current transformer operable to sense the current through said FET when conducting, which current is proportional to said DC voltage, and means for utilizing said current to generate said feedback signal.

9. A DC-DC converter according to claim 8, wherein said means for utilizing includes a resistor connected across said current transformer to provide a corresponding voltage signal.