GB2026260A - DC-to-DC converters - Google Patents

Abstract

A DC-to-DC Forward Converter of which the input circuit incorporates a DC input voltage source (D.C. IN), a switching device (SW1), the primary winding of a transformer and transfer reset means to limit the rise in reset voltage across the primary winding when the switching device is off, is characterised in that the transfer reset means consists of means to provide an effectively constant voltage (CONSTANT VOLT. LOAD) connected via a unidirectional device (D4) to the primary transformer winding whereby the reset voltage across the primary winding is the difference between said effectively constant voltage and the input voltage. <IMAGE>

Description

SPECIFICATION DC-to-DC converters This invention relates to DC-to-DC converters and particularly relates to the control of the transformer reset voltage in such converters.

A DC-to-DC converter, of the type known as a forward converter, is shown in Fig. 1 to consist of an input D.C. source, which may be derived from either rectified A.C. mains or a battery, connected in series with a switching device SW1 and the primary winding of a transformer. A diode D1 and inductance L1 are connected in series to the secondary winding of the transformer. The D.C. output voltage from the converter is controlled, for example for variations in input voltage, by varying the ratio of "on" to "off" time of the switching device SW1. When the switching device SW1 is turned "on" power is transferred to the secondary winding of the transformer and thence through diode D1 and inductance L1 to the load.When the switching device SW1 is turned "off" the voltage at point A rapidly rises to more than the input supply voltage V1 due to the stored energy in the magnetic field of the transformer. The voltage level (V2) risen to must be limited by some means to prevent damage to the switching device SW1. In the secondary circuit, diode D1 becomes nonconducting and the load current circulates through a diode D2, connected across the secondary winding, for as long as the voltage at point A remains high.

The flux in the core of the transformer is reset to its initial value during the period that the point A is at a higher potential than the input voltage V1.

A known method of limiting this voltage rise, or resetting the transformer, is shown in Fig. 2 to consist of a third transformer winding connected via a diode D3 to the input D.C. supply. When the switch ing device SW1 is "on" the diode D3 is reverse biased and no currentflows in the third, reset, wind ing. When the switching device SW1 is turned "off", the voltage at point A rises until the diode D3 con ducts. The voltage (V2) to which point A rises is N1 given by V2 = V1 (1±), where N1 and N2 are the N2 numbers of turns of the primary transformer winding and the reset winding respectively. This method of resetting the transformer thus uses a voltage proportional to the input voltage.

It is an object of the present invention to provide a method of resetting the transformer using an effectively constant reset voltage which provides improved utilisation of the switching device and the transformer.

According to the present invention, a DC-to-DC converter incorporates a D.C. input voltage source, a switching device, the primary winding ofatrans- former and transformer reset means to limit the rise in reset voltage across the primary winding when the switching device is off, and is characterised in that the transformer reset means consists of means to provide an effectively constant voltage connected via a diode to the primary transformer winding whereby the reset voltage across the primary winding is the difference between the constant and input voltages.

The following formula is true for all methods of resetting the transformer flux to its original value:

where tl is the "on" time and t2 the "off" time for the switching device, (tl -t2) is the reset time, VF is the voltage across the transformer primary winding when the switching device is "on" and VR is the voltage across the primary during the reset period.

With reset means in accordance with the invention, the actual reset voltage is the constant voltage minus the input voltage which means that, for the same output voltage from the converter, at high input voltages, where smaller switch "on" times are needed, the reset voltage to the transformer is low and therefore longer switch "off" times are required to restore the transformer flux; whereas at low input voltages, where longer switch "on" times are needed, a higher reset voltage is obtainable and hence a shorter switch "off" time required.

Thus, a more nearly constant total required cycle time is obtained with a forward converter with constant voltage reset in accordance with the invention than with a forward converter with conventional proportional voltage reset.

The invention is illustrated, by way of example, on the accompanying drawings, wherein: Fig. 3 is a diagram of the basic circuit of the invention, Figs. 4a, b and c are diagrams of alternative circuits in accordance with the invention, and Figs. 5 and 6 are graphs of typical waveforms for conventional and constant voltage reset respectively.

The basic circuit is shown by Fig. 3 to consist of a constant voltage load connected via a diode D4 to the transformer primary winding. During the "off" periods of the switching device SWl,the potential at point A will rise until it is greater than the constant load voltage and the diode D4 will then conduct.

Fig. 4a illustrates the use of a non-linear device D5, such as a zener diode, which has characteristics such that negligible current is drawn until a certain voltage (the constant voltage) is reached and, at that voltage, any current can pass through the device D5 without significantly altering its voltage.

Fig. 4b shows a capacitor C1 and a resistor R1 connected in parallel and instead of the diode D5.

The action of this circuit is that the magnetising cur rent of the transformer will rise to such a value that the capacitor C1 will, after several cycles, be charged to that voltage which enables full resetting of the transformer to occur. If the "on" to "off" ratio of the switching device is controlled in such a manner as to keep the output voltage from the converter constant for a varying input voltage, then the voltage on The drawings originally filed were informal and the print here reproduced is taken from a later filed formal copy.

capacitor C1 will also be constant, thus achieving the required result. The resistor R1 is used to dissipate the energy which is stored in the transformer during the "on" period of the switching device SW1.

In Fig. 4e a DC-to-DC converter or a like device having input characteristics similar to that of the diode device D5 in the circuit of Fig. aS, is employed to convert power fed into it into a suitable form to feed back into the incoming power supply, indicated by I out. Alternativeiy, the converted power could be fed to the output supply. Either way would increase the efficiency of the forward converter.

Example Comparison between a conventional reset and a constant voltage reset forward converter. The waveforms for point A for conventional reset and constant voltage reset are respectively illustrated by Figs. 5 and 6.

The converter conditions are: Maximum input voltage = 400 V Minimum input voltage = 200 V Reset voltage = switch voltage rating = 600 V maximum Operating frequency- constant Mean output voltage - constant.

Conventional Reset To limit the switch rating to 600 V (for an input of 400 V) the reset transformer winding to primary transformer winding ratio must be 2:1. This would give a conduction angle (switch "on" time divided by time for one complete cycle) of 33% maximum at any input voltage.

The conduction angle at high input voltage must be half that at low input voltage to keep a constant output and therefore, the conduction angle can only be 16.6% for a 400 V input as the conduction angle for a 200 V input is 33%.

The switch peak current rating is therefore 15 mA per watt of output and the switch peak voltage rating is 600 V, i.e. a "V.A." rating of 9 VA/watt.

Constant Voftage Reset For a reset voltage of 600 V at an input of 200 V, the conduction angle is 66%, and for 400 V input it is 33%. The switch peak current rating is, therefore, 7.5 mA per watt of output and the "V./1" rating is 4.5 VAlwatt.

Thus there is a significant advantage in using constant voltage reset in accordance with the invention when compared with conventional reset. A forward converter with a single switching device and con stantvoltage reset in accordance with the invention also compares favourably, when input changes have to be accommodated, with other converter circuits using two switching devices.

Improvement of the utilisation of the power transformer also occurs when constant voltage reset is used.

Claims (7)

1. A DC-to-DC forward converter of which the input circuit incorporates a D.C. input voltage source, a switching device, the primary winding of a transformer and transformer reset means to limit the rise in reset voltage across the primary winding when the switching device is off, characterised in that the transformer reset means consists of means to provide an effectively constant voltage connected via a unidirectional device to the primarytransformer winding whereby the reset voltage across the primary winding is the difference between said effectively constant voltage and the input voltage.

2. Aforward converter as claimed in claim 1, wherein the unidirectional device is a diode.

3. Aforwardconverter as claimed in claim 1 or claim 2, wherein the constant voltage means is a non-linear device having the characteristic that negligible current is drawn until a certain voltage, said effectively constantvoltage, is reached and, at that voltage, any currentcan pass through the device without significantly altering the voltage.

4. Forward converter as claimed in claim 3, wherein the non-linear device is a zener diode.

15. Aforward converter as claimed in claim 3, wherein the non-linear device is a secondary DCto-DC converter employed to convert power fed into it into a suitable form to be fed back either into the incoming power supply or to the output supply of the forward converter.

6. A forward converter as claimed in claim 1 or claim 2, wherein the constant voltage means consists of a capacitor and a resistor connected in parallel.

7. A DC-to-DC forward converter substantially as described with reference to or as shown by Fig. 3, or Fig. 4a, or Fig. 4b, or Fig. 4c, or Fig. 4d of the Drawings.