GB2478284A

GB2478284A - AC-DC converter having adjustable phase chopping angle

Info

Publication number: GB2478284A
Application number: GB1003380A
Authority: GB
Inventors: Peter Gordon Davy
Original assignee: Multiload Technology Ltd
Current assignee: Multiload Technology Ltd
Priority date: 2010-03-01
Filing date: 2010-03-01
Publication date: 2011-09-07
Anticipated expiration: 2030-03-01
Also published as: GB201003380D0; WO2011107780A3; GB2478284B; WO2011107780A2

Abstract

Power supply apparatus comprises an input for connection to a AC power source 1, an output for connection to a DC load, a chain comprising a transformer 2, rectifier 3 and smoothing circuit 4 connected in series between the input and the output. A phase chopping arrangement 6 is connected prior to the smoothing circuit in the chain and a phase chopping angle setting arrangement 29 controls a phase chopping angle dependent on a phase angle representation stored in the phase chopping angle setting arrangement. A control circuit is configured: to extract a parameter of a signal provided at the output, to compare the parameter with a target, and to provide an adjusting signal for adjusting the phase angle representation stored in the phase chopping angle. The DC load may be a bank of light emitting diodes (LEDs).

Description

Power Supply Apparatus

Description

The following general description is with reference to Figure 1. The invention is applicable to all situations in which A.C. power (1) provided from the Imains, a generator or otherwise, is rectified to D.C. power by a rectifftg arrangement (3) which uses one or a plurality of diodes or other rectifying devices, which rectification is possibly in combination with a transformer (2) initially connected to the A.C. supply to step down, step up and/or isolate the A.C. voltage; a single diode provides a half-wave rectified D.C. voltage waveform, two diodes in combination with a dual secondary winding on the transformer provide full-wave rectification, as do four diodes [possibly direcdy connected as a bridge rectifier to the A.C. power supply (1)]. To provide a useful D.C. voltage, the half-wave or full-wave rectified voltage is usually smoothed by a smoothing circuit (4), consisting of an arrangement of resistors, capacitors and possibly inductors, the simplest such arrangement being a single capacitor connected across the rectified voltage, possibly with a resistor interposed in one of the lines to limit the (surge) current into the capacitor. The resultant D.C. voltage is a D.C. ripple voltage for connection to a load (5) with the same frequency as the AC. supply for half-wave rectification or double that of the A.C. supply for full-wave rectification and with a ripple amplitude dependent on the relative values of the capacitance of the capacitor employed and the load current.

Apart from the ripple amplitude varying when the capacitance value drifts with temperature or time or when the load current varies, the overall amplitude of the load voltage (i.e. the voltage across the load resulting from the attenuation of the D.C. ripple voltage by its transmission to the load) can vary as a result of A.C.

supply voltage variation and as a result of variable voltage drops in the transformer (if used) and in the transmission wiring between all the elements (1) (2) (3) (4) (5) [as these may be spatially separated in different enclosures if convenient] as the load current varies.

According to the present invention, a phase-chopping circuit (6), which will most conveniendy be a leading-edge ditntner circuit (although trailing-edge circuits may be possible) is interposed in the A.C. voltage path either before or after the transformer (if used) or is combined with the rectifying arrangement (3) by substituting its diode or diodes with thyristors or other rectifying/switching devices; whichever position is chosen for the phase-chopping circuit, it will have access to an A.C. voltage whose zero crossing can act as a time-point reference for the phase timing of the phase-chopping. The result of the phase-chopping is that both the overall amplitude and the ripple amplitude of the D.C. ripple voltage and load voltage are modified and provided the parameter value increases/decreases monotonically as the phase conduction angle increases/decreases, the value of a specific relevant parameter of the load voltage can be adjusted by varying the phase-chopping angle.

According to the present invention, the value of at least one relevant parameter of the load voltage is extracted from the load voltage by applying the load voltage to at least one parameter-extraction circuit (7), which derives from the load voltage a reasonably accurate analogue voltage or digital representation of the value of the parameter. If an analogue voltage representation of a parameter consisting of the instantaneous value of the load voltage is required, this circuit (7) may be a direct connection or voltage divided (by a resistor network) connection to the load voltage. If this parameter of the load voltage is not to be adjusted below the value required for circuit (7)'s correct operation, the load voltage can also provide a D.C.

power supply for the circuit (7); otherwise a floating D.C. power supply (8) is provided for circuit (7).

According to the present invention, the representation of the actual value of the relevant parameter is compared with an equivalent representation of a target value of the relevant parameter in (at least one) comparator means (9), whose output sends a signal to a single holding means (10), which holds a (phase-angle) representation of the phase-chopping angle [which controls the phase-chopping circuit (6)], possibly via an electrical isolation means (11). Elements (9), (10), (11) may use the same D.C. power supply as element (7) if convenient. If the target is a lower target, the output signal from the comparator is an up-signal which is present if the actual value is less than the target value and which changes the phase angle representation so as to increase the phase conduction angle at a certain up-rate (with respect to time). If the target is an upper target, the signal from the comparator is a down-signal which is present if the actual value is more than the target value and which changes the phase-angle representation so as to decrease the phase conduction angle at a certain down-rate. The comparator means (9) may be modified so that their output signals vary the up-rate and/or down-rate depending on the magnitude of the deviation of the actual value from the target value. In the absence of up-signal or down-signal the holding means (10) in the phase-angle representation block will usually impose a slow drift on the value of the representation either up or down in the direction of a target value. Since the capacitor of the smoothing circuit (4) necessarily introduces a delay in the response of the load voltage to a change in phase angle, and the parameter-extraction circuit (7) can introduce a further delay, the drift rate is held at a low value such as to avoid hunting in the feedback loop formed by the various elements.

If the single target value of the parameter is a minimum requirement (lower target) for the parameter (say a minimum value of the instantaneous load voltage required to properly run a regulated power supply, i.e. regulator) a fast up-rate will be preferable to correct the representation (10) if the actual instantaneous load voltage goes below that required (by the regulator); in the absence of the up-signal, the holding means (10) will preferably impose a slow drift down to the minimum requirement. If in addition (as say when only up to a maximum input voltage can be tolerated by the regulator) a second target value of the parameter (upper target) is a maximum allowable value, it can be applied to a second comparator (9) which whenever the actual instantaneous load voltage exceeds the (maximum allowable) upper target value, can send an additional fast down-rate producing down-signal to the same single phase-angle representation [held in (1 0)] .The accuracy in the relationship between this representation and the phase angle of the phase chopper is unimportant (provided it is roughly linear), since it is part of the feedback system; accuracy is more important in the extraction of the parameter's actual value and the comparison of this with the target value in the comparator (9).

Figure 1 shows a generalised arrangement according to the invention showing the numbered functional block elements heretofore described in the general description.

Figure 2 shows an embodiment of the invention for supplying a D.C. input voltage to a load consisting of a voltage regulatoq some features of this embodiment are used as examples in the general description. The advantage of the invention in this application is that the instantaneous value of the load voltage is held just above the minimum requirement for the voltage regulator which then has to dissipate less heat; furthermore since the regulation dining the conversion of A.C. to D.C. is by on/off switching of current by the phase chopper, most of the heat saved in the regulator is not dissipated elsewhere. This means that the complete arrangement draws a lower amount of power from the A.C. supply (thus saving energy) and that the heatsinfring arrangements (for the voltage regulator) need to be less substantial.

This advantage of minimsi heat loss in the phase-chopper and lower heat dissipation in the load will apply to most other embodiments and applications of the invention.

In Figure 2 the same functional block elements as in the general description and Figure 1 are shown as blocks around elements of the schematic diagram for the embodiment which is described below; the isolated analogue input leading-edge phase-chopper (6) and electrical isolation (11) are left as functional blocks, since the circuitry options for these are weli-known and their exact natures are nnimportant.

A msins supply (1), liv; L' is phase-chopped in the phase-chopper (6) to provide a variable liv; VL', for connection, with the msin s-supply neutral, N', to the primtry winding of the transformer (2). The split secondary winding of the transformer is rectified by the two diodes in (3) to provide the D.C. ripple voltage (12) with respect to the OV datum provided by the midpoint of this winding.

The D.C. ripple voltage is input to a voltage regulator (14) which provides a regulated voltage supply (15) which is the output to equipment powered by the embodiment. This voltage (15) will often be of a suitable magnitude (say +15V) to also provide power rails [(38) and (39)]for the succeeding circuitry consisting of comparators [(18) and (19)] and other elements (16) through to the O-IOV representation at (25). The voltage divider (13) consisting of two resistors scales down the instantaneous values of the D.C. ripple voltage so that the possible range of the voltage applied to the comparators (18) and (19) is weli within the power rails: OVand the regulated voltage (15).

Two more voltage dividers (16) and (17) provide respectively from the power rails lower and upper target values for comparison with the scaled down actual instantaneous value [at (13)] respectively in comparators (18) and (19) whose output voltages swing between high and low levels (near to the rail voltages) according to the relative magnitude of the (voltage) values on their non-inverting (marked +) and inverting (marked -) inputs.

Because as heretofore stated a low accuracy is required in the translation of the phase angle representation [at (10)] to the actual phase angle of the Variable live [VL at (6)], the range of voltage of the representation on capacitor (23) can be kept small (at approximately mid-rail voltage) and then increased to an approximately 0-I 0V range by the buffering and amplifying means (24).

When the actual value lies between target values, output (18) is at low level and output (19) is at high level and neither of the diodes at (21) and (22) conducts; the representation voltage on the capacitor (23) (which is always at approximately mid-rail voltage) is pulled slowly down (towards OV) by the high-value resistor (20) connected to OV [leakage current in the diodes (21) and (22) and into the non-inverting input of the OP AMP (in 24) is kept substantially below the current in resistor (20)]. The reduction in representation voltage at (23) is translated into an equivalent reduction in the 0-IOV signal at (25), isolated 0-bY signal (26) and ultimately in phase (conduction) angle of YL. This last progressively reduces the lowest value of the D.C. ripple voltage and its scaled down version at (13) until this latter value starts to fall below the lower target value at (16) for a short period of each ripple cycle. During these short periods the output of (18) changes to high level and the diode (in 21) conducts; although the periods are short, resistor (21) is arranged to have a value an order of magnitude or so lower than the value of resistor (20), so that eventually the charge flowing through resistor (21) to capacitor (23) during the short period balances the charge flowing from capacitor (23) [through resistor (20)] during the whole cycle, thus stabilising the representation, the actual phase angle of YL, and the lowest instantaneous value of D.C. ripple voltage [scaled down at (13)] at just below the lower target value.

If conditions such as the mains supply voltage or the current drawn by the equipment from the regulated supply vary, the relationship between the phase angle at VL and the lowest instantaneous value of the D.C. ripple voltage will vary, but this above-described first part of the feedback system [utilising comparator (18)] adjusts the former so that the latter remains substantially constant. [Circuit variables and values are arranged such that as long as the lowest instantaneous value of the D.C. ripple value is stabilised at its desired level, the highest instantaneous value of the D.C. ripple voltage, scaled down, will be well below the maximum target value and output (19) will stay at high level].

The second part of the feedback system uses comparator (19) and the diode etc at (22) to provide a fast down-signal to the representation at (10) in the following circumstances. If A.C. supply or load current conditions change quickly [e.g.

disconnection of equipment from the regulated voltage (15) suddenly reduces the load current drawn], the whole D.C. ripple voltage may suddenly jump up (over a short time) while the phase angle and its representation initially remain unchanged because the low current in the high-value resistor (20) can reduce the representation voltage at (23) only relatively slowly. During this short time the highest instantaneous value of the D.C. ripple voltage may well be jumping above the maximum permissible for the voltage regulator. If, given the possible range of mains voltages and load currents this is not possible, comparator (19) and diode etc at (22) are not required. By lowering the upper target value below this maximum permissible value (scaled down), the output of (19) will change to low level before the actual highest value reaches the maximum permissible, and if this change results in sufficient current in diode (22), the representation at (23) and the phase angle of VL may be reduced fast enough to stop this actual highest value reaching the maximum permissible value. The current in diode (22) is made sufficient by arranging that the resistors of the voltage divider [connected to the output of (19)] and the resistor (22) have a low value. The voltage divider at the output of (19) is provided so that, although the representation at (23) reduces rapidly [when the output of (19) goes low], it reduces only to a value at which the equivalent phase angle on VL ensures that, whatever the mains voltage and load current conditions, the highest actual value is safely below that which would damage the regulator; the fast reduction only to this safe value, which may still inadvertently reduce the lowest instantaneous value of the D.C. ripple voltage below the minimum required, shortens the time taken for the (limited) current through the resistor at (21) to restore this lowest value to this required value.

Because of the limited speed of response of the ripple voltage to a change in phase angle at VL [due to the delay introduced by the smoothing circuit (4)], the magnitudes of the charging and discharging currents to capacitor (23) are restricted to avoid excessive overshoot and hunting in the feedback loop; substituting constant-current sources for the resistors at (21) and (22) so that the changes in the representation at (23) are linear rather than exponential may allow the intentionally introduced delays caused by these restricted currents to be lessened.

The (low leakage-current) non-inverting input of the op-amp at (24) buffers the voltage on capacitor (23) and the three-resistor arrangement on its inverting input (which is held at approximately mid-rail voltage by the two approximately equal of these resistors forming the voltage divider across the rails) amplifies the representation at (23) to approximately a O-IOV range at (25). Circuitry at (25) provides a signal for an opto-isolator (such as linear opto-coupler 1L300 from Vishay) and circuitry at (26) receives this optical signal and decodes it at (26) back to an approximately O-IOV range, but at the potential of the input to the phase-chopper (6), which may also supply power rails (27) back to the circuitry at (26).

In a second embodiment of the invention (shown in Figure 3) the load (5) is an arrangement of light emitting diodes (LEDs). The D.C. ripple voltage is attenuated by the transmission wiring to produce a reduced voltage, i.e. load voltage (40), at the load. Since LEDs have a low forward voltage (only a few volts) across them when they are conducting, i.e. producing light as a light source, they are usually connected in series, possibly in conjunction with a current-limiting series resistor so that the voltage across the whole series string is more convenient (say 12V or 24V). In the load arrangement (5) several of these series strings may be connected in parallel across the load voltage, up to a number which keeps the total of the current in the various strings within the available load current from the arrangement. Since the current in LEDs is not linear with the forward voltage across them (the current starts to become very low as the forward voltage is reduced to about three quarters of the normal running forward voltage), the load current in a 24 volt series string arrangement (and thus the light emitted) will vary from full' to (effectively) 2ero' as the load voltage varies from 24V to say 17V. Since the load voltage (40), derived from the D.C. ripple voltage (12) also has a ripple, the LED currents and the intensity of light they produce also show a ripple: the consequent slight flicker in the light will normally be ignored by the eye and the intensity perceived will be roughly the average intensity of the light. Since the instantaneous intensity is approximately proportional to the instantaneous LED current, the average intensity is approximately proportional to the average load current. Thus, according to the invention, if conditions of mains voltage and/or load current vary, the perceived (average) intensity is kept constant by adjusting the phase angle so that the average LED current remains approximately constant.

In Figure 3 the functional block (28) which produces the ripple voltage (12) from the mains supply (1) is identical to block (28) identified in Figure 2. Functional block (29) in Figure 3 is also identical to functional block (29) identified in Figure 2, except that, in order to lessen overshoot, the value of the resistor at (22) may be increased to slow down the rate of reduction in the representation (23) produced by the current in diode (22); in addition the voltage divider at (19) may be modified to allow this reduction by this current to be to a lower voltage value. The load voltage (40) [which may be lower than the D.C. ripple voltage (12) owing to volt drops in the wiring between (28) and (5)] besides being applied to the load is also taken as an input to a modified parameter-extractor (7). A floating D.C. supply (8) provides power rails for the circuitry of (7) and also for the circuitry upstream of the opto-isolator in (29). According to the invention, an analogue voltage representation (30) of the average LED current [produced by the parameter-extraction circuit (7)] is produced in (7) by firstly having an OP-AMP arrangement (31) subtract from a scaled down version of the load voltage (13) a threshold voltage (31) derived by a voltage divider across the power rails and approximately equal to an equivalently scaled down version of the LED voltage for zero L.E.D. current (17V in the 24V example); the resulting voltage (33) is roughly proportional to the instantaneous LED current and is subsequently averaged by the resistor/capacitor arrangement (34) to produce the average current representation (30). This representation (30) is compared [in (29)] with the upper and lower target values (35) and (36) respectively which designate a band of values within which the average load current is held by the invention. This band may be lowered or raised by adjustment of a single D.C.

control voltage (37), which sets these values (35) and (36) (and thus the approximate perceived intensity of the light) to a desired level using voltage dividers returned to the positive (38) and negative (39) power rails respectively to derive values (35) and (36).

It will be understood that the two embodiments described and other embodiments can be implemented using other methods of representation of the various parameters (e.g. digital representation) and that the circuit elements will be appropriately modified or changed to handle these other representations

Claims

-10 -Claims 1. Power supply apparatus, comprising: an input for connection to a AC power source; an output for connection to a DC load; a chain comprising a transformer, a rectifier and smoothing circuit connected in series between the input and the output; a phase chopping arrangement connected prior to the smoothing circuit in the chain; a phase chopping angle setting arrangement configured to control a phase chopping angle of the phase chopping arrangement dependent on a phase angle representation stored in the phase chopping angle setting arrangement; and a control circuit configured: to extract a parameter of a signal provided at the output, to compare the parameter with a target, and to provide an adjusting signal for adjusting the phase angle representation stored in the phase chopping angle setting arrangement dependent on the result of the comparison.
2. Power supply apparatus as claimed in claim 1, wherein the phase angle representation is stored as a voltage at a terminal of a capacitor included in the phase chopping angle setting arrangement.
3. Power supply apparatus as claimed in claim I or claim 2, wherein the phase chopping angle setting arrangement is configured to provide a drift of the phase angle representation at a first rate in the absence of an adjusting signal from the control circuit, and wherein the control circuit is configured in response to determining that the parameter of the signal meets a predetermined relationship with respect to the target to provide an adjusting signal for adjusting the phase angle representation at a second rate, wherein the second rate is higher than the first rate.

-11 -
4. Power supply apparatus as claimed in any preceding claim, wherein the control circuit is configured: to compare the extracted parameter with an upper target and to provide an adjusting signal for adjusting the phase angle representation stored in the phase chopping angle setting arrangement downwards if the extracted parameter exceeds the upper target, and to compare the extracted parameter with a lower target and to provide an adjusting signal for adjusting the phase angle representation stored in the phase chopping angle setting arrangement upwards if the extracted parameter does not exceed the lower target.
5. Power supply apparatus as claimed in any preceding claim, wherein the output of the phase chopping angle setting arrangement is coupled to the phase chopping arrangement by an optoisolator.
6. Power supply apparatus as claimed in any preceding claim, wherein the parameter is a voltage of the signal provided at the output.
7. Power supply apparatus as claimed in any of claims I to 5, wherein the parameter of the signal provided at the output is a representation of a current drawn by a load connected to the output, and wherein the control circuit is configured to extract the parameter by subtracting a threshold voltage from a voltage detected at the output.
8. Power supply apparatus as claimed in claim 7, wherein the threshold voltage represents a scaled down approximate version of a voltage at which approximately zero current is drawn by the load and wherein the control circuit is configured to extract the parameter by subtracting the threshold voltage from a scaled version of a voltage detected at the output.