WO2009128025A1

WO2009128025A1 - Switched mode power supply

Info

Publication number: WO2009128025A1
Application number: PCT/IB2009/051549
Authority: WO
Inventors: Joan W. Strijker
Original assignee: Nxp B.V.
Priority date: 2008-04-16
Filing date: 2009-04-14
Publication date: 2009-10-22

Abstract

A switched mode power supply is disclosed which is operable from a power supply and comprises an isolated side which is electrically isolated, in use, from the power supply, a non-isolated side which is not electrically isolated, in use, from the power supply, and back communication means for passing information from the isolated side to the non-isolated side, characterised in that the switched mode power supply comprises a signal transformer, which signal transformer is comprised in the back communication means.

Description

DESCRIPTION

SWITCHED MODE POWER SUPPLY

Field of the Invention

This invention relates to switched mode power supplies, and in particular to switched mode power supplies having an isolated side and a nonisolated side and which incorporate means for communication of information from the isolated side to the non-isolated side.

Background of the Invention

Many conventional switched mode power supplies (SMPS) have a nonisolated side, which is connected in use to a power supply which may be for instance a mains power supply, and an isolated side, which is electrically isolated from the power supply. Such isolation is particularly useful to protect the output of the SMPS from high voltage or mains input on the occurrence of fault conditions. In some applications, isolation may be necessary or mandatory for safety reasons. For example, in applications such as notebook adaptors, the user may be able to touch the output, which thus must not be electrically connected to a mains supply. In other applications, isolation my be required from a lower voltage supply, such as for instance in power-over- ethernet applications, which typically use a power supply operable at a DC voltage of 48V.

Electrical isolation can be effected in several ways; one convenient conventional method of providing isolation is by means of opto-couplers. An opto-coupler is an electronic device in which a light emitter such as a light emitting diode is assembled in close proximity to a light receiver such as a photodiode. The light emitter is connected to the input of the opto-coupler, and the light receiver to the output. A signal to the input side of the device causes the light emitter to emit light, which is received by the light receiver to produce a signal at the output of the device. However, since the emitter and receiver form part of distinct and physically separate electrical circuits, there is no direct electrical connection between the input and output. Thus, in the event of a fault condition in which a high voltage or power is applied to the input, the output is electrically isolated and thus protected from the high voltage or power. Such opto-couplers find wide use in protection circuits, and in circuits in which the input side may become connected to potentially damaging or dangerous voltages or currents.

However, opto-couplers typically require to be biased with a direct current (DC) bias to operate, which results in unwanted power consumption. Furthermore, opto-couplers are not generally symmetrical. That is, although it is possible to use a light emitter as a light receiver and vice versa, (for example, an LED driven in reverse can act as a photodiode), in order to produce efficient opto-coupling, devices are normally manufactured with a dedicated "output side" having a dedicated light receiver, and a separate "input side" having a light emitter. Thus, to allow for two-way communication, two opto-couplers are required, oppositely arranged in the circuit. Use of multiple opto-couplers increases costs and requires additional space for implementation. Although dual opto-couplers are available, which include a pair of oppositely aligned opto-couplers in a single housing or package, these are more expensive and take up more space than standard single opto- couplers.

Use in a SMPS of a control transformer instead of an opto-coupler for communication of a single singal, in the forward direction of a DC-DC converter, has been proposed in Japanese patent application publication 2002-194158. In this disclosure two control transformers are used, one per signal, in one-way communication to control the sync transistor. This takes up more space than would a single device, and is more expensive. Thus there is an ongoing need for an improved SMPS, which does not suffer from the above disadvantages to the same extent.

Summary of the invention

It is an object of the present invention to provide an improved SMPS.

According to an aspect of the invention there is provided a switched mode power supply operable from a power supply and comprising an isolated side which in use is electrically isolated from the power supply, a non-isolated side which in use is not electrically isolated from the power supply, and back communication means for passing information from the isolated side to the non-isolated side, characterised in that the back communication means comprises a signal transformer having primary and secondary windings. Preferably the back communication means is adapted to pass protection information to the non-isolated side by means of the signal transformer. The protection information may be encoded by means of a Manchester coding. Advantageously, the average voltage across the transformer is zero over the Manchester coding interval; thus, there is no net energy storage in the transformer core. More preferably still, the back communication means is adapted to pass output information relating to at least one of output power, output voltage and output current to the non-isolated side by means of the signal transformer. Thus a transformer may be capable of communicating a variety of signals, and/or multiple signals, Beneficially, the output information may comprise a length of a delay between a signal received by the back communication means and a signal transmitted by the back communication means. Thus information may be accurately passed by use of the time domain.

Preferably, the switched mode power supply further comprises forward communication means for passing information from the non-isolated side to the isolated side, and the signal transformer also comprises part of the forward communication means. Thus two-way communication may be enabled, without the requirement for additional transformers or opto-couplers.

Beneficially, the forward communication means may be adapted to pass timing information to the isolated side by means of the signal transformer. Preferably, the switched mode power supply is operable from at least one of a high voltage power supply or a mains power supply.

According to a further aspect of the invention, there is provided in a switched mode power supply having an output power requirement and comprising an isolated side which in use is electrically isolated from a power supply, a non-isolated side which in use is not electrically isolated from the power supply, and a signal transformer having primary and secondary windings, a method of communicating information from the isolated side to the non-isolated side, the method comprising the steps of: generating a signal on the signal transformer on the non-isolated side, detecting a corresponding induced signal on the signal transformer on the isolated side, generating a reply signal on the signal transformer on the isolated side after a predetermined delay which is determined according to the output power requirement, detecting a corresponding induced reply signal on the signal transformer on the non-isolated side, and determining the output power requirement according to the delay between the signal and the corresponding induced reply signal.

Preferably the method further includes the steps of generating a further signal on the isolated side a predetermined time after generating the reply signal, detecting a corresponding induced further signal on the signal transformer on the non-isolated side, and determining a state of the switched mode power supply, other than the output requirement based on the further signal. Thus the method may be capable of communicating multiple signals.

Beneficially, the method may further comprise the steps of blanking the detection on the non-isolated side for a predetermined short time after generating the signal and blanking the detection on the isolated side for a predetermined short time after generating the reply signal. This has the advantage that the signals are definitely terminated, which reduces the risk of generating "false positive" errors.

Preferably, the method further comprises communication of information from the non-isolated side to the isolated side, by generating a forward signal on the signal transformer on the non-isolated side, and monitoring for a corresponding induced forward signal on the signal transformer on the isolated side.

These and other aspects of the invention will be apparent from, and elucidated with reference to, the embodiments described hereinafter.

Brief description of Drawings

Embodiments of the invention will be described, by way of example only, with reference to the drawings, in which

Fig. 1 shows a schematic representation of a prior art switched mode power supply;

Fig. 2 shows a prior art application of feedback and gate control using a opto-coupler; Fig. 3 shows an example of a prior art half bridge lamp driver circuit with several opto-couplers for communication across mains isolation;

Fig. 4 shows in block form an example SMPS according to an embodiment of the invention;

Fig. 5 shows part of the embodiment of Figure 4 in more detail; Fig. 6 and 7 show an example of a control signal for an SMPS according to the embodiment of Figure 4 under operation in high power and low power respectively;

Fig. 8 shows an example of a control signal for high power operation in combination with a protection signal; and Fig. 9 shows a block diagram of a multi-frequency communication means within an SMPS application. It should be noted that the Figures are diagrammatic and not drawn to scale. Relative dimensions and proportions of parts of these Figures have been shown exaggerated or reduced in size, for the sake of clarity and convenience in the drawings. The same reference signs are generally used to refer to corresponding or similar features in modified and different embodiments.

Detailed description of embodiments

Figure 1 shows in block form a typical prior art arrangement of a SMPS in which two opto-couplers are used to provide communication. In the figure, a primary side controller 1 controls the primary side circuit of the SMPS which has a voltage input Vin. A secondary side controller 3 controls the secondary side circuit. Between the primary and secondary side circuits, is a power transformer 2. The transformer output is rectified by diode 4 and smoothed by capacitor 5 to provide the output Vout. In order to pass information on the power control back from the isolated secondary side to the non-isolated primary side, a first opt-coupler 7 is used. A separate, second opto-coupler 8 is also provided, in order to pass protection information in the same direction - that is to say, from the secondary side to the primary side.

A more detailed schematic of another prior art circuit, providing communication within an SMPS, is shown in Figure2. The detailed layout and operation of this circuit is not critical for an understanding of the present invention, and so will not be described here; however, the circuit includes a power transformer 22, and a primary side controller integrated circuit (IC) 21 on the non-isolated side of the circuit (that is, to the left of the power transformer 22 as shown). The primary side controller IC 21 provides drive information to the control switch 24, which is this example comprises a control FET. The circuit further includes a secondary side controller IC 23 on the isolated side of the circuit (that is, to the right of the power transformer 22 as shown). The secondary side controller IC 23 provides drive information for the sync switch 25, which in this example is implemented as a sync FET. The controllers 21 and 23 combine to control operation of the power supply. Timing control for the secondary side control IC 23, is passed from the primary side control IC 21 , by means of a transformer 29. Thus information is passed from the non-isolated side to the isolated side by means of the transformer 29. In contrast, in order to pass information from the isolated side of the circuit to the non-isolated side of the circuit, an opto-coupler 27 is used. In this particular example, the opto-coupler is used to pass, that is to say to communicate, output power information back across the isolation, from the isolated side to the non-isolated side.

A further example of a prior art switched mode power supply, to which the invention can also beneficially be applied, is shown in Figure 3. This figure shows a half-bridge lamp driver circuit, and since detailed knowledge of the circuit is not necessary for an understanding of the invention, it will not be described in detail. A high voltage (400V in this instance) is input at 31 , and under the control of the non-isolated side controller 32, directed through a power FET 33, across a half-bridge node 35, and through a sync FET 34. Current from the half-bridge output node 35 is directed to the primary winding i3 of an inverting transformer. As shown in the figure, an auxiliary winding i2 on the transformer provides feed-back on the ouput voltage. However, in other examples the auxiliary winding may be used for timing information, or for both timing information and output voltage, power or current information; moreover, a supplementary signal transformer i4 is provided, to pass the lamp current information across the mains isolation barrier from the lamp, via the signal transformer, to the controller 32. In addition to the signal transformer, an opto-coupler i1 is provided to pass information relevant to lamp protection, that is to say, protection information, from the isolated side (i.e. the right side of the figure) to the non-isolated side (i.e. the left side of the figure) of the circuit. Furthermore, additional opto-couplers i5 and i6 are included, which pass small signal control board signals, that is to say signals relevant to a small signal control board, from the isolated to the non-isolated side. From consideration of the above Figure 3, it is clear that multiple opto- couplers may be required to pass information across the isolation barrier.

A block diagram of a SMPS incorporating a first embodiment of the present invention is shown in Figure 4. This figure may be compared to the prior art solution shown in Figure 1 , and like numerals are used to reference like elements of the circuit. Thus, primary side controller 1 controls the primary side circuit of the SMPS, having an input Vin. Secondary side controller 3 controls the secondary side of the power transformer 2. The transformer output is rectified by diode 4 and smoothed by capacitor 5 to provide the output voltage Vout. In this embodiment, however, in order both to pass information on the power control and protection information back from the isolated secondary side to the non-isolated primary side, and to pass, in the opposite direction, timing information relating to the timing of switch S1 from the primary side to the secondary side, a single signal transformer T1 is provided.

Thus, in comparison with the prior art circuit shown in Figure 1 , this embodiment of the invention utilises a single transformer to pass two sets of information (power control, and protection), in one direction, and a further set of information (timing control) in the opposite direction.

Since the signal transformer T1 is not transferring a high level of energy, but is concerned with signals, it may be small and inexpensive. There is a consequential reduction in the number of components, and thus the cost of, and space required by, the circuit. The transformer may be driven with pulses having a small duty cycle. This will require substantially less energy than a DC biased opto-coupler. Moreover, DC-biasing of opto-couplers can form a significant part of the consumed power in no-load situations, and in the absence of opto-couplers, this is avoided.

Figure 5 shows in more detail a part of this embodiment of the present invention. The primary side controller 51 includes a digital control unit 53, together with drive transistor M1 and detection transistor M3, appropriately biased through resistors R1 and R3 respectively. The equivalent secondary side controller 52 includes a digital control unit 54, together with drive transistor M2 and detection transistor M4, appropriately biased through resistors R2 and R4 respectively. On the primary side drive transistor M1 drives the signal transformer T1 with a signal. On the secondary side M4 is used to detect corresponding induced signals from T1. Equivalently, M2 drives the signal transformer T1 on the secondary side and M3 is used for primary side detection. The polarity of the transformer T1 on the primary side is opposite with respect to the secondary side. The 'blank' input on both sides (respectively at 55 and 56) is needed to distinguish between a transformer reset pulse after the drive pulse and a signal coming from the other side of the transformer, and will be discussed in more detail below. During the reset pulse the energy stored in the transformer falls back to zero. The 'blank' signal will blank the input signal for a certain time after the drive signal has gone low.

A method of effecting communication of power control information from the secondary side to the primary side, according to an aspect of the present invention will now be described, with reference to Figures 6 and 7.

Figure 6 shows an example of traces of the primary side transformer signal 62 and the secondary side transformer signal 63, for a switched mode power supply operating at a high power output. The digital controller 53, provides a signal to M1 , to provide an output current to the primary side of T1 , the start of which is shown at 64. After a short interval, an internal timer (not shown) in the controller 53 generates a signal that blanks the input for a certain short time which spans the transformer reset pulse 65. The transformer reset pulse 65, which resets the primary side transformer current to zero, is caused by the magnetic energy which is built up during the pulse inside the transformer core. Thus when the controller outputs a pulse, some magnetic energy is built up in the transformer core, and a blanking signal is required to blank the input during the resulting transformer reset period 65. In the absence of such a blanking, the controller would see an input signal, and could mistake the transformer reset signal 65, for the real return signal 70. The start of the blanking signal occurs just after the switch M1 is switched off. Trace 63 shows the corresponding signals 66 and 67 transferred to the secondary side of the transformer. Signals 66 and 67 are inverted relative to signals 64 and 65. The signal on the secondary side is detected by M4 and used as an input to the digital controller 54. After a delay period D1 , which is determined by the digital controller 54 and is dependant on the power output requirement, the digital controller on the secondary side provides a signal to M2. This produces a current output pulse 68 on the secondary side of the transformer, the input to which is similarly blanked for a short interval spanning reset pulse 69, by the controller 54 providing blanking 56. The corresponding pulse 70 on the primary side of the transformer is detected by means of transistor M3. The power output requirement is used by the digital controller 54 to code the delay between the primary side pulse and the return secondary side pulse - in this exemplary embodiment, the coding is such that this delay is directly dependant on, or proportional, to the power output requirement; thus by ensuring that the primary side digital controller can decode the delay back to the output power requirement, the information on the output power requirement has been transferred from the secondary side to the primary side. In other words, the power output requirement has been encoded into the time domain, transferred and then decoded. This aspect may be contrasted with conventional opto-coupler operation, in which a variable current is used to encode information, that is, the information is encoded in the current domain.

Figure 7 shows traces for the primary side transforming current 72 and secondary side transformer current 73 corresponding to those in Figure 6, but for a SMPS under low power operation. In this case the delay period D2 is shorter that the corresponding delay period D1 for the high power operation. Thus in this particular embodiment, the delay increases with the output power requirement. The delay may be directly proportional to the output power requirement, or include an offset. Alternatively, the delay may vary with the square of the requirement, or with its logarithm. Moreover, the delay may vary inversely with the requirement, again with or without an offset. Other arrangements for mapping the power output requirement onto the delay, which are suitable or appropriate for particular applications will be immediately apparent to the skilled person.

Figure 8 shows another method of effecting communication of information comprising the power control signal as described with reference to figure 8, in combination with a protection signal. Here the traces 82 and 83, of the primary and secondary side transformer signals respectively, include the same initial signals and blanking on the primary side at 64 and spanning 65 respectively, with the corresponding signals on the secondary side at 66 and 67, together with the return signal from the secondary side 68 and blanking spanning the reset signal 69 and corresponding signal 70 on the primary side. However, in this case, an additional signal 84 is generated by the digital controller on the secondary side, along with the associated reset signal 85 after the short interval, to communicate a protection signal from the secondary side (isolated side) to the primary side (non- isolated). This protection signal is relevant to protect the SMPS, or an attached load, from damage for example in the occurrence of a short circuit across the SMPS. The protection signal may be used on the primary side, for example to reduce the input power or switch off the SMPS.

In another embodiment, instead of a simple pulse a series of pulses may be used as, for example, the protection signal. Advantageously this allows for the passing of additional information. Moreover, by utilising coding such as a Manchester Coding, it can be ensured that the dc component across the transformer is zero. That is, the average voltage across the transformer is zero. Manchester coding, also known as phase encoding, will be well known to those skilled in the art. It consists of two variants which are the inverse of each other, one which follows the IEEE 802.3 standard, and the other being attributed to G. Thomas. The method of encoding comprises an exclusive or (XOR) of a clock signal and the original data. The phase of the coded signal thereby changes by π whenever there is a rising or falling edge to the original date.

It will be immediately evident to the skilled person that the interval between the return signal for the power control and the protection signal should be suitably controlled by (and in a preferred embodiment, predetermined by) the digital controller on the secondary side. In this way, information from multiple channels can be communicated across the isolation barrier using the transformer.

Moreover, as already mentioned in passing, the signals on the primary side of the transformer are inverted relative to those on the secondary side, by the use of an inverting transformer. This has the effect that the detection circuits of the primary and secondary side controllers can each distinguish between signals generated on their own side and those generated on the other side of the transformer.

In another embodiment of the invention, multiple signals can be communicated across the isolation barrier using frequency multiplexing, rather than the time multiplexing described above. A block diagram of such an embodiment is shown in Figure 9. The arrangement of figure 9 is similar to that of figure 5, in that it comprises digital controllers 53 and 54 on the primary and secondary sides respectively, and drive transistors M1 and M2 with associated bias resistors R1 and R2. However, in this case, the respective primary and secondary side controllers 91 and 92 include a filter, 95 and 96 respectively, instead of the detection transistors M3 and M4. The filters are frequency filters, and are set to distinguish between the various sets of information to be communicated. Thus in this case, a high frequency pulse may correspond to power control information, and a low frequency pulse to protection information. Alternatively, the frequency filter may be used with alternating current control signals, to distinguish between signals in forward communication (i.e. from the non-isolated side to the isolated side), from those in back communication (i.e. from the isolated side to the non-isolated side).

The embodiments above have been described with reference to digital controllers. However, equally, an analog controller may be used, or a hybrid controller. Moreover, the control may be carried out either entirely or in part by software means.

From reading the present disclosure, other variations and modifications will be apparent to the skilled person. Such variations and modifications may involve equivalent and other features which are already known in the art of switched mode power supplies and which may be used instead of, or in addition to, features already described herein.

Although the appended claims are directed to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalisation thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention.

Features which are described in the context of separate embodiments may also be provided in combination in a single embodiment. Conversely, various features which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination.

The applicant hereby gives notice that new claims may be formulated to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom. For the sake of completeness it is also stated that the term "comprising" does not exclude other elements or steps, the term "a" or "an" does not exclude a plurality, a single processor or other unit may fulfil the functions of several means recited in the claims and reference signs in the claims shall not be construed as limiting the scope of the claims.

Claims

1. A switched mode power supply operable from a power supply and comprising an isolated side (3,4,5) which in use is electrically isolated from the power supply, a non-isolated side (1 ) which is in use not electrically isolated from the power supply, and back communication means for passing information from the isolated side to the non-isolated side, characterised in that the back communication means comprises a signal transformer (T1 ) having primary and secondary windings.

2. A switched mode power supply according to claim 1 , wherein the back communication means is adapted to pass protection information to the nonisolated side by means of the signal transformer (T1 ).

3. A switched mode power supply according to claim 2, wherein the protection information is encoded by means of a Manchester coding.

4. A switched mode power supply according to any preceding claim, wherein the back communication means is adapted to pass output information relating to at least one of output power, output voltage and output current to the non-isolated side by means of the signal transformer.

5. A switched mode power supply according to claim 4, wherein said output information comprises a length of a delay (D1 , D2) between a signal received by the back communication means and a signal transmitted by the back communication means.

6. A switched mode power supply according to any preceding claim, further comprising forward communication means for passing information (84) from the non-isolated side to the isolated side, and wherein the signal transformer also comprises part of the forward communication means.

7. A switched mode power supply according to claim 6 wherein said forward communication means is adapted to pass timing information to the isolated side by means of the signal transformer.

8. A switched mode power supply according to any preceding claim operable from at least one of a high voltage power supply or a mains power supply.

9. In a switched mode power supply having an output power requirement and comprising an isolated side which in use is electrically isolated from a power supply, a non-isolated side which in use is not electrically isolated from the power supply, and a signal transformer having primary and secondary windings, a method of communicating information from the isolated side to the non-isolated side, the method comprising the steps of: generating a signal (64) on the signal transformer on the non-isolated side, detecting a corresponding induced signal (66) on the signal transformer on the isolated side, generating a reply signal (68) on the signal transformer on the isolated side after a predetermined delay which is determined according to the output power requirement, detecting a corresponding induced reply signal (70) on the signal transformer on the non-isolated side, and determining the output power requirement according to the delay (D1 , D2) between the signal and the corresponding induced reply signal.

10. A method according to claim 9, further including the steps of generating a further signal (84) on the isolated side a predetermined time after generating the reply signal, detecting a corresponding induced further signal (86) on the signal transformer on the non-isolated side, and determining a state of the switched mode power supply, other than the output power requirement, based on the further signal.

11. A method according to claim 9, further comprising the steps of blanking the detection on the non-isolated side for a predetermined short time after generating the signal and blanking the detection on the isolated side for a predetermined short time after generating the reply signal.

12. A method according to claim 9, further comprising communication of information from the non-isolated side to the isolated side, by generating a forward signal on the signal transformer on the non-isolated side and monitoring for a corresponding induced forward signal on the signal transformer on the isolated side.