GB2455987A - A power supply unit incorporating a current monitoring device - Google Patents
A power supply unit incorporating a current monitoring device Download PDFInfo
- Publication number
- GB2455987A GB2455987A GB0725231A GB0725231A GB2455987A GB 2455987 A GB2455987 A GB 2455987A GB 0725231 A GB0725231 A GB 0725231A GB 0725231 A GB0725231 A GB 0725231A GB 2455987 A GB2455987 A GB 2455987A
- Authority
- GB
- United Kingdom
- Prior art keywords
- current
- transistor
- supply unit
- power supply
- output
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Granted
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Classifications
-
- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
- H02H7/10—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for converters; for rectifiers
- H02H7/12—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for converters; for rectifiers for static converters or rectifiers
- H02H7/125—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for converters; for rectifiers for static converters or rectifiers for rectifiers
- H02H7/1257—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for converters; for rectifiers for static converters or rectifiers for rectifiers responsive to short circuit or wrong polarity in output circuit
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H3/00—Emergency protective circuit arrangements for automatic disconnection directly responsive to an undesired change from normal electric working condition with or without subsequent reconnection ; integrated protection
- H02H3/08—Emergency protective circuit arrangements for automatic disconnection directly responsive to an undesired change from normal electric working condition with or without subsequent reconnection ; integrated protection responsive to excess current
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03K—PULSE TECHNIQUE
- H03K17/00—Electronic switching or gating, i.e. not by contact-making and –breaking
- H03K17/08—Modifications for protecting switching circuit against overcurrent or overvoltage
- H03K17/082—Modifications for protecting switching circuit against overcurrent or overvoltage by feedback from the output to the control circuit
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03K—PULSE TECHNIQUE
- H03K17/00—Electronic switching or gating, i.e. not by contact-making and –breaking
- H03K17/08—Modifications for protecting switching circuit against overcurrent or overvoltage
- H03K17/082—Modifications for protecting switching circuit against overcurrent or overvoltage by feedback from the output to the control circuit
- H03K17/0828—Modifications for protecting switching circuit against overcurrent or overvoltage by feedback from the output to the control circuit in composite switches
Landscapes
- Engineering & Computer Science (AREA)
- Power Engineering (AREA)
- Dc-Dc Converters (AREA)
Abstract
A power supply unit is arranged to receive ac power (VIN) and to output a number of different do output voltages (V1 ,V2). The unit includes a power transformer (T1) and a feedback control device (8) arranged to limit the primary side current in response to high currents to the lower rated dc output (V2). The current to output (V2) is sensed by a bipolar transistor (Q2) connected so that, in normal conditions, it is switched off. If the current supplied to output (V2) is too high, the transistor (Q2) switches on and supplies current to the feedback control (8) whereby regulation of the output voltage (V2) is achieved. The transistor (Q2) can be arranged in a single package with a diode (D2) provided in the rectifying circuit for the lower rated output (V2). This protects the circuit against variations due to differences in temperature between the diode and the transistor. The packaged arrangement of these components can be used as a current monitoring device in other contexts.
Description
A POWER SUPPLY UNIT INCORPORATING
A CURRENT MONITORING DEVICE
The present invention relates to a current monitoring device, and to a power supply unit incorporating such a current monitoring device.
Power supply units, which transform the available ac mains power into dc outputs, are used or incorporated in many devices. For example, they may be utilised to power computers and set top boxes. Frequently, more than one dc output is provided, with differing dc outputs having different power ratings. In this way the power supply unit is able to meet the requirements of the powered device which may require high power to drive fans and disc drives and the like, as well as low voltage outputs to power logic circuits.
Where a power supply unit has a number of dc outputs, with different voltage ratings, there is a potential problem that any overload current on the lower rated voltage outputs can be very high and damaging if, for example, there is no load or a fault on the highest voltage output. The present invention seeks to deal with this problem and proposes a secondary side current limiting technique. However, it has now been appreciated that the current limiting technique has broader application to current monitoring generally.
The present invention seeks to provide a current monitoring device.
The invention also seeks to provide a power supply unit with secondary side current limiting According to a first aspect of the present invention there is provided a current monitoring device comprising a rectifier connected such that the current to be monitored is arranged to flow through the rectifier, and a transistor coupled to the output of the rectifier, the transistor being arranged to be in an off condition when a current below a predetermined threshold flows through the rectifier and to be turned on when the current flow is above the predetermined threshold, whereby the output of the transistor indicates the current level. (
Embodiments of a current monitoring device of the invention provide a simple, low cost circuit which is advantageous in many current monitoring applications.
In an embodiment, the rectifier is a junction diode and is formed in a common package with the transistor such that the temperature of the diode and of the transistor is substantially the same.
The provision of a junction diode and the transistor in the same package ensures that the temperature of the diode and of the transistor remain substantially the same. Whilst the diode and the transistor have dissimilar typical power levels, as their temperatures remain the same, variations of the current being monitored with ambient temperature will be acceptably low.
Preferably, the current monitoring device comprises a voltage divider arranged to determine the current to be flowed through the rectifier In an embodiment, the transistor is an npn device and has its emitter connected to the rectifier output, and the base-collector current of the transistor indicates the current level of the rectifier.
The present invention also extends to a power supply unit for providing at least two dc outputs from an ac input, where the dc outputs have different output voltages, the power supply unit comprising a power transformer, the primary of the power transformer being arranged to be coupled to the ac input, and the dc outputs being coupled to secondaries of the power transformer by respective rectifying means, wherein at least one of the dc outputs is coupled to a secondary of the power transformer by way of a current monitoring device as defined above.
In an embodiment, the current output of the transistor of the current monitoring device is applied to feedback means arranged to control the current flowing through the rectifier of the current monitoring device. (
According to a further aspect of the present invention there is provided a power supply unit for providing at least two dc outputs from an ac input, where the dc outputs have different output voltages, the power supply unit comprising a power transformer, the primary of the power transformer being arranged to be coupled to the ac input, and the dc outputs being coupled to secondaries of the power transformer by respective rectifying means, wherein a transistor is coupled to the output of one of the rectifying means, the transistor being arranged to be in an off condition when a current below a predetermined threshold flows through the rectifying means and to be turned on when the current flow is above the predetermined threshold, and wherein the current flow of the transistor is applied to a feedback control of the power supply unit which is arranged to control the current flow of the unit.
Embodiments of a power supply unit as defined above provide secondary side current limiting which is able to protect against current overloads on low voltage dc outputs. The components used for current limiting, namely the rectifying means and the transistor, are relatively low cost and the circuit is simple.
Preferably, said one of the rectifying means is a junction diode and is formed in a common package with the transistor such that the temperature of the diode and of the transistor is substantially the same.
The junction diode and the transistor may be provided in a single package which forms part of the simple low cost current limiting circuit outputting a voltage which is a function of the diode's forward current. This voltage may not be very accurate nor fundamentally linear, but the circuit is non-intrusive and is non-dissipative which can be advantageous in the circumstances. The provision of the diode and transistor in a package enables the current limiting circuit to be substantially unaffected by temperature rises within the unit.
Preferably, a voltage divider for setting the voltage at each output of the power supply unit is provided, the base of the transistor being coupled to a node of said voltage divider
I -4-.
In an embodiment, the transistor is an npn device and has its emitter connected to the output of said one rectifying means, and the base-collector current of the transistor is apphed to the feedback control of the power supply unit.
Embodiments of the present invention will hereinafter be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a power supply unit incorporating a known circuit providing primary side current limiting: Figure 2 shows a power supply unit using a known secondary side current limiting technique; Figure 3 shows a power supply unit incorporating a further known secondary side current limiting technique; and Figure 4 shows an embodiment of a power supply unit of the present invention.
Figure 1 shows a power supply unit having input terminals 2 to which the mains ac power VIN is applied. The power supply unit includes a transformer Ti. On its secondary side, the power supply unit is provided with rectifying means Di and D2 so that dc voltages are available at output terminals 4 and 6. As indicated, a high dc voltage V1 may be provided across the output terminals 4, 4, and a lower output voltage V2 provided across the output terminals 4, 6. By way of example, a power supply unit for a set top box may be provided with four dc outputs capable of delivering power at 22 volts, 12 volts, 5 volts and 3.3 volts. In a set top box, the hard disc drive has to be powered from a relatively high voltage whilst memories and logic circuits should not be subjected to high currents.
There is a potential difficulty with a power supply unit as shown in Figure 1 which is able to deliver relatively high power at voltage V1 across its highest rated voltage output terminals 4, 4. If there is no load, or a relatively light load, across the terminals 4, 4 relatively high current will appear on the lower rated voltage output 4, 6. The high current has the capability of damaging the components in the circuit connected to the low voltage output.
It is, of course, possible to construct all of the circuits within the power
I
supply unit using components capable of withstanding high currents, but this is an expensive option. There is, therefore, a balance to be drawn between the risk of high currents being generated and the need to have high current components.
In the power supply unit shown in Figure 1, primary side current limiting is provided to give overload protection. Thus, an FET 01 is provided as a smart switch on the primary side and is connected to a resistor R3 which acts to limit the current in the primary side. Each secondary side voltage output is supplied by way of a respective diode Dl, D2 and its associated smoothing capacitor Ci, C2. Resistors Ri and R2 enable the output voltage V1 on terminals 4, 4 to be sensed and the input current on the primary side can be regulated by way of a feedback control 8 if the current on that output is too high. It will be seen that the feedback control 8 acts on the smart switch 01. The feedback control 8 acts to change the duty cycle of the primary of the transformer T1.
However, if the voltage V2 on terminals 4, 6 is significantly lower than the voltage on V1 on terminals 4, 4 there is the potential problem that the overload current on output 4, 6 can be very high compared with what is maximum for normal running, should the current taken from the other output 4, 4 just happen to be light. This means that the components D2, C2, and the corresponding secondary sec. 2 of the transformer Ti will all have to be designed to handle much higher currents than would otherwise be the case.
Figure 2 shows a power supply unit with primary side current limiting as in Figure 1, but with the addition of a current transformer T2 to provide secondary side current limiting. This technique for secondary side current limiting is known. The current transformer T2 picks up the ac component of the current to be applied to terminal 4, 6. If this current is higher than its rating a feedback current by way of diodes D4 and D3 is applied to the feedback control 8 and hence to the current limiting means Qi, R3 on the primary side.
This technique has the disadvantage that current transformers as T2 are relatively expensive compared with other electronic components.
C
Furthermore, the current transformer T2 can actually worsen the regulation of voltage V2 unless it has a high turns ratio and, in that event, the transformer is even more costly. In addition, a resistor R4 which is provided in the transformer/feedback circuit will need to be a high powered component with consequent expense. Therefore, this technique uses relatively expensive components, and is complex.
Figure 3 shows another example of a known technique for secondary side current limiting. In this technique, an operational amplifier 10 is connected between the ground terminal 4 and an auxiliary terminal 12. The operational amplifier 10 is connected to a voltage divider of resistors R4, R5, and R6 and effectively senses the voltage drop across R4. If the operational amplifier 10 is drawing too much current, the diode D3 is switched on and a feedback current is applied to the feedback control 8 to provide current limiting.
The arrangement shown in Figure 3 is complex and may require the provision of the auxiliary rail 12 to provide the necessary power for the operation of the operational amplifier 10. The operational amplifier 10, being an integrated circuit, is quite a complex inclusion. The resistor R4 has to withstand high currents and is therefore expensive. Furthermore, the existence of the drop across resistor R4 worsens the regulation of voltage V2 at the output 4, 6.
Figure 4 shows a power supply unit incorporating a current limiting technique according to an embodiment of the present invention.
In the arrangement shown in Figure 4, a bipolar npn transistor Q2 is connected such that its base is coupled to a voltage divider R4, R5 and R6 arranged to set the voltage at each dc output 4, 4 and 4, 6. The emitter of the transistor 02 is connected to the output of the diode D2. The resistors of the voltage divider R4, R5 and R6 determine the current flowing towards output 4, 6 and the voltage on the base of the transistor Q2 increases as the voltage on the emitter increases. When loads are normal, transistor 02 is off. When the current rises, the voltage rise on the emitter turns the transistor Q2 on so that a current is applied to the feedback control 8 to
I
cause current limiting of the primary side current as previously.
It may be seen that, as compared with the other techniques for secondary side limiting, there is no impedance added in series with the diode 02. This means that the current limiting circuit has no impact on the regulation of the voltage V2 Furthermore, the added components Q2 and R4, R5 and R6, which provide the secondary side current limiting, are low cost, low power, small size, and few in number. This is very advantageous.
It is proposed that the bipolar transistor Q2 and the junction diode D2 should be constructed physically to be within the same package. This means that there will be minimal temperature difference between the diode junction and the transistor junctions even though the two devices have quite dissimilar power levels. Thus, the variation of the current flowing to the output 4, 6 with ambient temperature internal to the power supply unit will be kept acceptably low.
The combination of the diode D2 with the transistor 02 provides a current monitoring circuit which is able to control or monitor the current through the diode D2 and which might be used in other circumstances.
Thus, the combined package of transistor and diode is a simple low cost circuit which outputs an analogue voltage which is a function of the rectifier's forward current. Whilst this voltage may not be very accurate nor fundamentally linear, it can be used to enable monitoring of the current or its control. The advantage of the packaged circuit is that it is non-dissipative.
It will be appreciated that variations in and modifications to the invention as described and illustrated may be made within the scope of the accompanying claims. In particular, it is noted that all of the drawings show two dc outputs only for the power supply unit. Clearly, further outputs may be provided.
The drawings all show one particular way of limiting the primary side current in response to feedback. Alternative techniques for limiting the primary side current and/or limiting the current through the secondary coils, can also be utilised. (
The topologies of the power supply unit as illustrated primarily use off line fly back. The technique of the invention may be used with any other power supply unit topology (
Claims (10)
1. A current monitoring device comprising a rectifier connected such that the current to be monitored is arranged to flow through the rectifier, and a transistor coupled to the output of the rectifier, the transistor being arranged to be in an off condition when a current below a predetermined threshold flows through the rectifier and to be turned on when the current flow is above the predetermined threshold, whereby the output of the transistor indicates the current level.
2. A current monitoring device as claimed in Claim 1, wherein the rectifier is a junction diode and is formed in a common package with the transistor such that the temperature of the diode and of the transistor is substantially the same.
3. A current monitoring device as claimed in Claim 1 or Claim 2, further comprising a voltage divider arranged to determine the current to be flowed through the rectifier.
4. A current monitoring device as claimed in any preceding claim, wherein the transistor is an npn device and has its emitter connected to the rectifier output, the base-collector current of the transistor indicating the current level of the rectifier.
5. A power supply unit for providing at least two dc outputs from an ac input, where the dc outputs have different output voltages, the power supply unit comprising a power transformer, the primary of the power transformer being arranged to be coupled to the ac input, and the dc outputs being coupled to secondaries of the power transformer by respective rectifying means, wherein at least one of the dc outputs is coupled to a secondary of the power transformer by way of a current monitoring device as claimed in any of Claims 1 to 4.
6. A power supply unit as claimed in Claim 5, wherein the current output of the transistor of the current monitoring device is applied to feedback means arranged to control the current flowing through the rectifier of the
I
current monitoring device.
7. A power supply unit for providing at least two dc outputs from an ac input, where the dc outputs have different output voltages, the power supply unit comprising a power transformer, the primary of the power transformer being arranged to be coupled to the ac input, and the dc outputs being coupled to secondaries of the power transformer by respective rectifying means, wherein a transistor is coupled to the output of one of the rectifying means, the transistor being arranged to be in an off condition when a current below a predetermined threshold flows through the rectifying means and to be turned on when the current flow is above the predetermined threshold, and wherein the current flow of the transistor is applied to a feedback control of the power supply unit which is arranged to control the current flow of the unit.
8. A power supply unit as claimed in Claim 7, wherein said one of the rectifying means is a junction diode and is formed in a common package with the transistor such that the temperature of the diode and of the transistor is substantially the same.
9. A power supply unit as claimed in Claim 7 or Claim 8, further comprising a voltage divider for sethng the voltage at each output of the power supply unit, the base of the transistor being coupled to a node of said voltage divider.
10. A power supply unit as claimed in any of Claims 7 to 9, wherein the transistor is an npn device and has its emitter connected to the output of said one rectifying means, the base-collector current of the transistor being applied to the feedback control of the power supply unit.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB0725231A GB2455987B (en) | 2007-12-24 | 2007-12-24 | A power supply unit incorporating a current monitoring device |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB0725231A GB2455987B (en) | 2007-12-24 | 2007-12-24 | A power supply unit incorporating a current monitoring device |
Publications (3)
| Publication Number | Publication Date |
|---|---|
| GB0725231D0 GB0725231D0 (en) | 2008-02-06 |
| GB2455987A true GB2455987A (en) | 2009-07-01 |
| GB2455987B GB2455987B (en) | 2012-07-25 |
Family
ID=39092376
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB0725231A Expired - Fee Related GB2455987B (en) | 2007-12-24 | 2007-12-24 | A power supply unit incorporating a current monitoring device |
Country Status (1)
| Country | Link |
|---|---|
| GB (1) | GB2455987B (en) |
Citations (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE2402091A1 (en) * | 1973-11-09 | 1975-05-15 | Bbc Brown Boveri & Cie | Signal delay line for overload protection ccts. - has input switch as comparator feeding trigger via integrator |
| US4323961A (en) * | 1980-09-12 | 1982-04-06 | Astec Components, Ltd. | Free-running flyback DC power supply |
-
2007
- 2007-12-24 GB GB0725231A patent/GB2455987B/en not_active Expired - Fee Related
Patent Citations (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE2402091A1 (en) * | 1973-11-09 | 1975-05-15 | Bbc Brown Boveri & Cie | Signal delay line for overload protection ccts. - has input switch as comparator feeding trigger via integrator |
| US4323961A (en) * | 1980-09-12 | 1982-04-06 | Astec Components, Ltd. | Free-running flyback DC power supply |
Also Published As
| Publication number | Publication date |
|---|---|
| GB0725231D0 (en) | 2008-02-06 |
| GB2455987B (en) | 2012-07-25 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| 732E | Amendments to the register in respect of changes of name or changes affecting rights (sect. 32/1977) |
Free format text: REGISTERED BETWEEN 20170601 AND 20170607 |
|
| PCNP | Patent ceased through non-payment of renewal fee |
Effective date: 20201224 |