GB2316556A - Power supply with protection arrangement - Google Patents

Abstract

A power supply for a television set has a switching transistor 27 coupled to a transformer 29. A feedback voltage Vfb from the transformer is compared 35 with a reference voltage Vref to control drive transistors 24,25 for pulse width modulation of transistor 27. The drive transistors 24,25 are supplied with a voltage Vcc derived from transformer 29 by a peak detector 30. The voltage Vcc is also supplied to an overvoltage detector 37 coupled to drive control 21. If one of the drive transistors 24,25 shorts, voltage Vcc is effectively grounded and a zener diode 32 breaks down to reduce the level of the feedback voltage so that the output voltage exceeds a limit, causing overvoltage detector 37 to operate, resulting in shutdown of the circuit.

Description

Power Supply The present invention relates to a power supply, which can be realised as a switch mode power supply (SMPS).

SMPS power supplies are already well known. Such a power supply includes a power transistor, which is driven by an according driver stage. It is also known to realise such a driver stage by a push-pull configuration, which includes a pair of a first transistor and a second transistor which are altematively switched on and off during the normal operation. These transistors can be connected such that the emitter of the first transistor is connected to the collector of the second transistor, where this connection is also used as output for driving the power transistor. The collector of the first transistor is then connected via a collector resistor with a supply voltage. The emitter of the second transistor is connected to ground.

It is also known that such a driver stage is realised as part of an integrated circuit (IC), e.g. in the IC TEA2261 of SGS-THOMSON. This IC also includes an error amplifier having an input (at pin 6). A voltage provided at this input is compared with a reference voltage. When the input voltage is below a value defined by the reference voltage, the driver stage is such controlled that the duty cycle of the pulse width modulation (PWM) signal, which is used for controlling the first and the second transistor, is increased.

Is has been shown that it may happen that one of the transistors can be damaged whereby it may be shortened. This has the effect that a current through the resistor will exceed a given value, whereby an according overheating may occur. This can increase the risc of a malfunction or even of a fire hazard.

It is already known that overheating can be reduced or even be avoided when a collector resistor is taken with a high power resistance. Such resistors are quite expensive.

So, it is the object of the present invention to avoid the overheating of the collector resistor used for the driver stage of a power supply in a simple and cost effective way.

According to this invention a driver stage of a power supply, or more precisely a supply connection of said driver stage which is connected to a supply voltage may be directly or via some electrical means, is connected with the input of a comparison stage, which compares the voltage of said connection of the driver stage with a given reference value and which controls the driver stage such that its duty cycle can be varied.

The invention is based on the following principle. The transistors of the driver stage working in push-pull configuration are altematively switched on and off in normal operation. This means that at any time one of the transistors is open and the other is connecting.

In case of a short of one of both transistors, e.g. the first one, it happens at that times when the second transistor is controlled such that it is connecting, that voltage at the supply connection is dropped to a low value. This value is lower than a given value of the comparison stage, which is comparable with the error amplifier of IC TEA2261.

In this case the driver stage is such controlled that it works at a very high duty cycle. This has the effect that within a short period of time the supply voltage tends to drastically increase. Thereby an overvoltage protection stage becomes active and causes that the PWM mode will be switched off. In addition the power transistor is damaged by biasing the power transistor when the driver stage is completely shut down. Thereby no further current can flow from the supply voltage through the resistor and the transistors to ground. As no further overcurrent flows through the resistor, its overheating and thereby an according firerisk can be avoided.

In a further embodiment the connection between said supply connection and said further comparison stage include rectifying means, like a diode, a zener diode or the like. This means that rectifying means may be the only conductive means, but also other means, like e.g. resistors1 capacitors, may be included.

This has also following advantage. Electrical devices which are nowadays brought on the market, should often be able to work with different values of mains voltages, e.g. in the range from 80VAC to 276VAC. The supply voltage which is lead to a supply pin of the driver circuit, e.g. pin 16 of IC TEA2261, is usually also controlled in view of overvoltage and undervoltage. As said supply voltage also depends on the mains voltage, it may happen that its value is in the correct range for a low mains voltage (e.g lower than 100VAC), but exceeds a value which is detected as overvalue for a high mains voltage (e.g. higher than 22OVAC). This may happen especially during the standby mode of a device, e.g.

a TV set, said SMPS is used for.

Providing a zener diode such that its cathode is connected to said supply connection and that its anode is connected to said comparison stage has the following effect. When the supply voltage increases over a value which is essentially the sum of the zener voltage and the reference voltage of the comparison means, said zener diode will become conductive. The according zener current will be added on to the comparison means input and hence its input voltage tends to increase. Thereby the PWM signal is such controlled that the ON-times are reduced, i.e. the duty cycle is reduced. Due to the closed-loop control the supply voltage is clamped and thereby limited to a value below the overvoltage protection.

Further details and advantages of the present invention will now be explained by means of preferred embodiments and an according drawing where Fig. 1 shows a block diagram of a preferred embodiment Fig. 2 shows a current path during a short circuit Fig. 3 shows a current path to clamp supply voltage Vcc Fig. 4 shows a circuit diagram of the preferred embodiment Fig. 1 shows a preferred embodiment using an integrated circuit (IC) 20 which is of the type TEA2261 from SGS-THOMSON. The IC 20 includes some logic stages which are indicated by a block 21. The logic stages 21 control a positive output stage 22 and a negative output stage 23. The positive output stage 22 further controls a first transistor 24 and the negative output stage 23 controls a second transistor 25. Transistors 24, 25 are such connected that they work in push-pull configuration. A common connection is the output which is led via a pin 14 (pin number according to TEA2261 configuration) and via further stages 26 to a power transistor 27, emitter of which is connected via a resistor 28 to ground and collector of which is connected with a first terminal 29a of a transformer 29.

A second terminal 29b the transformer 29 supplies an output voltage Vout and is connected to a peak detector 30, output of which is called supply voltage Vcc and is lead to a pin 16 and to a current limiter resistor 31, which is also connected to a pin 15 and to the cathode of a rectifying means 32, which is realised in this embodiment as zener diode 32. The anode of diode 32 is connected to the center point of a voltage divider which is realised by resistors 33, 34 and said anode is also connected via pin 6 to a first input of an error amplifier 35, which compares the input signal with a reference voltage Vref at its second input. The output of the error amplifier 35 is connected to the logic stages 21.

The second terminal 29b is also connected with an average stage 36, output of which is called feedback voltage Vfb and connected to resistor 33.

Pin 16 which receives the supply voltage Vcc is also connected to the input of an overvoltage protection 37, output of which leads to the logic stages 21.

The normal operation of the embodiment of Fig. 1 is like follow. The transistors 24, 25 work in push-pull configuration and are controlled by means of a pulse width modulated (PWM) signal by the logic stages 21. Through the current limiter resistor flows in this preferred embodiment an average current of about 0.1 44A, which results in a dissipated power of 0.4 Watts for a resisance of 18 Ohm. The PWM signal is generated in dependence on the output of the error amplifier 35, which compares a voltage which is derived from the signal Vfb with the reference voltage Vref. The overvoltage protection 37 can also influence the stages 21 when the supply voltage Vcc exceeds a given value Vccmax, which is about 15.7V. During the normal operation, which is already well known and which is here described only as far as it is necessary for the understanding of the invention, there is no real effect by the zener diode 32.

When a short occurs in one of the transistors 24, 25, that means that emitter and collector are short-circuited, the zener diode 32 has the following effect, which is also indicated in Fig. 2 showing the current path in case of a short of one of the transistors 24, 25.

Assuming there is a short in the first transistor 24. When transistor 25 is driven by the stages 21, 23 pin 15 is nearly connected to ground at pin 13. Thereby the average current through resistor 31 is about 0.459A which results in a dissipated power of 3.8 Watts. But concurrently the input of the error amplifier 35 is also drastically dropped by the connection between pin 6 and pin 15 through the zener diode 32. Thereby the duty cycle of the PWM signal is increased which results also in an increase of Vcc. Thereby the overvoltage protection 37 becomes active whereby all PWM switching is stopped. Due to fault pin 15-14 DC biasing the power transistor 27 finally leads to the fact that the power transistor 27 is such damaged that it stops operating when the IC is completely shut down. Thereby a further overcurrent through the resistor 31 can be avoided.

The embodiment also solves the problem that an overvoltage is sensed by the overvoltage protection 37 due to high mains voltage. This will now be explained also with the help of Fig. 3.

When an increase of the voltage Vcc occurs, the zener diode 32 will conduct in the zener direction when the value of Vcc reaches approximately 14.55 Volts1 which is the sum of the zener voltage, which is in this embodiment 12 Volts and the reference voltage Vref, which is in the IC 20 (TEA2261) 2.55 Volts. This zener current will be added onto the input divider circuit 33, 34 of the error amplifier 35 and therefore the voltage at pin 6 tends to increase. The logic stages 21 react such that the switching ON time, i.e. the duty cycle, will be reduced. Thereby the voltage Vcc is clamped to 1 4.55V and the system voltage is limited to an acceptable voltage.

It may be mentioned that the combination of the resistor 31 together with the zener diode 32 has the effect that only a slow increase of the voltage Vcc is clamped. If it happened that a fast increase of Vcc occurs, which is mostly the case when an error in the circuitry occurs, like e.g. the short of one of the transistors 24, 25, the voltage Vcc will not be clamped fast enough. Thereby an according overvoltage is detected by the overvoltage protection 37 and the IC 20 will be switched in an according mode.

Fig. 4 shows the circuit diagram of the preferred embodiment. Only such means are included which are necessary for the understanding of the present invention.

Concerning the detailed structure of IC 20 (TEA2261) it is referred to according data books of the company SGS-THOMSON. Means which have been mentioned already are marked with the same reference numbers. In addition there are given references and values which are used in an actual circuit diagram.

So, to summarize the invention it may be mentioned that it is the object to present a power supply with a protection against a short circuit in the driver stage.

Therefore conductive means are provided between a supply terminal of the driver stage and an input of an control amplifier, which allows a feedback control by comparing an output voltage of a transformer with a given reference voltage.

When the conductive means include a zener diode also a malfunction due to an allowable overvoltage e.g. in standby mode can be avoided.

The invention is not restricted to SMPS power supplies, but can be used for all power supplies which work in an feedback mode.

Claims (5)

1. Power supply having a power transistor (27) which is driven by a driver stage (24, 25) which has a first connection to ground (13) and a second connection (15) to a supply voltage (Vcc), where said driver stage (24, 25) is controlled by control means (21, 22, 23) also including a control stage (35) which compares a second voltage (Vfb) which is derived from an output voltage of a transformer (29) with a reference voltage (Vref), characterized in that conductive means (32) are provided between the second connection (15) of the driver stage (24, 25) and the input of the control stage (35).

2. Power supply according to claim 1, characterized in that said driver stage (24, 25) is controlled by said control means (21, 22, 23) such that it works in pushpull configuration.

3. Power supply according to claim 1 or 2, characterized in that said driver stage (24, 25) is controlled by said control means (21, 22, 23) with a pulse width modulated signal.

4. Power supply according to one of the preceeding claims, characterized in that said conductive means (32) include rectifying means.

5. Power supply according to claim 4, characterized in that said rectifying means include a zener diode (32).