SE511444C2 - Procedure for limiting the output current of switched flyback-type power units in overload situations and switched flyback-type power units - Google Patents

Abstract

The invention relates to a method for limiting the output current (Iout) of a switched-mode power supply of flyback type in overload situations and to a switched-mode power supply of flyback type. In accordance with the method, the output current (Iout) is limited by means of pulse width modulation (PWM) by adjusting, by means of a control circuit (13) known per se, the ratio of the duration of the ON and OFF phases of the switch (SW) of the primary circuit. In order for the output current not to increase inordinately in overload situations, the voltage (Vs) which is present on the primary side of the transformer (10) and which is dependant on the output voltage (Uout) of the power supply is utilized to control a current generator (21), and output current (Icc) of which is used to form a control signal for the control circuit (13). <IMAGE>

Description

10 l5 20 25 30 35 511 444 2 often excessively high, especially when the energy transferred for the other outputs is transferred to the loaded the output. Separate current measuring circuits seen for solving the problem becomes excessively expensive and requires special arrangements, as the governing body today for economic reasons are usually in the primary side.

U.S. Patent 4,908,755 discloses a method for limiting the output current from a power supply of flyback type. The control is performed by controlling the primary the peak value of the current as a function of the input and output voltage arna. Also in this case, the control is realized by a rather complicated circuit, in which the parameters should dimensioned so that the formal interdependence between the input and output voltages and the peak current of the primary current value in the flyback type power unit can be simulated.

The object of the present invention is to eliminate the above-mentioned disadvantages by means of a solution, which guarantees such a cost-effective practical realizing as possible. This is achieved with the procedure and the power unit according to the invention, which is characterized by what is stated in the subscribing part of appended claims J. and which power supply is characterized by what is performed in it characterizing part of the appended claims 5.

The idea of the invention is to utilize the customer voltage reflected by the transformer back to the primary side, by from this voltage form a control signal, which controls the circuit which controls the current. the switch.

Due to the solution according to the invention, the cooling and foil of rectifiers do not exceed dimensioned, and further there is in the planning of the secondary windings of the transformer no need to guard against excessively high currents in each wind 10 15 20 25 30 511 444 3 ning. In addition, there will be a possible damage to the load side in case of a short circuit to be minor.

In the following, the invention and its preferred embodiments are described in more detail with reference to the examples according to the accompanying drawings, where figure 1 shows a power unit of flyback type according to the invention, Figure 2 is a block diagram of the current boundary circuit shown in Figure 1, Figure 3 shows that of the current limiting circuit according to figure “2 delivered the control current dependence on power unit output voltage, Figure 4 shows a more detailed embodiment of the power unit according to Figure 1, and Figure 5 shows the output current and the output voltage behavior in a power unit according to the state of the art and in the power unit according to figure 4.

Figure 1 shows a power unit of the flyback type according to the invention, which unit converts a rectifier tad voltage Uin, which is fed to an input capacitor Cin terminals, to another DC voltage Uout, which is being in an output capacitor's Cout terminals. Force- the unit presents in a manner known per se one transformer 10, through which the energy is transferred from primary side to secondary side, a switch SW in the primary circuit, which switch disconnects the primary but, passing through a primary winding 10a, and a control circuit 13, which controls the switch, which circuit controls the output voltage Uout by regulating the operation of the switch bet cycle (duty cycle). The control is done by means of pulse width modulation (PWM), i.e. by adjusting the the country between the lengths of the ON and OFF switches OFF phases. In the secondary circuit, a rectifier diode is D1 and 10 15 20 25 30 35 511 444 4 an output capacitor Cout connected in series with a secondary winding l0b.

The flyback-type power unit works according to following. When the switch Sw is closed (ON), occurs a positive voltage at the point ends of the transformer. IN in that case, a blocking voltage across the rectifier the Dl at the output, and therefore the diode is non-conductive. Hence follows that the secondary current.srm fl J.under the switch ON mode. On the primary side, it increases through the switch however, the current passes linearly during the ON position.

The transformer stores energy in its magnetic flux (air gap) during this phase, and therefore the transformation tower in fact an inductance provided with a secondary winding. Then the switch is controlled to a non-conductive one (open, ie OFF) mode, it reverses in the transformer magnetic flux stored the energy voltage of the winding (flyback phenomenon), in which situation secondary the rectifier diode D1 of the side becomes conductive and a current begins pass through the secondary winding of the transformer. In return charge to the primary current, the secondary current decreases linearly. pea during the OFF mode. At the same time, secondary current, the required output voltage across the condenser Cout.

If the load on the output increases, one dast the time during which the switch is in is extended ON mode, which leads to the primary current having sufficient time to increase, and therefore secondary where the current during the OFF position is correspondingly higher.

The flyback-type power unit can operate either in continuous state (secondary energy does not have time to become complete: discharged after the flyback state) or in a continuous state, in which the energy is completed at the end of each period. Also such flyback-type power supplies are available that work in continuous and uncontinuous state, dependent 10 15 20 25 30 35 511444 5 on the load. The power supply according to the present invention may be of any of the above type.

The number of winding turns of the transformer merlind is denoted by reference Np and the number winding turns of the secondary winding in a corresponding manner with reference Ns in the drawing. The SW switch is displayed in the figure only as an ideal element, which illustrates its function; in practice, the switch is realized typically with a MOSFET (metal oxide semiconductor field effect) transistor) or a bipolar transistor. The control circuit 13 which controls the width of a switching pulse can operate either in voltage mode, which is based on on the output voltage, or in current state (current mode), which is based on the primary current and the output voltage one. The majority (approximately 80%) of today's switched power flyback-type units utilize power circuits (by current state control a better phase margin for the control than by voltage state steering). For this reason, the control circuit shown in In the embodiment according to Figure 1, a control circuit 13 in the current state and performs the control in response on the voltage information obtained from a differential potential amplifier 15 and the current information obtained kept from the power switch. The voltage information is formed by comparing the output voltage with a reference voltage in the differential amplifier and by feeding a differential cleaning signal e.g. through an optocoupler 14 to the control circuit sens differential voltage input EV. The current information is is held from the switch SW by a resistor Rcs to control circuit current measurement input CS. The information is held as a voltage acting across a current supply resistance R7 (which has a small value compared to the Rcs value of the resistor). The control circuit 13 may be of the type UC 3843 (or any other family of the same family), 10 15 20 25 30 35 511 444 6 workers Unitrode Corporation, U.S.A. Other manufacturers also has corresponding circuits.

The solution according to the invention utilizes it secondary voltage reflecting back to the primary dan. As is well known, the voltage Vs across the switch is in the OFF position in a flyback-type power supply: Vs = Uin + go (Um + Uout) (1) Um is the voltage that acts across the rectifier diode Dl on the secondary side. Because this voltage is small compared to the output voltage Uout, it does not need to poetically considered. In accordance with the invention, a separate power limiting circuitJle connected to the primary side, and the voltage across the switch is applied to the input A of this circuit. Output signal of the current limiting circuit 12 (output current) Icc is in turn connected to the control circuit current measurement output CS, where a control voltage is formed for control circuit 13 from the current Icc at the resistor Rcs.

The control circuit has a high input impedance, and therefore runs no current into the circuit.

Current limiting circuit output signal Icca active only in congestion situations, whereby the sar power output Iout, as in the following to be described.

Figure 2 shows the two main blocks of current the boundary circuit 12 according to the invention, i.e. a top- value rectifier circuit 22 and a controllable current generator 21, which is controlled by said peak value rectifier circuit. the rectifier circuit 22 receives at its input the voltage Vs, previously referred to. The power generator is à sin side connected to the Uin positive terminal and the car in its output a control current Icc, which is reversed portion.with the output of the power unit Uout. (After- as the current generator is connected to the Uin of the input voltage plus clamp, seems across it a tension Ug, which 10 15 20 25 30 35 511 444 7 corresponds to the latter part of formula (1), which is depending on the input voltage Uin.) Figure 3 shows that of the current limiting circuit 12 emitted the control current Icc as a function of ningen Uout. As the outflow has fallen from its nomi- value Uoutl to a predetermined value at * Uout1, the power generator 21 starts working. If the tension nominal value Uoutl in the power supply is to example 5 V, the starting point could correspond to, for example, 80 % of the nominal voltage (kl = 0.8). Power generation tower thus begins to function when the output voltage has dropped to 4 V. Furthermore, the current generator is dimensioned so that its maximum current (corresponding to full short-circuit Uout = 0 V) is not capable of completely stopping circuit 13. The control circuit 13 has a threshold value. those that completely disconnect the control, leaving no energy obtained from the power supply. Present description uses as an example a typical control circuit threshold value 1 V, which corresponds to a current Ith = 1 mA, then of the resistor Rc value is 1 kQ (the R7 value of the resistor is very small, for example 1Q, and thus has no effect). The control circuit Icc maximum value is thus a predetermined share, for example about 75% (k2 = 0.75), of said current threshold value. de Ith, which completely closes the circle of control 13.

Figure 4 shows a more detailed embodiment of the power supply according to Figures 1 and 2. For single For the sake of clarity, Figure 4 shows only the configuration of the primary circuit. ration, since the secondary circuit in this case corresponds to the configuration shown in Figure 1. Furthermore, it has feedback loop formed by dif- the differential amplifier and the optocoupler have been left unattended.

A zener diode Z1 and a resistor R3 are connected in series between the input terminals (input capacitor Cin is displayed not in Figure 4). A resistor R2 leads from their common same node to the base of a p-n-p transistor Trl, to 10 15 20 25 30 35 511444 8 point Pl. The emitter of the transistor is connected via a stand Rg to the Uin plus terminal of the input voltage. Transistors collector is connected to the common node for control point 13 current measurement input CS and resistor Rcs.

The base of the transistor is also connected via a resistor R1 to point P in the rectifier circuit 22, which point is connected to the negative terminal of the input voltage via a capacitor tor Cl. The SW of the primary winding and switch SW the same terminal is also connected to point P by serial coupling of a resistor R5 and a rectifier diode D2.

The resistor RS, the diode D2 and the capacitor C1 form one peak value rectifier circuit 22, and point P thus constitutes a supply point, from which the voltage according to the formula (1) above is fed to the current generator 21, which is composed of zener diode Z1, resistors R1 - R4 and Rg and trans torn Trl. As can be seen from formula (1), this decreases to point P supplied voltage when the output voltage Uout is reduced cut (short circuit).

As for its other parts, it corresponds the primary circuit the configuration shown in Figure 1, which means that the current measuring input of the control circuit 13 CS is connected via the resistor Rcs to the switch second terminal, which terminal is connected via a resistor R7 to the Uin negative terminal of the input voltage.

In order for the current generator 21 to be able to function in the manner described above, the resistance of the circuit values be correctly dimensioned. In the following the same example values are reversed as in connection with Figure 3, and it is further assumed that the number of winding turns of the the primary winding of the feeder is 13, the number of winding turns of the secondary winding is 3 and the resistor R2 biases the the diode Z1 so that the voltage across the zener diode is 6.2 V. In a balance position (ie in a position when the output voltage Uout has decreased to the threshold (4 V) at which the current the generator starts working), the transistor Trl starts just 10 15 20 25 30 511 444 9 become conductive, with its base-emitter voltage being approx O V (0 - 0.2 V). The current through the resistor Rg is still zero, and therefore the voltage must cross the resistor R2 corresponds to the voltage across the zener diode. About it it is assumed that the resistor R2 has a value of, for example, 56 kQ, the current I2 through the resistor R2 is about 110 uA. IN a balance position is the voltage at the base of the transistor Tr1 (at point P1) = Uin and the base current of the transistor zero, and therefore, the current I2 can be obtained only via the resistor R1. Since the voltage across the resistor R1 is = l7,3 V (l3ß3 * 4 V), the value R1 is obtained for the resistor R1 Rl = l5O kQ.

When the output voltage Uout is higher than 4 V, it is via the resistor R1 passing the current to a corresponding degree higher, and the base-emitter voltage holds the transistor in closed state. When the output voltage drops to 4 V, the control current Icc flows, and the more the output current decreases the smaller the corresponding cancellation effect which is obtained via the resistor R1, and thus is the control current Icc correspondingly higher.

In the second extreme situation, it is total short circuit at the output of the power supply (Uout = 0 V), and thus the voltage at point P is VP = Uin (assumed that the diode D1 is ideal, i.e. the tension across it is zero). In that situation, the Rg of the resistor is the value Rg = 5.7 kQ (assuming that the gain of the transistor is on example 30), when the desired maximum value of the current Icc is about 750 pA (the voltage across the resistor Rg is 6.2 V minus the base-emitter voltage of the transistor, which is approx 0.5 V, and the voltage across the resistor R2, which voltage is about 1.4 V).

The R3 value of the resistor should be dimensioned as such that the current passing through the resistor R2 cannot disturb the Z1 bias voltage of the zener diode. 10 15 20 25 30 35 511 444 lO By changing the relationship between the resistors R1 and the current generator of the current generator (Uout) = 4 V) is varied. On the other hand, different curves are obtained by changing the Rg value of the resistor.

These curves are described in the following.

Figure 5 shows the output current Iout from the power unit as shown in Figure 4, as a function of the output voltage Uout. The balance point is denoted by reference B. It the normal area of operation is an area within which the voltage Uout is maintained at its nominal value Uoutl (for example 5 V). Angle point L corresponds to it threshold of the control circuit 13 at which a sensing of the primary current of the control circuit begins to reduce the pulse width.and the power supply transitions to an almost constant supply state in which the output voltage and the output current are reciprocal dependence is illustrated by the curve D. According to the invention however, a balance point B of the type described above has and, on the basis of this point, the the current is further supplied by means of the control current Icc of the current generator 21. When the output voltage Uout drops to balance point, the outflow is thus limited more efficiently than hitherto, whereby the interdependence between the voltage and the output current are denoted, for example, by a of the straight lines F1 - F5. The direction mentioned limitation curve will assume depends on the resistance Rg value. The useful area is denoted in the figure by an arrow H. If the Rg value of the resistor increases so that it the value corresponding to the straight line F5 rises, so there is a transition from the area H to the curve D, whereby the solution according to the invention is of little use. Oh on the other hand, the power supply remains locked if the resistor its Rg value falls below the value corresponding to it straight line Fl. As previously stated, ba- the position of the lance point B on curve D is changed by change the ratio between resistors R1 and R2. 10 15 20 25 511 444 ll Although the invention has been described above with reference to embodiments according to the accompanying drawings It is obvious that the invention should not be limited in addition, but it can be varied within the scope of the above and inventions described in the appended claims. idea. In practice, the power unit can, for example, have one several outputs, although the embodiments described above the examples show only one output. The power generator Detailed realizations can also vary on many way. However, the structure described above allows an closure of the additional features of the invention offers to switched power units of flyback type so financially as possible. In principle, it is also possible to utilize the solution according to the invention in connection with a control circuit that operates in the voltage state, too if the invention has been described above only in connection with a circuit that operates in the current state. If it is it is desirable to use a circuit which operates in voltage condition, however, one must for a voltage condition circuit late (eg UC 3524, manufacturer Unitrode Corporation, USA) suitable control signal is formed from the control current Icc.

In that case, the solution will be more complicated than the one described above, and at the same time the benefits go with a control circuit operating in the current state is lost compared to a control circuit operating in voltage supply stand.

Claims (7)

10 15 20 25 30 35 511444 12 Patent claims: A method for limiting the output current (Iout) from a flyback-type switched power supply in overload situations, according to a method for limiting the output current (Iout) to mean pulse width modulation (PWM) by adjusting the ratio between the lengths of the ON and OFF phases of the switch (SW) in the primary circuit, characterized in that the voltage (Vs) which is present on the primary side of a transformer (10) and which is dependent on the output voltage (Uout) of the power supply, is used - a jack for controlling a current generator (21), the output current (Icc) of which is used to form a control signal for the control circuit (13). Method according to claim 1, characterized in that an output current (Icc), which deviates from zero and is inversely proportional to the output voltage (Uout) of the power unit, is supplied from the current generator (21), when the output voltage has dropped to a predetermined share (kl) of its nominal value (Uoutl). Method according to Claim 1, characterized in that the maximum current supplied by the current generator (21) is kept below the threshold current (Ith) which completely closes the control circuit (13). Method according to claim 3, in which the control circuit (13) operates in a current state, characterized in that the output current (Icc) from the current generator (Z1) is supplied to the current measuring input of the control circuit (13), in which in a manner known per se, a signal proportional to the primary current of the transformer (10) is also provided. A flyback-type switched power unit, which unit has a transformer (10) provided with a primary and a secondary winding (10a, 10b) and through which energy is transferred from the primary side to the secondary side, a switch (SW) in the primary circuit, which switch breaks the primary current passing through the primary winding (10a) of the transformer, and a control circuit (13) which controls the switch, which circuit controls the output voltage (Uout) from the power supply by pulse width modulation by adjusting the ratio of the lengths of the ON and OFF phases of the switch (SW), characterized in that it has means (21, 22) for forming a discrete control signal (Icc) in response to the voltage (Vs). ) which are present on the primary side of the transformer and which are dependent on the output voltage (Uout) of the power supply, which means connect said control signal to the control circuit (13). Power supply according to claim 5, whose control circuit (13) is a current state circuit, characterized in that said means has a peak value rectifier circuit (22), to which said voltage (Vs) is connected, and a current generator (21 ), to which the output of the rectifier circuit is connected, the output of said current generator being connected to the current measuring input (CS) of said control circuit (13). Power supply according to Claim 6, characterized in that the current generator (21) is connected to the positive terminal of the input voltage (Uin).