Switched-mode power supply

The secondary winding is connected, via the series circuit of a capacitor and a second diode, to the charging capacitor which delivers the operating voltage. The centre point of the series circuit is connected to earth via a third diode. This enables energy to be recovered, in that the energy stored in the capacitor is delivered back to the charging capacitor during the inhibiting phase of the switching transistor.

BACKGROUND 
The invention is based on a switched-mode power supply. 
In the case of a switched-mode power supply of this type, a network 
comprising a capacitor, a resistor and a diode is generally connected 
between an end, carrying the pulse voltage, of a winding of the 
transformer and a point which is dead in terms of AC voltage, for example 
earth or operating voltage. This network, also referred to as a snubber, 
is used to reduce the amplitude of interfering voltage spikes across the 
winding when the switching transistor is turned off. When the switching 
transistor is turned off, energy is stored in the capacitor of this 
network. Since this power is delivered by the transformer, the desirable 
result is a relatively slow rise in the collector voltage. When the 
switching transistor is turned on again, the energy stored in the 
capacitor is transmitted to the resistor of the network and converted 
there into heat. This power loss is considerable and can be of the order 
of magnitude of 1-5 watts. 
The invention is based on the object of modifying a network of this type 
for reducing the gradient of the voltage rise when the switching 
transistor is turned of, in such a way that the power generated in the 
network at the resistor and lost is reduced. 
In the case of the invention, the secondary winding is consequently 
connected to the charging capacitor via the series circuit of a capacitor 
and a second diode, and the centre point of the series circuit is 
connected to earth via a third diode. 
In the case of the solution according to the invention, the energy stored 
in the capacitor of the network when the switching transistor is turned 
off is consequently not converted into heat in a resistor when the 
switching transistor is turned on, but is advantageously delivered back to 
the charging capacitor of the rectifier circuit in the sense of energy 
recovery. Consequently, the network according to the invention essentially 
contains only components such as diodes and capacitors, which operate 
virtually without any losses. The resistor which was used hitherto in the 
said snubber network, for converting the energy into heat, is no longer 
required. The circuit according to the invention also permits the use of a 
larger capacitor for the said network and hence an improvement in the 
reduction of the amplitude of the pulse spikes at the transformer. 
Preferably, an inductance is also in series with the third diode. This has 
the effect that the current through the third diode does not increase 
abruptly, but increases slowly beginning at zero. A resistor is preferably 
in parallel with the inductance. This is used for suppressing interfering 
oscillations.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows a switched-mode power supply which generates from a mains 
voltage UN an operating voltage U1, for example for a television receiver 
or video recorder. The following are illustrated: the mains rectifier 1, 
the charging capacitor C3, the transformer Tr having the primary winding 
W1 and the secondary winding W2, the switching transistor T1, the diode D1 
for generating the operating voltage U1 across the charging capacitor C1, 
as well as a control generator 2 which generates the switching voltage 3 
for the switching transistor T1. The switching voltage 3 is usually 
pulse-width-modulated (PWM). The pulse width modulation is controlled by a 
control voltage in such a way that the amplitude of the operating voltage 
U1 generated is stabilized. A customary snubber network having the 
capacitor Cs, the diode Ds and the resistor Rs is connected to the primary 
winding W1. The circuit described so far is known. 
In addition to the primary snubber network described, it is also known to 
connect an identically constructed snubber network to a secondary winding 
as well. A secondary snubber network is also present in FIG. 1, on the 
winding W2. However, this network has a different design from the snubber 
network Cs, Rs and Ds on the primary winding W1. The secondary winding W2 
is connected to earth via the series circuit of the capacitor C2, the 
inductance L1 and the diode D3. The centre point a of the series circuit 
is connected, via the diode D2, to the terminal 6 carrying the operating 
voltage U1. A resistor R3 is in parallel with the inductance L1. 
The secondary network according to FIG. 1 operates as follows: during the 
conducting phase of the switching transistor T1, the capacitor C2 is 
charged up to the negative voltage, via the diode D3 and the inductance 
L1, while the winding W2 is being driven by the transformer. The 
inductance L1 limits the current rise, starting from 0 A. During this 
time, energy is therefore stored in the capacitor C2. 
When the switching transistor T1 is turned off, the voltage Us across the 
secondary winding W2 rises rapidly, until finally the voltage across the 
anode of the diode D2 reaches the value of the voltage of U1 across the 
charging capacitor C1. At this point in time on, the energy stored in the 
capacitor C2 is transmitted to the charging capacitor C1 via the diode D2. 
The reduction, aimed for by the network, in the rise of the voltage Us, 
that is to say dUs/dt, is achieved by this operation. Since the network 
contains no resistor comparable with the resistor Rs, in contrast to the 
primary network, oscillations are produced during this operation. These 
oscillations are attenuated by the resistor R3. 
In FIG. 2, the period t1-t2 designates the forward sweep. During this time, 
the switching transistor T1 is controlled to conduct by the switching 
voltage 3. The collector current ic rises approximately linearly, while 
the collector voltage Uc is virtually zero. The voltage Us across the 
secondary winding W2 then has its negative value. The so-called flyback 
begins at the instant t2. The switching transistor T1 is then inhibited, 
with the result that the collector current ic falls rapidly. This results 
in a steep rise of the collector voltage Uc with a voltage spike 5, the 
amplitude of which is limited by the snubber networks described. The 
voltage Us has a similar characteristic to Uc, but has no DC voltage 
component. FIG. 2c shows the current i2 through the capacitor C2. The 
capacitor C2 is charged by this current from t1-t3. The current i2 flows 
via the diode D2 from t2-t4, with te result that energy stored in the 
capacitor C2 is transmitted to the charging capacitor C1. FIG. 2d shows 
the current i4 through the diode D2, which current corresponds to the 
current i2 during this time.