Wholly integrated switch-on control loop of a high voltage power transistor of a quasi resonant flyback converter

A flyback DC--DC converter employs a flyback transformer for storing and transferring energy to a load having an auxiliary winding whose voltage is compared by a comparator with a threshold to detect its crossing. As a consequence, a power transistor driving the primary winding of the transformer is switched on through a control flip-flop, for a new phase of conduction and accumulation of energy, whose duration is established by a secondary control loop of the output voltage producing the switching off of the power transistor for a successive energy transfer phase toward the load of the energy stored in the transformer during the preceding conduction phase. The converter has a wholly integrated control circuit that includes a second comparator of the voltage existing on the current terminal of the power transistor connected to the primary winding of the transformer with respect to the ground potential of the circuit. Furthermore, a delay network is coupled in cascade to the output of a first comparator and has an output coupled to a second input of a logic gate, so that under steady state functioning conditions of the converter, the setting of the flip-flop is done by the second comparator rather than by the first comparator.

FIELD OF THE INVENTION 
The present invention relates to flyback DC--DC converters, and, more 
particularly, to flyback DC--DC converters which operate in a quasi 
resonant or "zero voltage" switch-on mode that switches on when the 
voltage of the conductive terminal of the switching element is in the 
vicinity of zero volts. 
BACKGROUND OF THE INVENTION 
The quasi resonant functioning mode of flyback DC--DC converters at steady 
state conditions is particularly efficient because, compared to 
traditional flyback applications (hard switching mode), it allows for a 
reduction of power dissipation during the switching phases and a reduction 
of electromagnetic noise. 
FIG. 1 shows the basic scheme of a flyback converter for quasi resonant 
applications. The switching element Q1 is indicated as being a bipolar 
junction transistor though it may be of a different type. The D1 and C1 
components allow for a quasi resonant functioning, also called QRC mode. 
In traditional applications, such as in hard-switching applications, their 
function is performed by dedicated snubber or clamper circuits. 
The type of control of the switchings of the (Q1) power switch is similar 
to that of selfoscillating circuits, commonly named SOPS (Self Oscillating 
Power Supply), because the switch-on is commanded always in the vicinity 
of the instant at which the current on the secondary winding of the 
flyback transformer becomes null. Hence, the converter always functions in 
a discontinuous manner, that is, with the current becoming null at every 
cycle, though remaining at the border between continuous and discontinuous 
functioning conditions. 
During the ON phase of Q1, the D2 diode is OFF and there is an accumulation 
of energy in the primary winding of the transformer, which is transferred 
to the secondary during the OFF phase of Q1. In this phase the voltage Vc 
on the Q1 terminals is 
EQU Vc=Valim+(N1:N2) V2 (being V2 Vout) 
When the energy is completely transferred (I.sub.F(D2) =0), the voltage Vc 
oscillates at the resonant frequency given by 
##EQU1## 
By suitably sizing the electrical parameters it is possible to produce an 
oscillation capable of allowing the diode D1 to conduct for a short period 
of time in order to realize a control transistor of the Q1 power during 
this phase, thus eliminating of the switch-on losses. 
Therefore, the flyback converter belongs to the class of the so-called 
"zero-voltage quasi resonant" converters. These converters are frequently 
used in TV and VCR power supplies, wherein the input voltage Valim is 
obtained by rectifying and filtering the main voltage. Such a preference 
is due also to the fact that the architecture of these converters allows 
for multiple outputs by simply increasing the number of secondary windings 
of the flyback transformer. The auxiliary winding AUS is used to self 
power the control circuit during steady state functioning. 
For such applications, during the switch-off phase, the power transistor Q1 
that implements the switch must withstand voltages that may reach or even 
exceed a thousand volts. In case of a completely monolithic realization 
(control circuits and power device realized on the same chip), a 
fabrication technology usually referred to as "Smart Power", suitable for 
high voltage applications, must be used. 
Traditional QRC flyback converters are realized with discrete components, 
or in the form of an integrated device containing a low voltage control 
circuit, a high voltage power MOS transistor and eventually some of the 
passive components, later realized in so-called SMD technology. As 
depicted by way of an example in FIG. 3, the switch-on under a quasi 
resonant condition is obtained through an external Tdelay network 
connected to the auxiliary winding Aus and dimensioned so as to 
synchronize the condition Vc=0 with the switching of a comparator of the 
control circuit contained in the CONTROL-IC block, within the time 
interval indicated as Tdelay in the diagrams of FIG. 2. 
Thus, the integrated circuit of the block CONTROL IC controls a 
selfoscillating or SOPS functioning mode of the converter. Replacing the 
delay block TDELAY with a resistor would produce a classic hard-switching 
flyback application. 
In traditional circuits, the QRC function is thus obtained through external 
networks, which especially in TV applications, where there are large 
variations of either the supply voltage and the load, necessitate a 
substantial number of components, as illustrated in the detail of the 
block Tdelay of FIG. 3. However, other circuit arrangements of the delay 
network may be used, depending on the specific characteristics of the 
application. 
Therefore, known circuits have the drawback of requiring the realization of 
a Tdelay network with discrete elements external to the integrated 
circuit. Moreover, the switching under zero voltage conditions is tied to 
the precision of the Tdelay network (that is, to the spread of the actual 
values of the network components) as well as to the electrical parameters 
that establish the resonance frequency (the spread of the values of Lp and 
Cr). 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a QRC quasi resonant 
selfoscillating flyback DC--DC converter with the synchronization of the 
switch-on of the power transistor, when the voltage thereon becomes null, 
realized with a wholly integrated circuit. 
According to the present invention, this objective is obtained by 
monitoring, rather than the voltage existing on one auxiliary winding of 
the flyback transformer, as normally done in known circuits, the voltage 
existing on the current terminal of the power transistor connected to the 
primary winding of the flyback transformer. Furthermore, this is done by 
controlling the "set" terminal of a driving flip-flop of the power 
transistor through a OR logic gate, to an input of which is coupled the 
output of the comparator of the voltage present on the current node of the 
power transistor, while the other input of which is coupled, through an 
integrated Tdelay network, to the output of the comparator of the voltage 
present on the auxiliary winding in respect to a reference voltage 
generated by a control circuit of the converter. 
The comparator of the voltage on the current terminal of the power 
transistor in respect to the ground potential of the circuit is integrated 
with the same high voltage fabrication technology that is used to realize 
high voltage power transistors. Specifically, the components of the input 
stage of the comparator, which are subject to voltages that may reach and 
exceed thousands of volts are integrated with such high voltage 
fabrication technology.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 4 shows the basic scheme of an embodiment of a quasi resonant 
selfoscillating flyback converter in accordance with the present 
invention. All the components included within the rectangular perimeter 
are integrated on a single chip. 
The integrated circuit is operated at steady state through the V.sub.DD pin 
by charging a supply capacitor C of high capacitance through a diode D, 
whose anode, according to a common practice, is coupled to the voltage 
induced on the Aus auxiliary winding of the transformer. Although not 
expressly shown in FIG. 4, the converter includes internal or external 
means to ensure the charging of the capacitor C during the power-on phase. 
The secondary regulating loop of the output voltage commonly uses, an 
error amplifier ERROR AMP whose output is coupled, by way of the 
photo-diode and the photo-transistor, to the relative pin COMP of the 
integrated circuit to which is connected a compensation capacitor CCOMP of 
the secondary regulating loop of the output voltage. 
A primary regulating loop, whose function is explained in the following 
description, is realized through the comparator COMP1 which compares the 
voltage of the auxiliary winding Aus with a pre-established threshold 
Vref1, fixed by the control circuitry, represented as a whole by the 
CONTROL block of the diagram of FIG. 4. The secondary control loop of the 
output voltage switches off the switch POWER by driving the reset of the 
command flip-flop FF according to a traditional control scheme. 
According to a fundamental aspect of the invention, the synchronization of 
the switch-on of the POWER switch with the zero crossing instant of the 
HVC voltage on the current terminal of the power transistor POWER 
connected to the primary winding of the flyback transformer is effected by 
controlling the set command of the flip-flop FF through an OR logic gate. 
A first input of the OR gate is coupled to the output of a high voltage 
comparator HVCOMP of the voltage present on the current terminal of the 
POWER in respect to the ground potential of the circuit. Such a voltage 
coincides with the potential of the substrate of the integrated circuit. 
The output of the first comparator Comp1 is coupled to the second input of 
the OR logic gate, through a delay network On Delay functionally connected 
in cascade to the output of the COMP1 comparator. 
From FIG. 4 it may be easily verified that for a substrate voltage 
Vsub=V.sub.HVC .ltoreq.0V, the output voltage of the high voltage 
comparator V.sub.OUT HVCOMP switches from a low logic state to a high 
logic state, provoking the setting of the flip-flop FF and thereby the 
switching on of the power transistor POWER. This realizes the 
synchronization of the switching on during the quasi resonant steady state 
functioning of the converter, without external components. 
In any case, the quasi resonant condition only exists during steady state 
operation but not during the start-up and recovery phases. During these 
phases, the output voltage is reduced and may be zero at the power on 
instant. Therefore, the voltage induced on the auxiliary winding is 
insufficient to allow for oscillations of sufficient amplitude. Under such 
conditions, the control circuit must guarantee the switching on of the 
switch POWER and this function is performed by the comparator COMP1 
through the ON DELAY block and the OR logic gate. 
The On DELAY block is designed to ensure that during the steady state 
selfoscillating function, the set of the command flip-flop FF is caused by 
the switching of the high voltage comparator HVCOMP rather than by the 
switching of the comparator COMP1. This ensures a POWER switching on of 
the power transistor POWER in a quasi resonant condition. This delay may 
also be fixed when designing the circuit, similarly to what was done for 
the delay block Tdelay of the traditional scheme of FIG. 3, in order to 
switch on the power switch at zero voltage conditions on its terminals. 
However, it is evident that without a second high voltage comparator 
HVCOMP a perfect synchronization of the switching cannot be guaranteed, 
and the circuit would be subject to the effects of the deviations from the 
nominal design values of the different components due to the fabricating 
process. 
Thus, the high voltage comparator HVCOMP ensures the correct functioning of 
the zero voltage quasi resonant converter of the invention, regardless of 
process spreads of the Lp and Cr values which fix the converter's 
resonance frequency. 
Main waveforms from a computer simulation of the circuit of FIG. 4, with a 
resistive load coupled to the circuit output OUT, are shown in FIGS. 5 and 
6. Upon observing the current and voltage waveforms on the power 
transistors POWER shown in FIG. 5, relative to a start-up phase and to a 
successive steady state condition, initially, the hard-switching condition 
(current peaks at the switch-on due to the discharge of the capacitor Cr) 
and the successive passage to quasi resonant switchings (elimination of 
the peaks and thereby of the switching losses) appear evident. 
FIG. 6 shows the waveforms of the preceding FIG. 5 in greater detail, 
together with the control voltage of the power transistor switchings and 
of the output voltages of the high voltage comparator HVCOMP and of the 
block ON DELAY, respectively, in correspondence of the passage from the 
hard-switching mode and the QRC mode.