DC to DC converter employing a free-running single stage blocking oscillator

A DC to DC converter employs a single ended blocking oscillator which employs a transformer having a primary winding and a plurality of secondary windings. The transformer has a core which is of a square loop hysteresis type. The blocking oscillator is supplied operating voltage by means of a voltage regulator circuit which operates to regulate the voltage applied to the blocking oscillator according to both input voltage variations and output load variations. Due to circuit operation, the output voltage is extremely well regulated while having low ripple. An output secondary winding of the transformer is coupled to a rectifier. The rectifier output is fed back to the regulator to control the voltage applied to the blocking oscillator to cause the blocking oscillator to provide a waveform to maintain the output voltage constant.

BACKGROUND OF THE INVENTION 
This invention generally relates to power supplies and more particularly to 
a DC to DC converter employing a single stage blocking oscillator. 
DC to DC converters are well known and operate to convert an available DC 
input voltage to a DC output voltage or multiple output voltages with 
isolation between the input and the output. Such converters normally 
utilize high frequency switching techniques to enable one to achieve small 
transformer size which, in turn, provides isolation and voltage scaling. 
There are many different types of converters each having specific 
advantages and disadvantages. For example, square wave inverter stages may 
be used where the secondary voltage of a transformer is rectified and 
filtered to provide a desired DC output. With the square wave inverter the 
output voltage is not regulated and is a function of the input voltage and 
load current. However, for fixed input voltages and fixed loads, the 
output voltage may be maintained to a desired voltage tolerance. Where 
voltage output regulation is required due to input voltage and output load 
variations, regulation is conveniently achieved by pulse width modulation 
which controls the conduction period or duty cycle of an active device. As 
indicated a DC input source may be employed but most frequently a 
rectified and filtered AC input source serves as the DC source as in the 
case of off-line switch mode power supplies. Low power converters 
typically employ bipolar transistors or MOSFETs while very high power 
converters normally employ SCRs as switching elements. Each different 
approach has certain advantages. In the prior art it has been known, for 
example, to employ a free-running oscillator or a free-running circuit 
configuration to convert the DC voltage to an AC voltage which AC voltage 
is then rectified to produce a final output DC voltage. The term 
"free-running converter" implies that the operation of the converter is 
controlled by volt-second parameters of magnetic components. The switching 
frequency and the output voltage vary as a function of input voltage and 
load current. These converters are ideal for low power requirements and 
for operation from a fixed voltage source and into a fixed load. 
Essentially the prior art employed various free-running configurations to 
implement such converters including blocking oscillators and other circuit 
configurations. For examples of typical converter operation reference is 
made to a text entitled PRINCIPLES OF SOLID STATE POWER CONVERSION by 
Ralph E. Tarter (1985) published by Howard W. Sams & Company, Inc. As one 
can ascertain, while DC to DC converters are well known, there are many 
problems associated with each different approach and such problems have to 
be resolved accordingly. These problems relate to regulation of the output 
voltage, the amount of ripple, volumetric size, as well as the overall 
economics. 
It is therefore an object of the present invention to provide a compact DC 
output voltage source having a regulated output voltage including low 
ripple. 
It is a further object of the present invention to provide a regulated 
output DC voltage source having low ripple, small size, and which is 
relatively inexpensive to produce. 
SUMMARY OF THE INVENTION 
A DC to DC converter apparatus, comprising: a blocking oscillator including 
a transistor having a base, collector and emitter electrode, with the 
emitter electrode coupled to a point of reference potential, a transformer 
having a primary winding and a first and second secondary winding, with a 
first terminal of said primary winding coupled to said collector electrode 
of said transistor, and with the second terminal of said primary winding 
adapted to receive operating potential, with the base electrode of said 
transistor coupled to one terminal of said first secondary winding and 
with the other terminal of said first secondary winding adapted to receive 
operating potential, to cause said blocking oscillator to provide a high 
frequency output waveform; rectifier means coupled to said second 
secondary winding to provide an output voltage by rectifying said high 
frequency output waveform as present on said second secondary winding; 
voltage regulator means coupled to said second terminal of said primary 
winding and said other terminal of said first secondary winding and 
adapted to apply regulated voltage thereto; and means coupled between said 
rectifier means and said voltage regulator means and operative to vary the 
value of said regulated voltage according to the value of said output 
voltage and according to the temperature of operation and always in a 
direction to maintain said output voltage constant.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1 there is shown a compact regulated voltage source which 
exhibits output regulation and low ripple using a DC to DC converter 
which, in this case, is a single stage blocking oscillator. The voltage 
source of FIG. 1 is particularly useful in systems with volumetric 
limitations, where line and load regulation is required, line to load and 
load to line noise isolation is required, and where electrical noise would 
cause degradation of signal quality of the system. For example, the power 
supply described in FIG. 1 can be utilized in telephone systems which 
utilize voice or audio signals and due to the frequency of operation of 
the supply, these signals will not be interfered with. Essentially, the 
supply of FIG. 1 consists of an input regulator stage which supplies a 
regulated voltage to a single stage blocking oscillator including a 
transistor 30. The blocking oscillator generates a square waveform which 
is rectified at the secondary winding by means of a rectifier 35 to 
produce an output DC voltage. The circuit shown in FIG. receives an input 
voltage at terminals 10 and 11 also designated as "+" and "-". The input 
voltage may come from a DC source or from an AC supply which is rectified 
and converted to a suitable DC. The purpose of a DC to DC converter is to 
convert DC at terminals 10 and 11 to a different level output DC across 
output capacitor 36 which is coupled across the secondary winding 34 of 
the transformer T1 as will be explained. The positive terminal 1 is 
coupled to the collector electrode of an NPN transistor 14 arranged in a 
common emitter configuration. The emitter electrode of transistor 14, as 
will be explained, supplies a regulated voltage for operation and control 
of the blocking oscillator including transistor 30. In this manner the 
emitter electrode of transistor 14 is coupled to one terminal of resistor 
26 and to one terminal of the primary winding 32 of the transformer T1. 
Transformer T1 is associated with the blocking oscillator. The emitter 
electrode of transistor 14 is returned to the point of reference potential 
via resistor 15 in series with capacitor 16. Transistor 14 is biased in 
the "ON" condition by means of the resistor 13 which is coupled between 
terminal 10 and the base electrode of transistor 14. The base electrode of 
transistor 14 is also coupled to the collector electrode of NPN transistor 
17 having its emitter electrode coupled to the point of reference 
potential through the Zener diode 18. Capacitor 16 is in shunt with the 
Zener diode 18 and serves to aid in providing a stable potential at the 
emitter electrode. The base electrode of transistor 17 is coupled to the 
collector electrode of PNP transistor 20 having the emitter electrode 
coupled via resistor 21 to the emitter electrode of transistor 14. The 
base electrode of transistor 20 is directed to the emitter electrode of 
transistor 14 via resistor 22. The base electrode is further coupled to a 
feedback circuit consisting of resistor 23 in series with Zener diode 24. 
The anode electrode of Zener diode 24 is coupled to the anode electrode of 
rectifier 35 to provide feedback from the output rectifier circuit to the 
voltage regulator. As indicated, the emitter electrode of transistor 14 
supplies operating potential to the blocking oscillator including 
transistor 30. The emitter electrode of transistor 14 is shunted by means 
of capacitor 25 which acts as a filter capacitor. Transistor 30 has its 
collector electrode coupled to one terminal of the primary winding 32 of 
transformer T1. The other terminal of the primary winding 32 is coupled to 
the emitter electrode of transistor 14. The base electrode of transistor 
30 is coupled through secondary winding 33 of transformer T1 to the 
junction between resistors 26 and 27, forming a voltage divider between 
the emitter electrode of transistor 14 and the point of reference 
potential. Capacitor 34 by-passes resistor 27 to stabilize the voltage at 
the junction and therefore at the base electrode. The transformer T1 is of 
the type having a square loop hysteresis curve and also has an output 
secondary winding 34. The output secondary winding 34 has one terminal 
coupled to the point of reference potential with the other terminal 
coupled to the cathode of the diode 35. The anode of diode 35 is coupled 
to the anode of diode 24 which, as indicated, is coupled to the base 
electrode of transistor 20 forming part of the voltage regulator. The 
diode 35 in conjunction with capacitor 36 and the secondary winding 34 
operates to rectify the square wave produced by the blocking oscillator 3 
and to provide a DC voltage across capacitor 36. This voltage provides the 
DC output voltage for the system. As indicated in FIG. 1, the dots 
associated with the transformer windings are indicative of the direction 
of current flow as is conventional. The operation of the circuit is as 
follows. The application of voltage to the primary winding 32 of 
transformer T1 and the application of this voltage to resistor 26 will 
bias transistor 30 into conduction. As transistor 30 conducts the base 
drive to the transistor is increased by the secondary winding 33. The 
phase of the voltage at the secondary winding 34 is such that diode 35 is 
reverse biased. As the transformer approaches saturation due to the 
increased current flow through transistor 30, the change in the magnetic 
field decreases and the voltage produced by the secondary winding 33 
decreases. This decrease in drive voltage causes transistor 30 to decrease 
conduction. As the current in the transformer begins to decrease the 
magnetic field starts to collapse causing the voltage at the base 
electrode of transistor 30 to reverse, thereby driving the transistor into 
cut-off. The magnetic field in the transformer would normally collapse at 
a very high rate but diode 35 is now forward biased. In this manner some 
current will flow as capacitor 36 is charged, limiting the collapse rate. 
When the current into capacitor 36 approaches zero, the magnetic field 
ceases to change and transistor 30 will be biased on by means of resistors 
26 and 33. Once more energy is stored in the magnetic field and the cycle 
resumes. The frequency at which the circuit runs is related to the voltage 
level across capacitor 36. Each cycle charges capacitor 36 a smaller 
amount than the previous cycle. This operation is shown in FIG. 3 whereby 
the voltage across capacitor 36 is shown with respect to time, indicating 
the charging of capacitor 36. It is understood that diode 35 is employed 
as a half-wave voltage rectifier. Since the level of the output voltage 
remains reasonably square, the diode 35 virtually conducts through the 
entire half-cycle. The waveforms developed by the circuit are shown in 
FIGS. 2A-2D. FIG. 2A shows the voltage between the collector and emitter 
electrode of transistor 30. FIG. 2B shows the voltage between the base and 
emitter junction of transistor 30. FIG. 2C shows the output voltage which 
is developed across the transformer winding 34 while FIG. 2D shows the 
current through diode 35. As one can ascertain, the voltage across an 
inductance is equal to the value of the inductance times the rate of 
change of current with respect to time. By this relationship it is noted 
that if the current is large and the voltage is low, then the pulse period 
is long, hence providing a low operating frequency. If, however, the 
current is close to zero and the voltage is large, then the pulse period 
tends to be very short, thus specifying a higher operating frequency. The 
circuit shown in FIG. 1 changes its operating frequency as a function of 
the load it is required to supply power to. The energy stored in the 
magnetic field is easily determined which, in turn, specifies the size of 
the transformer T1 and the related components relative to the load energy 
requirements. In this manner the supply operates at a frequency beyond the 
passband of the system and produces virtually no ripple in its output. In 
general terms, if the power supply is developed from a sine wave source, 
the ripple voltage appears as shown in FIG. 4. As one can see, the voltage 
drops between peaks of the sine wave because the charge on the capacitor 
36 will cause diode 35 to be reverse biased. Therefore, diode 35 conducts 
for a very short period of time as shown in FIG. 5. During the interval 
between conduction of the diode, the voltage across capacitor 36 will 
start to follow a logarithmic decay causing a high ripple voltage to 
appear on the output. As the output voltage increases, diode 24 eventually 
will avalanche. This causes the base voltage of transistor 20 to move in a 
negative direction turning transistor 20 on. When transistor 20 starts to 
conduct, transistor 17 will start to conduct. The current normally flowing 
in the base to emitter junction of transistor 14 will be reduced by 
becoming the collector current for transistor 17. In this manner the 
voltage at the emitter electrode of transistor 14 is controlled to keep 
the output voltage across capacitor 36 constant. Regulation is achieved 
such that if the output voltage rises due to load shifts or input voltage 
increases, then transistor 20 conducts more as does transistor 17 which 
causes transistor 14 to conduct less, reducing the input voltage at 
transformer T1. If the output voltage decreases due to increased loading 
or a reduction in the input voltage, then transistor 20 conducts less as 
does transistor 17. Conduction in transistor 14 increases and the voltage 
at T1 is increased. Diode 18 and the large Vbe of transistors 17 and 20 
form a temperature compensation network in conjunction with diode 24. As 
the temperature increases, the Zener voltage and the voltage between the 
base and emitter of transistors 20 and 17 decreases. This effectively 
causes transistor 14 to conduct less. However, most high voltage Zener 
diodes tend to drift in a positive direction with temperature. Since less 
current is required to cause the regulator to decrease the output of the 
system, the operating point on diode 24's curve shifts towards a lower 
voltage point. This operation is shown in FIG. 6. This compensates, in 
part, for the increase in the Zener voltage of diode 24. Properly 
balanced, the net effect is a minimal change in the output voltage over 
the operating temperature range. It is indicated that if the output 
voltage were to remain steady, changes in the input voltage would cause 
the proper corrections to take place by means of transistors 17 and 20. 
That is, if the voltage increased, then transistor 20 would increase 
conduction causing transistor 17 to conduct more, decreasing the 
conduction of transistor 14 and hence keeping the system stable. The 
voltage between the base and emitter electrodes of transistors 14, 17, and 
20 decrease about 2.2 millivolts per degree Centigrade. Feedback from the 
output is obtained by means of temperature stable components and diode 18 
should have a positive temperature coefficient. Typical drift values for 
Zener diodes in a range of 6.3 to 6.8 volts is an increase of about 2.2 
millivolts per degree Centigrade. These values virtually offset each other 
and all temperature drifts. The use of diode 24 in the feedback path 
provides a higher percentage of the variation in the output when using 
only resistors. It can be shown that the use of the diode 24 in the 
feedback path improves the control over output variations without an 
excessive gain required in the regulator circuit. The ripple voltage in 
line powered power supplies can be calculated and it can be seen that the 
higher the frequency of operation, the smaller the size of the output 
capacitor and hence the smaller the power supply can be. Enormous 
reductions in capacitor size, hence reduced volume requirements and cost 
can be achieved if the frequency of operation is increased. An operating 
frequency of 25 Khz or greater represents a reasonable choice. This 
frequency range was selected based on availability of high speed 
rectifiers, transformer cores, and also to be sufficiently high above the 
audio range as to not interfere with these signals. An additional 
advantage of this type of power supply is that the switching action of the 
blocking oscillator provides filtering. In this manner only signals of a 
frequency equal to the operating frequency and within a certain phase 
relationship will be able to get through the DC to DC converter. This 
provides for electromagnetic interference isolation of the equipment from 
the line and the line from the equipment. The circuit provides a high 
frequency of operation while reducing the ripple in the output section. 
The output voltage is compensated for changes in input voltage, the load 
applied to the circuit, as well as changes in ambient temperature. 
Switching action acts as a synchronous demodulator such that only signals 
of the same frequency having proper phase angles will be passed. This 
provides a very high level of isolation between the load and the line as 
required for FCC compliance.