Resonant switching power supply

A resonant switching power supply has a zero voltage and zero current switch feature which can be operated in a half-bridge or a full-bridge scheme. This enables power consumption to be reduced and electromagnetic radiation to be minimized, and provides for low cost and convenient manufacture, in mass production. The power supply is not influenced by parasitic capacitance and leak inductance. The resonant switching power supply is disclosed in several different embodiments.

FIELD OF THE INVENTION 
The present invention relates to a resonant switching power supply, more 
particularly, to a resonant switching power supply with zero voltage and 
zero current switch feature, whereby the power consumption is reduced and 
electromagnetic radiation is minimized. 
BACKGROUND OF THE INVENTION 
The shrinking size of electronic equipment demands increasing power density 
of the supplying system. The switching power supply based on the principle 
of pulse width modulation is better suited for efficient power control and 
has become more prevailing. FIGS. 1 and 2 show two types of conventional 
resonant switching power supplies. Those resonant switching power supplies 
use resonant circuits composed of inductor and capacitor to generate 
sinusoid wave, and the zero cross points of the sinusoid wave to provide 
zero-voltage switching (ZVS) or zero-current switching (ZCS). This 
technique can be roughly classified as serial load resonant (SLR) as shown 
in FIG. 1 and parallel load resonant (PLR) as shown in FIG. 2. In the 
above two types of circuit, the frequency of the input voltage is designed 
to be the same as the resonant frequency as the resonant circuit to 
provide the most efficient output. However, the optimal operation relies 
on the assumption of constant load, this is rare in practical situations. 
Another conventional resonant switching power supply, is a full bridge ZVS 
PWM converter. The full bridge ZVS PWM converter uses four sets of 
switches. Therefore, the circuit is complicated and the parasitic 
capacitance and leak inductance is hard to manipulate, thus being 
difficult to mass produce. Moreover, this full bridge ZVS PWM converter is 
not economic in light load application, especially hard to ensure 
zero-voltage switching (ZVS) or zero-current switching (ZCS) in a light 
load application, 
Therefore, the present invention is intended to provide a resonant 
switching power supply with zero voltage and zero current switch feature, 
which can be operated in a half-bridge or a full-bridge scheme. 
It is an object of the present invention to provide a resonant switching 
power supply with zero voltage and zero current switch features, whereby 
the power consumption is reduced and electromagnetic radiation is 
minimized. 
It is another object of the present invention to provide a resonant 
switching power supply which has the advantages of low-cost and convenient 
manufacture and can be mass-produced to meet the requirement of various 
loads. 
It is still another object of the present invention to provide a resonant 
switching power supply which will not be influenced by the parasitic 
capacitance and leak inductance.

DETAILED DESCRIPTION OF THE PRESENT INVENTION 
FIG. 3ashows the circuit diagram of a resonant switching power supply 
according to a first preferred embodiment of the present invention. It 
should be noted that the circuit shown in FIG. 3ais a half-bridge 
switching power supply; however, the principle of the present invention 
can also be applied to the full-bridge switching power supply as shown in 
FIG. 3b. As shown in this figure, the symbols SW1 and SW2 denote switching 
elements and can be implemented by FET, transistor or IGBT. The symbols C1 
and C2 denote the parasitic capacitance of the switching elements SW1 and 
SW2, respectively. D1 and D2 are by-pass diodes, C3: resonant capacitor, 
C4 and C5: by-pass capacitors for power supply, L1: resonant inductor, T1: 
transformer, D3 and D4: output rectifying diode, L2: flywheel inductor for 
storing and removing energy, C6: output filtering capacitor and parallel 
with load to smooth the output voltage. Moreover, the resonant switching 
power supply according to the first preferred embodiment of the present 
invention further comprises a feedback control circuit F, which can detect 
the load condition and generate a control pulse to control the operation 
of the first switch SW1 and the second switch SW2. 
With reference to FIGS. 5 to 7, the operation principle of the resonant 
switching power supply according to the first preferred embodiment of the 
present invention is described in detail. FIG. 5a shows a simplified 
circuit of the circuit shown in FIG. 3a. FIG. 5b shows the waveforms of 
switch driving voltage and resonant current in the circuit of FIG. 3a. 
FIG. 6 shows the voltage or current waveforms of several elements or nodes 
in the circuit of FIG. 3a. FIG. 7 also shows the voltage or current 
waveforms of several elements or nodes in the circuit of FIG. 3a. 
The first switch SW1 and the second switch SW2 have timing diagram as shown 
in curve 1 and 2 in FIG. 6. The theoretical value of voltage between the 
nodes A and C in FIG. 3a is shown in curve 3 of FIG. 6. The resonant 
current is shown in curve 4 of FIG. 6. On the assumption of constant load, 
the first switch SW1 and the second switch SW2 are square waves with duty 
ratio smaller than 50%. It will be explained below that the choice of duty 
ratio, in conjunction with the particular choice of switch driving 
frequency, will induce the effect of zero voltage and zero current switch. 
More particularly, in the resonant switching power supply shown in FIG. 
3a, the frequencies of the driving pulses for the first switch SW1 and the 
second switch SW2 are selected to slightly deviate from the resonant 
frequency of the resonant frequency .omega..sub.0 decided by the resonant 
inductor L1 and the resonant capacitor C3, and the driving pulses are 
continuously generated as shown in the curves 1 and 2. As shown in the 
curves 4 and 5 of FIG. 6, the voltage between nodes A and C (curve 5) 
slightly leads the resonant current (curve 4). In other words, the 
resonant switching power supply according to the present invention uses a 
switching signal with duty ratio smaller than 50% and particular selected 
frequency of the switching signal to induce advantageous interaction 
between the remaining current in the resonant circuit and the parasitic 
capacitance C1 and C2 of the switching elements SW1 and SW2, thus 
achieving zero voltage and zero current switch. This will be explained in 
more detail below. 
With reference now to FIGS. 5a and 5b, and the reference arrow denoting the 
current flowing direction, FIG. 5a is the simplified circuit of that in 
FIG. 3a and FIG. 5b shows the voltage between nodes A and C (square wave) 
induced by the switching signal and the resonant current (sinusoid wave). 
When time approaches time t1, the resonant current I.sub.L1 is positive, 
switch SW1 is shorted circuit and switch SW2 is open circuit. The voltage 
at node A is Vcc, the voltage across the capacitor C1 is zero and the 
voltage across the capacitor C2 is 2 Vcc. The diodes D1 and D2 are reverse 
bias and conduct no current. The current I.sub.L1 decreases gradually. As 
time reached t1, the current I.sub.L1 is near zero and the SW1 becomes 
open circuit. Because the current in the inductor L1 should be continuous, 
a certain amount of positive current still flows into the resonant 
circuit. At this time, the switch SW1 is nearly zero current switching. 
Moreover, the voltage of the switch SW1 is established by charging the 
parasitic capacitance C1 thereof with the infinitesimal resonant current, 
thus achieving zero voltage switch. As time goes by, the capacitor C1 is 
charged and the capacitor C2 is discharged. The voltage at node A changes 
from +Vcc to -Vcc rapidly and the voltage of SW2 is dropped from 2 Vcc to 
0. At this time, if the switch SW2 is not turned on, an infinitesimal 
amount of current Id2 flows into the resonant inductor L1, wherein the 
diode D2 is used to protect the switch SW2. As time elapses, the resonant 
current becomes zero and the switch SW2 is turned on, and the resonant 
current I.sub.L1 reversely flows into the switch SW2. It should be noted 
that the SW2 is turned on in a zero voltage and zero current switch state. 
This feature is symptomed by the fact that the voltage between nodes A and 
C (curve 5) slightly leads the resonant current (curve 6). This is also 
verified in the experiment conducted by the inventor. 
When next switching time (t3, t4) is reached, the operation principle of 
the resonant switching power supply is similar to previously described. 
The switch SW1 replaces the role of switch SW2, C1 replaces C2, D1 
replaces D2, and vice versa. Even though the frequency of the switch 
driving signal may have slight drift, the resonant switching power supply 
still has high efficiency and the diodes D1 and D2 have infinitesimal 
current flowing therethrough if the phase of the switch driving signal 
leads the phase of the resonant current. Moreover, a phase control circuit 
(not shown) can be incorporated into the resonant switching power supply 
to stabilize the frequency of the switching driving signal. Therefore, the 
switching time will not have error. 
Moreover, the resonant switching power supply in the present invention is 
not sensitive to the value of parasitic capacitance and leak inductance, 
which hinders the conventional full bridge ZVS PWM converter. The 
conventional full bridge ZVS PWM converters, while switch at zero voltage, 
have considerable switching current. Therefore, the parasitic capacitance 
has rapid charging time and the discharging time of the leak inductance 
should be well controlled, or the performance of the conventional full 
bridge ZVS PWM converter is degraded. The resonant switching power supply 
in the present invention is operated in a zero voltage and zero current 
switch manner. The influence of parasitic capacitance and leak inductance 
is minimized. 
Moreover, to overcome the problem of output voltage fluctuation caused by 
the load variation, a feedback control circuit F is incorporated into the 
resonant switching power supply in the present invention. The feedback 
control circuit F detects the load condition and generates a lower 
frequency control pulse to control the operation of the switch driving 
signals for the first switch SW1 and the second switch SW2. More 
particularly, the switch driving signals for the first switch SW1 and the 
second switch SW2 generally have relatively high frequency such as 100 
KHz. The feedback control circuit F generates a control pulse with lower 
frequency, e.g. 1 KHz (as shown in the curve 6 in FIG. 7) to modulate the 
switch driving signals for SW1 and SW2 in response to the load condition. 
Therefore, the switch driving signals for the first switch SW1 and the 
second switch SW2 become intermittent rather than continuous. More 
particularly, as shown in FIG. 7, the switch driving signals for the first 
switch SW1 and the second switch SW2 are enabled when the control pulse 
(curve 6) is high, and disabled when the control pulse is low. Moreover, 
if the control signal is not carefully applied, the control signal will 
truncate the switch driving signals as indicated by the time period t5-t6. 
As a result, noise will be generated (as shown in the curve 10 of FIG. 7) 
and the efficiency of the inventive resonant switching power supply is 
degraded. Therefore, the control pulse according to the present invention 
should cover switch driving signals of integer number and does truncates 
the switch driving signals as indicated by the time period t5'-t6'. The 
resonant switching power supply generates noise-free resonant current as 
indicated by curve 9 of FIG. 7. The effeteness of the feedback control 
circuit is also experimentally validated. 
Moreover, the inventive resonant switching power supply is designed to have 
special output circuit. The conventional output circuit shown in FIG. 8a 
(for full-wave rectification circuit, the output circuit for half-wave 
rectification is shown in FIG. 8b) has two problems. Firstly, the diodes 
D3 and D4 conduct instantaneously when the output voltage of the 
transformer T1 exceeds the voltage Vc of the capacitor C6. The transformer 
T1 "see" a low output impedance instantaneously and large amount of 
current flows through the diodes D3 and D4. As a result, the waveform of 
the resonant current and resonant voltage is distorted and unwanted 
high-frequency noise is generated as shown in the curve 10 of the FIG. 7. 
Secondly, the diodes D3 and D4 are open circuit instantaneously when the 
output voltage of the transformer T1 is below the voltage Vc of the 
capacitor C6. . The transformer T1 "see" a high output impedance 
instantaneously, and the resonant circuit has a high Q value. In other 
word, the remaining current will keep oscillating within the high-Q 
resonant circuit such that energy is dissipated in the resonant circuit 
and can not output, as shown in the curve 9 of FIG. 7. As a result, the 
efficiency of the switching power supply is degraded, thermal energy is 
generated and the remaining oscillation may encounter the next switch 
driving pulse to generate noise. In this invention, a flywheel inductor L2 
for storing and removing energy is incorporated in the output circuit as 
shown in FIG. 10a (for full-wave rectification circuit, the output circuit 
for half-wave rectification is shown in FIG. 10b). This is different to 
the inductor used in the forward switching power supply as shown in FIG. 
9a (for full-wave rectification circuit, the output circuit for 
half-bridge is shown in FIG. 9b), wherein the inductor is used to isolate 
the output end of the transformer with the capacitor and a flywheel diode 
is required to remove the flywheel current of the inductor forward 
current. On the contrary, the flywheel inductor L2 shown in FIG. 10a does 
not require a flywheel diode to conduct the remaining current in the 
resonant circuit. The forward flywheel current will conduct the remaining 
current of the resonant circuit to the capacitor such that the remaining 
current in the resonant circuit is rapidly diminished as shown in the 
curve 11 of FIG. 7. 
FIG. 4a shows the circuit diagram of a resonant switching power supply 
according to second preferred embodiment of the present invention. It 
should be noted that the circuit shown in FIG. 4a is a half-bridge 
switching power supply, however, the principle of the present invention 
can also be applied to the full-bridge switching power supply as shown in 
FIG. 4b. The resonant switching power supply shown in FIG. 4a is similar 
to that shown in FIG. 3a except that the transformer T1 replaces the 
function of the resonant inductor L1, i.e., the resonant inductor L1 is 
eliminated to save cost. The switching power supply shown in FIG. 4a still 
has high efficiency if the frequency of the switch driving signal is 
selected to enable the phase of the switch driving signal lead the phase 
of the resonant current. Moreover, switching signal has duty ratio smaller 
than 50% to induce advantageous interaction between the remaining current 
in the resonant circuit and the parasitic capacitance C1 and C2 of the 
switching elements SW1 and SW2, thus achieving zero voltage and zero 
current switch. 
From above description, the present invention provides a resonant switching 
power supply with high efficiency and low electromagnetic radiation. The 
inventive resonant switching power supply is insensitive to the parasitic 
capacitance and leak inductance, thus eliminating complicated calibration 
process. The feedback control circuit can be easily implemented by simple 
digital circuit to reduce noise due to fluctuate load. The zero voltage 
and zero current switch feature can be realized by engineer the operation 
frequency of the switch driving signal to deviate form the resonant 
frequency such that the phase of the switch driving signal leads the 
resonant current. Moreover, the inventive resonant switching power supply 
has simple circuit and can be easily adapted to use in half-bridge circuit 
or full-bridge circuit. 
Although the present invention has been described with reference to the 
preferred embodiment thereof, it will be understood that the invention is 
not limited to the details thereof. Various substitutions and 
modifications have suggested in the foregoing description, and other will 
occur to those of ordinary skill in the art. Therefore, all such 
substitutions and modifications are intended to be embraced within the 
scope of the invention as defined in the appended claims.